Merge branch 'master' into indexed-store-path-outputs

.github/CODEOWNERS

@@ -0,0 +1,15 @@
+# Pull requests concerning the listed files will automatically invite the respective maintainers as reviewers.
+# This file is not used for denoting any kind of ownership, but is merely a tool for handling notifications.
+#
+# Merge permissions are required for maintaining an entry in this file.
+# For documentation on this mechanism, see https://help.github.com/articles/about-codeowners/
+
+# Default reviewers if nothing else matches
+* @edolstra @thufschmitt
+
+# This file
+.github/CODEOWNERS @edolstra
+
+# Public documentation
+/doc @fricklerhandwerk
+*.md @fricklerhandwerk

.github/ISSUE_TEMPLATE/bug_report.md

@@ -30,3 +30,7 @@ A clear and concise description of what you expected to happen.
 **Additional context**
 
 Add any other context about the problem here.
+
+**Priorities**
+
+Add :+1: to [issues you find important](https://github.com/NixOS/nix/issues?q=is%3Aissue+is%3Aopen+sort%3Areactions-%2B1-desc).

.github/ISSUE_TEMPLATE/feature_request.md

@@ -18,3 +18,7 @@ A clear and concise description of any alternative solutions or features you've
 
 **Additional context**
 Add any other context or screenshots about the feature request here.
+
+**Priorities**
+
+Add :+1: to [issues you find important](https://github.com/NixOS/nix/issues?q=is%3Aissue+is%3Aopen+sort%3Areactions-%2B1-desc).

.github/ISSUE_TEMPLATE/installer.md

@@ -0,0 +1,36 @@
+---
+name: Installer issue
+about: Report problems with installation
+title: ''
+labels: installer
+assignees: ''
+
+---
+
+## Platform
+
+<!-- select the platform on which you tried to install Nix -->
+
+- [ ] Linux: <!-- state your distribution, e.g. Arch Linux, Ubuntu, ... -->
+- [ ] macOS
+- [ ] WSL
+
+## Additional information
+
+<!-- state special circumstances on your system or additional steps you have taken prior to installation -->
+
+## Output
+
+<details><summary>Output</summary>
+
+```log
+
+<!-- paste console output here and remove this comment -->
+
+```
+
+</details>
+
+## Priorities
+
+Add :+1: to [issues you find important](https://github.com/NixOS/nix/issues?q=is%3Aissue+is%3Aopen+sort%3Areactions-%2B1-desc).

.github/ISSUE_TEMPLATE/missing_documentation.md

@@ -26,3 +26,6 @@ assignees: ''
 
 <!-- propose a solution -->
 
+## Priorities
+
+Add :+1: to [issues you find important](https://github.com/NixOS/nix/issues?q=is%3Aissue+is%3Aopen+sort%3Areactions-%2B1-desc).

.github/PULL_REQUEST_TEMPLATE/pull_request_template.md

@@ -5,3 +5,7 @@ Please include relevant [release notes](https://github.com/NixOS/nix/blob/master
 **Testing**
 
 If this issue is a regression or something that should block release, please consider including a test either in the [testsuite](https://github.com/NixOS/nix/tree/master/tests) or as a [hydraJob]( https://github.com/NixOS/nix/blob/master/flake.nix#L396) so that it can be part of the [automatic checks](https://hydra.nixos.org/jobset/nix/master).
+
+**Priorities**
+
+Add :+1: to [pull requests you find important](https://github.com/NixOS/nix/pulls?q=is%3Aopen+sort%3Areactions-%2B1-desc).

.github/workflows/backport.yml

@@ -21,7 +21,7 @@ jobs:
           fetch-depth: 0
       - name: Create backport PRs
         # should be kept in sync with `version`
-        uses: zeebe-io/backport-action@v0.0.8
+        uses: zeebe-io/backport-action@v0.0.9
         with:
           # Config README: https://github.com/zeebe-io/backport-action#backport-action
           github_token: ${{ secrets.GITHUB_TOKEN }}

.version

@@ -1 +1 @@
-2.12.0
+2.13.0

doc/manual/generate-builtins.nix

@@ -1,16 +1,20 @@
-with builtins;
+builtinsDump:
-with import ./utils.nix;
+let
+  showBuiltin = name:
+    let
+      inherit (builtinsDump.${name}) doc args;
+    in
+    ''
+      <dt id="builtins-${name}">
+        <a href="#builtins-${name}"><code>${name} ${listArgs args}</code></a>
+      </dt>
+      <dd>
 
-builtins:
+        ${doc}
+
+      </dd>
+    '';
+  listArgs = args: builtins.concatStringsSep " " (map (s: "<var>${s}</var>") args);
+in
+with builtins; concatStringsSep "\n" (map showBuiltin (attrNames builtinsDump))
 
-concatStrings (map
-  (name:
-    let builtin = builtins.${name}; in
-    "<dt id=\"builtins-${name}\"><a href=\"#builtins-${name}\"><code>${name} "
-    + concatStringsSep " " (map (s: "<var>${s}</var>") builtin.args)
-    + "</code></a></dt>"
-    + "<dd>\n\n"
-    + builtin.doc
-    + "\n\n</dd>"
-  )
-  (attrNames builtins))

doc/manual/generate-manpage.nix

@@ -99,6 +99,7 @@ let
     in [ cmd ] ++ concatMap subcommand (attrNames details.commands or {});
 
   parsedToplevel = builtins.fromJSON toplevel;
+  
   manpages = processCommand {
     command = "nix";
     details = parsedToplevel;

doc/manual/generate-options.nix

@@ -1,29 +1,41 @@
-with builtins;
+let
-with import ./utils.nix;
+  inherit (builtins) attrNames concatStringsSep isAttrs isBool;
+  inherit (import ./utils.nix) concatStrings squash splitLines;
+in
 
-options:
+optionsInfo:
+let
+  showOption = name:
+    let
+      inherit (optionsInfo.${name}) description documentDefault defaultValue aliases;
+      result = squash ''
+          - <span id="conf-${name}">[`${name}`](#conf-${name})</span>
 
-concatStrings (map
+          ${indent "  " body}
-  (name:
+        '';
-    let option = options.${name}; in
+      # separate body to cleanly handle indentation
-    "  - [`${name}`](#conf-${name})"
+      body = ''
-    + "<p id=\"conf-${name}\"></p>\n\n"
+          ${description}
-    + concatStrings (map (s: "    ${s}\n") (splitLines option.description)) + "\n\n"
+
-    + (if option.documentDefault
+          **Default:** ${showDefault documentDefault defaultValue}
-       then "    **Default:** " + (
+
-       if option.defaultValue == "" || option.defaultValue == []
+          ${showAliases aliases}
-       then "*empty*"
+        '';
-       else if isBool option.defaultValue
+      showDefault = documentDefault: defaultValue:
-       then (if option.defaultValue then "`true`" else "`false`")
+        if documentDefault then
-       else
+          # a StringMap value type is specified as a string, but
-         # n.b. a StringMap value type is specified as a string, but
+          # this shows the value type. The empty stringmap is `null` in
-         # this shows the value type.  The empty stringmap is "null" in
+          # JSON, but that converts to `{ }` here.
-         # JSON, but that converts to "{ }" here.
+          if defaultValue == "" || defaultValue == [] || isAttrs defaultValue
-         (if isAttrs option.defaultValue then "`\"\"`"
+            then "*empty*"
-          else "`" + toString option.defaultValue + "`")) + "\n\n"
+            else if isBool defaultValue then
-       else "    **Default:** *machine-specific*\n")
+              if defaultValue then "`true`" else "`false`"
-    + (if option.aliases != []
+            else "`${toString defaultValue}`"
-       then "    **Deprecated alias:** " + (concatStringsSep ", " (map (s: "`${s}`") option.aliases)) + "\n\n"
+        else "*machine-specific*";
-       else "")
+      showAliases = aliases:
-    )
+          if aliases == [] then "" else
-  (attrNames options))
+            "**Deprecated alias:** ${(concatStringsSep ", " (map (s: "`${s}`") aliases))}";
+      indent = prefix: s:
+        concatStringsSep "\n" (map (x: if x == "" then x else "${prefix}${x}") (splitLines s));
+      in result;
+in concatStrings (map showOption (attrNames optionsInfo))

doc/manual/local.mk

@@ -29,19 +29,19 @@ nix-eval = $(dummy-env) $(bindir)/nix eval --experimental-features nix-command -
 $(d)/%.1: $(d)/src/command-ref/%.md
 	@printf "Title: %s\n\n" "$$(basename $@ .1)" > $^.tmp
 	@cat $^ >> $^.tmp
-	$(trace-gen) lowdown -sT man -M section=1 $^.tmp -o $@
+	$(trace-gen) lowdown -sT man --nroff-nolinks -M section=1 $^.tmp -o $@
 	@rm $^.tmp
 
 $(d)/%.8: $(d)/src/command-ref/%.md
 	@printf "Title: %s\n\n" "$$(basename $@ .8)" > $^.tmp
 	@cat $^ >> $^.tmp
-	$(trace-gen) lowdown -sT man -M section=8 $^.tmp -o $@
+	$(trace-gen) lowdown -sT man --nroff-nolinks -M section=8 $^.tmp -o $@
 	@rm $^.tmp
 
 $(d)/nix.conf.5: $(d)/src/command-ref/conf-file.md
 	@printf "Title: %s\n\n" "$$(basename $@ .5)" > $^.tmp
 	@cat $^ >> $^.tmp
-	$(trace-gen) lowdown -sT man -M section=5 $^.tmp -o $@
+	$(trace-gen) lowdown -sT man --nroff-nolinks -M section=5 $^.tmp -o $@
 	@rm $^.tmp
 
 $(d)/src/SUMMARY.md: $(d)/src/SUMMARY.md.in $(d)/src/command-ref/new-cli

doc/manual/redirects.js

@@ -35,7 +35,6 @@ const redirects = {
     "conf-build-max-jobs": "command-ref/conf-file.html#conf-build-max-jobs",
     "conf-build-max-log-size": "command-ref/conf-file.html#conf-build-max-log-size",
     "conf-build-max-silent-time": "command-ref/conf-file.html#conf-build-max-silent-time",
-    "conf-build-repeat": "command-ref/conf-file.html#conf-build-repeat",
     "conf-build-timeout": "command-ref/conf-file.html#conf-build-timeout",
     "conf-build-use-chroot": "command-ref/conf-file.html#conf-build-use-chroot",
     "conf-build-use-sandbox": "command-ref/conf-file.html#conf-build-use-sandbox",
@@ -47,7 +46,6 @@ const redirects = {
     "conf-connect-timeout": "command-ref/conf-file.html#conf-connect-timeout",
     "conf-cores": "command-ref/conf-file.html#conf-cores",
     "conf-diff-hook": "command-ref/conf-file.html#conf-diff-hook",
-    "conf-enforce-determinism": "command-ref/conf-file.html#conf-enforce-determinism",
     "conf-env-keep-derivations": "command-ref/conf-file.html#conf-env-keep-derivations",
     "conf-extra-binary-caches": "command-ref/conf-file.html#conf-extra-binary-caches",
     "conf-extra-platforms": "command-ref/conf-file.html#conf-extra-platforms",
@@ -74,7 +72,6 @@ const redirects = {
     "conf-plugin-files": "command-ref/conf-file.html#conf-plugin-files",
     "conf-post-build-hook": "command-ref/conf-file.html#conf-post-build-hook",
     "conf-pre-build-hook": "command-ref/conf-file.html#conf-pre-build-hook",
-    "conf-repeat": "command-ref/conf-file.html#conf-repeat",
     "conf-require-sigs": "command-ref/conf-file.html#conf-require-sigs",
     "conf-restrict-eval": "command-ref/conf-file.html#conf-restrict-eval",
     "conf-run-diff-hook": "command-ref/conf-file.html#conf-run-diff-hook",

doc/manual/src/SUMMARY.md.in

@@ -65,6 +65,7 @@
   - [CLI guideline](contributing/cli-guideline.md)
 - [Release Notes](release-notes/release-notes.md)
   - [Release X.Y (202?-??-??)](release-notes/rl-next.md)
+  - [Release 2.12 (2022-12-06)](release-notes/rl-2.12.md)
   - [Release 2.11 (2022-08-25)](release-notes/rl-2.11.md)
   - [Release 2.10 (2022-07-11)](release-notes/rl-2.10.md)
   - [Release 2.9 (2022-05-30)](release-notes/rl-2.9.md)

doc/manual/src/advanced-topics/diff-hook.md

@@ -121,37 +121,3 @@ error:
     are not valid, so checking is not possible
 
 Run the build without `--check`, and then try with `--check` again.
-
-# Automatic and Optionally Enforced Determinism Verification
-
-Automatically verify every build at build time by executing the build
-multiple times.
-
-Setting `repeat` and `enforce-determinism` in your `nix.conf` permits
-the automated verification of every build Nix performs.
-
-The following configuration will run each build three times, and will
-require the build to be deterministic:
-
-    enforce-determinism = true
-    repeat = 2
-
-Setting `enforce-determinism` to false as in the following
-configuration will run the build multiple times, execute the build
-hook, but will allow the build to succeed even if it does not build
-reproducibly:
-
-    enforce-determinism = false
-    repeat = 1
-
-An example output of this configuration:
-
-```console
-$ nix-build ./test.nix -A unstable
-this derivation will be built:
-  /nix/store/ch6llwpr2h8c3jmnf3f2ghkhx59aa97f-unstable.drv
-building '/nix/store/ch6llwpr2h8c3jmnf3f2ghkhx59aa97f-unstable.drv' (round 1/2)...
-building '/nix/store/ch6llwpr2h8c3jmnf3f2ghkhx59aa97f-unstable.drv' (round 2/2)...
-output '/nix/store/6xg356v9gl03hpbbg8gws77n19qanh02-unstable' of '/nix/store/ch6llwpr2h8c3jmnf3f2ghkhx59aa97f-unstable.drv' differs from '/nix/store/6xg356v9gl03hpbbg8gws77n19qanh02-unstable.check' from previous round
-/nix/store/6xg356v9gl03hpbbg8gws77n19qanh02-unstable
-```

doc/manual/src/advanced-topics/post-build-hook.md

@@ -33,12 +33,17 @@ distribute the public key for verifying the authenticity of the paths.
 example-nix-cache-1:1/cKDz3QCCOmwcztD2eV6Coggp6rqc9DGjWv7C0G+rM=
 ```
 
-Then, add the public key and the cache URL to your `nix.conf`'s
+Then update [`nix.conf`](../command-ref/conf-file.md) on any machine that will access the cache.
-`trusted-public-keys` and `substituters` options:
+Add the cache URL to [`substituters`](../command-ref/conf-file.md#conf-substituters) and the public key to [`trusted-public-keys`](../command-ref/conf-file.md#conf-trusted-public-keys):
 
     substituters = https://cache.nixos.org/ s3://example-nix-cache
     trusted-public-keys = cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= example-nix-cache-1:1/cKDz3QCCOmwcztD2eV6Coggp6rqc9DGjWv7C0G+rM=
 
+Machines that build for the cache must sign derivations using the private key.
+On those machines, add the path to the key file to the [`secret-key-files`](../command-ref/conf-file.md#conf-secret-key-files) field in their [`nix.conf`](../command-ref/conf-file.md):
+
+    secret-key-files = /etc/nix/key.private
+
 We will restart the Nix daemon in a later step.
 
 # Implementing the build hook
@@ -52,10 +57,8 @@ set -eu
 set -f # disable globbing
 export IFS=' '
 
-echo "Signing paths" $OUT_PATHS
-nix store sign --key-file /etc/nix/key.private $OUT_PATHS
 echo "Uploading paths" $OUT_PATHS
-exec nix copy --to 's3://example-nix-cache' $OUT_PATHS
+exec nix copy --to "s3://example-nix-cache" $OUT_PATHS
 ```
 
 > **Note**

doc/manual/src/command-ref/nix-build.md

@@ -53,16 +53,18 @@ All options not listed here are passed to `nix-store
 --realise`, except for `--arg` and `--attr` / `-A` which are passed to
 `nix-instantiate`.
 
-  - [`--no-out-link`]{#opt-no-out-link}\
+  - <span id="opt-no-out-link">[`--no-out-link`](#opt-no-out-link)<span>
+
     Do not create a symlink to the output path. Note that as a result
     the output does not become a root of the garbage collector, and so
-    might be deleted by `nix-store
+    might be deleted by `nix-store --gc`.
-                    --gc`.
+
+  - <span id="opt-dry-run">[`--dry-run`](#opt-dry-run)</span>
 
-  - [`--dry-run`]{#opt-dry-run}\
     Show what store paths would be built or downloaded.
 
-  - [`--out-link`]{#opt-out-link} / `-o` *outlink*\
+  - <span id="opt-out-link">[`--out-link`](#opt-out-link)</span> / `-o` *outlink*
+
     Change the name of the symlink to the output path created from
     `result` to *outlink*.
 

doc/manual/src/command-ref/nix-store.md

@@ -22,7 +22,8 @@ This section lists the options that are common to all operations. These
 options are allowed for every subcommand, though they may not always
 have an effect.
 
-  - [`--add-root`]{#opt-add-root} *path*\
+  - <span id="opt-add-root">[`--add-root`](#opt-add-root)</span> *path*
+
     Causes the result of a realisation (`--realise` and
     `--force-realise`) to be registered as a root of the garbage
     collector. *path* will be created as a symlink to the resulting
@@ -104,10 +105,6 @@ The following flags are available:
     previous build, the new output path is left in
     `/nix/store/name.check.`
 
-    See also the `build-repeat` configuration option, which repeats a
-    derivation a number of times and prevents its outputs from being
-    registered as “valid” in the Nix store unless they are identical.
-
 Special exit codes:
 
   - `100`\

doc/manual/src/installation/installing-binary.md

@@ -88,6 +88,29 @@ extension. The installer will also create `/etc/profile.d/nix.sh`.
 
 ### Linux
 
+If you are on Linux with systemd:
+
+1. Remove the Nix daemon service:
+
+   ```console
+   sudo systemctl stop nix-daemon.service
+   sudo systemctl disable nix-daemon.socket nix-daemon.service
+   sudo systemctl daemon-reload
+   ```
+
+1. Remove systemd service files:
+
+   ```console
+   sudo rm /etc/systemd/system/nix-daemon.service /etc/systemd/system/nix-daemon.socket
+   ```
+
+1. The installer script uses systemd-tmpfiles to create the socket directory.
+    You may also want to remove the configuration for that:
+
+   ```console
+   sudo rm /etc/tmpfiles.d/nix-daemon.conf
+   ```
+
 Remove files created by Nix:
 
 ```console
@@ -103,16 +126,6 @@ done
 sudo groupdel 30000
 ```
 
-If you are on Linux with systemd, remove the Nix daemon service:
-
-```console
-sudo systemctl stop nix-daemon.socket
-sudo systemctl stop nix-daemon.service
-sudo systemctl disable nix-daemon.socket
-sudo systemctl disable nix-daemon.service
-sudo systemctl daemon-reload
-```
-
 There may also be references to Nix in
 
 - `/etc/profile`

doc/manual/src/release-notes/rl-2.12.md

@@ -0,0 +1,43 @@
+# Release 2.12 (2022-12-06)
+
+* On Linux, Nix can now run builds in a user namespace where they run
+  as root (UID 0) and have 65,536 UIDs available.
+  <!-- FIXME: move this to its own section about system features -->
+  This is primarily useful for running containers such as `systemd-nspawn`
+  inside a Nix build. For an example, see [`tests/systemd-nspawn/nix`][nspawn].
+
+  [nspawn]: https://github.com/NixOS/nix/blob/67bcb99700a0da1395fa063d7c6586740b304598/tests/systemd-nspawn.nix.
+
+  A build can enable this by setting the derivation attribute:
+
+  ```
+  requiredSystemFeatures = [ "uid-range" ];
+  ```
+
+  The `uid-range` [system feature] requires the [`auto-allocate-uids`]
+  setting to be enabled.
+
+  [system feature]: (../command-ref/conf-file.md#conf-system-features)
+
+* Nix can now automatically pick UIDs for builds, removing the need to
+  create `nixbld*` user accounts. See [`auto-allocate-uids`].
+
+  [`auto-allocate-uids`]: (../command-ref/conf-file.md#conf-auto-allocate-uids)
+
+* On Linux, Nix has experimental support for running builds inside a
+  cgroup. See
+  [`use-cgroups`](../command-ref/conf-file.md#conf-use-cgroups).
+
+* `<nix/fetchurl.nix>` now accepts an additional argument `impure` which
+  defaults to `false`.  If it is set to `true`, the `hash` and `sha256`
+  arguments will be ignored and the resulting derivation will have
+  `__impure` set to `true`, making it an impure derivation.
+
+* If `builtins.readFile` is called on a file with context, then only
+  the parts of the context that appear in the content of the file are
+  retained.  This avoids a lot of spurious errors where strings end up
+  having a context just because they are read from a store path
+  ([#7260](https://github.com/NixOS/nix/pull/7260)).
+
+* `nix build --json` now prints some statistics about top-level
+  derivations, such as CPU statistics when cgroups are enabled.

doc/manual/src/release-notes/rl-next.md

@@ -1,14 +1,6 @@
 # Release X.Y (202?-??-??)
 
