Merge remote-tracking branch 'upstream/master' into path-info

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,28 @@
+# Motivation
+<!-- Briefly explain what the change is about and why it is desirable. -->
+
+# Context
+<!-- Provide context. Reference open issues if available. -->
+
+<!-- Non-trivial change: Briefly outline the implementation strategy. -->
+
+<!-- Invasive change: Discuss alternative designs or approaches you considered. -->
+
+<!-- Large change: Provide instructions to reviewers how to read the diff. -->
+
+# Checklist for maintainers
+
+<!-- Contributors: please leave this as is -->
+
+Maintainers: tick if completed or explain if not relevant
+
+ - [ ] agreed on idea
+ - [ ] agreed on implementation strategy
+ - [ ] tests, as appropriate
+   - functional tests - `tests/**.sh`
+   - unit tests - `src/*/tests`
+   - integration tests
+ - [ ] documentation in the manual
+ - [ ] code and comments are self-explanatory
+ - [ ] commit message explains why the change was made
+ - [ ] new feature or bug fix: updated release notes

.version

@@ -1 +1 @@
-2.13.0
+2.14.0

configure.ac

@@ -274,6 +274,12 @@ fi
 PKG_CHECK_MODULES([GTEST], [gtest_main])
 
 
+# Look for rapidcheck.
+# No pkg-config yet, https://github.com/emil-e/rapidcheck/issues/302
+AC_CHECK_HEADERS([rapidcheck/gtest.h], [], [], [#include <gtest/gtest.h>])
+AC_CHECK_LIB([rapidcheck], [])
+
+
 # Look for nlohmann/json.
 PKG_CHECK_MODULES([NLOHMANN_JSON], [nlohmann_json >= 3.9])
 

doc/manual/src/SUMMARY.md.in

@@ -67,6 +67,7 @@
   - [CLI guideline](contributing/cli-guideline.md)
 - [Release Notes](release-notes/release-notes.md)
   - [Release X.Y (202?-??-??)](release-notes/rl-next.md)
+  - [Release 2.13 (2023-01-17)](release-notes/rl-2.13.md)
   - [Release 2.12 (2022-12-06)](release-notes/rl-2.12.md)
   - [Release 2.11 (2022-08-25)](release-notes/rl-2.11.md)
   - [Release 2.10 (2022-07-11)](release-notes/rl-2.10.md)

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

@@ -66,11 +66,11 @@ The operation `--realise` essentially “builds” the specified store
 paths. Realisation is a somewhat overloaded term:
 
   - If the store path is a *derivation*, realisation ensures that the
-    output paths of the derivation are [valid](../glossary.md) (i.e.,
+    output paths of the derivation are [valid] (i.e.,
     the output path and its closure exist in the file system). This
     can be done in several ways. First, it is possible that the
     outputs are already valid, in which case we are done
-    immediately. Otherwise, there may be [substitutes](../glossary.md)
+    immediately. Otherwise, there may be [substitutes]
     that produce the outputs (e.g., by downloading them). Finally, the
     outputs can be produced by running the build task described
     by the derivation.
@@ -82,6 +82,9 @@ paths. Realisation is a somewhat overloaded term:
     produced through substitutes. If there are no (successful)
     substitutes, realisation fails.
 
+[valid]: ../glossary.md#validity
+[substitutes]: ../glossary.md#substitute
+
 The output path of each derivation is printed on standard output. (For
 non-derivations argument, the argument itself is printed.)
 
@@ -295,8 +298,8 @@ error: cannot delete path `/nix/store/zq0h41l75vlb4z45kzgjjmsjxvcv1qk7-mesa-6.4'
 
 ## Description
 
-The operation `--query` displays various bits of information about the
+The operation `--query` displays information about [store path]s.
-store paths . The queries are described below. At most one query can be
+The queries are described below. At most one query can be
 specified. The default query is `--outputs`.
 
 The paths *paths* may also be symlinks from outside of the Nix store, to
@@ -316,12 +319,12 @@ symlink.
 ## Queries
 
   - `--outputs`\
-    Prints out the [output paths](../glossary.md) of the store
+    Prints out the [output path]s of the store
     derivations *paths*. These are the paths that will be produced when
     the derivation is built.
 
