build-time: hide boost stacktrace in a .cc file

Saves about 16s of CPU time. Not a lot but not nothing. Feels more like
the principle of the thing.

Change-Id: I0992d4024317c20d6985a7977d5649edfb9f46bb

.gitignore

@@ -9,6 +9,10 @@ GTAGS
 # ccls
 /.ccls-cache
 
+# auto-generated compilation database
+compile_commands.json
+rust-project.json
+
 result
 result-*
 
@@ -29,3 +33,6 @@ buildtime.bin
 /.pre-commit-config.yaml
 /.nocontribmsg
 /release
+
+# Rust build files when using Cargo (not actually supported for building but it spews the files anyway)
+/target/

Cargo.lock

@@ -2,12 +2,6 @@
 # It is not intended for manual editing.
 version = 3
 
-[[package]]
-name = "autocfg"
-version = "1.1.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d468802bab17cbc0cc575e9b053f41e72aa36bfa6b7f55e3529ffa43161b97fa"
-
 [[package]]
 name = "countme"
 version = "3.0.1"
@@ -16,15 +10,15 @@ checksum = "7704b5fdd17b18ae31c4c1da5a2e0305a2bf17b5249300a9ee9ed7b72114c636"
 
 [[package]]
 name = "dissimilar"
-version = "1.0.7"
+version = "1.0.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "86e3bdc80eee6e16b2b6b0f87fbc98c04bee3455e35174c0de1a125d0688c632"
+checksum = "59f8e79d1fbf76bdfbde321e902714bf6c49df88a7dda6fc682fc2979226962d"
 
 [[package]]
 name = "expect-test"
-version = "1.4.1"
+version = "1.5.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "30d9eafeadd538e68fb28016364c9732d78e420b9ff8853fa5e4058861e9f8d3"
+checksum = "9e0be0a561335815e06dab7c62e50353134c796e7a6155402a64bcff66b6a5e0"
 dependencies = [
  "dissimilar",
  "once_cell",
@@ -45,15 +39,6 @@ dependencies = [
  "rowan",
 ]
 
-[[package]]
-name = "memoffset"
-version = "0.9.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "488016bfae457b036d996092f6cb448677611ce4449e970ceaf42695203f218a"
-dependencies = [
- "autocfg",
-]
-
 [[package]]
 name = "once_cell"
 version = "1.19.0"
@@ -71,13 +56,12 @@ dependencies = [
 
 [[package]]
 name = "rowan"
-version = "0.15.15"
+version = "0.15.16"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "32a58fa8a7ccff2aec4f39cc45bf5f985cec7125ab271cf681c279fd00192b49"
+checksum = "0a542b0253fa46e632d27a1dc5cf7b930de4df8659dc6e720b647fc72147ae3d"
 dependencies = [
  "countme",
  "hashbrown",
- "memoffset",
  "rustc-hash",
  "text-size",
 ]

Cargo.toml

@@ -0,0 +1,6 @@
+[workspace]
+resolver = "2"
+members = ["src/lix-doc"]
+
+[workspace.package]
+edition = "2021"

doc/manual/generate-features-shortlist.nix

@@ -1,9 +1,14 @@
+# Usually "experimental" or "deprecated"
+kind:
+# "xp" or "dp"
+kindShort:
+
 with builtins;
 with import ./utils.nix;
 
 let
   showExperimentalFeature = name: doc: ''
-    - [`${name}`](@docroot@/contributing/experimental-features.md#xp-feature-${name})
+    - [`${name}`](@docroot@/contributing/${kind}-features.md#${kindShort}-feature-${name})
   '';
 in
 xps: indent "  " (concatStrings (attrValues (mapAttrs showExperimentalFeature xps)))

doc/manual/generate-features.nix

@@ -0,0 +1,18 @@
+# Usually "experimental" or "deprecated"
+_kind:
+# "xp" or "dp"
+kindShort:
+
+with builtins;
+with import ./utils.nix;
+
+let
+  showFeature =
+    name: doc:
+    squash ''
+      ## [`${name}`]{#${kindShort}-feature-${name}}
+
+      ${doc}
+    '';
+in
+xps: (concatStringsSep "\n" (attrValues (mapAttrs showFeature xps)))

doc/manual/generate-xp-features.nix

@@ -1,13 +0,0 @@
-with builtins;
-with import ./utils.nix;
-
-let
-  showExperimentalFeature =
-    name: doc:
-    squash ''
-      ## [`${name}`]{#xp-feature-${name}}
-
-      ${doc}
-    '';
-in
-xps: (concatStringsSep "\n" (attrValues (mapAttrs showExperimentalFeature xps)))

doc/manual/meson.build

@@ -20,6 +20,8 @@ conf_file_json = custom_target(
   capture : true,
   output : 'conf-file.json',
   env : nix_env_for_docs,
+  # FIXME: put the actual lib targets in here? meson have introspection challenge 2024 though.
+  build_always_stale : true,
 )
 
 nix_conf_file_md_body = custom_target(
@@ -50,6 +52,8 @@ nix_exp_features_json = custom_target(
   command : [ nix, '__dump-xp-features' ],
   capture : true,
   output : 'xp-features.json',
+  # FIXME: put the actual lib targets in here? meson have introspection challenge 2024 though.
+  build_always_stale : true,
 )
 
 language_json = custom_target(
@@ -57,6 +61,8 @@ language_json = custom_target(
   output : 'language.json',
   capture : true,
   env : nix_env_for_docs,
+  # FIXME: put the actual lib targets in here? meson have introspection challenge 2024 though.
+  build_always_stale : true,
 )
 
 nix3_cli_json = custom_target(
@@ -64,6 +70,8 @@ nix3_cli_json = custom_target(
   capture : true,
   output : 'nix.json',
   env : nix_env_for_docs,
+  # FIXME: put the actual lib targets in here? meson have introspection challenge 2024 though.
+  build_always_stale : true,
 )
 
 generate_manual_deps = files(
@@ -72,9 +80,9 @@ generate_manual_deps = files(
 
 # Generates builtins.md and builtin-constants.md.
 subdir('src/language')
-# Generates new-cli pages, experimental-features-shortlist.md, and conf-file.md.
+# Generates new-cli pages, {experimental,deprecated}-features-shortlist.md, and conf-file.md.
 subdir('src/command-ref')
-# Generates experimental-feature-descriptions.md.
+# Generates {experimental,deprecated}-feature-descriptions.md.
 subdir('src/contributing')
 # Generates rl-next-generated.md.
 subdir('src/release-notes')
@@ -106,6 +114,8 @@ manual = custom_target(
     nix3_cli_files,
     experimental_features_shortlist_md,
     experimental_feature_descriptions_md,
+    deprecated_features_shortlist_md,
+    deprecated_feature_descriptions_md,
     conf_file_md,
     builtins_md,
     builtin_constants_md,

doc/manual/rl-next/allowsubstitutes-errors.md

@@ -0,0 +1,21 @@
+---
+synopsis: "Build failures caused by `allowSubstitutes = false` while being the wrong system now produce a decent error"
+issues: [fj#484]
+cls: [1841]
+category: Fixes
+credits: jade
+---
+
+Nix allows derivations to set `allowSubstitutes = false` in order to force them to be built locally without querying substituters for them.
+This is useful for derivations that are very fast to build (especially if they produce large output).
+However, this can shoot you in the foot if the derivation *has* to be substituted such as if the derivation is for another architecture, which is what `--always-allow-substitutes` is for.
+
+Perhaps such derivations that are known to be impossible to build locally should ignore `allowSubstitutes` (irrespective of remote builders) in the future, but this at least reports the failure and solution directly.
+
+```
+$ nix build -f fail.nix
+error: a 'unicornsandrainbows-linux' with features {} is required to build '/nix/store/...-meow.drv', but I am a 'x86_64-linux' with features {...}
+
+       Hint: the failing derivation has allowSubstitutes set to false, forcing it to be built rather than substituted.
+       Passing --always-allow-substitutes to force substitution may resolve this failure if the path is available in a substituter.
+```

doc/manual/rl-next/deprecated-features.md

@@ -0,0 +1,17 @@
+---
+synopsis: Deprecated language features
+issues: [fj#437]
+cls: [1785, 1736, 1735, 1744]
+category: Breaking Changes
+credits: [piegames, horrors]
+---
+
+A system for deprecation (and then the planned removal) of undesired language features has been put into place.
+It is controlled via feature flags much like experimental features, except that the deprecations are enabled default,
+and can be disabled via the flags for backwards compatibility (opt-out with `--extra-deprecated-features` or the Nix configuration file).
+
+- `url-literals`: **URL literals** have long been obsolete and discouraged of use, and now they are officially deprecated.
+  This means that all URLs must be properly put within quotes like all other strings.
+- `rec-set-overrides`: **__overrides** is an old arcane syntax which has not been in use for more than a decade.
+  It is soft-deprecated with a warning only, with the plan to turn that into an error in a future release.
+- `ancient-let`: **The old `let` syntax** (`let { body = …; … }`) is soft-deprecated with a warning as well. Use the regular `let … in` instead.

doc/manual/rl-next/http-proxy-for-s3.md

@@ -0,0 +1,10 @@
+---
+synopsis: HTTP proxy environment variables are now respected for S3 binary cache stores
+issues: [fj#433]
+cls: [1788]
+category: Fixes
+credits: jade
+---
+
+Due to "legacy reasons" (according to the AWS C++ SDK docs), the AWS SDK ignores system proxy configuration by default.
+We turned it back on.

doc/manual/src/SUMMARY.md

@@ -192,6 +192,7 @@
   - [Hacking](contributing/hacking.md)
   - [Testing](contributing/testing.md)
   - [Experimental Features](contributing/experimental-features.md)
+  - [Deprecated Features](contributing/deprecated-features.md)
   - [CLI guideline](contributing/cli-guideline.md)
   - [C++ style guide](contributing/cxx.md)
 - [Release Notes](release-notes/release-notes.md)

doc/manual/src/command-ref/meson.build

@@ -1,23 +1,37 @@
-xp_features_json = custom_target(
-  command : [nix, '__dump-xp-features'],
-  capture : true,
-  output : 'xp-features.json',
-)
-
 experimental_features_shortlist_md = custom_target(
   command : nix_eval_for_docs + [
     '--expr',
-    'import @INPUT0@ (builtins.fromJSON (builtins.readFile @INPUT1@))',
+    'import @INPUT0@ "experimental" "xp" (builtins.fromJSON (builtins.readFile @INPUT1@))',
   ],
   input : [
-    '../../generate-xp-features-shortlist.nix',
-    xp_features_json,
+    '../../generate-features-shortlist.nix',
+    nix_exp_features_json,
   ],
   capture : true,
   output : 'experimental-features-shortlist.md',
   env : nix_env_for_docs,
 )
 
+dp_features_json = custom_target(
+  command : [nix, '__dump-dp-features'],
+  capture : true,
+  output : 'dp-features.json',
+)
+
+deprecated_features_shortlist_md = custom_target(
+  command : nix_eval_for_docs + [
+    '--expr',
+    'import @INPUT0@ "deprecated" "dp" (builtins.fromJSON (builtins.readFile @INPUT1@))',
+  ],
+  input : [
+    '../../generate-features-shortlist.nix',
+    dp_features_json,
+  ],
+  capture : true,
+  output : 'deprecated-features-shortlist.md',
+  env : nix_env_for_docs,
+)
+
 # Intermediate step for manpage generation.
 # This splorks the output of generate-manpage.nix as JSON,
 # which gets written as a directory tree below.
@@ -60,6 +74,7 @@ conf_file_md = custom_target(
     '../../utils.nix',
     conf_file_json,
     experimental_features_shortlist_md,
+    deprecated_features_shortlist_md,
   ],
   output : 'conf-file.md',
   env : nix_env_for_docs,

doc/manual/src/contributing/deprecated-features.md

@@ -0,0 +1,37 @@
+This section describes the notion of *deprecated features*, and how it fits into the big picture of the development of Lix.
+
+# What are deprecated features?
+
+Deprecated features are disabled by default, with the intent to eventually remove them.
+Users must explicitly enable them to keep using them, by toggling the associated [deprecated feature flags](@docroot@/command-ref/conf-file.md#conf-deprecated-features).
+This allows backwards compatibility and a graceful transition away from undesired features.
+
+# Which features can be deprecated?
+
+Undesired features should be soft-deprecated by yielding a warning when used for a significant amount of time before the can be deprecated.
+Legacy obsolete feature with little to no usage may go through this process faster.
+Deprecated features should have a migration path to a preferred alternative.
+
+# Lifecycle of a deprecated feature
+
+This description is not normative, but a feature removal may roughly happen like this:
+
+1. Add a warning when the feature is being used.
+2. Disable the feature by default, putting it behind a deprecated feature flag.
+  - If disabling the feature started out as an opt-in experimental feature, turn that experimental flag into a no-op or remove it entirely.
+    For example, `--extra-experimental-features=no-url-literals` becomes `--extra-deprecated-features=url-literals`.
+3. Decide on a time frame for how long that feature will still be supported for backwards compatibility, and clearly communicate that in the error messages.
+  - Sometimes, automatic migration to alternatives is possible, and such should be provided if possible
+  - At least one NixOS release cycle should be the minimum
+4. Finally remove the feature entirely, only keeping the error message for those still using it.
+
+# Relation to language versioning
+
+Obviously, removing anything breaks backwards compatibility.
+In an ideal world, we'd have SemVer controls over the language and its features, cleanly allowing us to make breaking changes.
+See https://wiki.lix.systems/books/lix-contributors/page/language-versioning and [RFC 137](https://github.com/nixos/rfcs/pull/137) for efforts on that.
+However, we do not live in such an ideal world, and currently this goal is so far away, that "just disable it with some back-compat for a couple of years" is the most realistic solution, especially for comparatively minor changes.
+
+# Currently available deprecated features
+
+{{#include @generated@/contributing/deprecated-feature-descriptions.md}}

doc/manual/src/contributing/meson.build

@@ -4,12 +4,25 @@
 experimental_feature_descriptions_md = custom_target(
   command : nix_eval_for_docs + [
     '--expr',
-    'import @INPUT0@ (builtins.fromJSON (builtins.readFile @INPUT1@))',
+    'import @INPUT0@ "experimental" "xp" (builtins.fromJSON (builtins.readFile @INPUT1@))',
   ],
   input : [
-    '../../generate-xp-features.nix',
-    xp_features_json,
+    '../../generate-features.nix',
+    nix_exp_features_json,
   ],
   capture : true,
   output : 'experimental-feature-descriptions.md',
 )
+
+deprecated_feature_descriptions_md = custom_target(
+  command : nix_eval_for_docs + [
+    '--expr',
+    'import @INPUT0@ "deprecated" "dp" (builtins.fromJSON (builtins.readFile @INPUT1@))',
+  ],
+  input : [
+    '../../generate-features.nix',
+    dp_features_json,
+  ],
+  capture : true,
+  output : 'deprecated-feature-descriptions.md',
+)