-* `<nix/fetchurl.nix>` now accepts an additional argument `impure` which
+* The `repeat` and `enforce-determinism` options have been removed
-  defaults to `false`.  If it is set to `true`, the `hash` and `sha256`
+  since they had been broken under many circumstances for a long time.
-  arguments will be ignored and the resulting derivation will have
-  `__impure` set to `true`, making it an impure derivation.
-
-* If `builtins.readFile` is called on a file with context, then only the parts
-  of that context that appear in the content of the file are retained.
-  This avoids a lot of spurious errors where some benign strings end-up having
-  a context just because they are read from a store path
-  ([#7260](https://github.com/NixOS/nix/pull/7260)).
 
 * Allow explicitly selecting outputs with *store derivations* installable syntax too.

docker.nix

@@ -36,6 +36,17 @@ let
       shell = "${pkgs.bashInteractive}/bin/bash";
       home = "/root";
       gid = 0;
+      groups = [ "root" ];
+      description = "System administrator";
+    };
+
+    nobody = {
+      uid = 65534;
+      shell = "${pkgs.shadow}/bin/nologin";
+      home = "/var/empty";
+      gid = 65534;
+      groups = [ "nobody" ];
+      description = "Unprivileged account (don't use!)";
     };
 
   } // lib.listToAttrs (
@@ -57,6 +68,7 @@ let
   groups = {
     root.gid = 0;
     nixbld.gid = 30000;
+    nobody.gid = 65534;
   };
 
   userToPasswd = (

flake.nix

@@ -9,14 +9,14 @@
 
     let
 
-      version = builtins.readFile ./.version + versionSuffix;
+      officialRelease = false;
+
+      version = nixpkgs.lib.fileContents ./.version + versionSuffix;
       versionSuffix =
         if officialRelease
         then ""
         else "pre${builtins.substring 0 8 (self.lastModifiedDate or self.lastModified or "19700101")}_${self.shortRev or "dirty"}";
 
-      officialRelease = false;
-
       linux64BitSystems = [ "x86_64-linux" "aarch64-linux" ];
       linuxSystems = linux64BitSystems ++ [ "i686-linux" ];
       systems = linuxSystems ++ [ "x86_64-darwin" "aarch64-darwin" ];
@@ -506,6 +506,12 @@
           overlay = self.overlays.default;
         });
 
+        tests.containers = (import ./tests/containers.nix rec {
+          system = "x86_64-linux";
+          inherit nixpkgs;
+          overlay = self.overlays.default;
+        });
+
         tests.setuid = nixpkgs.lib.genAttrs
           ["i686-linux" "x86_64-linux"]
           (system:

maintainers/README.md

@@ -0,0 +1,79 @@
+# Nix maintainers team
+
+## Motivation
+
+The goal of the team is to help other people to contribute to Nix.
+
+## Members
+
+- Eelco Dolstra (@edolstra) – Team lead
+- Théophane Hufschmitt (@thufschmitt)
+- Valentin Gagarin (@fricklerhandwerk)
+- Thomas Bereknyei (@tomberek)
+- Robert Hensing (@roberth)
+
+## Meeting protocol
+
+The team meets twice a week:
+
+- Discussion meeting: [Fridays 13:00-14:00 CET](https://calendar.google.com/calendar/event?eid=MHNtOGVuNWtrZXNpZHR2bW1sM3QyN2ZjaGNfMjAyMjExMjVUMTIwMDAwWiBiOW81MmZvYnFqYWs4b3E4bGZraGczdDBxZ0Bn)
+
+  1. Triage issues and pull requests from the _No Status_ column (30 min)
+  2. Discuss issues and pull requests from the _To discuss_ column (30 min)
+
+- Work meeting: [Mondays 13:00-15:00 CET](https://calendar.google.com/calendar/event?eid=NTM1MG1wNGJnOGpmOTZhYms3bTB1bnY5cWxfMjAyMjExMjFUMTIwMDAwWiBiOW81MmZvYnFqYWs4b3E4bGZraGczdDBxZ0Bn)
+
+  1. Code review on pull requests from _In review_.
+  2. Other chores and tasks.
+
+Meeting notes are collected on a [collaborative scratchpad](https://pad.lassul.us/Cv7FpYx-Ri-4VjUykQOLAw), and published on Discourse under the [Nix category](https://discourse.nixos.org/c/dev/nix/50).
+
+## Project board protocol
+
+The team uses a [GitHub project board](https://github.com/orgs/NixOS/projects/19/views/1) for tracking its work.
+
+Issues on the board progress through the following states:
+
+- No Status
+
+  Team members can add pull requests or issues to discuss or review together.
+
+  During the discussion meeting, the team triages new items.
+  If there is disagreement on the general idea behind an issue or pull request, it is moved to _To discuss_, otherwise to _In review_.
+
+- To discuss
+
+  Pull requests and issues that are important and controversial are discussed by the team during discussion meetings.
+
+  This may be where the merit of the change itself or the implementation strategy is contested by a team member.
+
+- In review
+
+  Pull requests in this column are reviewed together during work meetings.
+  This is both for spreading implementation knowledge and for establishing common values in code reviews.
+
+  When the overall direction is agreed upon, even when further changes are required, the pull request is assigned to one team member.
+
+- Assigned for merging
+
+  One team member is assigned to each of these pull requests.
+  They will communicate with the authors, and make the final approval once all remaining issues are addressed.
+
+  If more substantive issues arise, the assignee can move the pull request back to _To discuss_ to involve the team again.
+
+The process is illustrated in the following diagram:
+
+```mermaid
+flowchart TD
+    discuss[To discuss]
+
+    review[To review]
+
+    New --> |Disagreement on idea| discuss
+    New & discuss --> |Consensus on idea| review
+
+    review --> |Consensus on implementation| Assigned
+
+    Assigned --> |Implementation issues arise| review
+    Assigned --> |Remaining issues fixed| Merged
+```

misc/launchd/org.nixos.nix-daemon.plist.in

@@ -28,7 +28,7 @@
     <key>SoftResourceLimits</key>
     <dict>
       <key>NumberOfFiles</key>
-      <integer>4096</integer>
+      <integer>1048576</integer>
     </dict>
   </dict>
 </plist>

misc/systemd/nix-daemon.service.in

@@ -9,7 +9,7 @@ ConditionPathIsReadWrite=@localstatedir@/nix/daemon-socket
 [Service]
 ExecStart=@@bindir@/nix-daemon nix-daemon --daemon
 KillMode=process
-LimitNOFILE=4096
+LimitNOFILE=1048576
 
 [Install]
 WantedBy=multi-user.target

scripts/install-multi-user.sh

@@ -97,13 +97,10 @@ is_os_darwin() {
 }
 
 contact_us() {
-    echo "You can open an issue at https://github.com/nixos/nix/issues"
+    echo "You can open an issue at"
+    echo "https://github.com/NixOS/nix/issues/new?labels=installer&template=installer.md"
     echo ""
-    echo "Or feel free to contact the team:"
+    echo "Or get in touch with the community: https://nixos.org/community"
-    echo " - Matrix: #nix:nixos.org"
-    echo " - IRC: in #nixos on irc.libera.chat"
-    echo " - twitter: @nixos_org"
-    echo " - forum: https://discourse.nixos.org"
 }
 get_help() {
     echo "We'd love to help if you need it."

src/libcmd/installables.cc

@@ -958,7 +958,7 @@ std::vector<std::pair<std::shared_ptr<Installable>, BuiltPathWithResult>> Instal
 
     case Realise::Outputs: {
         if (settings.printMissing)
-          printMissing(store, pathsToBuild, lvlInfo);
+            printMissing(store, pathsToBuild, lvlInfo);
 
         for (auto & buildResult : store->buildPathsWithResults(pathsToBuild, bMode, evalStore)) {
             if (!buildResult.success())

src/libcmd/repl.cc

@@ -270,6 +270,7 @@ void NixRepl::mainLoop()
             // ctrl-D should exit the debugger.
             state->debugStop = false;
             state->debugQuit = true;
+            logger->cout("");
             break;
         }
         try {

src/libexpr/eval.cc

@@ -7,7 +7,6 @@
 #include "globals.hh"
 #include "eval-inline.hh"
 #include "filetransfer.hh"
-#include "json.hh"
 #include "function-trace.hh"
 
 #include <algorithm>
@@ -21,6 +20,7 @@
 #include <functional>
 
 #include <sys/resource.h>
+#include <nlohmann/json.hpp>
 
 #if HAVE_BOEHMGC
 
@@ -35,6 +35,8 @@
 
 #endif
 
+using json = nlohmann::json;
+
 namespace nix {
 
 static char * allocString(size_t size)
@@ -69,15 +71,11 @@ static char * dupString(const char * s)
 // empty string.
 static const char * makeImmutableStringWithLen(const char * s, size_t size)
 {
-    char * t;
     if (size == 0)
         return "";
-#if HAVE_BOEHMGC
+    auto t = allocString(size + 1);
-    t = GC_STRNDUP(s, size);
+    memcpy(t, s, size);
-#else
+    t[size] = 0;
-    t = strndup(s, size);
-#endif
-    if (!t) throw std::bad_alloc();
     return t;
 }
 
@@ -1648,7 +1646,7 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                 auto dts = debugRepl
                     ? makeDebugTraceStacker(
                         *this, *lambda.body, env2, positions[lambda.pos],
-                        "while evaluating %s",
+                        "while calling %s",
                         lambda.name
                         ? concatStrings("'", symbols[lambda.name], "'")
                         : "anonymous lambda")
@@ -1657,7 +1655,7 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                 lambda.body->eval(*this, env2, vCur);
             } catch (Error & e) {
                 if (loggerSettings.showTrace.get()) {
-                    addErrorTrace(e, lambda.pos, "while evaluating %s",
+                    addErrorTrace(e, lambda.pos, "while calling %s",
                         (lambda.name
                             ? concatStrings("'", symbols[lambda.name], "'")
                             : "anonymous lambda"));
@@ -2441,97 +2439,97 @@ void EvalState::printStats()
         std::fstream fs;
         if (outPath != "-")
             fs.open(outPath, std::fstream::out);
-        JSONObject topObj(outPath == "-" ? std::cerr : fs, true);
+        json topObj = json::object();
-        topObj.attr("cpuTime",cpuTime);
+        topObj["cpuTime"] = cpuTime;
-        {
+        topObj["envs"] = {
-            auto envs = topObj.object("envs");
+            {"number", nrEnvs},
-            envs.attr("number", nrEnvs);
+            {"elements", nrValuesInEnvs},
-            envs.attr("elements", nrValuesInEnvs);
+            {"bytes", bEnvs},
-            envs.attr("bytes", bEnvs);
+        };
-        }
+        topObj["list"] = {
-        {
+            {"elements", nrListElems},
-            auto lists = topObj.object("list");
+            {"bytes", bLists},
-            lists.attr("elements", nrListElems);
+            {"concats", nrListConcats},
-            lists.attr("bytes", bLists);
+        };
-            lists.attr("concats", nrListConcats);
+        topObj["values"] = {
-        }
+            {"number", nrValues},
-        {
+            {"bytes", bValues},
-            auto values = topObj.object("values");
+        };
-            values.attr("number", nrValues);
+        topObj["symbols"] = {
-            values.attr("bytes", bValues);
+            {"number", symbols.size()},
-        }
+            {"bytes", symbols.totalSize()},
-        {
+        };
-            auto syms = topObj.object("symbols");
+        topObj["sets"] = {
-            syms.attr("number", symbols.size());
+            {"number", nrAttrsets},
-            syms.attr("bytes", symbols.totalSize());
+            {"bytes", bAttrsets},
-        }
+            {"elements", nrAttrsInAttrsets},
-        {
+        };
-            auto sets = topObj.object("sets");
+        topObj["sizes"] = {
-            sets.attr("number", nrAttrsets);
+            {"Env", sizeof(Env)},
-            sets.attr("bytes", bAttrsets);
+            {"Value", sizeof(Value)},
-            sets.attr("elements", nrAttrsInAttrsets);
+            {"Bindings", sizeof(Bindings)},
-        }
+            {"Attr", sizeof(Attr)},
-        {
+        };
-            auto sizes = topObj.object("sizes");
+        topObj["nrOpUpdates"] = nrOpUpdates;
-            sizes.attr("Env", sizeof(Env));
+        topObj["nrOpUpdateValuesCopied"] = nrOpUpdateValuesCopied;
-            sizes.attr("Value", sizeof(Value));
+        topObj["nrThunks"] = nrThunks;
-            sizes.attr("Bindings", sizeof(Bindings));
+        topObj["nrAvoided"] = nrAvoided;
-            sizes.attr("Attr", sizeof(Attr));
+        topObj["nrLookups"] = nrLookups;
-        }
+        topObj["nrPrimOpCalls"] = nrPrimOpCalls;
-        topObj.attr("nrOpUpdates", nrOpUpdates);
+        topObj["nrFunctionCalls"] = nrFunctionCalls;
-        topObj.attr("nrOpUpdateValuesCopied", nrOpUpdateValuesCopied);
-        topObj.attr("nrThunks", nrThunks);
-        topObj.attr("nrAvoided", nrAvoided);
-        topObj.attr("nrLookups", nrLookups);
-        topObj.attr("nrPrimOpCalls", nrPrimOpCalls);
-        topObj.attr("nrFunctionCalls", nrFunctionCalls);
 #if HAVE_BOEHMGC
-        {
+        topObj["gc"] = {
-            auto gc = topObj.object("gc");
+            {"heapSize", heapSize},
-            gc.attr("heapSize", heapSize);
+            {"totalBytes", totalBytes},
-            gc.attr("totalBytes", totalBytes);
+        };
-        }
 #endif
 
         if (countCalls) {
+            topObj["primops"] = primOpCalls;
             {
-                auto obj = topObj.object("primops");
+                auto& list = topObj["functions"];
-                for (auto & i : primOpCalls)
+                list = json::array();
-                    obj.attr(i.first, i.second);
-            }
-            {
-                auto list = topObj.list("functions");
                 for (auto & [fun, count] : functionCalls) {
-                    auto obj = list.object();
+                    json obj = json::object();
                     if (fun->name)
-                        obj.attr("name", (std::string_view) symbols[fun->name]);
+                        obj["name"] = (std::string_view) symbols[fun->name];
                     else
-                        obj.attr("name", nullptr);
+                        obj["name"] = nullptr;
                     if (auto pos = positions[fun->pos]) {
-                        obj.attr("file", (std::string_view) pos.file);
+                        obj["file"] = (std::string_view) pos.file;
-                        obj.attr("line", pos.line);
+                        obj["line"] = pos.line;
-                        obj.attr("column", pos.column);
+                        obj["column"] = pos.column;
                     }
-                    obj.attr("count", count);
+                    obj["count"] = count;
+                    list.push_back(obj);
                 }
             }
             {
-                auto list = topObj.list("attributes");
+                auto list = topObj["attributes"];
+                list = json::array();
                 for (auto & i : attrSelects) {
-                    auto obj = list.object();
+                    json obj = json::object();
                     if (auto pos = positions[i.first]) {
-                        obj.attr("file", (const std::string &) pos.file);
+                        obj["file"] = (const std::string &) pos.file;
-                        obj.attr("line", pos.line);
+                        obj["line"] = pos.line;
-                        obj.attr("column", pos.column);
+                        obj["column"] = pos.column;
                     }
-                    obj.attr("count", i.second);
+                    obj["count"] = i.second;
+                    list.push_back(obj);
                 }
             }
         }
 
         if (getEnv("NIX_SHOW_SYMBOLS").value_or("0") != "0") {
-            auto list = topObj.list("symbols");
+            // XXX: overrides earlier assignment
-            symbols.dump([&](const std::string & s) { list.elem(s); });
+            topObj["symbols"] = json::array();
+            auto &list = topObj["symbols"];
+            symbols.dump([&](const std::string & s) { list.emplace_back(s); });
+        }
+        if (outPath == "-") {
+            std::cerr << topObj.dump(2) << std::endl;
+        } else {
+            fs << topObj.dump(2) << std::endl;
         }
     }
 }

src/libexpr/primops.cc

@@ -8,12 +8,12 @@
 #include "references.hh"
 #include "store-api.hh"
 #include "util.hh"
-#include "json.hh"
 #include "value-to-json.hh"
 #include "value-to-xml.hh"
 #include "primops.hh"
 
 #include <boost/container/small_vector.hpp>
+#include <nlohmann/json.hpp>
 
 #include <sys/types.h>
 #include <sys/stat.h>
@@ -1011,6 +1011,7 @@ static void prim_second(EvalState & state, const PosIdx pos, Value * * args, Val
    derivation. */
 static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
+    using nlohmann::json;
     state.forceAttrs(*args[0], pos);
 
     /* Figure out the name first (for stack backtraces). */
@@ -1032,11 +1033,10 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
     }
 
     /* Check whether attributes should be passed as a JSON file. */
-    std::ostringstream jsonBuf;
+    std::optional<json> jsonObject;
-    std::unique_ptr<JSONObject> jsonObject;
     attr = args[0]->attrs->find(state.sStructuredAttrs);
     if (attr != args[0]->attrs->end() && state.forceBool(*attr->value, pos))
-        jsonObject = std::make_unique<JSONObject>(jsonBuf);
+        jsonObject = json::object();
 
     /* Check whether null attributes should be ignored. */
     bool ignoreNulls = false;
@@ -1138,8 +1138,7 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
 
                     if (i->name == state.sStructuredAttrs) continue;
 
-                    auto placeholder(jsonObject->placeholder(key));
+                    (*jsonObject)[key] = printValueAsJSON(state, true, *i->value, pos, context);
-                    printValueAsJSON(state, true, *i->value, pos, placeholder, context);
 
                     if (i->name == state.sBuilder)
                         drv.builder = state.forceString(*i->value, context, posDrvName);
@@ -1183,8 +1182,8 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
     }
 
     if (jsonObject) {
+        drv.env.emplace("__json", jsonObject->dump());
         jsonObject.reset();
-        drv.env.emplace("__json", jsonBuf.str());
     }
 
     /* Everything in the context of the strings in the derivation
@@ -2421,12 +2420,18 @@ static RegisterPrimOp primop_listToAttrs({
       Construct a set from a list specifying the names and values of each
       attribute. Each element of the list should be a set consisting of a
       string-valued attribute `name` specifying the name of the attribute,
-      and an attribute `value` specifying its value. Example:
+      and an attribute `value` specifying its value.
+
+      In case of duplicate occurrences of the same name, the first
+      takes precedence.
+
+      Example:
 
       ```nix
       builtins.listToAttrs
         [ { name = "foo"; value = 123; }
           { name = "bar"; value = 456; }
+          { name = "bar"; value = 420; }
         ]
       ```
 

src/libexpr/value-to-json.cc

@@ -1,84 +1,82 @@
 #include "value-to-json.hh"
-#include "json.hh"
 #include "eval-inline.hh"
 #include "util.hh"
 
 #include <cstdlib>
 #include <iomanip>
+#include <nlohmann/json.hpp>
 
 
 namespace nix {
-
+using json = nlohmann::json;
-void printValueAsJSON(EvalState & state, bool strict,
+json printValueAsJSON(EvalState & state, bool strict,
-    Value & v, const PosIdx pos, JSONPlaceholder & out, PathSet & context, bool copyToStore)
+    Value & v, const PosIdx pos, PathSet & context, bool copyToStore)
 {
     checkInterrupt();
 
     if (strict) state.forceValue(v, pos);
 