   - `--requisites`; `-R`\
-    Prints out the [closure](../glossary.md) of the store path *paths*.
+    Prints out the [closure] of the given *paths*.
 
     This query has one option:
 
@@ -338,10 +341,12 @@ symlink.
     derivation and specifying the option `--include-outputs`.
 
   - `--references`\
-    Prints the set of [references](../glossary.md) of the store paths
+    Prints the set of [references]s of the store paths
     *paths*, that is, their immediate dependencies. (For *all*
     dependencies, use `--requisites`.)
 
+    [reference]: ../glossary.md#gloss-reference
+
   - `--referrers`\
     Prints the set of *referrers* of the store paths *paths*, that is,
     the store paths currently existing in the Nix store that refer to
@@ -356,11 +361,13 @@ symlink.
     in the Nix store that are dependent on *paths*.
 
   - `--deriver`; `-d`\
-    Prints the [deriver](../glossary.md) of the store paths *paths*. If
+    Prints the [deriver] of the store paths *paths*. If
     the path has no deriver (e.g., if it is a source file), or if the
     deriver is not known (e.g., in the case of a binary-only
     deployment), the string `unknown-deriver` is printed.
 
+    [deriver]: ../glossary.md#gloss-deriver
+
   - `--graph`\
     Prints the references graph of the store paths *paths* in the format
     of the `dot` tool of AT\&T's [Graphviz

doc/manual/src/contributing/hacking.md

@@ -92,7 +92,8 @@ $ nix develop
 
 The unit-tests for each Nix library (`libexpr`, `libstore`, etc..) are defined
 under `src/{library_name}/tests` using the
-[googletest](https://google.github.io/googletest/) framework.
+[googletest](https://google.github.io/googletest/) and
+[rapidcheck](https://github.com/emil-e/rapidcheck) frameworks.
 
 You can run the whole testsuite with `make check`, or the tests for a specific component with `make libfoo-tests_RUN`. Finer-grained filtering is also possible using the [--gtest_filter](https://google.github.io/googletest/advanced.html#running-a-subset-of-the-tests) command-line option.
 

doc/manual/src/glossary.md

@@ -19,6 +19,17 @@
 
     [store derivation]: #gloss-store-derivation
 
+  - [realise]{#gloss-realise}, realisation\
+    Ensure a [store path] is [valid][validity].
+
+    This means either running the `builder` executable as specified in the corresponding [derivation] or fetching a pre-built [store object] from a [substituter].
+
+    See [`nix-build`](./command-ref/nix-build.md) and [`nix-store --realise`](./command-ref/nix-store.md#operation---realise).
+
+    See [`nix build`](./command-ref/new-cli/nix3-build.md) (experimental).
+
+    [realise]: #gloss-realise
+
   - [content-addressed derivation]{#gloss-content-addressed-derivation}\
     A derivation which has the
     [`__contentAddressed`](./language/advanced-attributes.md#adv-attr-__contentAddressed)
@@ -101,6 +112,8 @@
     copy store objects it doesn't have.  For details, see the
     [`substituters` option](./command-ref/conf-file.md#conf-substituters).
 
+    [substituter]: #gloss-substituter
+
   - [purity]{#gloss-purity}\
     The assumption that equal Nix derivations when run always produce
     the same output. This cannot be guaranteed in general (e.g., a
@@ -149,13 +162,15 @@
     [output path]: #gloss-output-path
 
   - [deriver]{#gloss-deriver}\
-    The deriver of an *output path* is the store
+    The [store derivation] that produced an [output path].
-    derivation that built it.
 
   - [validity]{#gloss-validity}\
-    A store path is considered *valid* if it exists in the file system,
+    A store path is valid if all [store object]s in its [closure] can be read from the [store].
-    is listed in the Nix database as being valid, and if all paths in
+
-    its closure are also valid.
+    For a local store, this means:
+    - The store path leads to an existing [store object] in that [store].
+    - The store path is listed in the Nix database as being valid.
+    - All paths in the store path's [closure] are valid.
 
   - [user environment]{#gloss-user-env}\
     An automatically generated store object that consists of a set of

doc/manual/src/language/index.md

@@ -191,12 +191,12 @@ This is an incomplete overview of language features, by example.
  <tr>
   <td>
 
-   <nixpkgs>
+   `<nixpkgs>`
 
   </td>
   <td>
 
-   Search path. Value determined by [`$NIX_PATH` environment variable](../command-ref/env-common.md#env-NIX_PATH).
+   Search path for Nix files. Value determined by [`$NIX_PATH` environment variable](../command-ref/env-common.md#env-NIX_PATH).
 