doc/manual/src/language/values.md

@@ -77,12 +77,6 @@
   }
   ```
 
-  Finally, as a convenience, *URIs* as defined in appendix B of
-  [RFC 2396](http://www.ietf.org/rfc/rfc2396.txt) can be written *as
-  is*, without quotes. For instance, the string
-  `"http://example.org/foo.tar.bz2"` can also be written as
-  `http://example.org/foo.tar.bz2`.
-
 - <a id="type-number" href="#type-number">Number</a>
 
   Numbers, which can be *integers* (like `123`) or *floating point*

meson.build

@@ -30,6 +30,14 @@
 # FIXME: This hack should be removed when https://git.lix.systems/lix-project/lix/issues/359
 # is fixed.
 #
+# lix-doc is built with Meson in lix-doc/meson.build, and linked into libcmd in
+# src/libcmd/meson.build. When building outside the Nix sandbox, Meson will use the .wrap
+# files in subprojects/ to download and extract the dependency crates into subprojects/.
+# When building inside the Nix sandbox, Lix's derivation in package.nix uses a
+# fixed-output derivation to fetch those crates in advance instead, and then symlinks
+# them into subprojects/ with the same names that Meson uses when downloading them
+# itself -- perfect for --wrap-mode=nodownload, which mesonConfigurePhase uses.
+#
 # Unit tests are setup in tests/unit/meson.build, under the test suite "check".
 #
 # Functional tests are a bit more complicated. Generally they're defined in
@@ -38,10 +46,11 @@
 # be placed in specific directories' meson.build files to create the right directory tree
 # in the build directory.
 
-project('lix', 'cpp',
+project('lix', 'cpp', 'rust',
   version : run_command('bash', '-c', 'echo -n $(jq -r .version < ./version.json)$VERSION_SUFFIX', check : true).stdout().strip(),
   default_options : [
     'cpp_std=c++2a',
+    'rust_std=2021',
     # TODO(Qyriad): increase the warning level
     'warning_level=1',
     'debug=true',
@@ -138,6 +147,17 @@ if should_pch
   # Unlike basically everything else that takes a file, Meson requires the arguments to
   # cpp_pch : to be strings and doesn't accept files(). So absolute path it is.
   cpp_pch = [meson.project_source_root() / 'src/pch/precompiled-headers.hh']
+
+  # Saves about 400s (30% at time of writing) from compile time on-cpu, mostly
+  # by removing instantiations of nlohmann from every single damned compilation
+  # unit.
+  # There is no equivalent in gcc.
+  if cxx.get_id() == 'clang'
+    add_project_arguments(
+      '-fpch-instantiate-templates',
+      language : 'cpp',
+    )
+  endif
 else
   cpp_pch = []
 endif
@@ -322,13 +342,6 @@ pegtl = dependency(
 
 nlohmann_json = dependency('nlohmann_json', required : true, include_type : 'system')
 
-# lix-doc is a Rust project provided via buildInputs and unfortunately doesn't have any way to be detected.
-# Just declare it manually to resolve this.
-#
-# FIXME: build this with meson in the future after we drop Make (with which we
-# *absolutely* are not going to make it work)
-lix_doc = declare_dependency(link_args : [ '-llix_doc' ])
-
 if is_freebsd
   libprocstat = declare_dependency(link_args : [ '-lprocstat' ])
 endif
@@ -417,7 +430,7 @@ check_funcs = [
   'strsignal',
   'sysconf',
 ]
-if target_machine.kernel() in ['linux', 'freebsd']
+if is_linux or is_freebsd
   # musl does not have close_range as of 2024-08-10
   # patch: https://www.openwall.com/lists/musl/2024/08/01/9
   check_funcs += [ 'close_range' ]
@@ -543,6 +556,33 @@ if cxx.get_id() in ['clang', 'gcc']
   )
 endif
 
+# Until Meson 1.5¹, we can't just give Meson a Cargo.lock file and be done with it.
+# Meson will *detect* what dependencies are needed from Cargo files; it just won't
+# fetch them. The Meson 1.5 feature essentially internally translates Cargo.lock entries
+# to .wrap files, and that translation is incredibly straightforward, so let's just
+# use a simple Python script to generate the .wrap files ourselves while we wait for
+# Meson 1.5. Weirdly, it seems Meson will only detect dependencies from other
+# dependency() calls, so we have to specify lix-doc's two top-level dependencies,
+# rnix and rowan, manually, and then their dependencies will be recursively translated
+# into more dependency() calls.
+#
+# When Meson translates a Cargo dependency, the string passed to `dependency()` follows
+# a fixed format, which is important as the .wrap files' basenames must match the string
+# passed to `dependency()` exactly.
+# In Meson 1.4, this format is `$packageName-rs`. Meson 1.5 changes this to
+# `$packageName-$shortenedVersionString-rs`, because of course it does, but we'll cross
+# that bridge when we get there...
+#
+# [1]: https://github.com/mesonbuild/meson/commit/9b8378985dbdc0112d11893dd42b33b7bc8d1e62
+# FIXME: remove (along with its generated wrap files) when we get rid of meson 1.4
+run_command(
+  python,
+  meson.project_source_root() / 'meson/cargo-lock-to-wraps.py',
+  meson.project_source_root() / 'Cargo.lock',
+  meson.project_source_root() / 'subprojects',
+  check : true,
+)
+
 if is_darwin
   configure_file(
     input : 'misc/launchd/org.nixos.nix-daemon.plist.in',

meson/cargo-lock-to-wraps.py

@@ -0,0 +1,43 @@
+#!/usr/bin/env python3
+
+import argparse
+import tomllib
+import sys
+
+DOWNLOAD_URI_FORMAT = 'https://crates.io/api/v1/crates/{crate}/{version}/download'
+
+WRAP_TEMPLATE = """
+[wrap-file]
+method = cargo
+directory = {crate}-{version}
+source_url = {url}
+source_filename = {crate}-{version}.tar.gz
+source_hash = {hash}
+""".lstrip()
+
+parser = argparse.ArgumentParser()
+parser.add_argument('lockfile', help='path to the Cargo lockfile to generate wraps from')
+parser.add_argument('outdir', help="the 'subprojects' directory to write .wrap files to")
+
+args = parser.parse_args()
+
+with open(args.lockfile, 'rb') as f:
+    lock_toml = tomllib.load(f)
+
+for dependency in lock_toml['package']:
+    try:
+        hash = dependency['checksum']
+    except KeyError:
+        # The base package, e.g. lix-doc, won't have a checksum, and conveniently
+        # the base package is also not something we want a wrap file for.
+        # Doesn't that work out nicely?
+        continue
+
+    crate = dependency['name']
+    version = dependency['version']
+
+    url = DOWNLOAD_URI_FORMAT.format(crate=crate, version=version)
+
+    wrap_text = WRAP_TEMPLATE.format(crate=crate, version=version, url=url, hash=hash)
+    with open(f'{args.outdir}/{crate}-rs.wrap', 'w') as f:
+        f.write(wrap_text)

meson/clang-tidy/clang-tidy-runner.py

@@ -0,0 +1,89 @@
+#!/usr/bin/env python3
+"""
+Runs run-clang-tidy. A bit meta. Maybe it will replace run-clang-tidy one day
+because the run-clang-tidy UX is so questionable.
+"""
+
+# I hereby dedicate this script to fuck you meson.
+# I cannot simply write my code to invoke a subprocess in a meson file because
+# Meson corrupts backslashes in command line args to subprocesses.
+# This is allegedly for "Windows support", but last time I checked Windows
+# neither needs nor wants you to corrupt its command lines.
+# https://github.com/mesonbuild/meson/issues/1564
+
+import multiprocessing
+import subprocess
+import os
+import sys
+from pathlib import Path
+
+
+def default_concurrency():
+    return min(multiprocessing.cpu_count(),
+               int(os.environ.get("NIX_BUILD_CORES", "16")))
+
+
+def go(exe: str, plugin_path: Path, compile_commands_json_dir: Path, jobs: int,
+       paths: list[Path], werror: bool, fix: bool):
+    args = [
+        # XXX: This explicitly invokes it with python because of a nixpkgs bug
+        # where clang-unwrapped does not patch interpreters in run-clang-tidy.
+        # However, making clang-unwrapped depend on python is also silly, so idk.
+        sys.executable,
+        exe,
+        '-quiet',
+        '-load',
+        plugin_path,
+        '-p',
+        compile_commands_json_dir,
+        '-j',
+        str(jobs),
+        '-header-filter',
+        r'src/[^/]+/.*\.hh'
+    ]
+    if werror:
+        args += ['-warnings-as-errors', '*']
+    if fix:
+        args += ['-fix']
+    args += ['--']
+    args += paths
+    os.execvp(sys.executable, args)
+
+
+def main():
+    import argparse
+
+    ap = argparse.ArgumentParser(description='Runs run-clang-tidy for you')
+    ap.add_argument('--jobs',
+                    '-j',
+                    type=int,
+                    default=default_concurrency(),
+                    help='Parallel linting jobs to run')
+    ap.add_argument('--plugin-path',
+                    type=Path,
+                    help='Path to the Lix clang-tidy plugin')
+    # FIXME: maybe we should integrate this so it just fixes the compdb for you and throws it in a tempdir?
+    ap.add_argument(
+        '--compdb-path',
+        type=Path,
+        help=
+        'Path to the directory containing the fixed-up compilation database from clean_compdb'
+    )
+    ap.add_argument('--werror',
+                    action='store_true',
+                    help='Warnings get turned into errors')
+    ap.add_argument('--fix',
+                    action='store_true',
+                    help='Apply fixes for warnings')
+    ap.add_argument('--run-clang-tidy-path',
+                    default='run-clang-tidy',
+                    help='Path to run-clang-tidy')
+    ap.add_argument('paths', nargs='*', help='Source paths to check')
+    args = ap.parse_args()
+
+    go(args.run_clang_tidy_path, args.plugin_path, args.compdb_path, args.jobs,
+       args.paths, args.werror, args.fix)
+
+
+if __name__ == '__main__':
+    main()

meson/clang-tidy/clean_compdb.py

@@ -13,8 +13,8 @@ def process_compdb(compdb: list[dict]) -> list[dict]:
         out = []
         eat_next = False
         for i, arg in enumerate(args):
-            if arg == '-fpch-preprocess':
-                # as used with gcc
+            if arg in ['-fpch-preprocess', '-fpch-instantiate-templates']:
+                # -fpch-preprocess as used with gcc, -fpch-instantiate-templates as used by clang
                 continue
             elif arg == '-include-pch' or (arg == '-include' and args[i + 1] == 'precompiled-headers.hh'):
                 # -include-pch some-pch (clang), or -include some-pch (gcc)
@@ -30,7 +30,14 @@ def process_compdb(compdb: list[dict]) -> list[dict]:
         item['command'] = shlex.join(munch_command(shlex.split(item['command'])))
         return item
 
-    return [chomp(x) for x in compdb if not x['file'].endswith('precompiled-headers.hh')]
+    def cmdfilter(item: dict) -> bool:
+        file = item['file']
+        return (
+            not file.endswith('precompiled-headers.hh')
+            and not file.endswith('.rs')
+        )
+
+    return [chomp(x) for x in compdb if cmdfilter(x)]
 
 
 def main():

meson/clang-tidy/meson.build

@@ -58,26 +58,17 @@ build_all_generated_headers = custom_target(
 
 if lix_clang_tidy_so_found
   run_clang_tidy_args = [
-    '-load',
-    lix_clang_tidy_so,
-    '-p',
-    # We have to workaround a run-clang-tidy bug too, so we must give the
-    # directory name rather than the actual compdb file.
-    # https://github.com/llvm/llvm-project/issues/101440
-    meson.current_build_dir(),
-    '-quiet',
+    meson.current_source_dir() / 'clang-tidy-runner.py',
+    '--run-clang-tidy-path', run_clang_tidy,
+    '--compdb-path', meson.current_build_dir(),
+    '--plugin-path', lix_clang_tidy_so,
   ]
   run_target(
     'clang-tidy',
     command : [
-      # XXX: This explicitly invokes it with python because of a nixpkgs bug
-      # where clang-unwrapped does not patch interpreters in run-clang-tidy.
-      # However, making clang-unwrapped depend on python is also silly, so idk.
       python,
-      run_clang_tidy,
       run_clang_tidy_args,
-      '-warnings-as-errors',
-      '*',
+      '--werror',
     ],
     depends : [
       build_all_generated_headers,
@@ -87,9 +78,8 @@ if lix_clang_tidy_so_found
     'clang-tidy-fix',
     command : [
       python,
-      run_clang_tidy,
       run_clang_tidy_args,
-      '-fix',
+      '--fix',
     ],
     depends : [
       build_all_generated_headers,

package.nix

@@ -41,6 +41,8 @@
   pkg-config,
   python3,
   rapidcheck,
+  rustPlatform,
+  rustc,
   sqlite,
   toml11,
   util-linuxMinimal ? utillinuxMinimal,
@@ -49,9 +51,6 @@
 
   busybox-sandbox-shell,
 
-  # internal fork of nix-doc providing :doc in the repl
-  lix-doc ? __forDefaults.lix-doc,
-
   pname ? "lix",
   versionSuffix ? "",
   officialRelease ? __forDefaults.versionJson.official_release,
@@ -83,7 +82,6 @@
       configureFlags = prev.configureFlags or [ ] ++ [ (lib.enableFeature true "sigstop") ];
     });
 
-    lix-doc = callPackage ./lix-doc/package.nix { };
     build-release-notes = callPackage ./maintainers/build-release-notes.nix { };
   },
 }:
@@ -139,6 +137,8 @@ let
     ./meson
     ./scripts/meson.build
     ./subprojects
+    # Required for meson to generate Cargo wraps
+    ./Cargo.lock
   ]);
 
   functionalTestFiles = fileset.unions [
@@ -219,6 +219,7 @@ stdenv.mkDerivation (finalAttrs: {
       meson
       ninja
       cmake
+      rustc
     ]
     ++ [
       (lib.getBin lowdown)
@@ -258,7 +259,6 @@ stdenv.mkDerivation (finalAttrs: {
       lowdown
       libsodium
       toml11
-      lix-doc
       pegtl
     ]
     ++ lib.optionals hostPlatform.isLinux [
@@ -288,8 +288,15 @@ stdenv.mkDerivation (finalAttrs: {
   env = {
     BOOST_INCLUDEDIR = "${lib.getDev boost}/include";
     BOOST_LIBRARYDIR = "${lib.getLib boost}/lib";
+
+    # Meson allows referencing a /usr/share/cargo/registry shaped thing for subproject sources.
+    # Turns out the Nix-generated Cargo dependencies are named the same as they
+    # would be in a Cargo registry cache.
+    MESON_PACKAGE_CACHE_DIR = finalAttrs.cargoDeps;
   };
 
+  cargoDeps = rustPlatform.importCargoLock { lockFile = ./Cargo.lock; };
+
   preConfigure =
     lib.optionalString (!finalAttrs.dontBuild && !hostPlatform.isStatic) ''
       # Copy libboost_context so we don't get all of Boost in our closure.
@@ -425,6 +432,10 @@ stdenv.mkDerivation (finalAttrs: {
         pre-commit-checks,
         contribNotice,
         check-syscalls,
+
+        # debuggers
+        gdb,
+        rr,
       }:
       let
         glibcFix = lib.optionalAttrs (buildPlatform.isLinux && glibcLocales != null) {
@@ -504,6 +515,8 @@ stdenv.mkDerivation (finalAttrs: {
             ]
             ++ lib.optional (pre-commit-checks ? enabledPackages) pre-commit-checks.enabledPackages
             ++ lib.optional (lib.meta.availableOn buildPlatform clangbuildanalyzer) clangbuildanalyzer
+            ++ lib.optional (!stdenv.isDarwin) gdb
+            ++ lib.optional (lib.meta.availableOn buildPlatform rr) rr
             ++ finalAttrs.checkInputs;
 
           shellHook = ''

src/build-remote/build-remote.cc

@@ -1,11 +1,7 @@
-#include <cstdlib>
-#include <cstring>
 #include <algorithm>
 #include <set>
 #include <memory>
-#include <string_view>
 #include <tuple>
-#include <iomanip>
 #if __APPLE__
 #include <sys/time.h>
 #endif
@@ -18,6 +14,7 @@
 #include "build-result.hh"
 #include "store-api.hh"
 #include "derivations.hh"
+#include "strings.hh"
 #include "local-store.hh"
 #include "legacy.hh"