+    json out;
+
     switch (v.type()) {
 
         case nInt:
-            out.write(v.integer);
+            out = v.integer;
             break;
 
         case nBool:
-            out.write(v.boolean);
+            out = v.boolean;
             break;
 
         case nString:
             copyContext(v, context);
-            out.write(v.string.s);
+            out = v.string.s;
             break;
 
         case nPath:
             if (copyToStore)
-                out.write(state.copyPathToStore(context, v.path));
+                out = state.copyPathToStore(context, v.path);
             else
-                out.write(v.path);
+                out = v.path;
             break;
 
         case nNull:
-            out.write(nullptr);
             break;
 
         case nAttrs: {
             auto maybeString = state.tryAttrsToString(pos, v, context, false, false);
             if (maybeString) {
-                out.write(*maybeString);
+                out = *maybeString;
                 break;
             }
             auto i = v.attrs->find(state.sOutPath);
             if (i == v.attrs->end()) {
-                auto obj(out.object());
+                out = json::object();
                 StringSet names;
                 for (auto & j : *v.attrs)
                     names.emplace(state.symbols[j.name]);
                 for (auto & j : names) {
                     Attr & a(*v.attrs->find(state.symbols.create(j)));
-                    auto placeholder(obj.placeholder(j));
+                    out[j] = printValueAsJSON(state, strict, *a.value, a.pos, context, copyToStore);
-                    printValueAsJSON(state, strict, *a.value, a.pos, placeholder, context, copyToStore);
                 }
             } else
-                printValueAsJSON(state, strict, *i->value, i->pos, out, context, copyToStore);
+                return printValueAsJSON(state, strict, *i->value, i->pos, context, copyToStore);
             break;
         }
 
         case nList: {
-            auto list(out.list());
+            out = json::array();
-            for (auto elem : v.listItems()) {
+            for (auto elem : v.listItems())
-                auto placeholder(list.placeholder());
+                out.push_back(printValueAsJSON(state, strict, *elem, pos, context, copyToStore));
-                printValueAsJSON(state, strict, *elem, pos, placeholder, context, copyToStore);
-            }
             break;
         }
 
         case nExternal:
-            v.external->printValueAsJSON(state, strict, out, context, copyToStore);
+            return v.external->printValueAsJSON(state, strict, context, copyToStore);
             break;
 
         case nFloat:
-            out.write(v.fpoint);
+            out = v.fpoint;
             break;
 
         case nThunk:
@@ -91,17 +89,17 @@ void printValueAsJSON(EvalState & state, bool strict,
             state.debugThrowLastTrace(e);
             throw e;
     }
+    return out;
 }
 
 void printValueAsJSON(EvalState & state, bool strict,
     Value & v, const PosIdx pos, std::ostream & str, PathSet & context, bool copyToStore)
 {
-    JSONPlaceholder out(str);
+    str << printValueAsJSON(state, strict, v, pos, context, copyToStore);
-    printValueAsJSON(state, strict, v, pos, out, context, copyToStore);
 }
 
-void ExternalValueBase::printValueAsJSON(EvalState & state, bool strict,
+json ExternalValueBase::printValueAsJSON(EvalState & state, bool strict,
-    JSONPlaceholder & out, PathSet & context, bool copyToStore) const
+    PathSet & context, bool copyToStore) const
 {
     state.debugThrowLastTrace(TypeError("cannot convert %1% to JSON", showType()));
 }

src/libexpr/value-to-json.hh

@@ -5,13 +5,12 @@
 
 #include <string>
 #include <map>
+#include <nlohmann/json_fwd.hpp>
 
 namespace nix {
 
-class JSONPlaceholder;
+nlohmann::json printValueAsJSON(EvalState & state, bool strict,
-
+    Value & v, const PosIdx pos, PathSet & context, bool copyToStore = true);
-void printValueAsJSON(EvalState & state, bool strict,
-    Value & v, const PosIdx pos, JSONPlaceholder & out, PathSet & context, bool copyToStore = true);
 
 void printValueAsJSON(EvalState & state, bool strict,
     Value & v, const PosIdx pos, std::ostream & str, PathSet & context, bool copyToStore = true);

src/libexpr/value.hh

@@ -7,6 +7,7 @@
 #if HAVE_BOEHMGC
 #include <gc/gc_allocator.h>
 #endif
+#include <nlohmann/json_fwd.hpp>
 
 namespace nix {
 
@@ -62,7 +63,6 @@ class StorePath;
 class Store;
 class EvalState;
 class XMLWriter;
-class JSONPlaceholder;
 
 
 typedef int64_t NixInt;
@@ -98,8 +98,8 @@ class ExternalValueBase
     virtual bool operator ==(const ExternalValueBase & b) const;
 
     /* Print the value as JSON. Defaults to unconvertable, i.e. throws an error */
-    virtual void printValueAsJSON(EvalState & state, bool strict,
+    virtual nlohmann::json printValueAsJSON(EvalState & state, bool strict,
-        JSONPlaceholder & out, PathSet & context, bool copyToStore = true) const;
+        PathSet & context, bool copyToStore = true) const;
 
     /* Print the value as XML. Defaults to unevaluated */
     virtual void printValueAsXML(EvalState & state, bool strict, bool location,

src/libmain/progress-bar.cc

@@ -180,10 +180,12 @@ public:
             auto machineName = getS(fields, 1);
             if (machineName != "")
                 i->s += fmt(" on " ANSI_BOLD "%s" ANSI_NORMAL, machineName);
-            auto curRound = getI(fields, 2);
+
-            auto nrRounds = getI(fields, 3);
+            // Used to be curRound and nrRounds, but the
-            if (nrRounds != 1)
+            // implementation was broken for a long time.
-                i->s += fmt(" (round %d/%d)", curRound, nrRounds);
+            if (getI(fields, 2) != 1 || getI(fields, 3) != 1) {
+                throw Error("log message indicated repeating builds, but this is not currently implemented");
+            }
             i->name = DrvName(name).name;
         }
 

src/libstore/binary-cache-store.cc

@@ -9,7 +9,6 @@
 #include "remote-fs-accessor.hh"
 #include "nar-info-disk-cache.hh"
 #include "nar-accessor.hh"
-#include "json.hh"
 #include "thread-pool.hh"
 #include "callback.hh"
 
@@ -194,19 +193,12 @@ ref<const ValidPathInfo> BinaryCacheStore::addToStoreCommon(
     /* Optionally write a JSON file containing a listing of the
        contents of the NAR. */
     if (writeNARListing) {
-        std::ostringstream jsonOut;
+        nlohmann::json j = {
+            {"version", 1},
+            {"root", listNar(ref<FSAccessor>(narAccessor), "", true)},
+        };
 
-        {
+        upsertFile(std::string(info.path.hashPart()) + ".ls", j.dump(), "application/json");
-            JSONObject jsonRoot(jsonOut);
-            jsonRoot.attr("version", 1);
-
-            {
-                auto res = jsonRoot.placeholder("root");
-                listNar(res, ref<FSAccessor>(narAccessor), "", true);
-            }
-        }
-
-        upsertFile(std::string(info.path.hashPart()) + ".ls", jsonOut.str(), "application/json");
     }
 
     /* Optionally maintain an index of DWARF debug info files

src/libstore/build-result.hh

@@ -5,7 +5,7 @@
 
 #include <string>
 #include <chrono>
-
+#include <optional>
 
 namespace nix {
 
@@ -78,6 +78,9 @@ struct BuildResult
        was repeated). */
     time_t startTime = 0, stopTime = 0;
 
+    /* User and system CPU time the build took. */
+    std::optional<std::chrono::microseconds> cpuUser, cpuSystem;
+
     bool success()
     {
         return status == Built || status == Substituted || status == AlreadyValid || status == ResolvesToAlreadyValid;

src/libstore/build/derivation-goal.cc

@@ -7,7 +7,6 @@
 #include "finally.hh"
 #include "util.hh"
 #include "archive.hh"
-#include "json.hh"
 #include "compression.hh"
 #include "worker-protocol.hh"
 #include "topo-sort.hh"
@@ -502,6 +501,14 @@ void DerivationGoal::inputsRealised()
                now-known results of dependencies. If so, we become a
                stub goal aliasing that resolved derivation goal. */
             std::optional attempt = fullDrv.tryResolve(worker.store, inputDrvOutputs);
+            if (!attempt) {
+              /* TODO (impure derivations-induced tech debt) (see below):
+                 The above attempt should have found it, but because we manage
+                 inputDrvOutputs statefully, sometimes it gets out of sync with
+                 the real source of truth (store). So we query the store
+                 directly if there's a problem. */
+              attempt = fullDrv.tryResolve(worker.store);
+            }
             assert(attempt);
             Derivation drvResolved { *std::move(attempt) };
 
@@ -564,10 +571,6 @@ void DerivationGoal::inputsRealised()
     /* What type of derivation are we building? */
     derivationType = drv->type();
 
-    /* Don't repeat fixed-output derivations since they're already
-       verified by their output hash.*/
-    nrRounds = derivationType.isFixed() ? 1 : settings.buildRepeat + 1;
-
     /* Okay, try to build.  Note that here we don't wait for a build
        slot to become available, since we don't need one if there is a
        build hook. */
@@ -582,12 +585,11 @@ void DerivationGoal::started()
     auto msg = fmt(
         buildMode == bmRepair ? "repairing outputs of '%s'" :
         buildMode == bmCheck ? "checking outputs of '%s'" :
-        nrRounds > 1 ? "building '%s' (round %d/%d)" :
+        "building '%s'", worker.store.printStorePath(drvPath));
-        "building '%s'", worker.store.printStorePath(drvPath), curRound, nrRounds);
     fmt("building '%s'", worker.store.printStorePath(drvPath));
     if (hook) msg += fmt(" on '%s'", machineName);
     act = std::make_unique<Activity>(*logger, lvlInfo, actBuild, msg,
-        Logger::Fields{worker.store.printStorePath(drvPath), hook ? machineName : "", curRound, nrRounds});
+        Logger::Fields{worker.store.printStorePath(drvPath), hook ? machineName : "", 1, 1});
     mcRunningBuilds = std::make_unique<MaintainCount<uint64_t>>(worker.runningBuilds);
     worker.updateProgress();
 }
@@ -887,6 +889,14 @@ void DerivationGoal::buildDone()
 
     cleanupPostChildKill();
 
+    if (buildResult.cpuUser && buildResult.cpuSystem) {
+        debug("builder for '%s' terminated with status %d, user CPU %.3fs, system CPU %.3fs",
+            worker.store.printStorePath(drvPath),
+            status,
+            ((double) buildResult.cpuUser->count()) / 1000000,
+            ((double) buildResult.cpuSystem->count()) / 1000000);
+    }
+
     bool diskFull = false;
 
     try {
@@ -933,14 +943,6 @@ void DerivationGoal::buildDone()
 
         cleanupPostOutputsRegisteredModeNonCheck();
 
-        /* Repeat the build if necessary. */
-        if (curRound++ < nrRounds) {
-            outputLocks.unlock();
-            state = &DerivationGoal::tryToBuild;
-            worker.wakeUp(shared_from_this());
-            return;
-        }
-
         /* It is now safe to delete the lock files, since all future
            lockers will see that the output paths are valid; they will
            not create new lock files with the same names as the old
@@ -1001,22 +1003,34 @@ void DerivationGoal::resolvedFinished()
                 throw Error(
                     "derivation '%s' doesn't have expected output '%s' (derivation-goal.cc/resolvedFinished,resolve)",
                     worker.store.printStorePath(drvPath), wantedOutput);
-            auto realisation = get(resolvedResult.builtOutputs, DrvOutput { *resolvedHash, wantedOutput });
+
-            if (!realisation)
+            auto realisation = [&]{
-                throw Error(
+              auto take1 = get(resolvedResult.builtOutputs, DrvOutput { *resolvedHash, wantedOutput });
-                    "derivation '%s' doesn't have expected output '%s' (derivation-goal.cc/resolvedFinished,realisation)",
+              if (take1) return *take1;
-                    worker.store.printStorePath(resolvedDrvGoal->drvPath), wantedOutput);
+
+              /* The above `get` should work. But sateful tracking of
+                 outputs in resolvedResult, this can get out of sync with the
+                 store, which is our actual source of truth. For now we just
+                 check the store directly if it fails. */
+              auto take2 = worker.evalStore.queryRealisation(DrvOutput { *resolvedHash, wantedOutput });
+              if (take2) return *take2;
+
+              throw Error(
+                  "derivation '%s' doesn't have expected output '%s' (derivation-goal.cc/resolvedFinished,realisation)",
+                  worker.store.printStorePath(resolvedDrvGoal->drvPath), wantedOutput);
+            }();
+
             if (drv->type().isPure()) {
-                auto newRealisation = *realisation;
+                auto newRealisation = realisation;
                 newRealisation.id = DrvOutput { initialOutput->outputHash, wantedOutput };
                 newRealisation.signatures.clear();
                 if (!drv->type().isFixed())
-                    newRealisation.dependentRealisations = drvOutputReferences(worker.store, *drv, realisation->outPath);
+                    newRealisation.dependentRealisations = drvOutputReferences(worker.store, *drv, realisation.outPath);
                 signRealisation(newRealisation);
                 worker.store.registerDrvOutput(newRealisation);
             }
-            outputPaths.insert(realisation->outPath);
+            outputPaths.insert(realisation.outPath);
-            builtOutputs.emplace(realisation->id, *realisation);
+            builtOutputs.emplace(realisation.id, realisation);
         }
 
         runPostBuildHook(

src/libstore/build/derivation-goal.hh

@@ -115,11 +115,6 @@ struct DerivationGoal : public Goal
 
     BuildMode buildMode;
 
-    /* The current round, if we're building multiple times. */
-    size_t curRound = 1;
-
-    size_t nrRounds;
-
     std::unique_ptr<MaintainCount<uint64_t>> mcExpectedBuilds, mcRunningBuilds;
 
     std::unique_ptr<Activity> act;

src/libstore/build/local-derivation-goal.cc

@@ -8,13 +8,13 @@
 #include "finally.hh"
 #include "util.hh"
 #include "archive.hh"
-#include "json.hh"
 #include "compression.hh"
 #include "daemon.hh"
 #include "worker-protocol.hh"
 #include "topo-sort.hh"
 #include "callback.hh"
 #include "json-utils.hh"
+#include "cgroup.hh"
 
 #include <regex>
 #include <queue>
@@ -56,6 +56,7 @@
 
 #include <pwd.h>
 #include <grp.h>
+#include <iostream>
 
 namespace nix {
 
@@ -129,26 +130,44 @@ void LocalDerivationGoal::killChild()
     if (pid != -1) {
         worker.childTerminated(this);
 
-        if (buildUser) {
+        /* If we're using a build user, then there is a tricky race
-            /* If we're using a build user, then there is a tricky
+           condition: if we kill the build user before the child has
-               race condition: if we kill the build user before the
+           done its setuid() to the build user uid, then it won't be
-               child has done its setuid() to the build user uid, then
+           killed, and we'll potentially lock up in pid.wait().  So
-               it won't be killed, and we'll potentially lock up in
+           also send a conventional kill to the child. */
-               pid.wait().  So also send a conventional kill to the
+        ::kill(-pid, SIGKILL); /* ignore the result */
-               child. */
-            ::kill(-pid, SIGKILL); /* ignore the result */
-            buildUser->kill();
-            pid.wait();
-        } else
-            pid.kill();
 
-        assert(pid == -1);
+        killSandbox(true);
+
+        pid.wait();
     }
 
     DerivationGoal::killChild();
 }
 
 
+void LocalDerivationGoal::killSandbox(bool getStats)
+{
+    if (cgroup) {
+        #if __linux__
+        auto stats = destroyCgroup(*cgroup);
+        if (getStats) {
+            buildResult.cpuUser = stats.cpuUser;
+            buildResult.cpuSystem = stats.cpuSystem;
+        }
+        #else
+        abort();
+        #endif
+    }
+
+    else if (buildUser) {
+        auto uid = buildUser->getUID();
+        assert(uid != 0);
+        killUser(uid);
+    }
+}
+
+
 void LocalDerivationGoal::tryLocalBuild() {
     unsigned int curBuilds = worker.getNrLocalBuilds();
     if (curBuilds >= settings.maxBuildJobs) {
@@ -158,28 +177,46 @@ void LocalDerivationGoal::tryLocalBuild() {
         return;
     }
 
-    /* If `build-users-group' is not empty, then we have to build as
+    /* Are we doing a chroot build? */
-       one of the members of that group. */
+    {
-    if (settings.buildUsersGroup != "" && getuid() == 0) {
+        auto noChroot = parsedDrv->getBoolAttr("__noChroot");
-#if defined(__linux__) || defined(__APPLE__)
+        if (settings.sandboxMode == smEnabled) {
-        if (!buildUser) buildUser = std::make_unique<UserLock>();
+            if (noChroot)
+                throw Error("derivation '%s' has '__noChroot' set, "
+                    "but that's not allowed when 'sandbox' is 'true'", worker.store.printStorePath(drvPath));
+#if __APPLE__
+            if (additionalSandboxProfile != "")
+                throw Error("derivation '%s' specifies a sandbox profile, "
+                    "but this is only allowed when 'sandbox' is 'relaxed'", worker.store.printStorePath(drvPath));
+#endif
+            useChroot = true;
+        }
+        else if (settings.sandboxMode == smDisabled)
+            useChroot = false;
+        else if (settings.sandboxMode == smRelaxed)
+            useChroot = derivationType.isSandboxed() && !noChroot;
+    }
 
-        if (buildUser->findFreeUser()) {
+    auto & localStore = getLocalStore();
-            /* Make sure that no other processes are executing under this
+    if (localStore.storeDir != localStore.realStoreDir.get()) {
-               uid. */
+        #if __linux__
-            buildUser->kill();
+            useChroot = true;
-        } else {
+        #else
+            throw Error("building using a diverted store is not supported on this platform");
+        #endif
+    }
+
+    if (useBuildUsers()) {
+        if (!buildUser)
+            buildUser = acquireUserLock(parsedDrv->useUidRange() ? 65536 : 1, useChroot);
+
+        if (!buildUser) {
             if (!actLock)
                 actLock = std::make_unique<Activity>(*logger, lvlWarn, actBuildWaiting,
                     fmt("waiting for UID to build '%s'", yellowtxt(worker.store.printStorePath(drvPath))));
             worker.waitForAWhile(shared_from_this());
             return;
         }
-#else
-        /* Don't know how to block the creation of setuid/setgid
-           binaries on this platform. */
-        throw Error("build users are not supported on this platform for security reasons");
-#endif
     }
 
     actLock.reset();
@@ -270,7 +307,7 @@ void LocalDerivationGoal::cleanupPostChildKill()
        malicious user from leaving behind a process that keeps files
        open and modifies them after they have been chown'ed to
        root. */
-    if (buildUser) buildUser->kill();
+    killSandbox(true);
 
     /* Terminate the recursive Nix daemon. */
     stopDaemon();
@@ -363,6 +400,64 @@ static void linkOrCopy(const Path & from, const Path & to)
 
 void LocalDerivationGoal::startBuilder()
 {
+    if ((buildUser && buildUser->getUIDCount() != 1)
+        #if __linux__
+        || settings.useCgroups
+        #endif
+        )
+    {
+        #if __linux__
+        settings.requireExperimentalFeature(Xp::Cgroups);
+
+        auto cgroupFS = getCgroupFS();
+        if (!cgroupFS)
+            throw Error("cannot determine the cgroups file system");
+
+        auto ourCgroups = getCgroups("/proc/self/cgroup");
+        auto ourCgroup = ourCgroups[""];
+        if (ourCgroup == "")
+            throw Error("cannot determine cgroup name from /proc/self/cgroup");
+
+        auto ourCgroupPath = canonPath(*cgroupFS + "/" + ourCgroup);
+
+        if (!pathExists(ourCgroupPath))
+            throw Error("expected cgroup directory '%s'", ourCgroupPath);
+
+        static std::atomic<unsigned int> counter{0};
+
+        cgroup = buildUser
+            ? fmt("%s/nix-build-uid-%d", ourCgroupPath, buildUser->getUID())
+            : fmt("%s/nix-build-pid-%d-%d", ourCgroupPath, getpid(), counter++);
+
+        debug("using cgroup '%s'", *cgroup);
+
+        /* When using a build user, record the cgroup we used for that
+           user so that if we got interrupted previously, we can kill
+           any left-over cgroup first. */
+        if (buildUser) {
+            auto cgroupsDir = settings.nixStateDir + "/cgroups";
+            createDirs(cgroupsDir);
+
+            auto cgroupFile = fmt("%s/%d", cgroupsDir, buildUser->getUID());
+
+            if (pathExists(cgroupFile)) {
+                auto prevCgroup = readFile(cgroupFile);
+                destroyCgroup(prevCgroup);
+            }
+
+            writeFile(cgroupFile, *cgroup);
+        }
+
+        #else
+        throw Error("cgroups are not supported on this platform");
+        #endif
+    }
+
+    /* Make sure that no other processes are executing under the
+       sandbox uids. This must be done before any chownToBuilder()
+       calls. */
+    killSandbox(false);
+
     /* Right platform? */
     if (!parsedDrv->canBuildLocally(worker.store))
         throw Error("a '%s' with features {%s} is required to build '%s', but I am a '%s' with features {%s}",
@@ -376,35 +471,6 @@ void LocalDerivationGoal::startBuilder()
     additionalSandboxProfile = parsedDrv->getStringAttr("__sandboxProfile").value_or("");
 #endif
 
-    /* Are we doing a chroot build? */
-    {
-        auto noChroot = parsedDrv->getBoolAttr("__noChroot");
-        if (settings.sandboxMode == smEnabled) {
-            if (noChroot)
-                throw Error("derivation '%s' has '__noChroot' set, "
-                    "but that's not allowed when 'sandbox' is 'true'", worker.store.printStorePath(drvPath));
-#if __APPLE__
-            if (additionalSandboxProfile != "")
-                throw Error("derivation '%s' specifies a sandbox profile, "
-                    "but this is only allowed when 'sandbox' is 'relaxed'", worker.store.printStorePath(drvPath));
-#endif
-            useChroot = true;
-        }
-        else if (settings.sandboxMode == smDisabled)
-            useChroot = false;
-        else if (settings.sandboxMode == smRelaxed)
-            useChroot = derivationType.isSandboxed() && !noChroot;
-    }
-
-    auto & localStore = getLocalStore();
-    if (localStore.storeDir != localStore.realStoreDir.get()) {
-        #if __linux__
-            useChroot = true;
-        #else
-            throw Error("building using a diverted store is not supported on this platform");
-        #endif
-    }
-
     /* Create a temporary directory where the build will take
        place. */
     tmpDir = createTempDir("", "nix-build-" + std::string(drvPath.name()), false, false, 0700);
@@ -580,10 +646,11 @@ void LocalDerivationGoal::startBuilder()
 
         printMsg(lvlChatty, format("setting up chroot environment in '%1%'") % chrootRootDir);
 
-        if (mkdir(chrootRootDir.c_str(), 0750) == -1)
+        // FIXME: make this 0700
+        if (mkdir(chrootRootDir.c_str(), buildUser && buildUser->getUIDCount() != 1 ? 0755 : 0750) == -1)
             throw SysError("cannot create '%1%'", chrootRootDir);
 
-        if (buildUser && chown(chrootRootDir.c_str(), 0, buildUser->getGID()) == -1)
+        if (buildUser && chown(chrootRootDir.c_str(), buildUser->getUIDCount() != 1 ? buildUser->getUID() : 0, buildUser->getGID()) == -1)
             throw SysError("cannot change ownership of '%1%'", chrootRootDir);
 
         /* Create a writable /tmp in the chroot.  Many builders need
@@ -597,6 +664,10 @@ void LocalDerivationGoal::startBuilder()
            nobody account.  The latter is kind of a hack to support
            Samba-in-QEMU. */
         createDirs(chrootRootDir + "/etc");
+        chownToBuilder(chrootRootDir + "/etc");
+
+        if (parsedDrv->useUidRange() && (!buildUser || buildUser->getUIDCount() < 65536))
+            throw Error("feature 'uid-range' requires the setting '%s' to be enabled", settings.autoAllocateUids.name);
 
         /* Declare the build user's group so that programs get a consistent
            view of the system (e.g., "id -gn"). */
@@ -647,12 +718,28 @@ void LocalDerivationGoal::startBuilder()
                 dirsInChroot.erase(worker.store.printStorePath(*i.second.second));
         }
 
-#elif __APPLE__
+        if (cgroup) {
-        /* We don't really have any parent prep work to do (yet?)
+            if (mkdir(cgroup->c_str(), 0755) != 0)
-           All work happens in the child, instead. */
+                throw SysError("creating cgroup '%s'", *cgroup);
+            chownToBuilder(*cgroup);
+            chownToBuilder(*cgroup + "/cgroup.procs");
+            chownToBuilder(*cgroup + "/cgroup.threads");
+            //chownToBuilder(*cgroup + "/cgroup.subtree_control");
+        }
+
 #else
-        throw Error("sandboxing builds is not supported on this platform");
+        if (parsedDrv->useUidRange())
+            throw Error("feature 'uid-range' is not supported on this platform");
+        #if __APPLE__
+            /* We don't really have any parent prep work to do (yet?)
+               All work happens in the child, instead. */
+        #else
+            throw Error("sandboxing builds is not supported on this platform");
+        #endif
 #endif
+    } else {
+        if (parsedDrv->useUidRange())
+            throw Error("feature 'uid-range' is only supported in sandboxed builds");
     }
 
     if (needsHashRewrite() && pathExists(homeDir))
@@ -913,14 +1000,16 @@ void LocalDerivationGoal::startBuilder()
                the calling user (if build users are disabled). */
             uid_t hostUid = buildUser ? buildUser->getUID() : getuid();
             uid_t hostGid = buildUser ? buildUser->getGID() : getgid();
+            uid_t nrIds = buildUser ? buildUser->getUIDCount() : 1;
 
             writeFile("/proc/" + std::to_string(pid) + "/uid_map",
-                fmt("%d %d 1", sandboxUid(), hostUid));
+                fmt("%d %d %d", sandboxUid(), hostUid, nrIds));
 
-            writeFile("/proc/" + std::to_string(pid) + "/setgroups", "deny");
+            if (!buildUser || buildUser->getUIDCount() == 1)
+                writeFile("/proc/" + std::to_string(pid) + "/setgroups", "deny");
 
             writeFile("/proc/" + std::to_string(pid) + "/gid_map",
-                fmt("%d %d 1", sandboxGid(), hostGid));
+                fmt("%d %d %d", sandboxGid(), hostGid, nrIds));
         } else {
             debug("note: not using a user namespace");
             if (!buildUser)
@@ -947,6 +1036,10 @@ void LocalDerivationGoal::startBuilder()
                 throw SysError("getting sandbox user namespace");
         }
 
+        /* Move the child into its own cgroup. */
+        if (cgroup)
+            writeFile(*cgroup + "/cgroup.procs", fmt("%d", (pid_t) pid));
+
         /* Signal the builder that we've updated its user namespace. */
         writeFull(userNamespaceSync.writeSide.get(), "1");
 
@@ -1779,6 +1872,13 @@ void LocalDerivationGoal::runChild()
             if (mount("none", (chrootRootDir + "/proc").c_str(), "proc", 0, 0) == -1)
                 throw SysError("mounting /proc");
 
+            /* Mount sysfs on /sys. */
+            if (buildUser && buildUser->getUIDCount() != 1) {
+                createDirs(chrootRootDir + "/sys");
+                if (mount("none", (chrootRootDir + "/sys").c_str(), "sysfs", 0, 0) == -1)
+                    throw SysError("mounting /sys");
+            }
+
             /* Mount a new tmpfs on /dev/shm to ensure that whatever
                the builder puts in /dev/shm is cleaned up automatically. */
             if (pathExists("/dev/shm") && mount("none", (chrootRootDir + "/dev/shm").c_str(), "tmpfs", 0,
@@ -1821,6 +1921,12 @@ void LocalDerivationGoal::runChild()
             if (unshare(CLONE_NEWNS) == -1)
                 throw SysError("unsharing mount namespace");
 
+            /* Unshare the cgroup namespace. This means
+               /proc/self/cgroup will show the child's cgroup as '/'
+               rather than whatever it is in the parent. */
+            if (cgroup && unshare(CLONE_NEWCGROUP) == -1)
+                throw SysError("unsharing cgroup namespace");
+
             /* Do the chroot(). */
             if (chdir(chrootRootDir.c_str()) == -1)
                 throw SysError("cannot change directory to '%1%'", chrootRootDir);
@@ -1906,9 +2012,8 @@ void LocalDerivationGoal::runChild()
         if (setUser && buildUser) {
             /* Preserve supplementary groups of the build user, to allow
                admins to specify groups such as "kvm".  */
-            if (!buildUser->getSupplementaryGIDs().empty() &&
+            auto gids = buildUser->getSupplementaryGIDs();
-                setgroups(buildUser->getSupplementaryGIDs().size(),
+            if (setgroups(gids.size(), gids.data()) == -1)
-                          buildUser->getSupplementaryGIDs().data()) == -1)
                 throw SysError("cannot set supplementary groups of build user");
 
             if (setgid(buildUser->getGID()) == -1 ||
@@ -2155,7 +2260,6 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
     InodesSeen inodesSeen;
 
     Path checkSuffix = ".check";
-    bool keepPreviousRound = settings.keepFailed || settings.runDiffHook;
 
     std::exception_ptr delayedException;
 
@@ -2237,7 +2341,10 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
         /* Canonicalise first.  This ensures that the path we're
            rewriting doesn't contain a hard link to /etc/shadow or
            something like that. */
-        canonicalisePathMetaData(actualPath, buildUser ? buildUser->getUID() : -1, inodesSeen);
+        canonicalisePathMetaData(
+            actualPath,
+            buildUser ? std::optional(buildUser->getUIDRange()) : std::nullopt,
+            inodesSeen);
 
         debug("scanning for references for output '%s' in temp location '%s'", outputName, actualPath);
 
@@ -2330,6 +2437,10 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
                 sink.s = rewriteStrings(sink.s, outputRewrites);
                 StringSource source(sink.s);
                 restorePath(actualPath, source);
+
+                /* FIXME: set proper permissions in restorePath() so
+                   we don't have to do another traversal. */
+                canonicalisePathMetaData(actualPath, {}, inodesSeen);
             }
         };
 
@@ -2492,7 +2603,7 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
 
         /* FIXME: set proper permissions in restorePath() so
             we don't have to do another traversal. */
-        canonicalisePathMetaData(actualPath, -1, inodesSeen);
+        canonicalisePathMetaData(actualPath, {}, inodesSeen);
 
         /* Calculate where we'll move the output files. In the checking case we
            will leave leave them where they are, for now, rather than move to
@@ -2576,10 +2687,8 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
                 debug("unreferenced input: '%1%'", worker.store.printStorePath(i));
         }
 
-        if (curRound == nrRounds) {
+        localStore.optimisePath(actualPath, NoRepair); // FIXME: combine with scanForReferences()
-            localStore.optimisePath(actualPath, NoRepair); // FIXME: combine with scanForReferences()
+        worker.markContentsGood(newInfo.path);
-            worker.markContentsGood(newInfo.path);
-        }
 
         newInfo.deriver = drvPath;
         newInfo.ultimate = true;
@@ -2608,61 +2717,6 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
     /* Apply output checks. */
     checkOutputs(infos);
 
-    /* Compare the result with the previous round, and report which
-       path is different, if any.*/
-    if (curRound > 1 && prevInfos != infos) {
-        assert(prevInfos.size() == infos.size());
-        for (auto i = prevInfos.begin(), j = infos.begin(); i != prevInfos.end(); ++i, ++j)
-            if (!(*i == *j)) {
-                buildResult.isNonDeterministic = true;
-                Path prev = worker.store.printStorePath(i->second.path) + checkSuffix;
-                bool prevExists = keepPreviousRound && pathExists(prev);
-                hintformat hint = prevExists
-                    ? hintfmt("output '%s' of '%s' differs from '%s' from previous round",
-                        worker.store.printStorePath(i->second.path), worker.store.printStorePath(drvPath), prev)
-                    : hintfmt("output '%s' of '%s' differs from previous round",
-                        worker.store.printStorePath(i->second.path), worker.store.printStorePath(drvPath));
-
-                handleDiffHook(
-                    buildUser ? buildUser->getUID() : getuid(),
-                    buildUser ? buildUser->getGID() : getgid(),
-                    prev, worker.store.printStorePath(i->second.path),
-                    worker.store.printStorePath(drvPath), tmpDir);
-
-                if (settings.enforceDeterminism)
-                    throw NotDeterministic(hint);
-
-                printError(hint);
-
-                curRound = nrRounds; // we know enough, bail out early
-            }
-    }
-
-    /* If this is the first round of several, then move the output out of the way. */
-    if (nrRounds > 1 && curRound == 1 && curRound < nrRounds && keepPreviousRound) {
-        for (auto & [_, outputStorePath] : finalOutputs) {
-            auto path = worker.store.printStorePath(outputStorePath);
-            Path prev = path + checkSuffix;
-            deletePath(prev);
-            Path dst = path + checkSuffix;
-            renameFile(path, dst);
-        }
-    }
-
-    if (curRound < nrRounds) {
-        prevInfos = std::move(infos);
-        return {};
-    }
-
-    /* Remove the .check directories if we're done. FIXME: keep them
-       if the result was not determistic? */
-    if (curRound == nrRounds) {
-        for (auto & [_, outputStorePath] : finalOutputs) {
-            Path prev = worker.store.printStorePath(outputStorePath) + checkSuffix;
-            deletePath(prev);
-        }
-    }
-
     /* Register each output path as valid, and register the sets of
        paths referenced by each of them.  If there are cycles in the
        outputs, this will fail. */

src/libstore/build/local-derivation-goal.hh

@@ -15,6 +15,9 @@ struct LocalDerivationGoal : public DerivationGoal
     /* The process ID of the builder. */
     Pid pid;
 
+    /* The cgroup of the builder, if any. */
+    std::optional<Path> cgroup;
+
     /* The temporary directory. */
     Path tmpDir;
 
@@ -92,8 +95,8 @@ struct LocalDerivationGoal : public DerivationGoal
        result. */
     std::map<Path, ValidPathInfo> prevInfos;
 
-    uid_t sandboxUid() { return usingUserNamespace ? 1000 : buildUser->getUID(); }
+    uid_t sandboxUid() { return usingUserNamespace ? (!buildUser || buildUser->getUIDCount() == 1 ? 1000 : 0) : buildUser->getUID(); }
-    gid_t sandboxGid() { return usingUserNamespace ?  100 : buildUser->getGID(); }
+    gid_t sandboxGid() { return usingUserNamespace ? (!buildUser || buildUser->getUIDCount() == 1 ? 100  : 0) : buildUser->getGID(); }
 
     const static Path homeDir;
 
@@ -197,6 +200,10 @@ struct LocalDerivationGoal : public DerivationGoal
     /* Forcibly kill the child process, if any. */
     void killChild() override;
 
+    /* Kill any processes running under the build user UID or in the
+       cgroup of the build. */
+    void killSandbox(bool getStats);
+
     /* Create alternative path calculated from but distinct from the
        input, so we can avoid overwriting outputs (or other store paths)
        that already exist. */

src/libstore/daemon.cc

@@ -238,7 +238,6 @@ struct ClientSettings
                 }
                 else if (trusted
                     || name == settings.buildTimeout.name
-                    || name == settings.buildRepeat.name
                     || name == settings.maxSilentTime.name
                     || name == settings.pollInterval.name
                     || name == "connect-timeout"

src/libstore/gc.cc

@@ -147,7 +147,7 @@ void LocalStore::addTempRoot(const StorePath & path)
         } catch (SysError & e) {
             /* The garbage collector may have exited, so we need to
                restart. */
-            if (e.errNo == EPIPE) {
+            if (e.errNo == EPIPE || e.errNo == ECONNRESET) {
                 debug("GC socket disconnected");
                 state->fdRootsSocket.close();
                 goto restart;
@@ -506,6 +506,7 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
 
         Finally cleanup([&]() {
             debug("GC roots server shutting down");
+            fdServer.close();
             while (true) {
                 auto item = remove_begin(*connections.lock());
                 if (!item) break;

src/libstore/globals.cc

@@ -130,6 +130,10 @@ StringSet Settings::getDefaultSystemFeatures()
        actually require anything special on the machines. */
     StringSet features{"nixos-test", "benchmark", "big-parallel"};
 
+    #if __linux__
+    features.insert("uid-range");
+    #endif
+
     #if __linux__
     if (access("/dev/kvm", R_OK | W_OK) == 0)
         features.insert("kvm");

src/libstore/globals.hh

@@ -46,6 +46,14 @@ struct PluginFilesSetting : public BaseSetting<Paths>
     void set(const std::string & str, bool append = false) override;
 };
 
+const uint32_t maxIdsPerBuild =
+    #if __linux__
+    1 << 16
+    #else
+    1
+    #endif
+    ;
+
 class Settings : public Config {
 
     unsigned int getDefaultCores();
@@ -275,6 +283,64 @@ public:
           multi-user settings with untrusted users.
         )"};
 
+    Setting<bool> autoAllocateUids{this, false, "auto-allocate-uids",
+        R"(
+          Whether to select UIDs for builds automatically, instead of using the
+          users in `build-users-group`.
+
+          UIDs are allocated starting at 872415232 (0x34000000) on Linux and 56930 on macOS.
+
+          > **Warning**
+          > This is an experimental feature.
+
+          To enable it, add the following to [`nix.conf`](#):
+
+          ```
+          extra-experimental-features = auto-allocate-uids
+          auto-allocate-uids = true
+          ```
+        )"};
+
+    Setting<uint32_t> startId{this,
+        #if __linux__
+        0x34000000,
+        #else
+        56930,
+        #endif
+        "start-id",
+        "The first UID and GID to use for dynamic ID allocation."};
+
+    Setting<uint32_t> uidCount{this,
+        #if __linux__
+        maxIdsPerBuild * 128,
+        #else
+        128,
+        #endif
+        "id-count",
+        "The number of UIDs/GIDs to use for dynamic ID allocation."};
+
+    #if __linux__
+    Setting<bool> useCgroups{
+        this, false, "use-cgroups",
+        R"(
+          Whether to execute builds inside cgroups.
+          This is only supported on Linux.
+
+          Cgroups are required and enabled automatically for derivations 
+          that require the `uid-range` system feature.
+
+          > **Warning**
+          > This is an experimental feature.
+
+          To enable it, add the following to [`nix.conf`](#):
+
+          ```
+          extra-experimental-features = cgroups
+          use-cgroups = true
+          ```
+        )"};
+    #endif
+
     Setting<bool> impersonateLinux26{this, false, "impersonate-linux-26",
         "Whether to impersonate a Linux 2.6 machine on newer kernels.",
         {"build-impersonate-linux-26"}};
@@ -307,11 +373,6 @@ public:
         )",
         {"build-max-log-size"}};
 
-    /* When buildRepeat > 0 and verboseBuild == true, whether to print
-       repeated builds (i.e. builds other than the first one) to
-       stderr. Hack to prevent Hydra logs from being polluted. */
-    bool printRepeatedBuilds = true;
-
     Setting<unsigned int> pollInterval{this, 5, "build-poll-interval",
         "How often (in seconds) to poll for locks."};
 
@@ -435,19 +496,6 @@ public:
     Setting<bool> sandboxFallback{this, true, "sandbox-fallback",
         "Whether to disable sandboxing when the kernel doesn't allow it."};
 
-    Setting<size_t> buildRepeat{
-        this, 0, "repeat",
-        R"(
-          How many times to repeat builds to check whether they are
-          deterministic. The default value is 0. If the value is non-zero,
-          every build is repeated the specified number of times. If the
-          contents of any of the runs differs from the previous ones and
-          `enforce-determinism` is true, the build is rejected and the
-          resulting store paths are not registered as “valid” in Nix’s
-          database.
-        )",
-        {"build-repeat"}};
-
 #if __linux__
     Setting<std::string> sandboxShmSize{
         this, "50%", "sandbox-dev-shm-size",
@@ -511,10 +559,6 @@ public:
           configuration file, and cannot be passed at the command line.
         )"};
 
-    Setting<bool> enforceDeterminism{
-        this, true, "enforce-determinism",
-        "Whether to fail if repeated builds produce different output. See `repeat`."};
-
     Setting<Strings> trustedPublicKeys{
         this,
         {"cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY="},

src/libstore/legacy-ssh-store.cc

@@ -255,8 +255,8 @@ private:
                 << settings.maxLogSize;
         if (GET_PROTOCOL_MINOR(conn.remoteVersion) >= 3)
             conn.to
-                << settings.buildRepeat
+                << 0 // buildRepeat hasn't worked for ages anyway
-                << settings.enforceDeterminism;
+                << 0;
 
         if (GET_PROTOCOL_MINOR(conn.remoteVersion) >= 7) {
             conn.to << ((int) settings.keepFailed);

src/libstore/local-store.cc

@@ -583,7 +583,10 @@ void canonicaliseTimestampAndPermissions(const Path & path)
 }
 
 
-static void canonicalisePathMetaData_(const Path & path, uid_t fromUid, InodesSeen & inodesSeen)
+static void canonicalisePathMetaData_(
+    const Path & path,
+    std::optional<std::pair<uid_t, uid_t>> uidRange,
+    InodesSeen & inodesSeen)
 {
     checkInterrupt();
 