 #include "experimental-features.hh"

src/libcmd/built-path.hh

@@ -20,13 +20,15 @@ struct SingleBuiltPathBuilt {
     DECLARE_CMP(SingleBuiltPathBuilt);
 };
 
-using _SingleBuiltPathRaw = std::variant<
+namespace built_path::detail {
+using SingleBuiltPathRaw = std::variant<
     DerivedPathOpaque,
     SingleBuiltPathBuilt
 >;
+}
 
-struct SingleBuiltPath : _SingleBuiltPathRaw {
-    using Raw = _SingleBuiltPathRaw;
+struct SingleBuiltPath : built_path::detail::SingleBuiltPathRaw {
+    using Raw = built_path::detail::SingleBuiltPathRaw;
     using Raw::Raw;
 
     using Opaque = DerivedPathOpaque;
@@ -65,17 +67,19 @@ struct BuiltPathBuilt {
     DECLARE_CMP(BuiltPathBuilt);
 };
 
-using _BuiltPathRaw = std::variant<
+namespace built_path::detail {
+using BuiltPathRaw = std::variant<
     DerivedPath::Opaque,
     BuiltPathBuilt
 >;
+}
 
 /**
  * A built path. Similar to a DerivedPath, but enriched with the corresponding
  * output path(s).
  */
-struct BuiltPath : _BuiltPathRaw {
-    using Raw = _BuiltPathRaw;
+struct BuiltPath : built_path::detail::BuiltPathRaw {
+    using Raw = built_path::detail::BuiltPathRaw;
     using Raw::Raw;
 
     using Opaque = DerivedPathOpaque;

src/libcmd/editor-for.cc

@@ -1,6 +1,7 @@
 #include "editor-for.hh"
 #include "environment-variables.hh"
 #include "source-path.hh"
+#include "strings.hh"
 
 namespace nix {
 

src/libcmd/meson.build

@@ -50,7 +50,7 @@ libcmd = library(
     editline,
     lowdown,
     nlohmann_json,
-    lix_doc
+    liblix_doc,
   ],
   cpp_pch : cpp_pch,
   install : true,

src/libcmd/repl.cc

@@ -926,7 +926,7 @@ void NixRepl::loadFiles()
 
 void NixRepl::loadReplOverlays()
 {
-    if (!evalSettings.replOverlays) {
+    if (evalSettings.replOverlays.get().empty()) {
         return;
     }
 

src/libexpr/eval-error.hh

@@ -1,9 +1,8 @@
 #pragma once
 ///@file
 
-#include <algorithm>
-
 #include "error.hh"
+#include "types.hh"
 #include "pos-idx.hh"
 
 namespace nix {

src/libexpr/eval-inline.hh

@@ -31,7 +31,7 @@ Value * EvalState::allocValue()
 #endif
 
     nrValues++;
-    return (Value *) p;
+    return static_cast<Value *>(p);
 }
 
 
@@ -54,10 +54,10 @@ Env & EvalState::allocEnv(size_t size)
         void * p = *env1AllocCache;
         *env1AllocCache = GC_NEXT(p);
         GC_NEXT(p) = nullptr;
-        env = (Env *) p;
+        env = static_cast<Env *>(p);
     } else
 #endif
-        env = (Env *) gcAllocBytes(sizeof(Env) + size * sizeof(Value *));
+        env = static_cast<Env *>(gcAllocBytes(sizeof(Env) + size * sizeof(Value *)));
 
     /* We assume that env->values has been cleared by the allocator; maybeThunk() and lookupVar fromWith expect this. */
 

src/libexpr/eval-settings.hh

@@ -151,7 +151,7 @@ struct EvalSettings : Config
           This is useful for debugging warnings in third-party Nix code.
         )"};
 
-    PathsSetting replOverlays{this, Paths(), "repl-overlays",
+    PathsSetting<Paths> replOverlays{this, Paths(), "repl-overlays",
         R"(
           A list of files containing Nix expressions that can be used to add
           default bindings to [`nix

src/libexpr/eval.cc

@@ -19,6 +19,7 @@
 #include "gc-small-vector.hh"
 #include "fetch-to-store.hh"
 #include "flake/flakeref.hh"
+#include "exit.hh"
 
 #include <algorithm>
 #include <iostream>
@@ -249,6 +250,7 @@ EvalState::EvalState(
         .findFile = symbols.create("__findFile"),
         .nixPath = symbols.create("__nixPath"),
         .body = symbols.create("body"),
+        .overrides = symbols.create("__overrides"),
     }
     , repair(NoRepair)
     , emptyBindings(0)
@@ -2710,20 +2712,29 @@ Expr & EvalState::parseExprFromFile(const SourcePath & path, std::shared_ptr<Sta
 }
 
 
-Expr & EvalState::parseExprFromString(std::string s_, const SourcePath & basePath, std::shared_ptr<StaticEnv> & staticEnv, const ExperimentalFeatureSettings & xpSettings)
+Expr & EvalState::parseExprFromString(
+    std::string s_,
+    const SourcePath & basePath,
+    std::shared_ptr<StaticEnv> & staticEnv,
+    const FeatureSettings & featureSettings
+)
 {
     // NOTE this method (and parseStdin) must take care to *fully copy* their input
     // into their respective Pos::Origin until the parser stops overwriting its input
     // data.
     auto s = make_ref<std::string>(s_);
     s_.append("\0\0", 2);
-    return *parse(s_.data(), s_.size(), Pos::String{.source = s}, basePath, staticEnv, xpSettings);
+    return *parse(s_.data(), s_.size(), Pos::String{.source = s}, basePath, staticEnv, featureSettings);
 }
 
 
-Expr & EvalState::parseExprFromString(std::string s, const SourcePath & basePath, const ExperimentalFeatureSettings & xpSettings)
+Expr & EvalState::parseExprFromString(
+    std::string s,
+    const SourcePath & basePath,
+    const FeatureSettings & featureSettings
+)
 {
-    return parseExprFromString(std::move(s), basePath, staticBaseEnv, xpSettings);
+    return parseExprFromString(std::move(s), basePath, staticBaseEnv, featureSettings);
 }
 
 

src/libexpr/eval.hh

@@ -12,6 +12,7 @@
 #include "experimental-features.hh"
 #include "search-path.hh"
 #include "repl-exit-status.hh"
+#include "backed-string-view.hh"
 
 #include <map>
 #include <optional>
@@ -344,8 +345,17 @@ public:
     /**
      * Parse a Nix expression from the specified string.
      */
-    Expr & parseExprFromString(std::string s, const SourcePath & basePath, std::shared_ptr<StaticEnv> & staticEnv, const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
-    Expr & parseExprFromString(std::string s, const SourcePath & basePath, const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
+    Expr & parseExprFromString(
+        std::string s,
+        const SourcePath & basePath,
+        std::shared_ptr<StaticEnv> & staticEnv,
+        const FeatureSettings & xpSettings = featureSettings
+    );
+    Expr & parseExprFromString(
+        std::string s,
+        const SourcePath & basePath,
+        const FeatureSettings & xpSettings = featureSettings
+    );
 
     Expr & parseStdin();
 
@@ -569,7 +579,7 @@ private:
         Pos::Origin origin,
         const SourcePath & basePath,
         std::shared_ptr<StaticEnv> & staticEnv,
-        const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
+        const FeatureSettings & xpSettings = featureSettings);
 
     /**
      * Current Nix call stack depth, used with `max-call-depth` setting to throw stack overflow hopefully before we run out of system stack.
@@ -782,4 +792,4 @@ static constexpr std::string_view corepkgsPrefix{"/__corepkgs__/"};
 
 }
 
-#include "eval-inline.hh"
+#include "eval-inline.hh" // IWYU pragma: keep

src/libexpr/flake/flake.cc

@@ -342,8 +342,21 @@ static void updateOverrides(std::map<InputPath, FlakeInput> & overrideMap, const
     for (auto & [id, input] : overrides) {
         auto inputPath(inputPathPrefix);
         inputPath.push_back(id);
-        // Do not override existing assignment from outer flake
-        overrideMap.insert({inputPath, input});
+
+        /* Given
+         *
+         * { inputs.hydra.inputs.nix-eval-jobs.inputs.lix.follows = "lix"; }
+         *
+         * then `nix-eval-jobs` doesn't have an override.
+         * It's neither replaced using follows nor by a different
+         * URL. Thus no need to add it to overrides and thus re-fetch
+         * it.
+         */
+        if (input.ref || input.follows) {
+            // Do not override existing assignment from outer flake
+            overrideMap.insert({inputPath, input});
+        }
+
         updateOverrides(overrideMap, input.overrides, inputPath);
     }
 }

src/libexpr/gc-alloc.hh

@@ -120,6 +120,7 @@ inline T * gcAllocType(size_t howMany = 1)
     // However, people can and do request zero sized allocations, so we need
     // to check that neither of our multiplicands were zero before complaining
     // about it.
+    // NOLINTNEXTLINE(bugprone-sizeof-expression): yeah we only seem to alloc pointers with this. the calculation *is* correct though!
     auto checkedSz = checked::Checked<size_t>(howMany) * sizeof(T);
     size_t sz = checkedSz.valueWrapping();
     if (checkedSz.overflowed()) {

src/libexpr/nixexpr.cc

@@ -11,6 +11,7 @@
 namespace nix {
 
 ExprBlackHole eBlackHole;
+Expr *eBlackHoleAddr = &eBlackHole;
 
 // FIXME: remove, because *symbols* are abstract and do not have a single
 //        textual representation; see printIdentifier()

src/libexpr/nixexpr.hh

@@ -11,6 +11,7 @@
 #include "eval-error.hh"
 #include "pos-idx.hh"
 #include "pos-table.hh"
+#include "strings.hh"
 
 namespace nix {
 
@@ -48,7 +49,7 @@ protected:
 
 public:
     struct AstSymbols {
-        Symbol sub, lessThan, mul, div, or_, findFile, nixPath, body;
+        Symbol sub, lessThan, mul, div, or_, findFile, nixPath, body, overrides;
     };
 
 

src/libexpr/parser/parser.cc

@@ -115,7 +115,7 @@ struct ExprState
 
     std::unique_ptr<Expr> pipe(PosIdx pos, State & state, bool flip = false)
     {
-        if (!state.xpSettings.isEnabled(Xp::PipeOperator))
+        if (!state.featureSettings.isEnabled(Xp::PipeOperator))
             throw ParseError({
                 .msg = HintFmt("Pipe operator is disabled"),
                 .pos = state.positions[pos]
@@ -656,10 +656,10 @@ template<> struct BuildAST<grammar::expr::path> : p::maybe_nothing {};
 
 template<> struct BuildAST<grammar::expr::uri> {
     static void apply(const auto & in, ExprState & s, State & ps) {
-       bool noURLLiterals = ps.xpSettings.isEnabled(Xp::NoUrlLiterals);
-       if (noURLLiterals)
+       bool URLLiterals = ps.featureSettings.isEnabled(Dep::UrlLiterals);
+       if (!URLLiterals)
            throw ParseError({
-               .msg = HintFmt("URL literals are disabled"),
+               .msg = HintFmt("URL literals are deprecated, allow using them with --extra-deprecated-features=url-literals"),
                .pos = ps.positions[ps.at(in)]
            });
        s.pushExpr<ExprString>(ps.at(in), in.string());
@@ -668,6 +668,16 @@ template<> struct BuildAST<grammar::expr::uri> {
 
 template<> struct BuildAST<grammar::expr::ancient_let> : change_head<BindingsState> {
     static void success(const auto & in, BindingsState & b, ExprState & s, State & ps) {
+        // Added 2024-09-18. Turn into an error at some point in the future.
+        // See the documentation on deprecated features for more details.
+        if (!ps.featureSettings.isEnabled(Dep::AncientLet))
+            warn(
+                "%s found at %s. This feature is deprecated and will be removed in the future. Use %s to silence this warning.",
+                "let {",
+                ps.positions[ps.at(in)],
+                "--extra-deprecated-features ancient-let"
+            );
+
         b.attrs.pos = ps.at(in);
         b.attrs.recursive = true;
         s.pushExpr<ExprSelect>(b.attrs.pos, b.attrs.pos, std::make_unique<ExprAttrs>(std::move(b.attrs)), ps.s.body);
@@ -676,6 +686,12 @@ template<> struct BuildAST<grammar::expr::ancient_let> : change_head<BindingsSta
 
 template<> struct BuildAST<grammar::expr::rec_set> : change_head<BindingsState> {
     static void success(const auto & in, BindingsState & b, ExprState & s, State & ps) {
+        // Before inserting new attrs, check for __override and throw an error
+        // (the error will initially be a warning to ease migration)
+        if (!featureSettings.isEnabled(Dep::RecSetOverrides) && b.attrs.attrs.contains(ps.s.overrides)) {
+            ps.overridesFound(ps.at(in));
+        }
+
         b.attrs.pos = ps.at(in);
         b.attrs.recursive = true;
         s.pushExpr<ExprAttrs>(b.attrs.pos, std::move(b.attrs));
@@ -858,7 +874,7 @@ Expr * EvalState::parse(
     Pos::Origin origin,
     const SourcePath & basePath,
     std::shared_ptr<StaticEnv> & staticEnv,
-    const ExperimentalFeatureSettings & xpSettings)
+    const FeatureSettings & featureSettings)
 {
     parser::State s = {
         symbols,
@@ -866,7 +882,7 @@ Expr * EvalState::parse(
         basePath,
         positions.addOrigin(origin, length),
         exprSymbols,
-        xpSettings
+        featureSettings,
     };
     parser::ExprState x;
 

src/libexpr/parser/state.hh

@@ -2,6 +2,7 @@
 ///@file
 
 #include "eval.hh"
+#include "logging.hh"
 
 namespace nix::parser {
 
@@ -19,10 +20,11 @@ struct State
     SourcePath basePath;
     PosTable::Origin origin;
     const Expr::AstSymbols & s;
-    const ExperimentalFeatureSettings & xpSettings;
+    const FeatureSettings & featureSettings;
 
     void dupAttr(const AttrPath & attrPath, const PosIdx pos, const PosIdx prevPos);
     void dupAttr(Symbol attr, const PosIdx pos, const PosIdx prevPos);
+    void overridesFound(const PosIdx pos);
     void addAttr(ExprAttrs * attrs, AttrPath && attrPath, std::unique_ptr<Expr> e, const PosIdx pos);
     std::unique_ptr<Formals> validateFormals(std::unique_ptr<Formals> formals, PosIdx pos = noPos, Symbol arg = {});
     std::unique_ptr<Expr> stripIndentation(const PosIdx pos,
@@ -58,6 +60,17 @@ inline void State::dupAttr(Symbol attr, const PosIdx pos, const PosIdx prevPos)
     });
 }
 
+inline void State::overridesFound(const PosIdx pos) {
+    // Added 2024-09-18. Turn into an error at some point in the future.
+    // See the documentation on deprecated features for more details.
+    warn(
+        "%s found at %s. This feature is deprecated and will be removed in the future. Use %s to silence this warning.",
+        "__overrides",
+        positions[pos],
+        "--extra-deprecated-features rec-set-overrides"
+    );
+}
+
 inline void State::addAttr(ExprAttrs * attrs, AttrPath && attrPath, std::unique_ptr<Expr> e, const PosIdx pos)
 {
     AttrPath::iterator i;
@@ -123,6 +136,12 @@ inline void State::addAttr(ExprAttrs * attrs, AttrPath && attrPath, std::unique_
                 dupAttr(attrPath, pos, j->second.pos);
             }
         } else {
+            // Before inserting new attrs, check for __override and throw an error
+            // (the error will initially be a warning to ease migration)
+            if (attrs->recursive && !featureSettings.isEnabled(Dep::RecSetOverrides) && i->symbol == s.overrides) {
+                overridesFound(pos);
+            }
+
             // This attr path is not defined. Let's create it.
             e->setName(i->symbol);
             attrs->attrs.emplace(std::piecewise_construct,

src/libexpr/value.hh

@@ -136,7 +136,9 @@ class ExternalValueBase
 
 std::ostream & operator << (std::ostream & str, const ExternalValueBase & v);
 
-extern ExprBlackHole eBlackHole;
+/** This is just the address of eBlackHole. It exists because eBlackHole has an
+ * incomplete type at usage sites so is not possible to cast. */
+extern Expr *eBlackHoleAddr;
 
 struct NewValueAs
 {
@@ -196,6 +198,7 @@ private:
 public:
 
     // Discount `using NewValueAs::*;`
+// NOLINTNEXTLINE(bugprone-macro-parentheses)
 #define USING_VALUETYPE(name) using name = NewValueAs::name
     USING_VALUETYPE(integer_t);
     USING_VALUETYPE(floating_t);
@@ -473,7 +476,7 @@ public:
     /// Constructs an evil thunk, whose evaluation represents infinite recursion.
     explicit Value(blackhole_t)
         : internalType(tThunk)