@@ -630,7 +633,7 @@ static void canonicalisePathMetaData_(const Path & path, uid_t fromUid, InodesSe
        However, ignore files that we chown'ed ourselves previously to
        ensure that we don't fail on hard links within the same build
        (i.e. "touch $out/foo; ln $out/foo $out/bar"). */
-    if (fromUid != (uid_t) -1 && st.st_uid != fromUid) {
+    if (uidRange && (st.st_uid < uidRange->first || st.st_uid > uidRange->second)) {
         if (S_ISDIR(st.st_mode) || !inodesSeen.count(Inode(st.st_dev, st.st_ino)))
             throw BuildError("invalid ownership on file '%1%'", path);
         mode_t mode = st.st_mode & ~S_IFMT;
@@ -663,14 +666,17 @@ static void canonicalisePathMetaData_(const Path & path, uid_t fromUid, InodesSe
     if (S_ISDIR(st.st_mode)) {
         DirEntries entries = readDirectory(path);
         for (auto & i : entries)
-            canonicalisePathMetaData_(path + "/" + i.name, fromUid, inodesSeen);
+            canonicalisePathMetaData_(path + "/" + i.name, uidRange, inodesSeen);
     }
 }
 
 
-void canonicalisePathMetaData(const Path & path, uid_t fromUid, InodesSeen & inodesSeen)
+void canonicalisePathMetaData(
+    const Path & path,
+    std::optional<std::pair<uid_t, uid_t>> uidRange,
+    InodesSeen & inodesSeen)
 {
-    canonicalisePathMetaData_(path, fromUid, inodesSeen);
+    canonicalisePathMetaData_(path, uidRange, inodesSeen);
 
     /* On platforms that don't have lchown(), the top-level path can't
        be a symlink, since we can't change its ownership. */
@@ -683,10 +689,11 @@ void canonicalisePathMetaData(const Path & path, uid_t fromUid, InodesSeen & ino
 }
 
 
-void canonicalisePathMetaData(const Path & path, uid_t fromUid)
+void canonicalisePathMetaData(const Path & path,
+    std::optional<std::pair<uid_t, uid_t>> uidRange)
 {
     InodesSeen inodesSeen;
-    canonicalisePathMetaData(path, fromUid, inodesSeen);
+    canonicalisePathMetaData(path, uidRange, inodesSeen);
 }
 
 
@@ -1331,7 +1338,7 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
 
             autoGC();
 
-            canonicalisePathMetaData(realPath, -1);
+            canonicalisePathMetaData(realPath, {});
 
             optimisePath(realPath, repair); // FIXME: combine with hashPath()
 
@@ -1444,7 +1451,7 @@ StorePath LocalStore::addToStoreFromDump(Source & source0, std::string_view name
                 narHash = narSink.finish();
             }
 
-            canonicalisePathMetaData(realPath, -1); // FIXME: merge into restorePath
+            canonicalisePathMetaData(realPath, {}); // FIXME: merge into restorePath
 
             optimisePath(realPath, repair);
 
@@ -1486,7 +1493,7 @@ StorePath LocalStore::addTextToStore(
 
             writeFile(realPath, s);
 
-            canonicalisePathMetaData(realPath, -1);
+            canonicalisePathMetaData(realPath, {});
 
             StringSink sink;
             dumpString(s, sink);

src/libstore/local-store.hh

@@ -310,9 +310,18 @@ typedef std::set<Inode> InodesSeen;
    - the permissions are set of 444 or 555 (i.e., read-only with or
      without execute permission; setuid bits etc. are cleared)
    - the owner and group are set to the Nix user and group, if we're
-     running as root. */
+     running as root.
-void canonicalisePathMetaData(const Path & path, uid_t fromUid, InodesSeen & inodesSeen);
+   If uidRange is not empty, this function will throw an error if it
-void canonicalisePathMetaData(const Path & path, uid_t fromUid);
+   encounters files owned by a user outside of the closed interval
+   [uidRange->first, uidRange->second].
+*/
+void canonicalisePathMetaData(
+    const Path & path,
+    std::optional<std::pair<uid_t, uid_t>> uidRange,
+    InodesSeen & inodesSeen);
+void canonicalisePathMetaData(
+    const Path & path,
+    std::optional<std::pair<uid_t, uid_t>> uidRange);
 
 void canonicaliseTimestampAndPermissions(const Path & path);
 

src/libstore/lock.cc

@@ -2,105 +2,197 @@
 #include "globals.hh"
 #include "pathlocks.hh"
 
-#include <grp.h>
 #include <pwd.h>
-
+#include <grp.h>
-#include <fcntl.h>
-#include <unistd.h>
 
 namespace nix {
 
-UserLock::UserLock()
+struct SimpleUserLock : UserLock
 {
-    assert(settings.buildUsersGroup != "");
+    AutoCloseFD fdUserLock;
-    createDirs(settings.nixStateDir + "/userpool");
+    uid_t uid;
-}
+    gid_t gid;
+    std::vector<gid_t> supplementaryGIDs;
 
-bool UserLock::findFreeUser() {
+    uid_t getUID() override { assert(uid); return uid; }
-    if (enabled()) return true;
+    uid_t getUIDCount() override { return 1; }
+    gid_t getGID() override { assert(gid); return gid; }
 
-    /* Get the members of the build-users-group. */
+    std::vector<gid_t> getSupplementaryGIDs() override { return supplementaryGIDs; }
-    struct group * gr = getgrnam(settings.buildUsersGroup.get().c_str());
-    if (!gr)
-        throw Error("the group '%1%' specified in 'build-users-group' does not exist",
-            settings.buildUsersGroup);
-    gid = gr->gr_gid;
 
-    /* Copy the result of getgrnam. */
+    static std::unique_ptr<UserLock> acquire()
-    Strings users;
+    {
-    for (char * * p = gr->gr_mem; *p; ++p) {
+        assert(settings.buildUsersGroup != "");
-        debug("found build user '%1%'", *p);
+        createDirs(settings.nixStateDir + "/userpool");
-        users.push_back(*p);
-    }
 
-    if (users.empty())
+        /* Get the members of the build-users-group. */
-        throw Error("the build users group '%1%' has no members",
+        struct group * gr = getgrnam(settings.buildUsersGroup.get().c_str());
-            settings.buildUsersGroup);
+        if (!gr)
+            throw Error("the group '%s' specified in 'build-users-group' does not exist", settings.buildUsersGroup);
 
-    /* Find a user account that isn't currently in use for another
+        /* Copy the result of getgrnam. */
-       build. */
+        Strings users;
-    for (auto & i : users) {
+        for (char * * p = gr->gr_mem; *p; ++p) {
-        debug("trying user '%1%'", i);
+            debug("found build user '%s'", *p);
-
+            users.push_back(*p);
-        struct passwd * pw = getpwnam(i.c_str());
-        if (!pw)
-            throw Error("the user '%1%' in the group '%2%' does not exist",
-                i, settings.buildUsersGroup);
-
-
-        fnUserLock = (format("%1%/userpool/%2%") % settings.nixStateDir % pw->pw_uid).str();
-
-        AutoCloseFD fd = open(fnUserLock.c_str(), O_RDWR | O_CREAT | O_CLOEXEC, 0600);
-        if (!fd)
-            throw SysError("opening user lock '%1%'", fnUserLock);
-
-        if (lockFile(fd.get(), ltWrite, false)) {
-            fdUserLock = std::move(fd);
-            user = i;
-            uid = pw->pw_uid;
-
-            /* Sanity check... */
-            if (uid == getuid() || uid == geteuid())
-                throw Error("the Nix user should not be a member of '%1%'",
-                    settings.buildUsersGroup);
-
-#if __linux__
-            /* Get the list of supplementary groups of this build user.  This
-               is usually either empty or contains a group such as "kvm".  */
-            int ngroups = 32; // arbitrary initial guess
-            supplementaryGIDs.resize(ngroups);
-
-            int err = getgrouplist(pw->pw_name, pw->pw_gid, supplementaryGIDs.data(),
-                                 &ngroups);
-
-            // Our initial size of 32 wasn't sufficient, the correct size has
-            // been stored in ngroups, so we try again.
-            if (err == -1) {
-                supplementaryGIDs.resize(ngroups);
-                err = getgrouplist(pw->pw_name, pw->pw_gid, supplementaryGIDs.data(),
-                                   &ngroups);
-            }
-
-            // If it failed once more, then something must be broken.
-            if (err == -1)
-                throw Error("failed to get list of supplementary groups for '%1%'",
-                            pw->pw_name);
-
-            // Finally, trim back the GID list to its real size
-            supplementaryGIDs.resize(ngroups);
-#endif
-
-            isEnabled = true;
-            return true;
         }
+
+        if (users.empty())
+            throw Error("the build users group '%s' has no members", settings.buildUsersGroup);
+
+        /* Find a user account that isn't currently in use for another
+           build. */
+        for (auto & i : users) {
+            debug("trying user '%s'", i);
+
+            struct passwd * pw = getpwnam(i.c_str());
+            if (!pw)
+                throw Error("the user '%s' in the group '%s' does not exist", i, settings.buildUsersGroup);
+
+            auto fnUserLock = fmt("%s/userpool/%s", settings.nixStateDir,pw->pw_uid);
+
+            AutoCloseFD fd = open(fnUserLock.c_str(), O_RDWR | O_CREAT | O_CLOEXEC, 0600);
+            if (!fd)
+                throw SysError("opening user lock '%s'", fnUserLock);
+
+            if (lockFile(fd.get(), ltWrite, false)) {
+                auto lock = std::make_unique<SimpleUserLock>();
+
+                lock->fdUserLock = std::move(fd);
+                lock->uid = pw->pw_uid;
+                lock->gid = gr->gr_gid;
+
+                /* Sanity check... */
+                if (lock->uid == getuid() || lock->uid == geteuid())
+                    throw Error("the Nix user should not be a member of '%s'", settings.buildUsersGroup);
+
+                #if __linux__
+                /* Get the list of supplementary groups of this build
+                   user.  This is usually either empty or contains a
+                   group such as "kvm".  */
+                int ngroups = 32; // arbitrary initial guess
+                std::vector<gid_t> gids;
+                gids.resize(ngroups);
+
+                int err = getgrouplist(
+                    pw->pw_name, pw->pw_gid,
+                    gids.data(),
+                    &ngroups);
+
+                /* Our initial size of 32 wasn't sufficient, the
+                   correct size has been stored in ngroups, so we try
+                   again. */
+                if (err == -1) {
+                    gids.resize(ngroups);
+                    err = getgrouplist(
+                        pw->pw_name, pw->pw_gid,
+                        gids.data(),
+                        &ngroups);
+                }
+
+                // If it failed once more, then something must be broken.
+                if (err == -1)
+                    throw Error("failed to get list of supplementary groups for '%s'", pw->pw_name);
+
+                // Finally, trim back the GID list to its real size.
+                for (auto i = 0; i < ngroups; i++)
+                    if (gids[i] != lock->gid)
+                        lock->supplementaryGIDs.push_back(gids[i]);
+                #endif
+
+                return lock;
+            }
+        }
+
+        return nullptr;
     }
+};
 
-    return false;
+struct AutoUserLock : UserLock
-}
-
-void UserLock::kill()
 {
-    killUser(uid);
+    AutoCloseFD fdUserLock;
+    uid_t firstUid = 0;
+    gid_t firstGid = 0;
+    uid_t nrIds = 1;
+
+    uid_t getUID() override { assert(firstUid); return firstUid; }
+
+    gid_t getUIDCount() override { return nrIds; }
+
+    gid_t getGID() override { assert(firstGid); return firstGid; }
+
+    std::vector<gid_t> getSupplementaryGIDs() override { return {}; }
+
+    static std::unique_ptr<UserLock> acquire(uid_t nrIds, bool useChroot)
+    {
+        settings.requireExperimentalFeature(Xp::AutoAllocateUids);
+        assert(settings.startId > 0);
+        assert(settings.uidCount % maxIdsPerBuild == 0);
+        assert((uint64_t) settings.startId + (uint64_t) settings.uidCount <= std::numeric_limits<uid_t>::max());
+        assert(nrIds <= maxIdsPerBuild);
+
+        createDirs(settings.nixStateDir + "/userpool2");
+
+        size_t nrSlots = settings.uidCount / maxIdsPerBuild;
+
+        for (size_t i = 0; i < nrSlots; i++) {
+            debug("trying user slot '%d'", i);
+
+            createDirs(settings.nixStateDir + "/userpool2");
+
+            auto fnUserLock = fmt("%s/userpool2/slot-%d", settings.nixStateDir, i);
+
+            AutoCloseFD fd = open(fnUserLock.c_str(), O_RDWR | O_CREAT | O_CLOEXEC, 0600);
+            if (!fd)
+                throw SysError("opening user lock '%s'", fnUserLock);
+
+            if (lockFile(fd.get(), ltWrite, false)) {
+
+                auto firstUid = settings.startId + i * maxIdsPerBuild;
+
+                auto pw = getpwuid(firstUid);
+                if (pw)
+                    throw Error("auto-allocated UID %d clashes with existing user account '%s'", firstUid, pw->pw_name);
+
+                auto lock = std::make_unique<AutoUserLock>();
+                lock->fdUserLock = std::move(fd);
+                lock->firstUid = firstUid;
+                if (useChroot)
+                    lock->firstGid = firstUid;
+                else {
+                    struct group * gr = getgrnam(settings.buildUsersGroup.get().c_str());
+                    if (!gr)
+                        throw Error("the group '%s' specified in 'build-users-group' does not exist", settings.buildUsersGroup);
+                    lock->firstGid = gr->gr_gid;
+                }
+                lock->nrIds = nrIds;
+                return lock;
+            }
+        }
+
+        return nullptr;
+    }
+};
+
+std::unique_ptr<UserLock> acquireUserLock(uid_t nrIds, bool useChroot)
+{
+    if (settings.autoAllocateUids)
+        return AutoUserLock::acquire(nrIds, useChroot);
+    else
+        return SimpleUserLock::acquire();
+}
+
+bool useBuildUsers()
+{
+    #if __linux__
+    static bool b = (settings.buildUsersGroup != "" || settings.startId.get() != 0) && getuid() == 0;
+    return b;
+    #elif __APPLE__
+    static bool b = settings.buildUsersGroup != "" && getuid() == 0;
+    return b;
+    #else
+    return false;
+    #endif
 }
 
 }

src/libstore/lock.hh

@@ -1,37 +1,38 @@
 #pragma once
 
-#include "sync.hh"
 #include "types.hh"
-#include "util.hh"
+
+#include <optional>
+
+#include <sys/types.h>
 
 namespace nix {
 
-class UserLock
+struct UserLock
 {
-private:
+    virtual ~UserLock() { }
-    Path fnUserLock;
-    AutoCloseFD fdUserLock;
 
-    bool isEnabled = false;
+    /* Get the first and last UID. */
-    std::string user;
+    std::pair<uid_t, uid_t> getUIDRange()
-    uid_t uid = 0;
+    {
-    gid_t gid = 0;
+        auto first = getUID();
-    std::vector<gid_t> supplementaryGIDs;
+        return {first, first + getUIDCount() - 1};
+    }
 
-public:
+    /* Get the first UID. */
-    UserLock();
+    virtual uid_t getUID() = 0;
 
-    void kill();
+    virtual uid_t getUIDCount() = 0;
 
-    std::string getUser() { return user; }
+    virtual gid_t getGID() = 0;
-    uid_t getUID() { assert(uid); return uid; }
-    uid_t getGID() { assert(gid); return gid; }
-    std::vector<gid_t> getSupplementaryGIDs() { return supplementaryGIDs; }
-
-    bool findFreeUser();
-
-    bool enabled() { return isEnabled; }
 
+    virtual std::vector<gid_t> getSupplementaryGIDs() = 0;
 };
 
+/* Acquire a user lock for a UID range of size `nrIds`. Note that this
+   may return nullptr if no user is available. */
+std::unique_ptr<UserLock> acquireUserLock(uid_t nrIds, bool useChroot);
+
+bool useBuildUsers();
+
 }

src/libstore/nar-accessor.cc

@@ -1,6 +1,5 @@
 #include "nar-accessor.hh"
 #include "archive.hh"
-#include "json.hh"
 
 #include <map>
 #include <stack>
@@ -243,42 +242,43 @@ ref<FSAccessor> makeLazyNarAccessor(const std::string & listing,
     return make_ref<NarAccessor>(listing, getNarBytes);
 }
 
-void listNar(JSONPlaceholder & res, ref<FSAccessor> accessor,
+using nlohmann::json;
-    const Path & path, bool recurse)
+json listNar(ref<FSAccessor> accessor, const Path & path, bool recurse)
 {
     auto st = accessor->stat(path);
 
-    auto obj = res.object();
+    json obj = json::object();
 
     switch (st.type) {
     case FSAccessor::Type::tRegular:
-        obj.attr("type", "regular");
+        obj["type"] = "regular";
-        obj.attr("size", st.fileSize);
+        obj["size"] = st.fileSize;
         if (st.isExecutable)
-            obj.attr("executable", true);
+            obj["executable"] = true;
         if (st.narOffset)
-            obj.attr("narOffset", st.narOffset);
+            obj["narOffset"] = st.narOffset;
         break;
     case FSAccessor::Type::tDirectory:
-        obj.attr("type", "directory");
+        obj["type"] = "directory";
         {
-            auto res2 = obj.object("entries");
+            obj["entries"] = json::object();
+            json &res2 = obj["entries"];
             for (auto & name : accessor->readDirectory(path)) {
                 if (recurse) {
-                    auto res3 = res2.placeholder(name);
+                    res2[name] = listNar(accessor, path + "/" + name, true);
-                    listNar(res3, accessor, path + "/" + name, true);
                 } else
-                    res2.object(name);
+                    res2[name] = json::object();
             }
         }
         break;
     case FSAccessor::Type::tSymlink:
-        obj.attr("type", "symlink");
+        obj["type"] = "symlink";
-        obj.attr("target", accessor->readLink(path));
+        obj["target"] = accessor->readLink(path);
         break;
     default:
         throw Error("path '%s' does not exist in NAR", path);
     }
+    return obj;
 }
 
 }

src/libstore/nar-accessor.hh

@@ -2,6 +2,7 @@
 
 #include <functional>
 
+#include <nlohmann/json_fwd.hpp>
 #include "fs-accessor.hh"
 
 namespace nix {
@@ -24,11 +25,8 @@ ref<FSAccessor> makeLazyNarAccessor(
     const std::string & listing,
     GetNarBytes getNarBytes);
 
-class JSONPlaceholder;
-
 /* Write a JSON representation of the contents of a NAR (except file
    contents). */
-void listNar(JSONPlaceholder & res, ref<FSAccessor> accessor,
+nlohmann::json listNar(ref<FSAccessor> accessor, const Path & path, bool recurse);
-    const Path & path, bool recurse);
 
 }

src/libstore/parsed-derivations.cc

@@ -2,7 +2,6 @@
 
 #include <nlohmann/json.hpp>
 #include <regex>
-#include "json.hh"
 
 namespace nix {
 
@@ -90,6 +89,7 @@ std::optional<Strings> ParsedDerivation::getStringsAttr(const std::string & name
 
 StringSet ParsedDerivation::getRequiredSystemFeatures() const
 {
+    // FIXME: cache this?
     StringSet res;
     for (auto & i : getStringsAttr("requiredSystemFeatures").value_or(Strings()))
         res.insert(i);
@@ -125,6 +125,11 @@ bool ParsedDerivation::substitutesAllowed() const
     return getBoolAttr("allowSubstitutes", true);
 }
 
+bool ParsedDerivation::useUidRange() const
+{
+    return getRequiredSystemFeatures().count("uid-range");
+}
+
 static std::regex shVarName("[A-Za-z_][A-Za-z0-9_]*");
 
 std::optional<nlohmann::json> ParsedDerivation::prepareStructuredAttrs(Store & store, const StorePathSet & inputPaths)
@@ -144,16 +149,11 @@ std::optional<nlohmann::json> ParsedDerivation::prepareStructuredAttrs(Store & s
     auto e = json.find("exportReferencesGraph");
     if (e != json.end() && e->is_object()) {
         for (auto i = e->begin(); i != e->end(); ++i) {
-            std::ostringstream str;
+            StorePathSet storePaths;
-            {
+            for (auto & p : *i)
-                JSONPlaceholder jsonRoot(str, true);
+                storePaths.insert(store.parseStorePath(p.get<std::string>()));
-                StorePathSet storePaths;
+            json[i.key()] = store.pathInfoToJSON(
-                for (auto & p : *i)
+                store.exportReferences(storePaths, inputPaths), false, true);
-                    storePaths.insert(store.parseStorePath(p.get<std::string>()));
-                store.pathInfoToJSON(jsonRoot,
-                    store.exportReferences(storePaths, inputPaths), false, true);
-            }
-            json[i.key()] = nlohmann::json::parse(str.str()); // urgh
         }
     }
 

src/libstore/parsed-derivations.hh

@@ -38,6 +38,8 @@ public:
 
     bool substitutesAllowed() const;
 
+    bool useUidRange() const;
+
     std::optional<nlohmann::json> prepareStructuredAttrs(Store & store, const StorePathSet & inputPaths);
 };
 

src/libstore/remote-fs-accessor.cc

@@ -1,6 +1,6 @@
+#include <nlohmann/json.hpp>
 #include "remote-fs-accessor.hh"
 #include "nar-accessor.hh"
-#include "json.hh"
 
 #include <sys/types.h>
 #include <sys/stat.h>
@@ -38,10 +38,8 @@ ref<FSAccessor> RemoteFSAccessor::addToCache(std::string_view hashPart, std::str
 
     if (cacheDir != "") {
         try {
-            std::ostringstream str;
+            nlohmann::json j = listNar(narAccessor, "", true);
-            JSONPlaceholder jsonRoot(str);
+            writeFile(makeCacheFile(hashPart, "ls"), j.dump());
-            listNar(jsonRoot, narAccessor, "", true);
-            writeFile(makeCacheFile(hashPart, "ls"), str.str());
         } catch (...) {
             ignoreException();
         }

src/libstore/store-api.cc

@@ -6,14 +6,16 @@
 #include "util.hh"
 #include "nar-info-disk-cache.hh"
 #include "thread-pool.hh"
-#include "json.hh"
 #include "url.hh"
 #include "archive.hh"
 #include "callback.hh"
 #include "remote-store.hh"
 
+#include <nlohmann/json.hpp>
 #include <regex>
 
+using json = nlohmann::json;
+
 namespace nix {
 
 
@@ -838,56 +840,53 @@ StorePathSet Store::exportReferences(const StorePathSet & storePaths, const Stor
     return paths;
 }
 
-
+json Store::pathInfoToJSON(const StorePathSet & storePaths,
-void Store::pathInfoToJSON(JSONPlaceholder & jsonOut, const StorePathSet & storePaths,
     bool includeImpureInfo, bool showClosureSize,
     Base hashBase,
     AllowInvalidFlag allowInvalid)
 {
-    auto jsonList = jsonOut.list();
+    json::array_t jsonList = json::array();
 
     for (auto & storePath : storePaths) {
-        auto jsonPath = jsonList.object();
+        auto& jsonPath = jsonList.emplace_back(json::object());
 
         try {
             auto info = queryPathInfo(storePath);
 
-            jsonPath.attr("path", printStorePath(info->path));
+            jsonPath["path"] = printStorePath(info->path);
-            jsonPath
+            jsonPath["narHash"] = info->narHash.to_string(hashBase, true);
-                .attr("narHash", info->narHash.to_string(hashBase, true))
+            jsonPath["narSize"] = info->narSize;
-                .attr("narSize", info->narSize);
 
             {
-                auto jsonRefs = jsonPath.list("references");
+                auto& jsonRefs = (jsonPath["references"] = json::array());
                 for (auto & ref : info->references)
-                    jsonRefs.elem(printStorePath(ref));
+                    jsonRefs.emplace_back(printStorePath(ref));
             }
 
             if (info->ca)
-                jsonPath.attr("ca", renderContentAddress(info->ca));
+                jsonPath["ca"] = renderContentAddress(info->ca);
 
             std::pair<uint64_t, uint64_t> closureSizes;
 
             if (showClosureSize) {
                 closureSizes = getClosureSize(info->path);
-                jsonPath.attr("closureSize", closureSizes.first);
+                jsonPath["closureSize"] = closureSizes.first;
             }
 
             if (includeImpureInfo) {
 
                 if (info->deriver)
-                    jsonPath.attr("deriver", printStorePath(*info->deriver));
+                    jsonPath["deriver"] = printStorePath(*info->deriver);
 
                 if (info->registrationTime)
-                    jsonPath.attr("registrationTime", info->registrationTime);
+                    jsonPath["registrationTime"] = info->registrationTime;
 
                 if (info->ultimate)
-                    jsonPath.attr("ultimate", info->ultimate);
+                    jsonPath["ultimate"] = info->ultimate;
 
                 if (!info->sigs.empty()) {
-                    auto jsonSigs = jsonPath.list("signatures");
                     for (auto & sig : info->sigs)
-                        jsonSigs.elem(sig);
+                        jsonPath["signatures"].push_back(sig);
                 }
 
                 auto narInfo = std::dynamic_pointer_cast<const NarInfo>(
@@ -895,21 +894,22 @@ void Store::pathInfoToJSON(JSONPlaceholder & jsonOut, const StorePathSet & store
 
                 if (narInfo) {
                     if (!narInfo->url.empty())
-                        jsonPath.attr("url", narInfo->url);
+                        jsonPath["url"] = narInfo->url;
                     if (narInfo->fileHash)
-                        jsonPath.attr("downloadHash", narInfo->fileHash->to_string(hashBase, true));
+                        jsonPath["downloadHash"] = narInfo->fileHash->to_string(hashBase, true);
                     if (narInfo->fileSize)
-                        jsonPath.attr("downloadSize", narInfo->fileSize);
+                        jsonPath["downloadSize"] = narInfo->fileSize;
                     if (showClosureSize)
-                        jsonPath.attr("closureDownloadSize", closureSizes.second);
+                        jsonPath["closureDownloadSize"] = closureSizes.second;
                 }
             }
 
         } catch (InvalidPath &) {
-            jsonPath.attr("path", printStorePath(storePath));
+            jsonPath["path"] = printStorePath(storePath);
-            jsonPath.attr("valid", false);
+            jsonPath["valid"] = false;
         }
     }
+    return jsonList;
 }
 
 

src/libstore/store-api.hh

@@ -14,6 +14,7 @@
 #include "path-info.hh"
 #include "repair-flag.hh"
 
+#include <nlohmann/json_fwd.hpp>
 #include <atomic>
 #include <limits>
 #include <map>
@@ -68,7 +69,6 @@ struct Derivation;
 class FSAccessor;
 class NarInfoDiskCache;
 class Store;
-class JSONPlaceholder;
 
 
 enum CheckSigsFlag : bool { NoCheckSigs = false, CheckSigs = true };
@@ -512,7 +512,7 @@ public:
        variable elements such as the registration time are
        included. If ‘showClosureSize’ is true, the closure size of
        each path is included. */
-    void pathInfoToJSON(JSONPlaceholder & jsonOut, const StorePathSet & storePaths,
+    nlohmann::json pathInfoToJSON(const StorePathSet & storePaths,
         bool includeImpureInfo, bool showClosureSize,
         Base hashBase = Base32,
         AllowInvalidFlag allowInvalid = DisallowInvalid);

src/libutil/cgroup.cc

@@ -0,0 +1,148 @@
+#if __linux__
+
+#include "cgroup.hh"
+#include "util.hh"
+#include "finally.hh"
+
+#include <chrono>
+#include <cmath>
+#include <regex>
+#include <unordered_set>
+#include <thread>
+
+#include <dirent.h>
+#include <mntent.h>
+
+namespace nix {
+
+std::optional<Path> getCgroupFS()
+{
+    static auto res = [&]() -> std::optional<Path> {
+        auto fp = fopen("/proc/mounts", "r");
+        if (!fp) return std::nullopt;
+        Finally delFP = [&]() { fclose(fp); };
+        while (auto ent = getmntent(fp))
+            if (std::string_view(ent->mnt_type) == "cgroup2")
+                return ent->mnt_dir;
+
+        return std::nullopt;
+    }();
+    return res;
+}
+
+// FIXME: obsolete, check for cgroup2
+std::map<std::string, std::string> getCgroups(const Path & cgroupFile)
+{
+    std::map<std::string, std::string> cgroups;
+
+    for (auto & line : tokenizeString<std::vector<std::string>>(readFile(cgroupFile), "\n")) {
+        static std::regex regex("([0-9]+):([^:]*):(.*)");
+        std::smatch match;
+        if (!std::regex_match(line, match, regex))
+            throw Error("invalid line '%s' in '%s'", line, cgroupFile);
+
+        std::string name = hasPrefix(std::string(match[2]), "name=") ? std::string(match[2], 5) : match[2];
+        cgroups.insert_or_assign(name, match[3]);
+    }
+
+    return cgroups;
+}
+
+static CgroupStats destroyCgroup(const Path & cgroup, bool returnStats)
+{
+    if (!pathExists(cgroup)) return {};
+
+    auto procsFile = cgroup + "/cgroup.procs";
+
+    if (!pathExists(procsFile))
+        throw Error("'%s' is not a cgroup", cgroup);
+
+    /* Use the fast way to kill every process in a cgroup, if
+       available. */
+    auto killFile = cgroup + "/cgroup.kill";
+    if (pathExists(killFile))
+        writeFile(killFile, "1");
+
+    /* Otherwise, manually kill every process in the subcgroups and
+       this cgroup. */
+    for (auto & entry : readDirectory(cgroup)) {
+        if (entry.type != DT_DIR) continue;
+        destroyCgroup(cgroup + "/" + entry.name, false);
+    }
+
+    int round = 1;
+
+    std::unordered_set<pid_t> pidsShown;
+
+    while (true) {
+        auto pids = tokenizeString<std::vector<std::string>>(readFile(procsFile));
+
+        if (pids.empty()) break;
+
+        if (round > 20)
+            throw Error("cannot kill cgroup '%s'", cgroup);
+
+        for (auto & pid_s : pids) {
+            pid_t pid;
+            if (auto o = string2Int<pid_t>(pid_s))
+                pid = *o;
+            else
+                throw Error("invalid pid '%s'", pid);
+            if (pidsShown.insert(pid).second) {
+                try {
+                    auto cmdline = readFile(fmt("/proc/%d/cmdline", pid));
+                    using namespace std::string_literals;
+                    warn("killing stray builder process %d (%s)...",
+                        pid, trim(replaceStrings(cmdline, "\0"s, " ")));
+                } catch (SysError &) {
+                }
+            }
+            // FIXME: pid wraparound
+            if (kill(pid, SIGKILL) == -1 && errno != ESRCH)
+                throw SysError("killing member %d of cgroup '%s'", pid, cgroup);
+        }
+
+        auto sleep = std::chrono::milliseconds((int) std::pow(2.0, std::min(round, 10)));
+        if (sleep.count() > 100)
+            printError("waiting for %d ms for cgroup '%s' to become empty", sleep.count(), cgroup);
+        std::this_thread::sleep_for(sleep);
+        round++;
+    }
+
+    CgroupStats stats;
+
+    if (returnStats) {
+        auto cpustatPath = cgroup + "/cpu.stat";
+
+        if (pathExists(cpustatPath)) {
+            for (auto & line : tokenizeString<std::vector<std::string>>(readFile(cpustatPath), "\n")) {
+                std::string_view userPrefix = "user_usec ";
+                if (hasPrefix(line, userPrefix)) {
+                    auto n = string2Int<uint64_t>(line.substr(userPrefix.size()));
+                    if (n) stats.cpuUser = std::chrono::microseconds(*n);
+                }
+
+                std::string_view systemPrefix = "system_usec ";
+                if (hasPrefix(line, systemPrefix)) {
+                    auto n = string2Int<uint64_t>(line.substr(systemPrefix.size()));
+                    if (n) stats.cpuSystem = std::chrono::microseconds(*n);
+                }
+            }
+        }
+
+    }
+
+    if (rmdir(cgroup.c_str()) == -1)
+        throw SysError("deleting cgroup '%s'", cgroup);
+
+    return stats;
+}
+
+CgroupStats destroyCgroup(const Path & cgroup)
+{
+    return destroyCgroup(cgroup, true);
+}
+
+}
+
+#endif

src/libutil/cgroup.hh

@@ -0,0 +1,29 @@
+#pragma once
+
+#if __linux__
+
+#include <chrono>
+#include <optional>
+
+#include "types.hh"
+
+namespace nix {
+
+std::optional<Path> getCgroupFS();
+
+std::map<std::string, std::string> getCgroups(const Path & cgroupFile);
+
+struct CgroupStats
+{
+    std::optional<std::chrono::microseconds> cpuUser, cpuSystem;
+};
+
+/* Destroy the cgroup denoted by 'path'. The postcondition is that
+   'path' does not exist, and thus any processes in the cgroup have
+   been killed. Also return statistics from the cgroup just before
+   destruction. */
+CgroupStats destroyCgroup(const Path & cgroup);
+
+}
+
+#endif

src/libutil/experimental-features.cc

@@ -14,6 +14,8 @@ std::map<ExperimentalFeature, std::string> stringifiedXpFeatures = {
     { Xp::NoUrlLiterals, "no-url-literals" },
     { Xp::FetchClosure, "fetch-closure" },
     { Xp::ReplFlake, "repl-flake" },
+    { Xp::AutoAllocateUids, "auto-allocate-uids" },
+    { Xp::Cgroups, "cgroups" },
 };
 
 const std::optional<ExperimentalFeature> parseExperimentalFeature(const std::string_view & name)

src/libutil/experimental-features.hh

@@ -23,6 +23,8 @@ enum struct ExperimentalFeature
     NoUrlLiterals,
     FetchClosure,
     ReplFlake,
+    AutoAllocateUids,
+    Cgroups,
 };
 
 /**

src/libutil/filesystem.cc

@@ -1,5 +1,6 @@
 #include <sys/time.h>
 #include <filesystem>
+#include <atomic>
 
 #include "finally.hh"
 #include "util.hh"
@@ -10,7 +11,7 @@ namespace fs = std::filesystem;
 namespace nix {
 
 static Path tempName(Path tmpRoot, const Path & prefix, bool includePid,
-    int & counter)
+    std::atomic<unsigned int> & counter)
 {
     tmpRoot = canonPath(tmpRoot.empty() ? getEnv("TMPDIR").value_or("/tmp") : tmpRoot, true);
     if (includePid)
@@ -22,9 +23,9 @@ static Path tempName(Path tmpRoot, const Path & prefix, bool includePid,
 Path createTempDir(const Path & tmpRoot, const Path & prefix,
     bool includePid, bool useGlobalCounter, mode_t mode)
 {
-    static int globalCounter = 0;
+    static std::atomic<unsigned int> globalCounter = 0;
-    int localCounter = 0;
+    std::atomic<unsigned int> localCounter = 0;
-    int & counter(useGlobalCounter ? globalCounter : localCounter);
+    auto & counter(useGlobalCounter ? globalCounter : localCounter);
 
     while (1) {
         checkInterrupt();

src/libutil/json.cc

@@ -1,203 +0,0 @@
-#include "json.hh"
-
-#include <iomanip>
-#include <cstdint>
-#include <cstring>
-
-namespace nix {
-
-template<>
-void toJSON<std::string_view>(std::ostream & str, const std::string_view & s)
-{
-    constexpr size_t BUF_SIZE = 4096;
-    char buf[BUF_SIZE + 7]; // BUF_SIZE + largest single sequence of puts
-    size_t bufPos = 0;
-
-    const auto flush = [&] {
-        str.write(buf, bufPos);
-        bufPos = 0;
-    };
-    const auto put = [&] (char c) {
-        buf[bufPos++] = c;
-    };
-
-    put('"');
-    for (auto i = s.begin(); i != s.end(); i++) {
-        if (bufPos >= BUF_SIZE) flush();
-        if (*i == '\"' || *i == '\\') { put('\\'); put(*i); }
-        else if (*i == '\n') { put('\\'); put('n'); }
-        else if (*i == '\r') { put('\\'); put('r'); }
-        else if (*i == '\t') { put('\\'); put('t'); }
-        else if (*i >= 0 && *i < 32) {
-            const char hex[17] = "0123456789abcdef";
-            put('\\');
-            put('u');
-            put(hex[(uint16_t(*i) >> 12) & 0xf]);
-            put(hex[(uint16_t(*i) >>  8) & 0xf]);
-            put(hex[(uint16_t(*i) >>  4) & 0xf]);
-            put(hex[(uint16_t(*i) >>  0) & 0xf]);
-        }
-        else put(*i);
-    }
-    put('"');
-    flush();
-}
-
-void toJSON(std::ostream & str, const char * s)
-{
-    if (!s) str << "null"; else toJSON(str, std::string_view(s));
-}
-
-template<> void toJSON<int>(std::ostream & str, const int & n) { str << n; }
-template<> void toJSON<unsigned int>(std::ostream & str, const unsigned int & n) { str << n; }
-template<> void toJSON<long>(std::ostream & str, const long & n) { str << n; }
-template<> void toJSON<unsigned long>(std::ostream & str, const unsigned long & n) { str << n; }
-template<> void toJSON<long long>(std::ostream & str, const long long & n) { str << n; }
-template<> void toJSON<unsigned long long>(std::ostream & str, const unsigned long long & n) { str << n; }
-template<> void toJSON<float>(std::ostream & str, const float & n) { str << n; }
-template<> void toJSON<double>(std::ostream & str, const double & n) { str << n; }
-template<> void toJSON<std::string>(std::ostream & str, const std::string & s) { toJSON(str, (std::string_view) s); }
-
-template<> void toJSON<bool>(std::ostream & str, const bool & b)
-{
-    str << (b ? "true" : "false");
-}
-
-template<> void toJSON<std::nullptr_t>(std::ostream & str, const std::nullptr_t & b)
-{
-    str << "null";
-}
-
-JSONWriter::JSONWriter(std::ostream & str, bool indent)
-    : state(new JSONState(str, indent))
-{
-    state->stack++;
-}
-
-JSONWriter::JSONWriter(JSONState * state)
-    : state(state)
-{
-    state->stack++;
-}
-
-JSONWriter::~JSONWriter()
-{
-    if (state) {
-        assertActive();
-        state->stack--;
-        if (state->stack == 0) delete state;
-    }
-}
-
-void JSONWriter::comma()
-{
-    assertActive();
-    if (first) {
-        first = false;
-    } else {
-        state->str << ',';
-    }
-    if (state->indent) indent();
-}
-
-void JSONWriter::indent()
-{
-    state->str << '\n' << std::string(state->depth * 2, ' ');
-}
-
-void JSONList::open()
-{
-    state->depth++;
-    state->str << '[';
-}
-
-JSONList::~JSONList()
-{
-    state->depth--;
-    if (state->indent && !first) indent();
-    state->str << "]";
-}
-
-JSONList JSONList::list()
-{
-    comma();
-    return JSONList(state);
-}
-
-JSONObject JSONList::object()
-{
-    comma();
-    return JSONObject(state);
-}
-
-JSONPlaceholder JSONList::placeholder()
-{
-    comma();
-    return JSONPlaceholder(state);
-}
-
-void JSONObject::open()
-{
-    state->depth++;
-    state->str << '{';
-}
-
-JSONObject::~JSONObject()
-{
-    if (state) {
-        state->depth--;
-        if (state->indent && !first) indent();
-        state->str << "}";
-    }
-}
-
-void JSONObject::attr(std::string_view s)
-{
-    comma();
-    toJSON(state->str, s);
-    state->str << ':';
-    if (state->indent) state->str << ' ';
-}
-
-JSONList JSONObject::list(std::string_view name)
-{
-    attr(name);
-    return JSONList(state);
-}
-
-JSONObject JSONObject::object(std::string_view name)
-{
-    attr(name);
-    return JSONObject(state);
-}
-
-JSONPlaceholder JSONObject::placeholder(std::string_view name)
-{
-    attr(name);
-    return JSONPlaceholder(state);
-}
-
-JSONList JSONPlaceholder::list()
-{
-    assertValid();
-    first = false;
-    return JSONList(state);
-}
-
-JSONObject JSONPlaceholder::object()
-{
-    assertValid();
-    first = false;
-    return JSONObject(state);
-}
-
-JSONPlaceholder::~JSONPlaceholder()
-{
-    if (first) {
-        assert(std::uncaught_exceptions());
-        if (state->stack != 0)
-            write(nullptr);
-    }
-}
-
-}