-        , thunk({ .env = nullptr, .expr = reinterpret_cast<Expr *>(&eBlackHole) })
+        , thunk({ .env = nullptr, .expr = eBlackHoleAddr })
     { }
 
     Value(Value const & rhs) = default;
@@ -513,7 +516,10 @@ public:
     // type() == nThunk
     inline bool isThunk() const { return internalType == tThunk; };
     inline bool isApp() const { return internalType == tApp; };
-    inline bool isBlackhole() const;
+    inline bool isBlackhole() const
+    {
+        return internalType == tThunk && thunk.expr == eBlackHoleAddr;
+    }
 
     // type() == nFunction
     inline bool isLambda() const { return internalType == tLambda; };
@@ -669,11 +675,6 @@ public:
 
     void mkStringMove(const char * s, const NixStringContext & context);
 
-    inline void mkString(const Symbol & s)
-    {
-        mkString(((const std::string &) s).c_str());
-    }
-
     void mkPath(const SourcePath & path);
 
     inline void mkPath(const char * path)
@@ -732,7 +733,11 @@ public:
         lambda.fun = f;
     }
 
-    inline void mkBlackhole();
+    inline void mkBlackhole()
+    {
+        internalType = tThunk;
+        thunk.expr = eBlackHoleAddr;
+    }
 
     void mkPrimOp(PrimOp * p);
 
@@ -832,18 +837,6 @@ public:
     }
 };
 
-
-bool Value::isBlackhole() const
-{
-    return internalType == tThunk && thunk.expr == (Expr*) &eBlackHole;
-}
-
-void Value::mkBlackhole()
-{
-    internalType = tThunk;
-    thunk.expr = (Expr*) &eBlackHole;
-}
-
 using ValueVector = GcVector<Value *>;
 using ValueMap = GcMap<Symbol, Value *>;
 using ValueVectorMap = std::map<Symbol, ValueVector>;

src/libfetchers/fetchers.hh

@@ -7,6 +7,8 @@
 #include "path.hh"
 #include "attrs.hh"
 #include "url.hh"
+#include "ref.hh"
+#include "strings.hh"
 
 #include <memory>
 

src/libmain/progress-bar.cc

@@ -1,8 +1,9 @@
 #include "progress-bar.hh"
+#include "file-system.hh"
 #include "sync.hh"
-#include "store-api.hh"
 #include "names.hh"
 #include "terminal.hh"
+#include "strings.hh"
 
 #include <map>
 #include <thread>

src/libmain/shared.cc

@@ -6,6 +6,8 @@
 #include "loggers.hh"
 #include "current-process.hh"
 #include "terminal.hh"
+#include "strings.hh"
+#include "exit.hh"
 
 #include <algorithm>
 #include <exception>

src/libmain/shared.hh

@@ -7,12 +7,10 @@
 #include "path.hh"
 #include "derived-path.hh"
 #include "processes.hh"
-#include "exit.hh"
+#include "strings.hh"
 
 #include <signal.h>
 
-#include <locale>
-
 
 namespace nix {
 

src/libstore/binary-cache-store.cc

@@ -10,6 +10,7 @@
 #include "nar-accessor.hh"
 #include "thread-pool.hh"
 #include "signals.hh"
+#include "strings.hh"
 
 #include <chrono>
 #include <regex>

src/libstore/build/derivation-goal.cc

@@ -1,22 +1,15 @@
 #include "derivation-goal.hh"
 #include "hook-instance.hh"
 #include "worker.hh"
-#include "builtins.hh"
-#include "builtins/buildenv.hh"
-#include "references.hh"
 #include "finally.hh"
-#include "archive.hh"
 #include "compression.hh"
 #include "common-protocol.hh"
-#include "common-protocol-impl.hh"
-#include "topo-sort.hh"
+#include "common-protocol-impl.hh" // IWYU pragma: keep
 #include "local-store.hh" // TODO remove, along with remaining downcasts
 #include "logging-json.hh"
 #include "substitution-goal.hh"
 #include "drv-output-substitution-goal.hh"
-
-#include <regex>
-#include <queue>
+#include "strings.hh"
 
 #include <fstream>
 #include <sys/types.h>
@@ -66,7 +59,7 @@ namespace nix {
 
 DerivationGoal::DerivationGoal(const StorePath & drvPath,
     const OutputsSpec & wantedOutputs, Worker & worker, BuildMode buildMode)
-    : Goal(worker, DerivedPath::Built { .drvPath = makeConstantStorePathRef(drvPath), .outputs = wantedOutputs })
+    : Goal(worker)
     , useDerivation(true)
     , drvPath(drvPath)
     , wantedOutputs(wantedOutputs)
@@ -84,7 +77,7 @@ DerivationGoal::DerivationGoal(const StorePath & drvPath,
 
 DerivationGoal::DerivationGoal(const StorePath & drvPath, const BasicDerivation & drv,
     const OutputsSpec & wantedOutputs, Worker & worker, BuildMode buildMode)
-    : Goal(worker, DerivedPath::Built { .drvPath = makeConstantStorePathRef(drvPath), .outputs = wantedOutputs })
+    : Goal(worker)
     , useDerivation(false)
     , drvPath(drvPath)
     , wantedOutputs(wantedOutputs)
@@ -127,6 +120,7 @@ std::string DerivationGoal::key()
 void DerivationGoal::killChild()
 {
     hook.reset();
+    builderOutFD = nullptr;
 }
 
 
@@ -137,9 +131,9 @@ Goal::Finished DerivationGoal::timedOut(Error && ex)
 }
 
 
-Goal::WorkResult DerivationGoal::work()
+Goal::WorkResult DerivationGoal::work(bool inBuildSlot)
 {
-    return (this->*state)();
+    return (this->*state)(inBuildSlot);
 }
 
 void DerivationGoal::addWantedOutputs(const OutputsSpec & outputs)
@@ -163,7 +157,7 @@ void DerivationGoal::addWantedOutputs(const OutputsSpec & outputs)
 }
 
 
-Goal::WorkResult DerivationGoal::getDerivation()
+Goal::WorkResult DerivationGoal::getDerivation(bool inBuildSlot)
 {
     trace("init");
 
@@ -171,7 +165,7 @@ Goal::WorkResult DerivationGoal::getDerivation()
        exists.  If it doesn't, it may be created through a
        substitute. */
     if (buildMode == bmNormal && worker.evalStore.isValidPath(drvPath)) {
-        return loadDerivation();
+        return loadDerivation(inBuildSlot);
     }
 
 
@@ -180,7 +174,7 @@ Goal::WorkResult DerivationGoal::getDerivation()
 }
 
 
-Goal::WorkResult DerivationGoal::loadDerivation()
+Goal::WorkResult DerivationGoal::loadDerivation(bool inBuildSlot)
 {
     trace("loading derivation");
 
@@ -207,11 +201,11 @@ Goal::WorkResult DerivationGoal::loadDerivation()
     }
     assert(drv);
 
-    return haveDerivation();
+    return haveDerivation(inBuildSlot);
 }
 
 
-Goal::WorkResult DerivationGoal::haveDerivation()
+Goal::WorkResult DerivationGoal::haveDerivation(bool inBuildSlot)
 {
     trace("have derivation");
 
@@ -239,7 +233,7 @@ Goal::WorkResult DerivationGoal::haveDerivation()
             });
         }
 
-        return gaveUpOnSubstitution();
+        return gaveUpOnSubstitution(inBuildSlot);
     }
 
     for (auto & i : drv->outputsAndOptPaths(worker.store))
@@ -268,34 +262,39 @@ Goal::WorkResult DerivationGoal::haveDerivation()
        through substitutes.  If that doesn't work, we'll build
        them. */
     WaitForGoals result;
-    if (settings.useSubstitutes && parsedDrv->substitutesAllowed())
-        for (auto & [outputName, status] : initialOutputs) {
-            if (!status.wanted) continue;
-            if (!status.known)
-                result.goals.insert(
-                    worker.makeDrvOutputSubstitutionGoal(
-                        DrvOutput{status.outputHash, outputName},
-                        buildMode == bmRepair ? Repair : NoRepair
-                    )
-                );
-            else {
-                auto * cap = getDerivationCA(*drv);
-                result.goals.insert(worker.makePathSubstitutionGoal(
-                    status.known->path,
-                    buildMode == bmRepair ? Repair : NoRepair,
-                    cap ? std::optional { *cap } : std::nullopt));
+    if (settings.useSubstitutes) {
+        if (parsedDrv->substitutesAllowed()) {
+            for (auto & [outputName, status] : initialOutputs) {
+                if (!status.wanted) continue;
+                if (!status.known)
+                    result.goals.insert(
+                        worker.makeDrvOutputSubstitutionGoal(
+                            DrvOutput{status.outputHash, outputName},
+                            buildMode == bmRepair ? Repair : NoRepair
+                        )
+                    );
+                else {
+                    auto * cap = getDerivationCA(*drv);
+                    result.goals.insert(worker.makePathSubstitutionGoal(
+                        status.known->path,
+                        buildMode == bmRepair ? Repair : NoRepair,
+                        cap ? std::optional { *cap } : std::nullopt));
+                }
             }
+        } else {
+            trace("skipping substitute because allowSubstitutes is false");
         }
+    }
 
     if (result.goals.empty()) { /* to prevent hang (no wake-up event) */
-        return outputsSubstitutionTried();
+        return outputsSubstitutionTried(inBuildSlot);
     } else {
         state = &DerivationGoal::outputsSubstitutionTried;
         return result;
     }
 }
 
-Goal::WorkResult DerivationGoal::outputsSubstitutionTried()
+Goal::WorkResult DerivationGoal::outputsSubstitutionTried(bool inBuildSlot)
 {
     trace("all outputs substituted (maybe)");
 
@@ -338,7 +337,7 @@ Goal::WorkResult DerivationGoal::outputsSubstitutionTried()
 
     if (needRestart == NeedRestartForMoreOutputs::OutputsAddedDoNeed) {
         needRestart = NeedRestartForMoreOutputs::OutputsUnmodifedDontNeed;
-        return haveDerivation();
+        return haveDerivation(inBuildSlot);
     }
 
     auto [allValid, validOutputs] = checkPathValidity();
@@ -354,13 +353,13 @@ Goal::WorkResult DerivationGoal::outputsSubstitutionTried()
             worker.store.printStorePath(drvPath));
 
     /* Nothing to wait for; tail call */
-    return gaveUpOnSubstitution();
+    return gaveUpOnSubstitution(inBuildSlot);
 }
 
 
 /* At least one of the output paths could not be
    produced using a substitute.  So we have to build instead. */
-Goal::WorkResult DerivationGoal::gaveUpOnSubstitution()
+Goal::WorkResult DerivationGoal::gaveUpOnSubstitution(bool inBuildSlot)
 {
     WaitForGoals result;
 
@@ -424,7 +423,7 @@ Goal::WorkResult DerivationGoal::gaveUpOnSubstitution()
     }
 
     if (result.goals.empty()) {/* to prevent hang (no wake-up event) */
-        return inputsRealised();
+        return inputsRealised(inBuildSlot);
     } else {
         state = &DerivationGoal::inputsRealised;
         return result;
@@ -495,7 +494,7 @@ Goal::WorkResult DerivationGoal::repairClosure()
 }
 
 
-Goal::WorkResult DerivationGoal::closureRepaired()
+Goal::WorkResult DerivationGoal::closureRepaired(bool inBuildSlot)
 {
     trace("closure repaired");
     if (nrFailed > 0)
@@ -505,7 +504,7 @@ Goal::WorkResult DerivationGoal::closureRepaired()
 }
 
 
-Goal::WorkResult DerivationGoal::inputsRealised()
+Goal::WorkResult DerivationGoal::inputsRealised(bool inBuildSlot)
 {
     trace("all inputs realised");
 
@@ -519,7 +518,7 @@ Goal::WorkResult DerivationGoal::inputsRealised()
 
     if (retrySubstitution == RetrySubstitution::YesNeed) {
         retrySubstitution = RetrySubstitution::AlreadyRetried;
-        return haveDerivation();
+        return haveDerivation(inBuildSlot);
     }
 
     /* Gather information necessary for computing the closure and/or
@@ -653,7 +652,7 @@ Goal::WorkResult DerivationGoal::inputsRealised()
     return ContinueImmediately{};
 }
 
-Goal::WorkResult DerivationGoal::started()
+void DerivationGoal::started()
 {
     auto msg = fmt(
         buildMode == bmRepair ? "repairing outputs of '%s'" :
@@ -664,10 +663,9 @@ Goal::WorkResult DerivationGoal::started()
     act = std::make_unique<Activity>(*logger, lvlInfo, actBuild, msg,
         Logger::Fields{worker.store.printStorePath(drvPath), hook ? machineName : "", 1, 1});
     mcRunningBuilds = std::make_unique<MaintainCount<uint64_t>>(worker.runningBuilds);
-    return StillAlive{};
 }
 
-Goal::WorkResult DerivationGoal::tryToBuild()
+Goal::WorkResult DerivationGoal::tryToBuild(bool inBuildSlot)
 {
     trace("trying to build");
 
@@ -739,25 +737,35 @@ Goal::WorkResult DerivationGoal::tryToBuild()
         && settings.maxBuildJobs.get() != 0;
 
     if (!buildLocally) {
-        switch (tryBuildHook()) {
-            case rpAccept:
-                /* Yes, it has started doing so.  Wait until we get
-                   EOF from the hook. */
-                actLock.reset();
-                buildResult.startTime = time(0); // inexact
-                state = &DerivationGoal::buildDone;
-                return started();
-            case rpPostpone:
-                /* Not now; wait until at least one child finishes or
-                   the wake-up timeout expires. */
-                if (!actLock)
-                    actLock = std::make_unique<Activity>(*logger, lvlTalkative, actBuildWaiting,
-                        fmt("waiting for a machine to build '%s'", Magenta(worker.store.printStorePath(drvPath))));
-                outputLocks.unlock();
-                return WaitForAWhile{};
-            case rpDecline:
-                /* We should do it ourselves. */
-                break;
+        auto hookReply = tryBuildHook(inBuildSlot);
+        auto result = std::visit(
+            overloaded{
+                [&](HookReply::Accept & a) -> std::optional<WorkResult> {
+                    /* Yes, it has started doing so.  Wait until we get
+                       EOF from the hook. */
+                    actLock.reset();
+                    buildResult.startTime = time(0); // inexact
+                    state = &DerivationGoal::buildDone;
+                    started();
+                    return WaitForWorld{std::move(a.fds), false};
+                },
+                [&](HookReply::Postpone) -> std::optional<WorkResult> {
+                    /* Not now; wait until at least one child finishes or
+                       the wake-up timeout expires. */
+                    if (!actLock)
+                        actLock = std::make_unique<Activity>(*logger, lvlTalkative, actBuildWaiting,
+                            fmt("waiting for a machine to build '%s'", Magenta(worker.store.printStorePath(drvPath))));
+                    outputLocks.unlock();
+                    return WaitForAWhile{};
+                },
+                [&](HookReply::Decline) -> std::optional<WorkResult> {
+                    /* We should do it ourselves. */
+                    return std::nullopt;
+                },
+            },
+            hookReply);
+        if (result) {
+            return std::move(*result);
         }
     }
 
@@ -767,7 +775,7 @@ Goal::WorkResult DerivationGoal::tryToBuild()
     return ContinueImmediately{};
 }
 
-Goal::WorkResult DerivationGoal::tryLocalBuild() {
+Goal::WorkResult DerivationGoal::tryLocalBuild(bool inBuildSlot) {
     throw Error(
         "unable to build with a primary store that isn't a local store; "
         "either pass a different '--store' or enable remote builds."
@@ -822,14 +830,16 @@ void replaceValidPath(const Path & storePath, const Path & tmpPath)
 
 int DerivationGoal::getChildStatus()
 {
+    builderOutFD = nullptr;
     return hook->pid.kill();
 }
 
 
 void DerivationGoal::closeReadPipes()
 {
-    hook->builderOut.readSide.reset();
-    hook->fromHook.readSide.reset();
+    hook->builderOut.reset();
+    hook->fromHook.reset();
+    builderOutFD = nullptr;
 }
 
 
@@ -925,7 +935,7 @@ void runPostBuildHook(
     proc.getStdout()->drainInto(sink);
 }
 
-Goal::WorkResult DerivationGoal::buildDone()
+Goal::WorkResult DerivationGoal::buildDone(bool inBuildSlot)
 {
     trace("build done");
 
@@ -1045,7 +1055,7 @@ Goal::WorkResult DerivationGoal::buildDone()
     }
 }
 
-Goal::WorkResult DerivationGoal::resolvedFinished()
+Goal::WorkResult DerivationGoal::resolvedFinished(bool inBuildSlot)