src/libutil/json.hh

@@ -1,185 +0,0 @@
-#pragma once
-
-#include <iostream>
-#include <vector>
-#include <cassert>
-
-namespace nix {
-
-void toJSON(std::ostream & str, const char * s);
-
-template<typename T>
-void toJSON(std::ostream & str, const T & n);
-
-class JSONWriter
-{
-protected:
-
-    struct JSONState
-    {
-        std::ostream & str;
-        bool indent;
-        size_t depth = 0;
-        size_t stack = 0;
-        JSONState(std::ostream & str, bool indent) : str(str), indent(indent) { }
-        ~JSONState()
-        {
-            assert(stack == 0);
-        }
-    };
-
-    JSONState * state;
-
-    bool first = true;
-
-    JSONWriter(std::ostream & str, bool indent);
-
-    JSONWriter(JSONState * state);
-
-    ~JSONWriter();
-
-    void assertActive()
-    {
-        assert(state->stack != 0);
-    }
-
-    void comma();
-
-    void indent();
-};
-
-class JSONObject;
-class JSONPlaceholder;
-
-class JSONList : JSONWriter
-{
-private:
-
-    friend class JSONObject;
-    friend class JSONPlaceholder;
-
-    void open();
-
-    JSONList(JSONState * state)
-        : JSONWriter(state)
-    {
-        open();
-    }
-
-public:
-
-    JSONList(std::ostream & str, bool indent = false)
-        : JSONWriter(str, indent)
-    {
-        open();
-    }
-
-    ~JSONList();
-
-    template<typename T>
-    JSONList & elem(const T & v)
-    {
-        comma();
-        toJSON(state->str, v);
-        return *this;
-    }
-
-    JSONList list();
-
-    JSONObject object();
-
-    JSONPlaceholder placeholder();
-};
-
-class JSONObject : JSONWriter
-{
-private:
-
-    friend class JSONList;
-    friend class JSONPlaceholder;
-
-    void open();
-
-    JSONObject(JSONState * state)
-        : JSONWriter(state)
-    {
-        open();
-    }
-
-    void attr(std::string_view s);
-
-public:
-
-    JSONObject(std::ostream & str, bool indent = false)
-        : JSONWriter(str, indent)
-    {
-        open();
-    }
-
-    JSONObject(const JSONObject & obj) = delete;
-
-    JSONObject(JSONObject && obj)
-        : JSONWriter(obj.state)
-    {
-        obj.state = 0;
-    }
-
-    ~JSONObject();
-
-    template<typename T>
-    JSONObject & attr(std::string_view name, const T & v)
-    {
-        attr(name);
-        toJSON(state->str, v);
-        return *this;
-    }
-
-    JSONList list(std::string_view name);
-
-    JSONObject object(std::string_view name);
-
-    JSONPlaceholder placeholder(std::string_view name);
-};
-
-class JSONPlaceholder : JSONWriter
-{
-
-private:
-
-    friend class JSONList;
-    friend class JSONObject;
-
-    JSONPlaceholder(JSONState * state)
-        : JSONWriter(state)
-    {
-    }
-
-    void assertValid()
-    {
-        assertActive();
-        assert(first);
-    }
-
-public:
-
-    JSONPlaceholder(std::ostream & str, bool indent = false)
-        : JSONWriter(str, indent)
-    {
-    }
-
-    ~JSONPlaceholder();
-
-    template<typename T>
-    void write(const T & v)
-    {
-        assertValid();
-        first = false;
-        toJSON(state->str, v);
-    }
-
-    JSONList list();
-
-    JSONObject object();
-};
-
-}

src/libutil/tests/json.cc

@@ -1,193 +0,0 @@
-#include "json.hh"
-#include <gtest/gtest.h>
-#include <sstream>
-
-namespace nix {
-
-    /* ----------------------------------------------------------------------------
-     * toJSON
-     * --------------------------------------------------------------------------*/
-
-    TEST(toJSON, quotesCharPtr) {
-        const char* input = "test";
-        std::stringstream out;
-        toJSON(out, input);
-
-        ASSERT_EQ(out.str(), "\"test\"");
-    }
-
-    TEST(toJSON, quotesStdString) {
-        std::string input = "test";
-        std::stringstream out;
-        toJSON(out, input);
-
-        ASSERT_EQ(out.str(), "\"test\"");
-    }
-
-    TEST(toJSON, convertsNullptrtoNull) {
-        auto input = nullptr;
-        std::stringstream out;
-        toJSON(out, input);
-
-        ASSERT_EQ(out.str(), "null");
-    }
-
-    TEST(toJSON, convertsNullToNull) {
-        const char* input = 0;
-        std::stringstream out;
-        toJSON(out, input);
-
-        ASSERT_EQ(out.str(), "null");
-    }
-
-
-    TEST(toJSON, convertsFloat) {
-        auto input = 1.024f;
-        std::stringstream out;
-        toJSON(out, input);
-
-        ASSERT_EQ(out.str(), "1.024");
-    }
-
-    TEST(toJSON, convertsDouble) {
-        const double input = 1.024;
-        std::stringstream out;
-        toJSON(out, input);
-
-        ASSERT_EQ(out.str(), "1.024");
-    }
-
-    TEST(toJSON, convertsBool) {
-        auto input = false;
-        std::stringstream out;
-        toJSON(out, input);
-
-        ASSERT_EQ(out.str(), "false");
-    }
-
-    TEST(toJSON, quotesTab) {
-        std::stringstream out;
-        toJSON(out, "\t");
-
-        ASSERT_EQ(out.str(), "\"\\t\"");
-    }
-
-    TEST(toJSON, quotesNewline) {
-        std::stringstream out;
-        toJSON(out, "\n");
-
-        ASSERT_EQ(out.str(), "\"\\n\"");
-    }
-
-    TEST(toJSON, quotesCreturn) {
-        std::stringstream out;
-        toJSON(out, "\r");
-
-        ASSERT_EQ(out.str(), "\"\\r\"");
-    }
-
-    TEST(toJSON, quotesCreturnNewLine) {
-        std::stringstream out;
-        toJSON(out, "\r\n");
-
-        ASSERT_EQ(out.str(), "\"\\r\\n\"");
-    }
-
-    TEST(toJSON, quotesDoublequotes) {
-        std::stringstream out;
-        toJSON(out, "\"");
-
-        ASSERT_EQ(out.str(), "\"\\\"\"");
-    }
-
-    TEST(toJSON, substringEscape) {
-        std::stringstream out;
-        std::string_view s = "foo\t";
-        toJSON(out, s.substr(3));
-
-        ASSERT_EQ(out.str(), "\"\\t\"");
-    }
-
-    /* ----------------------------------------------------------------------------
-     * JSONObject
-     * --------------------------------------------------------------------------*/
-
-    TEST(JSONObject, emptyObject) {
-        std::stringstream out;
-        {
-            JSONObject t(out);
-        }
-        ASSERT_EQ(out.str(), "{}");
-    }
-
-    TEST(JSONObject, objectWithList) {
-        std::stringstream out;
-        {
-            JSONObject t(out);
-            auto l = t.list("list");
-            l.elem("element");
-        }
-        ASSERT_EQ(out.str(), R"#({"list":["element"]})#");
-    }
-
-    TEST(JSONObject, objectWithListIndent) {
-        std::stringstream out;
-        {
-            JSONObject t(out, true);
-            auto l = t.list("list");
-            l.elem("element");
-        }
-        ASSERT_EQ(out.str(),
-R"#({
-  "list": [
-    "element"
-  ]
-})#");
-    }
-
-    TEST(JSONObject, objectWithPlaceholderAndList) {
-        std::stringstream out;
-        {
-            JSONObject t(out);
-            auto l = t.placeholder("list");
-            l.list().elem("element");
-        }
-
-        ASSERT_EQ(out.str(), R"#({"list":["element"]})#");
-    }
-
-    TEST(JSONObject, objectWithPlaceholderAndObject) {
-        std::stringstream out;
-        {
-            JSONObject t(out);
-            auto l = t.placeholder("object");
-            l.object().attr("key", "value");
-        }
-
-        ASSERT_EQ(out.str(), R"#({"object":{"key":"value"}})#");
-    }
-
-    /* ----------------------------------------------------------------------------
-     * JSONList
-     * --------------------------------------------------------------------------*/
-
-    TEST(JSONList, empty) {
-        std::stringstream out;
-        {
-            JSONList l(out);
-        }
-        ASSERT_EQ(out.str(), R"#([])#");
-    }
-
-    TEST(JSONList, withElements) {
-        std::stringstream out;
-        {
-            JSONList l(out);
-            l.elem("one");
-            l.object();
-            l.placeholder().write("three");
-        }
-        ASSERT_EQ(out.str(), R"#(["one",{},"three"])#");
-    }
-}
-

src/libutil/util.cc

@@ -2,6 +2,7 @@
 #include "sync.hh"
 #include "finally.hh"
 #include "serialise.hh"
+#include "cgroup.hh"
 
 #include <array>
 #include <cctype>
@@ -36,7 +37,6 @@
 #include <sys/prctl.h>
 #include <sys/resource.h>
 
-#include <mntent.h>
 #include <cmath>
 #endif
 
@@ -727,45 +727,22 @@ unsigned int getMaxCPU()
 {
     #if __linux__
     try {
-        FILE *fp = fopen("/proc/mounts", "r");
+        auto cgroupFS = getCgroupFS();
-        if (!fp)
+        if (!cgroupFS) return 0;
-            return 0;
 
-        Strings cgPathParts;
+        auto cgroups = getCgroups("/proc/self/cgroup");
+        auto cgroup = cgroups[""];
+        if (cgroup == "") return 0;
 
-        struct mntent *ent;
+        auto cpuFile = *cgroupFS + "/" + cgroup + "/cpu.max";
-        while ((ent = getmntent(fp))) {
-            std::string mountType, mountPath;
 
-            mountType = ent->mnt_type;
+        auto cpuMax = readFile(cpuFile);
-            mountPath = ent->mnt_dir;
+        auto cpuMaxParts = tokenizeString<std::vector<std::string>>(cpuMax, " \n");
-
+        auto quota = cpuMaxParts[0];
-            if (mountType == "cgroup2") {
+        auto period = cpuMaxParts[1];
-                cgPathParts.push_back(mountPath);
+        if (quota != "max")
-                break;
+                return std::ceil(std::stoi(quota) / std::stof(period));
-            }
+    } catch (Error &) { ignoreException(lvlDebug); }
-        }
-
-        fclose(fp);
-
-        if (cgPathParts.size() > 0 && pathExists("/proc/self/cgroup")) {
-            std::string currentCgroup = readFile("/proc/self/cgroup");
-            Strings cgValues = tokenizeString<Strings>(currentCgroup, ":");
-            cgPathParts.push_back(trim(cgValues.back(), "\n"));
-            cgPathParts.push_back("cpu.max");
-            std::string fullCgPath = canonPath(concatStringsSep("/", cgPathParts));
-
-            if (pathExists(fullCgPath)) {
-                std::string cpuMax = readFile(fullCgPath);
-                std::vector<std::string> cpuMaxParts = tokenizeString<std::vector<std::string>>(cpuMax, " ");
-                std::string quota = cpuMaxParts[0];
-                std::string period = trim(cpuMaxParts[1], "\n");
-
-                if (quota != "max")
-                    return std::ceil(std::stoi(quota) / std::stof(period));
-            }
-        }
-    } catch (Error &) { ignoreException(); }
     #endif
 
     return 0;
@@ -1427,7 +1404,7 @@ std::string shellEscape(const std::string_view s)
 }
 
 
-void ignoreException()
+void ignoreException(Verbosity lvl)
 {
     /* Make sure no exceptions leave this function.
        printError() also throws when remote is closed. */
@@ -1435,7 +1412,7 @@ void ignoreException()
         try {
             throw;
         } catch (std::exception & e) {
-            printError("error (ignored): %1%", e.what());
+            printMsg(lvl, "error (ignored): %1%", e.what());
         }
     } catch (...) { }
 }

src/libutil/util.hh

@@ -528,7 +528,7 @@ std::string shellEscape(const std::string_view s);
 
 /* Exception handling in destructors: print an error message, then
    ignore the exception. */
-void ignoreException();
+void ignoreException(Verbosity lvl = lvlError);
 
 
 

src/nix-env/nix-env.cc

@@ -12,7 +12,6 @@
 #include "local-fs-store.hh"
 #include "user-env.hh"
 #include "util.hh"
-#include "json.hh"
 #include "value-to-json.hh"
 #include "xml-writer.hh"
 #include "legacy.hh"
@@ -26,6 +25,7 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <unistd.h>
+#include <nlohmann/json.hpp>
 
 using namespace nix;
 using std::cout;
@@ -911,43 +911,47 @@ static VersionDiff compareVersionAgainstSet(
 
 static void queryJSON(Globals & globals, std::vector<DrvInfo> & elems, bool printOutPath, bool printMeta)
 {
-    JSONObject topObj(cout, true);
+    using nlohmann::json;
+    json topObj = json::object();
     for (auto & i : elems) {
         try {
             if (i.hasFailed()) continue;
 
-            JSONObject pkgObj = topObj.object(i.attrPath);
 
             auto drvName = DrvName(i.queryName());
-            pkgObj.attr("name", drvName.fullName);
+            json &pkgObj = topObj[i.attrPath];
-            pkgObj.attr("pname", drvName.name);
+            pkgObj = {
-            pkgObj.attr("version", drvName.version);
+                {"name", drvName.fullName},
-            pkgObj.attr("system", i.querySystem());
+                {"pname", drvName.name},
-            pkgObj.attr("outputName", i.queryOutputName());
+                {"version", drvName.version},
+                {"system", i.querySystem()},
+                {"outputName", i.queryOutputName()},
+            };
 
             {
                 DrvInfo::Outputs outputs = i.queryOutputs(printOutPath);
-                JSONObject outputObj = pkgObj.object("outputs");
+                json &outputObj = pkgObj["outputs"];
+                outputObj = json::object();
                 for (auto & j : outputs) {
                     if (j.second)
-                        outputObj.attr(j.first, globals.state->store->printStorePath(*j.second));
+                        outputObj[j.first] = globals.state->store->printStorePath(*j.second);
                     else
-                        outputObj.attr(j.first, nullptr);
+                        outputObj[j.first] = nullptr;
                 }
             }
 
             if (printMeta) {
-                JSONObject metaObj = pkgObj.object("meta");
+                json &metaObj = pkgObj["meta"];
+                metaObj = json::object();
                 StringSet metaNames = i.queryMetaNames();
                 for (auto & j : metaNames) {
                     Value * v = i.queryMeta(j);
                     if (!v) {
                         printError("derivation '%s' has invalid meta attribute '%s'", i.queryName(), j);
-                        metaObj.attr(j, nullptr);
+                        metaObj[j] = nullptr;
                     } else {
-                        auto placeholder = metaObj.placeholder(j);
                         PathSet context;
-                        printValueAsJSON(*globals.state, true, *v, noPos, placeholder, context);
+                        metaObj[j] = printValueAsJSON(*globals.state, true, *v, noPos, context);
                     }
                 }
             }
@@ -958,6 +962,7 @@ static void queryJSON(Globals & globals, std::vector<DrvInfo> & elems, bool prin
             throw;
         }
     }
+    std::cout << topObj.dump(2);
 }
 
 

src/nix-store/nix-store.cc

@@ -516,7 +516,7 @@ static void registerValidity(bool reregister, bool hashGiven, bool canonicalise)
         if (!store->isValidPath(info->path) || reregister) {
             /* !!! races */
             if (canonicalise)
-                canonicalisePathMetaData(store->printStorePath(info->path), -1);
+                canonicalisePathMetaData(store->printStorePath(info->path), {});
             if (!hashGiven) {
                 HashResult hash = hashPath(htSHA256, store->printStorePath(info->path));
                 info->narHash = hash.first;
@@ -808,14 +808,20 @@ static void opServe(Strings opFlags, Strings opArgs)
         if (GET_PROTOCOL_MINOR(clientVersion) >= 2)
             settings.maxLogSize = readNum<unsigned long>(in);
         if (GET_PROTOCOL_MINOR(clientVersion) >= 3) {
-            settings.buildRepeat = readInt(in);
+            auto nrRepeats = readInt(in);
-            settings.enforceDeterminism = readInt(in);
+            if (nrRepeats != 0) {
+                throw Error("client requested repeating builds, but this is not currently implemented");
+            }
+            // Ignore. It used to be true by default, but also only never had any effect when `nrRepeats == 0`.
+            // We have already asserted that `nrRepeats` in fact is 0, so we can safely ignore this without
+            // doing something other than what the client asked for.
+            auto _enforceDeterminism = readInt(in);
+
             settings.runDiffHook = true;
         }
         if (GET_PROTOCOL_MINOR(clientVersion) >= 7) {
             settings.keepFailed = (bool) readInt(in);
         }
-        settings.printRepeatedBuilds = false;
     };
 
     while (true) {

src/nix/build.cc

@@ -30,6 +30,10 @@ nlohmann::json builtPathsWithResultToJSON(const std::vector<BuiltPathWithResult>
             if (b.result) {
                 j["startTime"] = b.result->startTime;
                 j["stopTime"] = b.result->stopTime;
+                if (b.result->cpuUser)
+                    j["cpuUser"] = ((double) b.result->cpuUser->count()) / 1000000;
+                if (b.result->cpuSystem)
+                    j["cpuSystem"] = ((double) b.result->cpuSystem->count()) / 1000000;
             }
             res.push_back(j);
         }, b.path.raw());

src/nix/eval.cc

@@ -4,10 +4,11 @@
 #include "store-api.hh"
 #include "eval.hh"
 #include "eval-inline.hh"
-#include "json.hh"
 #include "value-to-json.hh"
 #include "progress-bar.hh"
 
+#include <nlohmann/json.hpp>
+
 using namespace nix;
 
 struct CmdEval : MixJSON, InstallableCommand
@@ -115,9 +116,7 @@ struct CmdEval : MixJSON, InstallableCommand
         }
 
         else if (json) {
-            JSONPlaceholder jsonOut(std::cout);
+            std::cout << printValueAsJSON(*state, true, *v, pos, context, false).dump() << std::endl;
-            printValueAsJSON(*state, true, *v, pos, jsonOut, context, false);
-            std::cout << std::endl;
         }
 
         else {

src/nix/flake.cc

@@ -11,7 +11,6 @@
 #include "attr-path.hh"
 #include "fetchers.hh"
 #include "registry.hh"
-#include "json.hh"
 #include "eval-cache.hh"
 #include "markdown.hh"
 
@@ -21,6 +20,7 @@
 
 using namespace nix;
 using namespace nix::flake;
+using json = nlohmann::json;
 
 class FlakeCommand : virtual Args, public MixFlakeOptions
 {
@@ -917,35 +917,44 @@ struct CmdFlakeArchive : FlakeCommand, MixJSON, MixDryRun
     {
         auto flake = lockFlake();
 
-        auto jsonRoot = json ? std::optional<JSONObject>(std::cout) : std::nullopt;
-
         StorePathSet sources;
 
         sources.insert(flake.flake.sourceInfo->storePath);
-        if (jsonRoot)
-            jsonRoot->attr("path", store->printStorePath(flake.flake.sourceInfo->storePath));
 
         // FIXME: use graph output, handle cycles.
-        std::function<void(const Node & node, std::optional<JSONObject> & jsonObj)> traverse;
+        std::function<nlohmann::json(const Node & node)> traverse;
-        traverse = [&](const Node & node, std::optional<JSONObject> & jsonObj)
+        traverse = [&](const Node & node)
         {
-            auto jsonObj2 = jsonObj ? jsonObj->object("inputs") : std::optional<JSONObject>();
+            nlohmann::json jsonObj2 = json ? json::object() : nlohmann::json(nullptr);
             for (auto & [inputName, input] : node.inputs) {
                 if (auto inputNode = std::get_if<0>(&input)) {
-                    auto jsonObj3 = jsonObj2 ? jsonObj2->object(inputName) : std::optional<JSONObject>();
                     auto storePath =
                         dryRun
                         ? (*inputNode)->lockedRef.input.computeStorePath(*store)
                         : (*inputNode)->lockedRef.input.fetch(store).first.storePath;
-                    if (jsonObj3)
+                    if (json) {
-                        jsonObj3->attr("path", store->printStorePath(storePath));
+                        auto& jsonObj3 = jsonObj2[inputName];
-                    sources.insert(std::move(storePath));
+                        jsonObj3["path"] = store->printStorePath(storePath);
-                    traverse(**inputNode, jsonObj3);
+                        sources.insert(std::move(storePath));
+                        jsonObj3["inputs"] = traverse(**inputNode);
+                    } else {
+                        sources.insert(std::move(storePath));
+                        traverse(**inputNode);
+                    }
                 }
             }
+            return jsonObj2;
         };
 
-        traverse(*flake.lockFile.root, jsonRoot);
+        if (json) {
+            nlohmann::json jsonRoot = {
+                {"path", store->printStorePath(flake.flake.sourceInfo->storePath)},
+                {"inputs", traverse(*flake.lockFile.root)},
+            };
+            std::cout << jsonRoot.dump() << std::endl;
+        } else {
+            traverse(*flake.lockFile.root);
+        }
 
         if (!dryRun && !dstUri.empty()) {
             ref<Store> dstStore = dstUri.empty() ? openStore() : openStore(dstUri);

src/nix/ls.cc

@@ -3,7 +3,7 @@
 #include "fs-accessor.hh"
 #include "nar-accessor.hh"
 #include "common-args.hh"
-#include "json.hh"
+#include <nlohmann/json.hpp>
 
 using namespace nix;
 
@@ -91,10 +91,9 @@ struct MixLs : virtual Args, MixJSON
         if (path == "/") path = "";
 
         if (json) {
-            JSONPlaceholder jsonRoot(std::cout);
             if (showDirectory)
                 throw UsageError("'--directory' is useless with '--json'");
-            listNar(jsonRoot, accessor, path, recursive);
+            std::cout << listNar(accessor, path, recursive);
         } else
             listText(accessor);
     }

src/nix/make-content-addressed.cc

@@ -2,10 +2,13 @@
 #include "store-api.hh"
 #include "make-content-addressed.hh"
 #include "common-args.hh"
-#include "json.hh"
+
+#include <nlohmann/json.hpp>
 
 using namespace nix;
 
+using nlohmann::json;
+
 struct CmdMakeContentAddressed : virtual CopyCommand, virtual StorePathsCommand, MixJSON
 {
     CmdMakeContentAddressed()
@@ -25,6 +28,7 @@ struct CmdMakeContentAddressed : virtual CopyCommand, virtual StorePathsCommand,
           ;
     }
 
+    using StorePathsCommand::run;
     void run(ref<Store> srcStore, StorePaths && storePaths) override
     {
         auto dstStore = dstUri.empty() ? openStore() : openStore(dstUri);
@@ -33,13 +37,15 @@ struct CmdMakeContentAddressed : virtual CopyCommand, virtual StorePathsCommand,
             StorePathSet(storePaths.begin(), storePaths.end()));
 
         if (json) {
-            JSONObject jsonRoot(std::cout);
+            auto jsonRewrites = json::object();
-            JSONObject jsonRewrites(jsonRoot.object("rewrites"));
             for (auto & path : storePaths) {
                 auto i = remappings.find(path);
                 assert(i != remappings.end());
-                jsonRewrites.attr(srcStore->printStorePath(path), srcStore->printStorePath(i->second));
+                jsonRewrites[srcStore->printStorePath(path)] = srcStore->printStorePath(i->second);
             }
+            auto json = json::object();
+            json["rewrites"] = jsonRewrites;
+            std::cout << json.dump();
         } else {
             for (auto & path : storePaths) {
                 auto i = remappings.find(path);

src/nix/path-info.cc

@@ -1,12 +1,13 @@
 #include "command.hh"
 #include "shared.hh"
 #include "store-api.hh"
-#include "json.hh"
 #include "common-args.hh"
 
 #include <algorithm>
 #include <array>
 
+#include <nlohmann/json.hpp>
+
 using namespace nix;
 
 struct CmdPathInfo : StorePathsCommand, MixJSON
@@ -86,11 +87,10 @@ struct CmdPathInfo : StorePathsCommand, MixJSON
             pathLen = std::max(pathLen, store->printStorePath(storePath).size());
 
         if (json) {
-            JSONPlaceholder jsonRoot(std::cout);
+            std::cout << store->pathInfoToJSON(
-            store->pathInfoToJSON(jsonRoot,
                 // FIXME: preserve order?
                 StorePathSet(storePaths.begin(), storePaths.end()),
-                true, showClosureSize, SRI, AllowInvalid);
+                true, showClosureSize, SRI, AllowInvalid).dump();
         }
 
         else {

src/nix/search.cc

@@ -5,7 +5,6 @@
 #include "names.hh"
 #include "get-drvs.hh"
 #include "common-args.hh"
-#include "json.hh"
 #include "shared.hh"
 #include "eval-cache.hh"
 #include "attr-path.hh"
@@ -13,8 +12,10 @@
 
 #include <regex>
 #include <fstream>
+#include <nlohmann/json.hpp>
 
 using namespace nix;
+using json = nlohmann::json;
 
 std::string wrap(std::string prefix, std::string s)
 {
@@ -84,7 +85,8 @@ struct CmdSearch : InstallableCommand, MixJSON
 
         auto state = getEvalState();
 
-        auto jsonOut = json ? std::make_unique<JSONObject>(std::cout) : nullptr;
+        std::optional<nlohmann::json> jsonOut;
+        if (json) jsonOut = json::object();
 
         uint64_t results = 0;
 
@@ -151,10 +153,11 @@ struct CmdSearch : InstallableCommand, MixJSON
                     {
                         results++;
                         if (json) {
-                            auto jsonElem = jsonOut->object(attrPath2);
+                            (*jsonOut)[attrPath2] = {
-                            jsonElem.attr("pname", name.name);
+                                {"pname", name.name},
-                            jsonElem.attr("version", name.version);
+                                {"version", name.version},
-                            jsonElem.attr("description", description);
+                                {"description", description},
+                            };
                         } else {
                             auto name2 = hiliteMatches(name.name, nameMatches, ANSI_GREEN, "\e[0;2m");
                             if (results > 1) logger->cout("");
@@ -193,6 +196,10 @@ struct CmdSearch : InstallableCommand, MixJSON
         for (auto & cursor : installable->getCursors(*state))
             visit(*cursor, cursor->getAttrPath(), true);
 
+        if (json) {
+            std::cout << jsonOut->dump() << std::endl;
+        }
+
         if (!json && !results)
             throw Error("no results for the given search term(s)!");
     }

src/nix/show-derivation.cc

@@ -5,10 +5,11 @@
 #include "common-args.hh"
 #include "store-api.hh"
 #include "archive.hh"
-#include "json.hh"
 #include "derivations.hh"
+#include <nlohmann/json.hpp>
 
 using namespace nix;
+using json = nlohmann::json;
 
 struct CmdShowDerivation : InstallablesCommand
 {
@@ -48,77 +49,63 @@ struct CmdShowDerivation : InstallablesCommand
             drvPaths = std::move(closure);
         }
 
-        {
+        json jsonRoot = json::object();
-
-        JSONObject jsonRoot(std::cout, true);
 
         for (auto & drvPath : drvPaths) {
             if (!drvPath.isDerivation()) continue;
 
-            auto drvObj(jsonRoot.object(store->printStorePath(drvPath)));
+            json& drvObj = jsonRoot[store->printStorePath(drvPath)];
 
             auto drv = store->readDerivation(drvPath);
 
             {
-                auto outputsObj(drvObj.object("outputs"));
+                json& outputsObj = drvObj["outputs"];
+                outputsObj = json::object();
                 for (auto & [_outputName, output] : drv.outputs) {
                     auto & outputName = _outputName; // work around clang bug
-                    auto outputObj { outputsObj.object(outputName) };
+                    auto& outputObj = outputsObj[outputName];
+                    outputObj = json::object();
                     std::visit(overloaded {
                         [&](const DerivationOutput::InputAddressed & doi) {
-                            outputObj.attr("path", store->printStorePath(doi.path));
+                            outputObj["path"] = store->printStorePath(doi.path);
                         },
                         [&](const DerivationOutput::CAFixed & dof) {
-                            outputObj.attr("path", store->printStorePath(dof.path(*store, drv.name, outputName)));
+                            outputObj["path"] = store->printStorePath(dof.path(*store, drv.name, outputName));
-                            outputObj.attr("hashAlgo", dof.hash.printMethodAlgo());
+                            outputObj["hashAlgo"] = dof.hash.printMethodAlgo();
-                            outputObj.attr("hash", dof.hash.hash.to_string(Base16, false));
+                            outputObj["hash"] = dof.hash.hash.to_string(Base16, false);
                         },
                         [&](const DerivationOutput::CAFloating & dof) {
-                            outputObj.attr("hashAlgo", makeFileIngestionPrefix(dof.method) + printHashType(dof.hashType));
+                            outputObj["hashAlgo"] = makeFileIngestionPrefix(dof.method) + printHashType(dof.hashType);
                         },
                         [&](const DerivationOutput::Deferred &) {},
                         [&](const DerivationOutput::Impure & doi) {
-                            outputObj.attr("hashAlgo", makeFileIngestionPrefix(doi.method) + printHashType(doi.hashType));
+                            outputObj["hashAlgo"] = makeFileIngestionPrefix(doi.method) + printHashType(doi.hashType);
-                            outputObj.attr("impure", true);
+                            outputObj["impure"] = true;
                         },
                     }, output.raw());
                 }
             }
 
             {
-                auto inputsList(drvObj.list("inputSrcs"));
+                auto& inputsList = drvObj["inputSrcs"];
+                inputsList = json::array();
                 for (auto & input : drv.inputSrcs)
-                    inputsList.elem(store->printStorePath(input));
+                    inputsList.emplace_back(store->printStorePath(input));
             }
 
             {
-                auto inputDrvsObj(drvObj.object("inputDrvs"));
+                auto& inputDrvsObj = drvObj["inputDrvs"];
-                for (auto & input : drv.inputDrvs) {
+                inputDrvsObj = json::object();
-                    auto inputList(inputDrvsObj.list(store->printStorePath(input.first)));
+                for (auto & input : drv.inputDrvs)
-                    for (auto & outputId : input.second)
+                    inputDrvsObj[store->printStorePath(input.first)] = input.second;
-                        inputList.elem(outputId);
-                }
             }
 
-            drvObj.attr("system", drv.platform);
+            drvObj["system"] = drv.platform;
-            drvObj.attr("builder", drv.builder);
+            drvObj["builder"] = drv.builder;
-
+            drvObj["args"] = drv.args;
-            {
+            drvObj["env"] = drv.env;
-                auto argsList(drvObj.list("args"));
-                for (auto & arg : drv.args)
-                    argsList.elem(arg);
-            }
-
-            {
-                auto envObj(drvObj.object("env"));
-                for (auto & var : drv.env)
-                    envObj.attr(var.first, var.second);
-            }
         }
-
+        std::cout << jsonRoot.dump(2) << std::endl;
-        }
-
-        std::cout << "\n";
     }
 };
 

tests/check.nix

@@ -44,7 +44,7 @@ with import ./config.nix;
   };
 
   hashmismatch = import <nix/fetchurl.nix> {
-    url = "file://" + builtins.getEnv "TMPDIR" + "/dummy";
+    url = "file://" + builtins.getEnv "TEST_ROOT" + "/dummy";
     sha256 = "0mdqa9w1p6cmli6976v4wi0sw9r4p5prkj7lzfd1877wk11c9c73";
   };
 

tests/check.sh

@@ -40,14 +40,6 @@ nix-build check.nix -A deterministic --argstr checkBuildId $checkBuildId \
 if grep -q 'may not be deterministic' $TEST_ROOT/log; then false; fi
 checkBuildTempDirRemoved $TEST_ROOT/log
 
-nix build -f check.nix deterministic --rebuild --repeat 1 \
-    --argstr checkBuildId $checkBuildId --keep-failed --no-link \
-    2> $TEST_ROOT/log
-if grep -q 'checking is not possible' $TEST_ROOT/log; then false; fi
-# Repeat is set to 1, ie. nix should build deterministic twice.
-if [ "$(grep "checking outputs" $TEST_ROOT/log | wc -l)" -ne 2 ]; then false; fi
-checkBuildTempDirRemoved $TEST_ROOT/log
-
 nix-build check.nix -A nondeterministic --argstr checkBuildId $checkBuildId \
     --no-out-link 2> $TEST_ROOT/log
 checkBuildTempDirRemoved $TEST_ROOT/log
@@ -58,12 +50,6 @@ grep 'may not be deterministic' $TEST_ROOT/log
 [ "$status" = "104" ]
 checkBuildTempDirRemoved $TEST_ROOT/log
 
-nix build -f check.nix nondeterministic --rebuild --repeat 1 \
-    --argstr checkBuildId $checkBuildId --keep-failed --no-link \
-    2> $TEST_ROOT/log || status=$?
-grep 'may not be deterministic' $TEST_ROOT/log
-checkBuildTempDirRemoved $TEST_ROOT/log
-
 nix-build check.nix -A nondeterministic --argstr checkBuildId $checkBuildId \
     --no-out-link --check --keep-failed 2> $TEST_ROOT/log || status=$?
 grep 'may not be deterministic' $TEST_ROOT/log
@@ -72,12 +58,6 @@ if checkBuildTempDirRemoved $TEST_ROOT/log; then false; fi
 
 clearStore
 
-nix-build dependencies.nix --no-out-link --repeat 3
-
-nix-build check.nix -A nondeterministic --no-out-link --repeat 1 2> $TEST_ROOT/log || status=$?
-[ "$status" = "1" ]
-grep 'differs from previous round' $TEST_ROOT/log
-
 path=$(nix-build check.nix -A fetchurl --no-out-link)
 
 chmod +w $path
@@ -91,13 +71,13 @@ nix-build check.nix -A fetchurl --no-out-link --check
 nix-build check.nix -A fetchurl --no-out-link --repair
 [[ $(cat $path) != foo ]]
 
-echo 'Hello World' > $TMPDIR/dummy
+echo 'Hello World' > $TEST_ROOT/dummy
 nix-build check.nix -A hashmismatch --no-out-link || status=$?
 [ "$status" = "102" ]
 
-echo -n > $TMPDIR/dummy
+echo -n > $TEST_ROOT/dummy
 nix-build check.nix -A hashmismatch --no-out-link
-echo 'Hello World' > $TMPDIR/dummy
+echo 'Hello World' > $TEST_ROOT/dummy
 
 nix-build check.nix -A hashmismatch --no-out-link --check || status=$?
 [ "$status" = "102" ]

tests/containers.nix

@@ -0,0 +1,68 @@
+# Test whether we can run a NixOS container inside a Nix build using systemd-nspawn.
+{ nixpkgs, system, overlay }:
+
+with import (nixpkgs + "/nixos/lib/testing-python.nix") {
+  inherit system;
+  extraConfigurations = [ { nixpkgs.overlays = [ overlay ]; } ];
+};
+
+makeTest ({
+  name = "containers";
+
+  nodes =
+    {
+      host =
+        { config, lib, pkgs, nodes, ... }:
+        { virtualisation.writableStore = true;
+          virtualisation.diskSize = 2048;
+          virtualisation.additionalPaths =
+            [ pkgs.stdenv
+              (import ./systemd-nspawn.nix { inherit nixpkgs; }).toplevel
+            ];
+          virtualisation.memorySize = 4096;
+          nix.binaryCaches = lib.mkForce [ ];
+          nix.extraOptions =
+            ''
+              extra-experimental-features = nix-command auto-allocate-uids cgroups
+              extra-system-features = uid-range
+            '';
+          nix.nixPath = [ "nixpkgs=${nixpkgs}" ];
+        };
+    };
+
+  testScript = { nodes }: ''
+    start_all()
+
+    host.succeed("nix --version >&2")
+
+    # Test that 'id' gives the expected result in various configurations.
+
+    # Existing UIDs, sandbox.
+    host.succeed("nix build --no-auto-allocate-uids --sandbox -L --offline --impure --file ${./id-test.nix} --argstr name id-test-1")
+    host.succeed("[[ $(cat ./result) = 'uid=1000(nixbld) gid=100(nixbld) groups=100(nixbld)' ]]")
+
+    # Existing UIDs, no sandbox.
+    host.succeed("nix build --no-auto-allocate-uids --no-sandbox -L --offline --impure --file ${./id-test.nix} --argstr name id-test-2")
+    host.succeed("[[ $(cat ./result) = 'uid=30001(nixbld1) gid=30000(nixbld) groups=30000(nixbld)' ]]")
+
+    # Auto-allocated UIDs, sandbox.
+    host.succeed("nix build --auto-allocate-uids --sandbox -L --offline --impure --file ${./id-test.nix} --argstr name id-test-3")
+    host.succeed("[[ $(cat ./result) = 'uid=1000(nixbld) gid=100(nixbld) groups=100(nixbld)' ]]")
+
+    # Auto-allocated UIDs, no sandbox.
+    host.succeed("nix build --auto-allocate-uids --no-sandbox -L --offline --impure --file ${./id-test.nix} --argstr name id-test-4")
+    host.succeed("[[ $(cat ./result) = 'uid=872415232 gid=30000(nixbld) groups=30000(nixbld)' ]]")
+
+    # Auto-allocated UIDs, UID range, sandbox.
+    host.succeed("nix build --auto-allocate-uids --sandbox -L --offline --impure --file ${./id-test.nix} --argstr name id-test-5 --arg uidRange true")
+    host.succeed("[[ $(cat ./result) = 'uid=0(root) gid=0(root) groups=0(root)' ]]")
+
+    # Auto-allocated UIDs, UID range, no sandbox.
+    host.fail("nix build --auto-allocate-uids --no-sandbox -L --offline --impure --file ${./id-test.nix} --argstr name id-test-6 --arg uidRange true")
+
+    # Run systemd-nspawn in a Nix build.
+    host.succeed("nix build --auto-allocate-uids --sandbox -L --offline --impure --file ${./systemd-nspawn.nix} --argstr nixpkgs ${nixpkgs}")
+    host.succeed("[[ $(cat ./result/msg) = 'Hello World' ]]")
+  '';
+
+})

tests/fetchClosure.sh

@@ -1,7 +1,6 @@
 source common.sh
 
 enableFeatures "fetch-closure"
-needLocalStore "'--no-require-sigs' can’t be used with the daemon"
 
 clearStore
 clearCacheCache
@@ -28,15 +27,19 @@ clearStore
 [ ! -e $nonCaPath ]
 [ -e $caPath ]
 
-# In impure mode, we can use non-CA paths.
+if [[ "$NIX_REMOTE" != "daemon" ]]; then
-[[ $(nix eval --raw --no-require-sigs --impure --expr "
-  builtins.fetchClosure {
-    fromStore = \"file://$cacheDir\";
-    fromPath = $nonCaPath;
-  }
-") = $nonCaPath ]]
 
-[ -e $nonCaPath ]
+    # In impure mode, we can use non-CA paths.
+    [[ $(nix eval --raw --no-require-sigs --impure --expr "
+      builtins.fetchClosure {
+        fromStore = \"file://$cacheDir\";
+        fromPath = $nonCaPath;
+      }
+    ") = $nonCaPath ]]
+
+    [ -e $nonCaPath ]
+
+fi
 
 # 'toPath' set to empty string should fail but print the expected path.
 nix eval -v --json --expr "

tests/id-test.nix

@@ -0,0 +1,8 @@
+{ name, uidRange ? false }:
+
+with import <nixpkgs> {};
+
+runCommand name
+  { requiredSystemFeatures = if uidRange then ["uid-range"] else [];
+  }
+  "id; id > $out"

tests/systemd-nspawn.nix

@@ -0,0 +1,78 @@
+{ nixpkgs }:
+
+let
+
+  machine = { config, pkgs, ... }:
+    {
+      system.stateVersion = "22.05";
+      boot.isContainer = true;
+      systemd.services.console-getty.enable = false;
+      networking.dhcpcd.enable = false;
+
+      services.httpd = {
+        enable = true;
+        adminAddr = "nixos@example.org";
+      };
+
+      systemd.services.test = {
+        wantedBy = [ "multi-user.target" ];
+        after = [ "httpd.service" ];
+        script = ''
+          source /.env
+          echo "Hello World" > $out/msg
+          ls -lR /dev > $out/dev
+          ${pkgs.curl}/bin/curl -sS --fail http://localhost/ > $out/page.html
+        '';
+        unitConfig = {
+          FailureAction = "exit-force";
+          FailureActionExitStatus = 42;
+          SuccessAction = "exit-force";
+        };
+      };
+    };
+
+  cfg = (import (nixpkgs + "/nixos/lib/eval-config.nix") {
+    modules = [ machine ];
+    system = "x86_64-linux";
+  });
+
+  config = cfg.config;
+
+in
+
+with cfg._module.args.pkgs;
+
+runCommand "test"
+  { buildInputs = [ config.system.path ];
+    requiredSystemFeatures = [ "uid-range" ];
+    toplevel = config.system.build.toplevel;
+  }
+  ''
+    root=$(pwd)/root
+    mkdir -p $root $root/etc
+
+    export > $root/.env
+
+    # Make /run a tmpfs to shut up a systemd warning.
+    mkdir /run
+    mount -t tmpfs none /run
+    chmod 0700 /run
+
+    mount -t cgroup2 none /sys/fs/cgroup
+
+    mkdir -p $out
+
+    touch /etc/os-release
+    echo a5ea3f98dedc0278b6f3cc8c37eeaeac > /etc/machine-id
+
+    SYSTEMD_NSPAWN_UNIFIED_HIERARCHY=1 \
+      ${config.systemd.package}/bin/systemd-nspawn \
+      --keep-unit \
+      -M ${config.networking.hostName} -D "$root" \
+      --register=no \
+      --resolv-conf=off \
+      --bind-ro=/nix/store \
+      --bind=$out \
+      --private-network \
+      $toplevel/init
+  ''