 {
     trace("resolved derivation finished");
 
@@ -1116,9 +1126,9 @@ Goal::WorkResult DerivationGoal::resolvedFinished()
     return done(status, std::move(builtOutputs));
 }
 
-HookReply DerivationGoal::tryBuildHook()
+HookReply DerivationGoal::tryBuildHook(bool inBuildSlot)
 {
-    if (!worker.hook.available || !useDerivation) return rpDecline;
+    if (!worker.hook.available || !useDerivation) return HookReply::Decline{};
 
     if (!worker.hook.instance)
         worker.hook.instance = std::make_unique<HookInstance>();
@@ -1128,7 +1138,7 @@ HookReply DerivationGoal::tryBuildHook()
         /* Send the request to the hook. */
         worker.hook.instance->sink
             << "try"
-            << (worker.getNrLocalBuilds() < settings.maxBuildJobs ? 1 : 0)
+            << (inBuildSlot ? 1 : 0)
             << drv->platform
             << worker.store.printStorePath(drvPath)
             << parsedDrv->getRequiredSystemFeatures();
@@ -1140,7 +1150,7 @@ HookReply DerivationGoal::tryBuildHook()
         while (true) {
             auto s = [&]() {
                 try {
-                    return readLine(worker.hook.instance->fromHook.readSide.get());
+                    return readLine(worker.hook.instance->fromHook.get());
                 } catch (Error & e) {
                     e.addTrace({}, "while reading the response from the build hook");
                     throw;
@@ -1161,14 +1171,14 @@ HookReply DerivationGoal::tryBuildHook()
         debug("hook reply is '%1%'", reply);
 
         if (reply == "decline")
-            return rpDecline;
+            return HookReply::Decline{};
         else if (reply == "decline-permanently") {
             worker.hook.available = false;
             worker.hook.instance.reset();
-            return rpDecline;
+            return HookReply::Decline{};
         }
         else if (reply == "postpone")
-            return rpPostpone;
+            return HookReply::Postpone{};
         else if (reply != "accept")
             throw Error("bad hook reply '%s'", reply);
 
@@ -1176,9 +1186,9 @@ HookReply DerivationGoal::tryBuildHook()
         if (e.errNo == EPIPE) {
             printError(
                 "build hook died unexpectedly: %s",
-                chomp(drainFD(worker.hook.instance->fromHook.readSide.get())));
+                chomp(drainFD(worker.hook.instance->fromHook.get())));
             worker.hook.instance.reset();
-            return rpDecline;
+            return HookReply::Decline{};
         } else
             throw;
     }
@@ -1186,7 +1196,7 @@ HookReply DerivationGoal::tryBuildHook()
     hook = std::move(worker.hook.instance);
 
     try {
-        machineName = readLine(hook->fromHook.readSide.get());
+        machineName = readLine(hook->fromHook.get());
     } catch (Error & e) {
         e.addTrace({}, "while reading the machine name from the build hook");
         throw;
@@ -1209,17 +1219,17 @@ HookReply DerivationGoal::tryBuildHook()
     }
 
     hook->sink = FdSink();
-    hook->toHook.writeSide.reset();
+    hook->toHook.reset();
 
     /* Create the log file and pipe. */
     Path logFile = openLogFile();
 
     std::set<int> fds;
-    fds.insert(hook->fromHook.readSide.get());
-    fds.insert(hook->builderOut.readSide.get());
-    worker.childStarted(shared_from_this(), fds, false, false);
+    fds.insert(hook->fromHook.get());
+    fds.insert(hook->builderOut.get());
+    builderOutFD = &hook->builderOut;
 
-    return rpAccept;
+    return HookReply::Accept{std::move(fds)};
 }
 
 
@@ -1279,24 +1289,23 @@ void DerivationGoal::closeLogFile()
 }
 
 
-bool DerivationGoal::isReadDesc(int fd)
-{
-    return fd == hook->builderOut.readSide.get();
-}
-
 Goal::WorkResult DerivationGoal::handleChildOutput(int fd, std::string_view data)
 {
+    assert(builderOutFD);
+
+    auto tooMuchLogs = [&] {
+        killChild();
+        return done(
+            BuildResult::LogLimitExceeded, {},
+            Error("%s killed after writing more than %d bytes of log output",
+                getName(), settings.maxLogSize));
+    };
+
     // local & `ssh://`-builds are dealt with here.
-    auto isWrittenToLog = isReadDesc(fd);
-    if (isWrittenToLog)
-    {
+    if (fd == builderOutFD->get()) {
         logSize += data.size();
         if (settings.maxLogSize && logSize > settings.maxLogSize) {
-            killChild();
-            return done(
-                BuildResult::LogLimitExceeded, {},
-                Error("%s killed after writing more than %d bytes of log output",
-                    getName(), settings.maxLogSize));
+            return tooMuchLogs();
         }
 
         for (auto c : data)
@@ -1311,9 +1320,10 @@ Goal::WorkResult DerivationGoal::handleChildOutput(int fd, std::string_view data
             }
 
         if (logSink) (*logSink)(data);
+        return StillAlive{};
     }
 
-    if (hook && fd == hook->fromHook.readSide.get()) {
+    if (hook && fd == hook->fromHook.get()) {
         for (auto c : data)
             if (c == '\n') {
                 auto json = parseJSONMessage(currentHookLine);
@@ -1321,11 +1331,17 @@ Goal::WorkResult DerivationGoal::handleChildOutput(int fd, std::string_view data
                     auto s = handleJSONLogMessage(*json, worker.act, hook->activities, true);
                     // ensure that logs from a builder using `ssh-ng://` as protocol
                     // are also available to `nix log`.
-                    if (s && !isWrittenToLog && logSink) {
+                    if (s && logSink) {
                         const auto type = (*json)["type"];
                         const auto fields = (*json)["fields"];
                         if (type == resBuildLogLine) {
-                            (*logSink)((fields.size() > 0 ? fields[0].get<std::string>() : "") + "\n");
+                            const std::string logLine =
+                                (fields.size() > 0 ? fields[0].get<std::string>() : "") + "\n";
+                            logSize += logLine.size();
+                            if (settings.maxLogSize && logSize > settings.maxLogSize) {
+                                return tooMuchLogs();
+                            }
+                            (*logSink)(logLine);
                         } else if (type == resSetPhase && ! fields.is_null()) {
                             const auto phase = fields[0];
                             if (! phase.is_null()) {
@@ -1530,7 +1546,7 @@ Goal::Finished DerivationGoal::done(
 
     return Finished{
         .result = buildResult.success() ? ecSuccess : ecFailed,
-        .ex = ex ? std::make_unique<Error>(std::move(*ex)) : nullptr,
+        .ex = ex ? std::make_shared<Error>(std::move(*ex)) : nullptr,
         .permanentFailure = buildResult.status == BuildResult::PermanentFailure,
         .timedOut = buildResult.status == BuildResult::TimedOut,
         .hashMismatch = anyHashMismatchSeen,

src/libstore/build/derivation-goal.hh

@@ -14,7 +14,21 @@ using std::map;
 
 struct HookInstance;
 
-typedef enum {rpAccept, rpDecline, rpPostpone} HookReply;
+struct HookReplyBase {
+    struct [[nodiscard]] Accept {
+        std::set<int> fds;
+    };
+    struct [[nodiscard]] Decline {};
+    struct [[nodiscard]] Postpone {};
+};
+
+struct [[nodiscard]] HookReply
+    : HookReplyBase,
+      std::variant<HookReplyBase::Accept, HookReplyBase::Decline, HookReplyBase::Postpone>
+{
+    HookReply() = delete;
+    using variant::variant;
+};
 
 /**
  * Unless we are repairing, we don't both to test validity and just assume it,
@@ -186,12 +200,19 @@ struct DerivationGoal : public Goal
      */
     std::unique_ptr<HookInstance> hook;
 
+    /**
+      * Builder output is pulled from this file descriptor when not null.
+      * Owned by the derivation goal or subclass, must not be reset until
+      * the build has finished and no more output must be processed by us
+      */
+    AutoCloseFD * builderOutFD = nullptr;
+
     /**
      * The sort of derivation we are building.
      */
     std::optional<DerivationType> derivationType;
 
-    typedef WorkResult (DerivationGoal::*GoalState)();
+    typedef WorkResult (DerivationGoal::*GoalState)(bool inBuildSlot);
     GoalState state;
 
     BuildMode buildMode;
@@ -224,7 +245,7 @@ struct DerivationGoal : public Goal
 
     std::string key() override;
 
-    WorkResult work() override;
+    WorkResult work(bool inBuildSlot) override;
 
     /**
      * Add wanted outputs to an already existing derivation goal.
@@ -234,23 +255,23 @@ struct DerivationGoal : public Goal
     /**
      * The states.
      */
-    WorkResult getDerivation();
-    WorkResult loadDerivation();
-    WorkResult haveDerivation();
-    WorkResult outputsSubstitutionTried();
-    WorkResult gaveUpOnSubstitution();
-    WorkResult closureRepaired();
-    WorkResult inputsRealised();
-    WorkResult tryToBuild();
-    virtual WorkResult tryLocalBuild();
-    WorkResult buildDone();
+    WorkResult getDerivation(bool inBuildSlot);
+    WorkResult loadDerivation(bool inBuildSlot);
+    WorkResult haveDerivation(bool inBuildSlot);
+    WorkResult outputsSubstitutionTried(bool inBuildSlot);
+    WorkResult gaveUpOnSubstitution(bool inBuildSlot);
+    WorkResult closureRepaired(bool inBuildSlot);
+    WorkResult inputsRealised(bool inBuildSlot);
+    WorkResult tryToBuild(bool inBuildSlot);
+    virtual WorkResult tryLocalBuild(bool inBuildSlot);
+    WorkResult buildDone(bool inBuildSlot);
 
-    WorkResult resolvedFinished();
+    WorkResult resolvedFinished(bool inBuildSlot);
 
     /**
      * Is the build hook willing to perform the build?
      */
-    HookReply tryBuildHook();
+    HookReply tryBuildHook(bool inBuildSlot);
 
     virtual int getChildStatus();
 
@@ -290,8 +311,6 @@ struct DerivationGoal : public Goal
     virtual void cleanupPostOutputsRegisteredModeCheck();
     virtual void cleanupPostOutputsRegisteredModeNonCheck();
 
-    virtual bool isReadDesc(int fd);
-
     /**
      * Callback used by the worker to write to the log.
      */
@@ -328,7 +347,7 @@ struct DerivationGoal : public Goal
 
     WorkResult repairClosure();
 
-    WorkResult started();
+    void started();
 
     Finished done(
         BuildResult::Status status,

src/libstore/build/drv-output-substitution-goal.cc

@@ -11,7 +11,7 @@ DrvOutputSubstitutionGoal::DrvOutputSubstitutionGoal(
     Worker & worker,
     RepairFlag repair,
     std::optional<ContentAddress> ca)
-    : Goal(worker, DerivedPath::Opaque { StorePath::dummy })
+    : Goal(worker)
     , id(id)
 {
     state = &DrvOutputSubstitutionGoal::init;
@@ -20,7 +20,7 @@ DrvOutputSubstitutionGoal::DrvOutputSubstitutionGoal(
 }
 
 
-Goal::WorkResult DrvOutputSubstitutionGoal::init()
+Goal::WorkResult DrvOutputSubstitutionGoal::init(bool inBuildSlot)
 {
     trace("init");
 
@@ -30,17 +30,14 @@ Goal::WorkResult DrvOutputSubstitutionGoal::init()
     }
 
     subs = settings.useSubstitutes ? getDefaultSubstituters() : std::list<ref<Store>>();
-    return tryNext();
+    return tryNext(inBuildSlot);
 }
 
-Goal::WorkResult DrvOutputSubstitutionGoal::tryNext()
+Goal::WorkResult DrvOutputSubstitutionGoal::tryNext(bool inBuildSlot)
 {
     trace("trying next substituter");
 
-    /* Make sure that we are allowed to start a substitution.  Note that even
-       if maxSubstitutionJobs == 0, we still allow a substituter to run. This
-       prevents infinite waiting. */
-    if (worker.runningSubstitutions >= std::max(1U, settings.maxSubstitutionJobs.get())) {
+    if (!inBuildSlot) {
         return WaitForSlot{};
     }
 
@@ -78,13 +75,11 @@ Goal::WorkResult DrvOutputSubstitutionGoal::tryNext()
             return sub->queryRealisation(id);
         });
 
-    worker.childStarted(shared_from_this(), {downloadState->outPipe.readSide.get()}, true, false);
-
     state = &DrvOutputSubstitutionGoal::realisationFetched;
-    return StillAlive{};
+    return WaitForWorld{{downloadState->outPipe.readSide.get()}, true};
 }
 
-Goal::WorkResult DrvOutputSubstitutionGoal::realisationFetched()
+Goal::WorkResult DrvOutputSubstitutionGoal::realisationFetched(bool inBuildSlot)
 {
     worker.childTerminated(this);
     maintainRunningSubstitutions.reset();
@@ -97,7 +92,7 @@ Goal::WorkResult DrvOutputSubstitutionGoal::realisationFetched()
     }
 
     if (!outputInfo) {
-        return tryNext();
+        return tryNext(inBuildSlot);
     }
 
     WaitForGoals result;
@@ -114,7 +109,7 @@ Goal::WorkResult DrvOutputSubstitutionGoal::realisationFetched()
                     worker.store.printStorePath(localOutputInfo->outPath),
                     worker.store.printStorePath(depPath)
                 );
-                return tryNext();
+                return tryNext(inBuildSlot);
             }
             result.goals.insert(worker.makeDrvOutputSubstitutionGoal(depId));
         }
@@ -123,14 +118,14 @@ Goal::WorkResult DrvOutputSubstitutionGoal::realisationFetched()
     result.goals.insert(worker.makePathSubstitutionGoal(outputInfo->outPath));
 
     if (result.goals.empty()) {
-        return outPathValid();
+        return outPathValid(inBuildSlot);
     } else {
         state = &DrvOutputSubstitutionGoal::outPathValid;
         return result;
     }
 }
 
-Goal::WorkResult DrvOutputSubstitutionGoal::outPathValid()
+Goal::WorkResult DrvOutputSubstitutionGoal::outPathValid(bool inBuildSlot)
 {
     assert(outputInfo);
     trace("output path substituted");
@@ -159,9 +154,9 @@ std::string DrvOutputSubstitutionGoal::key()
     return "a$" + std::string(id.to_string());
 }
 
-Goal::WorkResult DrvOutputSubstitutionGoal::work()
+Goal::WorkResult DrvOutputSubstitutionGoal::work(bool inBuildSlot)
 {
-    return (this->*state)();
+    return (this->*state)(inBuildSlot);
 }
 
 

src/libstore/build/drv-output-substitution-goal.hh

@@ -58,20 +58,20 @@ class DrvOutputSubstitutionGoal : public Goal {
 public:
     DrvOutputSubstitutionGoal(const DrvOutput& id, Worker & worker, RepairFlag repair = NoRepair, std::optional<ContentAddress> ca = std::nullopt);
 
-    typedef WorkResult (DrvOutputSubstitutionGoal::*GoalState)();
+    typedef WorkResult (DrvOutputSubstitutionGoal::*GoalState)(bool inBuildSlot);
     GoalState state;
 
-    WorkResult init();
-    WorkResult tryNext();
-    WorkResult realisationFetched();
-    WorkResult outPathValid();
+    WorkResult init(bool inBuildSlot);
+    WorkResult tryNext(bool inBuildSlot);
+    WorkResult realisationFetched(bool inBuildSlot);
+    WorkResult outPathValid(bool inBuildSlot);
     WorkResult finished();
 
     Finished timedOut(Error && ex) override { abort(); };
 
     std::string key() override;
 
-    WorkResult work() override;
+    WorkResult work(bool inBuildSlot) override;
 
     JobCategory jobCategory() const override {
         return JobCategory::Substitution;

src/libstore/build/entry-points.cc

@@ -2,6 +2,7 @@
 #include "substitution-goal.hh"
 #include "derivation-goal.hh"
 #include "local-store.hh"
+#include "strings.hh"
 
 namespace nix {
 
@@ -16,13 +17,13 @@ void Store::buildPaths(const std::vector<DerivedPath> & reqs, BuildMode buildMod
     worker.run(goals);
 
     StringSet failed;
-    std::optional<Error> ex;
+    std::shared_ptr<Error> ex;
     for (auto & i : goals) {
         if (i->ex) {
             if (ex)
                 logError(i->ex->info());
             else
-                ex = std::move(*i->ex);
+                ex = i->ex;
         }
         if (i->exitCode != Goal::ecSuccess) {
             if (auto i2 = dynamic_cast<DerivationGoal *>(i.get()))

src/libstore/build/goal.cc

@@ -1,5 +1,4 @@
 #include "goal.hh"
-#include "worker.hh"
 
 namespace nix {
 

src/libstore/build/goal.hh

@@ -51,7 +51,7 @@ enum struct JobCategory {
     Substitution,
 };
 
-struct Goal : public std::enable_shared_from_this<Goal>
+struct Goal
 {
     typedef enum {ecSuccess, ecFailed, ecNoSubstituters, ecIncompleteClosure} ExitCode;
 
@@ -112,9 +112,13 @@ public:
     struct [[nodiscard]] WaitForGoals {
         Goals goals;
     };
+    struct [[nodiscard]] WaitForWorld {
+        std::set<int> fds;
+        bool inBuildSlot;
+    };
     struct [[nodiscard]] Finished {
         ExitCode result;
-        std::unique_ptr<Error> ex;
+        std::shared_ptr<Error> ex;
         bool permanentFailure = false;
         bool timedOut = false;
         bool hashMismatch = false;
@@ -127,6 +131,7 @@ public:
                                           WaitForAWhile,
                                           ContinueImmediately,
                                           WaitForGoals,
+                                          WaitForWorld,
                                           Finished>
     {
         WorkResult() = delete;
@@ -136,9 +141,9 @@ public:
     /**
      * Exception containing an error message, if any.
      */
-    std::unique_ptr<Error> ex;
+    std::shared_ptr<Error> ex;
 
-    Goal(Worker & worker, DerivedPath path)
+    explicit Goal(Worker & worker)
         : worker(worker)
     { }
 
@@ -147,7 +152,7 @@ public:
         trace("goal destroyed");
     }
 
-    virtual WorkResult work() = 0;
+    virtual WorkResult work(bool inBuildSlot) = 0;
 
     virtual void waiteeDone(GoalPtr waitee) { }
 
@@ -160,6 +165,11 @@ public:
     {
     }
 
+    virtual bool respectsTimeouts()
+    {
+        return false;
+    }
+
     void trace(std::string_view s);
 
     std::string getName() const

src/libstore/build/hook-instance.cc

@@ -2,6 +2,7 @@
 #include "file-system.hh"
 #include "globals.hh"
 #include "hook-instance.hh"
+#include "strings.hh"
 
 namespace nix {
 
@@ -26,18 +27,21 @@ HookInstance::HookInstance()
     args.push_back(std::to_string(verbosity));
 
     /* Create a pipe to get the output of the child. */
-    fromHook.create();
+    Pipe fromHook_;
+    fromHook_.create();
 
     /* Create the communication pipes. */
-    toHook.create();
+    Pipe toHook_;
+    toHook_.create();
 
     /* Create a pipe to get the output of the builder. */
-    builderOut.create();
+    Pipe builderOut_;
+    builderOut_.create();
 
     /* Fork the hook. */
     pid = startProcess([&]() {
 
-        if (dup2(fromHook.writeSide.get(), STDERR_FILENO) == -1)
+        if (dup2(fromHook_.writeSide.get(), STDERR_FILENO) == -1)
             throw SysError("cannot pipe standard error into log file");
 
         commonChildInit();
@@ -45,16 +49,16 @@ HookInstance::HookInstance()
         if (chdir("/") == -1) throw SysError("changing into /");
 
         /* Dup the communication pipes. */
-        if (dup2(toHook.readSide.get(), STDIN_FILENO) == -1)
+        if (dup2(toHook_.readSide.get(), STDIN_FILENO) == -1)
             throw SysError("dupping to-hook read side");
 
         /* Use fd 4 for the builder's stdout/stderr. */
-        if (dup2(builderOut.writeSide.get(), 4) == -1)
+        if (dup2(builderOut_.writeSide.get(), 4) == -1)
             throw SysError("dupping builder's stdout/stderr");
 
         /* Hack: pass the read side of that fd to allow build-remote
            to read SSH error messages. */
-        if (dup2(builderOut.readSide.get(), 5) == -1)
+        if (dup2(builderOut_.readSide.get(), 5) == -1)
             throw SysError("dupping builder's stdout/stderr");
 
         execv(buildHook.c_str(), stringsToCharPtrs(args).data());
@@ -63,10 +67,11 @@ HookInstance::HookInstance()
     });
 
     pid.setSeparatePG(true);
-    fromHook.writeSide.reset();
-    toHook.readSide.reset();
+    fromHook = std::move(fromHook_.readSide);
+    toHook = std::move(toHook_.writeSide);
+    builderOut = std::move(builderOut_.readSide);
 
-    sink = FdSink(toHook.writeSide.get());
+    sink = FdSink(toHook.get());
     std::map<std::string, Config::SettingInfo> settings;
     globalConfig.getSettings(settings);
     for (auto & setting : settings)
@@ -78,7 +83,7 @@ HookInstance::HookInstance()
 HookInstance::~HookInstance()
 {
     try {
-        toHook.writeSide.reset();
+        toHook.reset();
         if (pid) pid.kill();
     } catch (...) {
         ignoreException();

src/libstore/build/hook-instance.hh

@@ -10,19 +10,19 @@ namespace nix {
 struct HookInstance
 {
     /**
-     * Pipes for talking to the build hook.
+     * Pipe for talking to the build hook.
      */
-    Pipe toHook;
+    AutoCloseFD toHook;
 
     /**
      * Pipe for the hook's standard output/error.
      */
-    Pipe fromHook;
+    AutoCloseFD fromHook;
 
     /**
      * Pipe for the builder's standard output/error.
      */
-    Pipe builderOut;
+    AutoCloseFD builderOut;
 
     /**
      * The process ID of the hook.

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

@@ -1,14 +1,12 @@
 #include "local-derivation-goal.hh"
 #include "indirect-root-store.hh"
-#include "hook-instance.hh"
+#include "machines.hh"
 #include "store-api.hh"
 #include "worker.hh"
 #include "builtins.hh"
 #include "builtins/buildenv.hh"
 #include "path-references.hh"
-#include "finally.hh"
 #include "archive.hh"
-#include "compression.hh"
 #include "daemon.hh"
 #include "topo-sort.hh"
 #include "json-utils.hh"
@@ -17,6 +15,8 @@
 #include "namespaces.hh"
 #include "child.hh"
 #include "unix-domain-socket.hh"
+#include "mount.hh"
+#include "strings.hh"
 
 #include <regex>
 #include <queue>
@@ -149,17 +149,30 @@ void LocalDerivationGoal::killSandbox(bool getStats)
 }
 
 
-Goal::WorkResult LocalDerivationGoal::tryLocalBuild()
+Goal::WorkResult LocalDerivationGoal::tryLocalBuild(bool inBuildSlot)
 {
 #if __APPLE__
     additionalSandboxProfile = parsedDrv->getStringAttr("__sandboxProfile").value_or("");
 #endif
 
-    unsigned int curBuilds = worker.getNrLocalBuilds();
-    if (curBuilds >= settings.maxBuildJobs) {
+    if (!inBuildSlot) {
         state = &DerivationGoal::tryToBuild;
         outputLocks.unlock();
-        return WaitForSlot{};
+        if (0U != settings.maxBuildJobs) {
+            return WaitForSlot{};
+        }
+        if (getMachines().empty()) {
+            throw Error(
+                "unable to start any build; either set '--max-jobs' to a non-zero value or enable "
+                "remote builds.\n"
+                "https://docs.lix.systems/manual/lix/stable/advanced-topics/distributed-builds.html"
+            );
+        } else {
+            throw Error(
+                "unable to start any build; remote machines may not have all required system features.\n"
+                "https://docs.lix.systems/manual/lix/stable/advanced-topics/distributed-builds.html"
+            );
+        }
     }
 
     assert(derivationType);
@@ -230,7 +243,14 @@ Goal::WorkResult LocalDerivationGoal::tryLocalBuild()
     try {
 
         /* Okay, we have to build. */
-        startBuilder();
+        auto fds = startBuilder();
+
+        /* This state will be reached when we get EOF on the child's
+           log pipe. */
+        state = &DerivationGoal::buildDone;
+
+        started();
+        return WaitForWorld{std::move(fds), true};
 
     } catch (BuildError & e) {
         outputLocks.unlock();
@@ -239,12 +259,6 @@ Goal::WorkResult LocalDerivationGoal::tryLocalBuild()
         report.permanentFailure = true;
         return report;
     }
-
-    /* This state will be reached when we get EOF on the child's
-       log pipe. */
-    state = &DerivationGoal::buildDone;
-
-    return started();
 }
 
 
@@ -279,8 +293,10 @@ void LocalDerivationGoal::closeReadPipes()
 {
     if (hook) {
         DerivationGoal::closeReadPipes();
-    } else
-        builderOut.close();
+    } else {
+        builderOutPTY.close();
+        builderOutFD = nullptr;
+    }
 }
 
 
@@ -372,40 +388,7 @@ void LocalDerivationGoal::cleanupPostOutputsRegisteredModeNonCheck()
     cleanupPostOutputsRegisteredModeCheck();
 }
 
-#if __linux__
-static void doBind(const Path & source, const Path & target, bool optional = false) {
-    debug("bind mounting '%1%' to '%2%'", source, target);
-
-    auto bindMount = [&]() {
-        if (mount(source.c_str(), target.c_str(), "", MS_BIND | MS_REC, 0) == -1)
-            throw SysError("bind mount from '%1%' to '%2%' failed", source, target);
-    };
-
-    auto maybeSt = maybeLstat(source);
-    if (!maybeSt) {
-        if (optional)
-            return;
-        else
-            throw SysError("getting attributes of path '%1%'", source);
-    }
-    auto st = *maybeSt;
-
-    if (S_ISDIR(st.st_mode)) {
-        createDirs(target);
-        bindMount();
-    } else if (S_ISLNK(st.st_mode)) {
-        // Symlinks can (apparently) not be bind-mounted, so just copy it
-        createDirs(dirOf(target));
-        copyFile(source, target, {});
-    } else {
-        createDirs(dirOf(target));
-        writeFile(target, "");
-        bindMount();
-    }
-};
-#endif
-
-void LocalDerivationGoal::startBuilder()
+std::set<int> LocalDerivationGoal::startBuilder()
 {
     if ((buildUser && buildUser->getUIDCount() != 1)
         #if __linux__
@@ -466,13 +449,23 @@ void LocalDerivationGoal::startBuilder()
     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}",
-            drv->platform,
-            concatStringsSep(", ", parsedDrv->getRequiredSystemFeatures()),
-            worker.store.printStorePath(drvPath),
-            settings.thisSystem,
-            concatStringsSep<StringSet>(", ", worker.store.systemFeatures));
+    if (!parsedDrv->canBuildLocally(worker.store)) {
+        HintFmt addendum{""};
+        if (settings.useSubstitutes && !parsedDrv->substitutesAllowed()) {
+            addendum = HintFmt("\n\nHint: the failing derivation has %s set to %s, forcing it to be built rather than substituted.\n"
+                "Passing %s to force substitution may resolve this failure if the path is available in a substituter.",
+                "allowSubstitutes", "false", "--always-allow-substitutes");
+        }
+        throw Error({
+            .msg = HintFmt("a '%s' with features {%s} is required to build '%s', but I am a '%s' with features {%s}%s",
+                drv->platform,
+                concatStringsSep(", ", parsedDrv->getRequiredSystemFeatures()),
+                worker.store.printStorePath(drvPath),
+                settings.thisSystem,
+                concatStringsSep<StringSet>(", ", worker.store.systemFeatures),
+                Uncolored(addendum))
+        });
+    }
 
     /* Create a temporary directory where the build will take
        place. */
@@ -704,12 +697,13 @@ void LocalDerivationGoal::startBuilder()
     Path logFile = openLogFile();
 
     /* Create a pseudoterminal to get the output of the builder. */
-    builderOut = AutoCloseFD{posix_openpt(O_RDWR | O_NOCTTY)};
-    if (!builderOut)
+    builderOutPTY = AutoCloseFD{posix_openpt(O_RDWR | O_NOCTTY)};
+    if (!builderOutPTY)
         throw SysError("opening pseudoterminal master");
+    builderOutFD = &builderOutPTY;
 
     // FIXME: not thread-safe, use ptsname_r
-    std::string slaveName = ptsname(builderOut.get());
+    std::string slaveName = ptsname(builderOutPTY.get());
 
     if (buildUser) {
         if (chmod(slaveName.c_str(), 0600))
@@ -720,12 +714,12 @@ void LocalDerivationGoal::startBuilder()
     }
 #if __APPLE__
     else {
-        if (grantpt(builderOut.get()))
+        if (grantpt(builderOutPTY.get()))
             throw SysError("granting access to pseudoterminal slave");
     }
 #endif
 
-    if (unlockpt(builderOut.get()))
+    if (unlockpt(builderOutPTY.get()))
         throw SysError("unlocking pseudoterminal");
 
     /* Open the slave side of the pseudoterminal and use it as stderr. */
@@ -756,14 +750,13 @@ void LocalDerivationGoal::startBuilder()
 
     /* parent */
     pid.setSeparatePG(true);
-    worker.childStarted(shared_from_this(), {builderOut.get()}, true, true);
 
     /* Check if setting up the build environment failed. */
     std::vector<std::string> msgs;
     while (true) {
         std::string msg = [&]() {
             try {
-                return readLine(builderOut.get());
+                return readLine(builderOutPTY.get());
             } catch (Error & e) {
                 auto status = pid.wait();
                 e.addTrace({}, "while waiting for the build environment for '%s' to initialize (%s, previous messages: %s)",
@@ -775,7 +768,7 @@ void LocalDerivationGoal::startBuilder()
         }();
         if (msg.substr(0, 1) == "\2") break;
         if (msg.substr(0, 1) == "\1") {
-            FdSource source(builderOut.get());
+            FdSource source(builderOutPTY.get());
             auto ex = readError(source);
             ex.addTrace({}, "while setting up the build environment");
             throw ex;
@@ -783,6 +776,8 @@ void LocalDerivationGoal::startBuilder()
         debug("sandbox setup: " + msg);
         msgs.push_back(std::move(msg));
     }
+
+    return {builderOutPTY.get()};
 }
 
 
@@ -1307,7 +1302,7 @@ void LocalDerivationGoal::addDependency(const StorePath & path)
             Path target = chrootRootDir + worker.store.printStorePath(path);
 
             if (pathExists(target)) {
-                // There is a similar debug message in doBind, so only run it in this block to not have double messages.
+                // There is a similar debug message in bindPath, so only run it in this block to not have double messages.
                 debug("bind-mounting %s -> %s", target, source);
                 throw Error("store path '%s' already exists in the sandbox", worker.store.printStorePath(path));
             }
@@ -1324,7 +1319,7 @@ void LocalDerivationGoal::addDependency(const StorePath & path)
                 if (setns(sandboxMountNamespace.get(), 0) == -1)
                     throw SysError("entering sandbox mount namespace");
 
-                doBind(source, target);
+                bindPath(source, target);
 
                 _exit(0);
             });
@@ -1516,7 +1511,7 @@ void LocalDerivationGoal::runChild()
                     chmodPath(dst, 0555);
                 } else
                 #endif
-                    doBind(i.second.source, chrootRootDir + i.first, i.second.optional);
+                    bindPath(i.second.source, chrootRootDir + i.first, i.second.optional);
             }
 
             /* Bind a new instance of procfs on /proc. */
@@ -1555,8 +1550,8 @@ void LocalDerivationGoal::runChild()
                 } else {
                     if (errno != EINVAL)
                         throw SysError("mounting /dev/pts");
-                    doBind("/dev/pts", chrootRootDir + "/dev/pts");
-                    doBind("/dev/ptmx", chrootRootDir + "/dev/ptmx");
+                    bindPath("/dev/pts", chrootRootDir + "/dev/pts");
+                    bindPath("/dev/ptmx", chrootRootDir + "/dev/ptmx");
                 }
             }
 
@@ -2596,13 +2591,6 @@ void LocalDerivationGoal::deleteTmpDir(bool force)
 }
 
 
-bool LocalDerivationGoal::isReadDesc(int fd)
-{
-    return (hook && DerivationGoal::isReadDesc(fd)) ||
-        (!hook && fd == builderOut.get());
-}
-
-
 StorePath LocalDerivationGoal::makeFallbackPath(OutputNameView outputName)
 {
     return worker.store.makeStorePath(

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

@@ -40,7 +40,7 @@ struct LocalDerivationGoal : public DerivationGoal
      * Master side of the pseudoterminal used for the builder's
      * standard output/error.
      */
-    AutoCloseFD builderOut;
+    AutoCloseFD builderOutPTY;
 
     /**
      * Pipe for synchronising updates to the builder namespaces.
@@ -211,12 +211,12 @@ struct LocalDerivationGoal : public DerivationGoal
     /**
      * The additional states.
      */
-    WorkResult tryLocalBuild() override;
+    WorkResult tryLocalBuild(bool inBuildSlot) override;
 
     /**
      * Start building a derivation.
      */
-    void startBuilder();
+    std::set<int> startBuilder();
 
     /**
      * Fill in the environment for the builder.
@@ -285,8 +285,6 @@ struct LocalDerivationGoal : public DerivationGoal
     void cleanupPostOutputsRegisteredModeCheck() override;
     void cleanupPostOutputsRegisteredModeNonCheck() override;
 
-    bool isReadDesc(int fd) override;
-
     /**
      * Delete the temporary directory, if we have one.
      */
@@ -359,6 +357,10 @@ protected:
         return false;
     }
 
+    virtual bool respectsTimeouts() override
+    {
+        return true;
+    }
 };
 
 }

src/libstore/build/substitution-goal.cc

@@ -7,7 +7,7 @@
 namespace nix {
 
 PathSubstitutionGoal::PathSubstitutionGoal(const StorePath & storePath, Worker & worker, RepairFlag repair, std::optional<ContentAddress> ca)
-    : Goal(worker, DerivedPath::Opaque { storePath })
+    : Goal(worker)
     , storePath(storePath)
     , repair(repair)
     , ca(ca)
@@ -39,13 +39,13 @@ Goal::Finished PathSubstitutionGoal::done(
 }
 
 
-Goal::WorkResult PathSubstitutionGoal::work()
+Goal::WorkResult PathSubstitutionGoal::work(bool inBuildSlot)
 {
-    return (this->*state)();
+    return (this->*state)(inBuildSlot);
 }
 
 
-Goal::WorkResult PathSubstitutionGoal::init()
+Goal::WorkResult PathSubstitutionGoal::init(bool inBuildSlot)
 {
     trace("init");
 
@@ -61,11 +61,11 @@ Goal::WorkResult PathSubstitutionGoal::init()
 
     subs = settings.useSubstitutes ? getDefaultSubstituters() : std::list<ref<Store>>();
 
-    return tryNext();
+    return tryNext(inBuildSlot);
 }
 
 
-Goal::WorkResult PathSubstitutionGoal::tryNext()
+Goal::WorkResult PathSubstitutionGoal::tryNext(bool inBuildSlot)
 {
     trace("trying next substituter");
 
@@ -97,23 +97,23 @@ Goal::WorkResult PathSubstitutionGoal::tryNext()
         if (sub->storeDir == worker.store.storeDir)
             assert(subPath == storePath);
     } else if (sub->storeDir != worker.store.storeDir) {
-        return tryNext();
+        return tryNext(inBuildSlot);
     }
 
     try {
         // FIXME: make async
         info = sub->queryPathInfo(subPath ? *subPath : storePath);
     } catch (InvalidPath &) {
-        return tryNext();
+        return tryNext(inBuildSlot);
     } catch (SubstituterDisabled &) {
         if (settings.tryFallback) {
-            return tryNext();
+            return tryNext(inBuildSlot);
         }
         throw;
     } catch (Error & e) {
         if (settings.tryFallback) {
             logError(e.info());
-            return tryNext();
+            return tryNext(inBuildSlot);
         }
         throw;
     }
@@ -126,7 +126,7 @@ Goal::WorkResult PathSubstitutionGoal::tryNext()
         } else {
             printError("asked '%s' for '%s' but got '%s'",
                 sub->getUri(), worker.store.printStorePath(storePath), sub->printStorePath(info->path));
-            return tryNext();
+            return tryNext(inBuildSlot);
         }
     }
 
@@ -147,7 +147,7 @@ Goal::WorkResult PathSubstitutionGoal::tryNext()
     {
         warn("ignoring substitute for '%s' from '%s', as it's not signed by any of the keys in 'trusted-public-keys'",
             worker.store.printStorePath(storePath), sub->getUri());
-        return tryNext();
+        return tryNext(inBuildSlot);
     }
 
     /* To maintain the closure invariant, we first have to realise the
@@ -158,7 +158,7 @@ Goal::WorkResult PathSubstitutionGoal::tryNext()
             result.goals.insert(worker.makePathSubstitutionGoal(i));
 
     if (result.goals.empty()) {/* to prevent hang (no wake-up event) */
-        return referencesValid();
+        return referencesValid(inBuildSlot);
     } else {
         state = &PathSubstitutionGoal::referencesValid;
         return result;
@@ -166,7 +166,7 @@ Goal::WorkResult PathSubstitutionGoal::tryNext()
 }
 
 
-Goal::WorkResult PathSubstitutionGoal::referencesValid()
+Goal::WorkResult PathSubstitutionGoal::referencesValid(bool inBuildSlot)
 {
     trace("all references realised");
 
@@ -186,14 +186,11 @@ Goal::WorkResult PathSubstitutionGoal::referencesValid()
 }
 
 
-Goal::WorkResult PathSubstitutionGoal::tryToRun()
+Goal::WorkResult PathSubstitutionGoal::tryToRun(bool inBuildSlot)
 {
     trace("trying to run");
 
-    /* Make sure that we are allowed to start a substitution.  Note that even
-       if maxSubstitutionJobs == 0, we still allow a substituter to run. This
-       prevents infinite waiting. */
-    if (worker.getNrSubstitutions() >= std::max(1U, (unsigned int) settings.maxSubstitutionJobs)) {
+    if (!inBuildSlot) {
         return WaitForSlot{};
     }
 
@@ -224,14 +221,12 @@ Goal::WorkResult PathSubstitutionGoal::tryToRun()
         }
     });
 
-    worker.childStarted(shared_from_this(), {outPipe.readSide.get()}, true, false);
-
     state = &PathSubstitutionGoal::finished;
-    return StillAlive{};
+    return WaitForWorld{{outPipe.readSide.get()}, true};
 }
 
 
-Goal::WorkResult PathSubstitutionGoal::finished()
+Goal::WorkResult PathSubstitutionGoal::finished(bool inBuildSlot)
 {
     trace("substitute finished");
 

src/libstore/build/substitution-goal.hh

@@ -66,7 +66,7 @@ struct PathSubstitutionGoal : public Goal
     std::unique_ptr<MaintainCount<uint64_t>> maintainExpectedSubstitutions,
         maintainRunningSubstitutions, maintainExpectedNar, maintainExpectedDownload;
 
-    typedef WorkResult (PathSubstitutionGoal::*GoalState)();
+    typedef WorkResult (PathSubstitutionGoal::*GoalState)(bool inBuildSlot);
     GoalState state;
 
     /**
@@ -94,17 +94,16 @@ public:
         return "a$" + std::string(storePath.name()) + "$" + worker.store.printStorePath(storePath);
     }
 
-    WorkResult work() override;
+    WorkResult work(bool inBuildSlot) override;
 
     /**
      * The states.
      */
-    WorkResult init();
-    WorkResult tryNext();
-    WorkResult gotInfo();
-    WorkResult referencesValid();
-    WorkResult tryToRun();
-    WorkResult finished();
+    WorkResult init(bool inBuildSlot);
+    WorkResult tryNext(bool inBuildSlot);
+    WorkResult referencesValid(bool inBuildSlot);
+    WorkResult tryToRun(bool inBuildSlot);
+    WorkResult finished(bool inBuildSlot);
 
     /**
      * Callback used by the worker to write to the log.

src/libstore/build/worker.cc

@@ -1,5 +1,4 @@
 #include "charptr-cast.hh"
-#include "machines.hh"
 #include "worker.hh"
 #include "substitution-goal.hh"
 #include "drv-output-substitution-goal.hh"
@@ -151,7 +150,7 @@ void Worker::goalFinished(GoalPtr goal, Goal::Finished & f)
         if (!goal->waiters.empty())
             logError(f.ex->info());
         else
-            goal->ex = std::move(f.ex);
+            goal->ex = f.ex;
     }
 
     for (auto & i : goal->waiters) {
@@ -198,6 +197,7 @@ void Worker::handleWorkResult(GoalPtr goal, Goal::WorkResult how)
                     dep->waiters.insert(goal);
                 }
             },
+            [&](Goal::WaitForWorld & w) { childStarted(goal, w.fds, w.inBuildSlot); },
             [&](Goal::Finished & f) { goalFinished(goal, f); },
         },
         how
@@ -232,20 +232,8 @@ void Worker::wakeUp(GoalPtr goal)
 }
 
 
-unsigned Worker::getNrLocalBuilds()
-{
-    return nrLocalBuilds;
-}
-
-
-unsigned Worker::getNrSubstitutions()
-{
-    return nrSubstitutions;
-}
-
-
 void Worker::childStarted(GoalPtr goal, const std::set<int> & fds,
-    bool inBuildSlot, bool respectTimeouts)
+    bool inBuildSlot)
 {
     Child child;
     child.goal = goal;
@@ -253,7 +241,6 @@ void Worker::childStarted(GoalPtr goal, const std::set<int> & fds,
     child.fds = fds;
     child.timeStarted = child.lastOutput = steady_time_point::clock::now();
     child.inBuildSlot = inBuildSlot;
-    child.respectTimeouts = respectTimeouts;
     children.emplace_back(child);
     if (inBuildSlot) {
         switch (goal->jobCategory()) {
@@ -307,8 +294,8 @@ void Worker::waitForBuildSlot(GoalPtr goal)
 {
     goal->trace("wait for build slot");
     bool isSubstitutionGoal = goal->jobCategory() == JobCategory::Substitution;
-    if ((!isSubstitutionGoal && getNrLocalBuilds() < settings.maxBuildJobs) ||
-        (isSubstitutionGoal && getNrSubstitutions() < settings.maxSubstitutionJobs))
+    if ((!isSubstitutionGoal && nrLocalBuilds < settings.maxBuildJobs) ||
+        (isSubstitutionGoal && nrSubstitutions < settings.maxSubstitutionJobs))
         wakeUp(goal); /* we can do it right away */
     else
         wantingToBuild.insert(goal);
@@ -364,7 +351,12 @@ void Worker::run(const Goals & _topGoals)
             awake.clear();
             for (auto & goal : awake2) {
                 checkInterrupt();
-                handleWorkResult(goal, goal->work());
+                /* Make sure that we are always allowed to run at least one substitution.
+                   This prevents infinite waiting. */
+                const bool inSlot = goal->jobCategory() == JobCategory::Substitution
+                    ? nrSubstitutions < std::max(1U, (unsigned int) settings.maxSubstitutionJobs)
+                    : nrLocalBuilds < settings.maxBuildJobs;
+                handleWorkResult(goal, goal->work(inSlot));
 
                 actDerivations.progress(
                     doneBuilds, expectedBuilds + doneBuilds, runningBuilds, failedBuilds
@@ -388,18 +380,6 @@ void Worker::run(const Goals & _topGoals)
         if (!children.empty() || !waitingForAWhile.empty())
             waitForInput();
         else {
-            if (awake.empty() && 0U == settings.maxBuildJobs)
-            {
-                if (getMachines().empty())
-                   throw Error("unable to start any build; either increase '--max-jobs' "
-                            "or enable remote builds."
-                            "\nhttps://docs.lix.systems/manual/lix/stable/advanced-topics/distributed-builds.html");
-                else
-                   throw Error("unable to start any build; remote machines may not have "
-                            "all required system features."
-                            "\nhttps://docs.lix.systems/manual/lix/stable/advanced-topics/distributed-builds.html");
-
-            }
             assert(!awake.empty());
         }
     }
@@ -434,11 +414,13 @@ void Worker::waitForInput()
         // Periodicallty wake up to see if we need to run the garbage collector.
         nearest = before + std::chrono::seconds(10);
     for (auto & i : children) {
-        if (!i.respectTimeouts) continue;
-        if (0 != settings.maxSilentTime)
-            nearest = std::min(nearest, i.lastOutput + std::chrono::seconds(settings.maxSilentTime));
-        if (0 != settings.buildTimeout)
-            nearest = std::min(nearest, i.timeStarted + std::chrono::seconds(settings.buildTimeout));
+        if (auto goal = i.goal.lock()) {
+            if (!goal->respectsTimeouts()) continue;
+            if (0 != settings.maxSilentTime)
+                nearest = std::min(nearest, i.lastOutput + std::chrono::seconds(settings.maxSilentTime));
+            if (0 != settings.buildTimeout)
+                nearest = std::min(nearest, i.timeStarted + std::chrono::seconds(settings.buildTimeout));
+        }
     }
     if (nearest != steady_time_point::max()) {
         timeout = std::max(1L, (long) std::chrono::duration_cast<std::chrono::seconds>(nearest - before).count());
@@ -491,7 +473,7 @@ void Worker::waitForInput()
 
         if (!goal->exitCode.has_value() &&
             0 != settings.maxSilentTime &&
-            j->respectTimeouts &&
+            goal->respectsTimeouts() &&
             after - j->lastOutput >= std::chrono::seconds(settings.maxSilentTime))
         {
             handleWorkResult(
@@ -507,7 +489,7 @@ void Worker::waitForInput()
 
         else if (!goal->exitCode.has_value() &&
             0 != settings.buildTimeout &&
-            j->respectTimeouts &&
+            goal->respectsTimeouts() &&
             after - j->timeStarted >= std::chrono::seconds(settings.buildTimeout))
         {
             handleWorkResult(

src/libstore/build/worker.hh

@@ -29,7 +29,6 @@ struct Child
     WeakGoalPtr goal;
     Goal * goal2; // ugly hackery
     std::set<int> fds;
-    bool respectTimeouts;
     bool inBuildSlot;
     /**
      * Time we last got output on stdout/stderr
@@ -153,6 +152,18 @@ private:
      */
     void waitForInput();
 
+    /**
+     * Remove a dead goal.
+     */
+    void removeGoal(GoalPtr goal);
+
+    /**
+     * Registers a running child process.  `inBuildSlot` means that
+     * the process counts towards the jobs limit.
+     */
+    void childStarted(GoalPtr goal, const std::set<int> & fds,
+        bool inBuildSlot);
+
 public:
 
     const Activity act;
@@ -224,29 +235,6 @@ public:
      */
     GoalPtr makeGoal(const DerivedPath & req, BuildMode buildMode = bmNormal);
 
-    /**
-     * Remove a dead goal.
-     */
-    void removeGoal(GoalPtr goal);
-
-    /**
-     * Return the number of local build processes currently running (but not
-     * remote builds via the build hook).
-     */
-    unsigned int getNrLocalBuilds();
-
-    /**
-     * Return the number of substitution processes currently running.
-     */
-    unsigned int getNrSubstitutions();
-
-    /**
-     * Registers a running child process.  `inBuildSlot` means that
-     * the process counts towards the jobs limit.
-     */
-    void childStarted(GoalPtr goal, const std::set<int> & fds,
-        bool inBuildSlot, bool respectTimeouts);
-
     /**
      * Unregisters a running child process.
      */

src/libstore/builtins/buildenv.cc

@@ -1,4 +1,5 @@
 #include "buildenv.hh"
+#include "strings.hh"
 
 #include <sys/stat.h>
 #include <sys/types.h>

src/libstore/builtins/fetchurl.cc

@@ -3,6 +3,7 @@
 #include "store-api.hh"
 #include "archive.hh"
 #include "compression.hh"
+#include "strings.hh"
 
 namespace nix {
 

src/libstore/common-protocol-impl.hh

@@ -20,6 +20,7 @@ namespace nix {
     { \
         return LengthPrefixedProtoHelper<CommonProto, T >::read(store, conn); \
     } \
+    /* NOLINTNEXTLINE(bugprone-macro-parentheses) */ \
     TEMPLATE [[nodiscard]] WireFormatGenerator CommonProto::Serialise< T >::write(const Store & store, CommonProto::WriteConn conn, const T & t) \
     { \
         return LengthPrefixedProtoHelper<CommonProto, T >::write(store, conn, t); \

src/libstore/content-address.cc

@@ -1,6 +1,7 @@
 #include "args.hh"
 #include "content-address.hh"
 #include "split.hh"
+#include "strings.hh"
 
 namespace nix {
 

src/libstore/daemon.cc

@@ -2,7 +2,7 @@
 #include "monitor-fd.hh"
 #include "worker-protocol.hh"
 #include "worker-protocol-impl.hh"
-#include "build-result.hh"
+#include "build-result.hh" // IWYU pragma: keep
 #include "store-api.hh"
 #include "store-cast.hh"
 #include "gc-store.hh"
@@ -12,6 +12,7 @@
 #include "finally.hh"
 #include "archive.hh"
 #include "derivations.hh"
+#include "strings.hh"
 #include "args.hh"
 
 #include <sstream>

src/libstore/derivations.cc

@@ -3,11 +3,12 @@
 #include "store-api.hh"
 #include "globals.hh"
 #include "types.hh"
-#include "split.hh"
 #include "common-protocol.hh"
 #include "common-protocol-impl.hh"
-#include "fs-accessor.hh"
 #include "json-utils.hh"
+#include "strings.hh"
+#include "backed-string-view.hh"
+
 #include <boost/container/small_vector.hpp>
 #include <nlohmann/json.hpp>
 

src/libstore/derived-path.hh

@@ -5,6 +5,7 @@
 #include "path.hh"
 #include "outputs-spec.hh"
 #include "comparator.hh"
+#include "ref.hh"
 
 #include <variant>
 
@@ -78,10 +79,12 @@ struct SingleDerivedPathBuilt {
     DECLARE_CMP(SingleDerivedPathBuilt);
 };
 
-using _SingleDerivedPathRaw = std::variant<
+namespace derived_path::detail {
+using SingleDerivedPathRaw = std::variant<
     DerivedPathOpaque,
     SingleDerivedPathBuilt
 >;
+}
 
 /**
  * A "derived path" is a very simple sort of expression (not a Nix
@@ -94,8 +97,8 @@ using _SingleDerivedPathRaw = std::variant<
  * - built, in which case it is a pair of a derivation path and an
  *   output name.
  */
-struct SingleDerivedPath : _SingleDerivedPathRaw {
-    using Raw = _SingleDerivedPathRaw;
+struct SingleDerivedPath : derived_path::detail::SingleDerivedPathRaw {
+    using Raw = derived_path::detail::SingleDerivedPathRaw;
     using Raw::Raw;
 
     using Opaque = DerivedPathOpaque;
@@ -201,10 +204,12 @@ struct DerivedPathBuilt {
     DECLARE_CMP(DerivedPathBuilt);
 };
 
-using _DerivedPathRaw = std::variant<
+namespace derived_path::detail {
+using DerivedPathRaw = std::variant<
     DerivedPathOpaque,
     DerivedPathBuilt
 >;
+}
 
 /**
  * A "derived path" is a very simple sort of expression that evaluates
@@ -216,8 +221,8 @@ using _DerivedPathRaw = std::variant<
  * - built, in which case it is a pair of a derivation path and some
  *   output names.
  */
-struct DerivedPath : _DerivedPathRaw {
-    using Raw = _DerivedPathRaw;
+struct DerivedPath : derived_path::detail::DerivedPathRaw {
+    using Raw = derived_path::detail::DerivedPathRaw;
     using Raw::Raw;
 
     using Opaque = DerivedPathOpaque;

src/libstore/dummy-store.cc

@@ -1,3 +1,4 @@
+#include "dummy-store.hh"
 #include "store-api.hh"
 
 namespace nix {
@@ -73,6 +74,8 @@ struct DummyStore : public virtual DummyStoreConfig, public virtual Store
     { unsupported("getFSAccessor"); }
 };
 
-static RegisterStoreImplementation<DummyStore, DummyStoreConfig> regDummyStore;
+void registerDummyStore() {
+    StoreImplementations::add<DummyStore, DummyStoreConfig>();
+}
 
 }

src/libstore/dummy-store.hh

@@ -0,0 +1,8 @@
+#pragma once
+///@file
+
+namespace nix {
+
+void registerDummyStore();
+
+}

src/libstore/filetransfer.cc

@@ -5,6 +5,7 @@
 #include "s3.hh"
 #include "signals.hh"
 #include "compression.hh"
+#include "strings.hh"
 
 #if ENABLE_S3
 #include <aws/core/client/ClientConfiguration.h>

src/libstore/filetransfer.hh

@@ -2,6 +2,7 @@
 ///@file
 
 #include "box_ptr.hh"
+#include "ref.hh"
 #include "logging.hh"
 #include "serialise.hh"
 #include "types.hh"

src/libstore/gc.cc

@@ -5,6 +5,7 @@
 #include "signals.hh"
 #include "finally.hh"
 #include "unix-domain-socket.hh"
+#include "strings.hh"
 
 #include <queue>
 #include <regex>

src/libstore/globals.cc

@@ -33,6 +33,16 @@
 #include <sys/sysctl.h>
 #endif
 
+// All built-in store implementations.
+#include "dummy-store.hh"
+#include "http-binary-cache-store.hh"
+#include "legacy-ssh-store.hh"
+#include "local-binary-cache-store.hh"
+#include "local-store.hh"
+#include "s3-binary-cache-store.hh"
+#include "ssh-store.hh"
+#include "uds-remote-store.hh"
+
 namespace nix {
 
 
@@ -396,6 +406,17 @@ static void preloadNSS()
     });
 }
 
+static void registerStoreImplementations() {
+  registerDummyStore();
+  registerHttpBinaryCacheStore();
+  registerLegacySSHStore();
+  registerLocalBinaryCacheStore();
+  registerLocalStore();
+  registerS3BinaryCacheStore();
+  registerSSHStore();
+  registerUDSRemoteStore();
+}
+
 static bool initLibStoreDone = false;
 
 void assertLibStoreInitialized() {
@@ -433,6 +454,8 @@ void initLibStore() {
         unsetenv("TMPDIR");
 #endif
 
+    registerStoreImplementations();
+
     initLibStoreDone = true;
 }
 

src/libstore/globals.hh

@@ -634,7 +634,7 @@ public:
           line.
         )"};
 
-    OptionalPathSetting diffHook{
+    PathsSetting<std::optional<Path>> diffHook{
         this, std::nullopt, "diff-hook",
         R"(
           Absolute path to an executable capable of diffing build

src/libstore/http-binary-cache-store.cc

@@ -1,3 +1,4 @@
+#include "http-binary-cache-store.hh"
 #include "binary-cache-store.hh"
 #include "filetransfer.hh"
 #include "globals.hh"
@@ -194,6 +195,8 @@ protected:
     }
 };
 
-static RegisterStoreImplementation<HttpBinaryCacheStore, HttpBinaryCacheStoreConfig> regHttpBinaryCacheStore;
+void registerHttpBinaryCacheStore() {
+    StoreImplementations::add<HttpBinaryCacheStore, HttpBinaryCacheStoreConfig>();
+}
 
 }

src/libstore/http-binary-cache-store.hh

@@ -0,0 +1,8 @@
+#pragma once
+///@file
+
+namespace nix {
+
+void registerHttpBinaryCacheStore();
+
+}

src/libstore/legacy-ssh-store.cc

@@ -1,4 +1,4 @@
-#include "ssh-store-config.hh"
+#include "legacy-ssh-store.hh"
 #include "archive.hh"
 #include "pool.hh"
 #include "remote-store.hh"
@@ -8,6 +8,8 @@
 #include "store-api.hh"
 #include "path-with-outputs.hh"
 #include "ssh.hh"
+#include "ssh-store.hh"
+#include "strings.hh"
 #include "derivations.hh"
 
 namespace nix {
@@ -412,6 +414,8 @@ public:
     { unsupported("queryRealisation"); }
 };
 
-static RegisterStoreImplementation<LegacySSHStore, LegacySSHStoreConfig> regLegacySSHStore;
+void registerLegacySSHStore() {
+    StoreImplementations::add<LegacySSHStore, LegacySSHStoreConfig>();
+}
 
 }

src/libstore/legacy-ssh-store.hh

@@ -0,0 +1,8 @@
+#pragma once
+///@file
+
+namespace nix {
+
+void registerLegacySSHStore();
+
+}

src/libstore/length-prefixed-protocol-helper.hh

@@ -61,9 +61,9 @@ template<class Inner, typename... Ts>
 LENGTH_PREFIXED_PROTO_HELPER(Inner, std::tuple<Ts...>);
 
 template<class Inner, typename K, typename V>
-#define _X std::map<K, V>
-LENGTH_PREFIXED_PROTO_HELPER(Inner, _X);
-#undef _X
+#define DONT_SUBSTITUTE_KV_TYPE std::map<K, V>
+LENGTH_PREFIXED_PROTO_HELPER(Inner, DONT_SUBSTITUTE_KV_TYPE);
+#undef DONT_SUBSTITUTE_KV_TYPE
 
 template<class Inner, typename T>
 std::vector<T>

src/libstore/local-binary-cache-store.cc

@@ -1,3 +1,4 @@
+#include "local-binary-cache-store.hh"
 #include "binary-cache-store.hh"
 #include "globals.hh"
 #include "nar-info-disk-cache.hh"
@@ -124,6 +125,8 @@ std::set<std::string> LocalBinaryCacheStore::uriSchemes()
         return {"file"};
 }
 
-static RegisterStoreImplementation<LocalBinaryCacheStore, LocalBinaryCacheStoreConfig> regLocalBinaryCacheStore;
+void registerLocalBinaryCacheStore() {
+    StoreImplementations::add<LocalBinaryCacheStore, LocalBinaryCacheStoreConfig>();
+}
 
 }

src/libstore/local-binary-cache-store.hh

@@ -0,0 +1,8 @@
+#pragma once
+///@file
+
+namespace nix {
+
+void registerLocalBinaryCacheStore();
+
+}

src/libstore/local-fs-store.hh

@@ -11,21 +11,21 @@ struct LocalFSStoreConfig : virtual StoreConfig
 {
     using StoreConfig::StoreConfig;
 
-    const OptionalPathSetting rootDir{this, std::nullopt,
+    const PathsSetting<std::optional<Path>>  rootDir{this, std::nullopt,
         "root",
         "Directory prefixed to all other paths."};
 
-    const PathSetting stateDir{this,
+    const PathsSetting<Path> stateDir{this,
         rootDir.get() ? *rootDir.get() + "/nix/var/nix" : settings.nixStateDir,
         "state",
         "Directory where Lix will store state."};
 
-    const PathSetting logDir{this,
+    const PathsSetting<Path> logDir{this,
         rootDir.get() ? *rootDir.get() + "/nix/var/log/nix" : settings.nixLogDir,
         "log",
         "directory where Lix will store log files."};
 
-    const PathSetting realStoreDir{this,
+    const PathsSetting<Path> realStoreDir{this,
         rootDir.get() ? *rootDir.get() + "/nix/store" : storeDir, "real",
         "Physical path of the Nix store."};
 };

src/libstore/local-store.cc

@@ -10,6 +10,7 @@
 #include "signals.hh"
 #include "finally.hh"
 #include "compression.hh"
+#include "strings.hh"
 
 #include <algorithm>
 #include <cstring>

src/libstore/local-store.hh

@@ -421,4 +421,7 @@ void canonicaliseTimestampAndPermissions(const Path & path);
 
 MakeError(PathInUse, Error);
 
+// Implemented by the relevant platform/ module being used.
+void registerLocalStore();
+
 }

src/libstore/lock.hh

@@ -1,11 +1,10 @@
 #pragma once
 ///@file
 
-#include "types.hh"
-
-#include <optional>
+#include <memory>
 
 #include <sys/types.h>
+#include <vector>
 
 namespace nix {
 

src/libstore/machines.hh

@@ -1,7 +1,9 @@
 #pragma once
 ///@file
 
-#include "types.hh"
+#include "ref.hh"
+#include <set>
+#include <vector>
 
 namespace nix {
 

src/libstore/make-content-addressed.cc

@@ -1,5 +1,6 @@
 #include "make-content-addressed.hh"
 #include "references.hh"
+#include "strings.hh"
 
 namespace nix {
 

src/libstore/meson.build

@@ -118,12 +118,16 @@ libstore_headers = files(
   'derived-path-map.hh',
   'derived-path.hh',
   'downstream-placeholder.hh',
+  'dummy-store.hh',
   'filetransfer.hh',
   'fs-accessor.hh',
   'gc-store.hh',
   'globals.hh',
+  'http-binary-cache-store.hh',
   'indirect-root-store.hh',
+  'legacy-ssh-store.hh',
   'length-prefixed-protocol-helper.hh',
+  'local-binary-cache-store.hh',
   'local-fs-store.hh',
   'local-store.hh',
   'lock.hh',
@@ -152,8 +156,8 @@ libstore_headers = files(
   'serve-protocol-impl.hh',
   'serve-protocol.hh',
   'sqlite.hh',
-  'ssh-store-config.hh',
   'ssh.hh',
+  'ssh-store.hh',
   'store-api.hh',
   'store-cast.hh',
   'uds-remote-store.hh',

src/libstore/misc.cc

@@ -1,12 +1,12 @@
 #include "derivations.hh"
 #include "parsed-derivations.hh"
 #include "globals.hh"
-#include "local-store.hh"
 #include "store-api.hh"
 #include "thread-pool.hh"
 #include "topo-sort.hh"
 #include "closure.hh"
 #include "filetransfer.hh"
+#include "strings.hh"
 
 namespace nix {
 

src/libstore/nar-accessor.hh

@@ -1,10 +1,12 @@
 #pragma once
 ///@file
 
+#include "fs-accessor.hh"
+#include "ref.hh"
+
 #include <functional>
 
 #include <nlohmann/json_fwd.hpp>
-#include "fs-accessor.hh"
 
 namespace nix {
 

src/libstore/nar-info-disk-cache.cc

@@ -4,6 +4,7 @@
 #include "sqlite.hh"
 #include "globals.hh"
 #include "users.hh"
+#include "strings.hh"
 
 #include <sqlite3.h>
 #include <nlohmann/json.hpp>

src/libstore/nar-info.cc

@@ -1,4 +1,4 @@
-#include "globals.hh"
+#include "strings.hh"
 #include "nar-info.hh"
 #include "store-api.hh"
 

src/libstore/optimise-store.cc

@@ -1,6 +1,7 @@
 #include "local-store.hh"
 #include "globals.hh"
 #include "signals.hh"
+#include "strings.hh"
 
 #include <cstring>
 #include <sys/types.h>

src/libstore/parsed-derivations.cc

@@ -1,4 +1,5 @@
 #include "parsed-derivations.hh"
+#include "strings.hh"
 
 #include <nlohmann/json.hpp>
 #include <regex>

src/libstore/path-info.cc

@@ -1,5 +1,6 @@
 #include "path-info.hh"
 #include "store-api.hh"
+#include "strings.hh"
 
 namespace nix {
 

src/libstore/path-with-outputs.cc

@@ -1,7 +1,6 @@
 #include "path-with-outputs.hh"
 #include "store-api.hh"
-
-#include <regex>
+#include "strings.hh"
 
 namespace nix {
 

src/libstore/path.cc

@@ -112,3 +112,13 @@ PathSet Store::printStorePathSet(const StorePathSet & paths) const
 }
 
 }
+
+std::size_t std::hash<nix::StorePath>::operator()(const nix::StorePath & path) const noexcept
+{
+    // It's already a cryptographic hash of 160 bits (assuming that nobody gives us bogus ones...), so just parse it.
+    auto h = nix::Hash::parseNonSRIUnprefixed(path.hashPart(), nix::HashType::SHA1);
+    // This need not be stable across machines, so bit casting the start of it is fine.
+    size_t r;
+    memcpy(&r, h.hash, sizeof(r));
+    return r;
+}

src/libstore/path.hh

@@ -2,8 +2,9 @@
 ///@file
 
 #include <string_view>
+#include <string>
 
-#include "types.hh"
+#include "types.hh" // IWYU pragma: keep
 
 namespace nix {
 
@@ -89,10 +90,7 @@ const std::string drvExtension = ".drv";
 namespace std {
 
 template<> struct hash<nix::StorePath> {
-    std::size_t operator()(const nix::StorePath & path) const noexcept
-    {
-        return * (std::size_t *) path.to_string().data();
-    }
+    std::size_t operator()(const nix::StorePath & path) const noexcept;
 };
 
 }

src/libstore/pathlocks.hh

@@ -49,8 +49,12 @@ struct FdLock
 
     ~FdLock()
     {
-        if (acquired)
-            lockFile(fd, ltNone, false);
+        try {
+            if (acquired)
+                lockFile(fd, ltNone, false);
+        } catch (SysError &) {
+            ignoreException();
+        }
     }
 };
 

src/libstore/platform/darwin.cc

@@ -2,6 +2,7 @@
 #include "signals.hh"
 #include "platform/darwin.hh"
 #include "regex.hh"
+#include "strings.hh"
 
 #include <sys/proc_info.h>
 #include <sys/sysctl.h>
@@ -261,4 +262,9 @@ void DarwinLocalDerivationGoal::execBuilder(std::string builder, Strings args, S
 
     posix_spawn(nullptr, builder.c_str(), nullptr, &attrp, stringsToCharPtrs(args).data(), stringsToCharPtrs(envStrs).data());
 }
+
+void registerLocalStore() {
+    StoreImplementations::add<DarwinLocalStore, LocalStoreConfig>();
+}
+
 }

src/libstore/platform/fallback.cc

@@ -1,5 +1,7 @@
 #include "platform/fallback.hh"
 
 namespace nix {
-static RegisterStoreImplementation<FallbackLocalStore, LocalStoreConfig> regLocalStore;
+void registerLocalStore() {
+    Implementations::add<FallbackLocalStore, LocalStoreConfig>();
+}
 }

src/libstore/platform/linux.cc

@@ -5,6 +5,7 @@
 #include "signals.hh"
 #include "platform/linux.hh"
 #include "regex.hh"
+#include "strings.hh"
 
 #include <grp.h>
 #include <regex>
@@ -25,7 +26,9 @@ namespace {
 constexpr const std::string_view nativeSystem = SYSTEM;
 }
 
-static RegisterStoreImplementation<LinuxLocalStore, LocalStoreConfig> regLocalStore;
+void registerLocalStore() {
+    StoreImplementations::add<LinuxLocalStore, LocalStoreConfig>();
+}
 
 static void readProcLink(const std::string & file, UncheckedRoots & roots)
 {