   </td>
  </tr>

doc/manual/src/language/operators.md

@@ -24,7 +24,7 @@
 | [Equality]                             | *expr* `==` *expr*                         | none          | 11         |
 | Inequality                             | *expr* `!=` *expr*                         | none          | 11         |
 | Logical conjunction (`AND`)            | *bool* `&&` *bool*                         | left          | 12         |
-| Logical disjunction (`OR`)             | *bool* `||` *bool*                         | left          | 13         |
+| Logical disjunction (`OR`)             | *bool* `\|\|` *bool*                       | left          | 13         |
 | [Logical implication]                  | *bool* `->` *bool*                         | none          | 14         |
 
 [string]: ./values.md#type-string
@@ -120,12 +120,12 @@ The result is a string.
 
 ## Update
 
-> *attrset1* + *attrset2*
+> *attrset1* // *attrset2*
 
 Update [attribute set] *attrset1* with names and values from *attrset2*.
 
-The returned attribute set will have of all the attributes in *e1* and *e2*.
+The returned attribute set will have of all the attributes in *attrset1* and *attrset2*.
-If an attribute name is present in both, the attribute value from the former is taken.
+If an attribute name is present in both, the attribute value from the latter is taken.
 
 [Update]: #update
 

doc/manual/src/package-management/binary-cache-substituter.md

@@ -32,13 +32,13 @@ which should print something like:
     Priority: 30
 
 On the client side, you can tell Nix to use your binary cache using
-`--option extra-binary-caches`, e.g.:
+`--substituters`, e.g.:
 
 ```console
-$ nix-env -iA nixpkgs.firefox --option extra-binary-caches http://avalon:8080/
+$ nix-env -iA nixpkgs.firefox --substituters http://avalon:8080/
 ```
 
-The option `extra-binary-caches` tells Nix to use this binary cache in
+The option `substituters` tells Nix to use this binary cache in
 addition to your default caches, such as <https://cache.nixos.org>.
 Thus, for any path in the closure of Firefox, Nix will first check if
 the path is available on the server `avalon` or another binary caches.
@@ -47,4 +47,4 @@ If not, it will fall back to building from source.
 You can also tell Nix to always use your binary cache by adding a line
 to the `nix.conf` configuration file like this:
 
-    binary-caches = http://avalon:8080/ https://cache.nixos.org/
+    substituters = http://avalon:8080/ https://cache.nixos.org/

doc/manual/src/release-notes/rl-2.13.md

@@ -0,0 +1,44 @@
+# Release 2.13 (2023-01-17)
+
+* The `repeat` and `enforce-determinism` options have been removed
+  since they had been broken under many circumstances for a long time.
+
+* You can now use [flake references] in the [old command line interface], e.g.
+
+   [flake references]: ../command-ref/new-cli/nix3-flake.md#flake-references
+   [old command line interface]: ../command-ref/main-commands.md
+
+  ```shell-session
+  # nix-build flake:nixpkgs -A hello
+  # nix-build -I nixpkgs=flake:github:NixOS/nixpkgs/nixos-22.05 \
+      '<nixpkgs>' -A hello
+  # NIX_PATH=nixpkgs=flake:nixpkgs nix-build '<nixpkgs>' -A hello
+  ```
+
+* Instead of "antiquotation", the more common term [string interpolation](../language/string-interpolation.md) is now used consistently.
+  Historical release notes were not changed.
+
+* Error traces have been reworked to provide detailed explanations and more
+  accurate error locations. A short excerpt of the trace is now shown by
+  default when an error occurs.
+
+* Allow explicitly selecting outputs in a store derivation installable, just like we can do with other sorts of installables.
+  For example,
+  ```shell-session
+  # nix-build /nix/store/gzaflydcr6sb3567hap9q6srzx8ggdgg-glibc-2.33-78.drv^dev
+  ```
+  now works just as
+  ```shell-session
+  # nix-build glibc^dev
+  ```
+  does already.
+
+* On Linux, `nix develop` now sets the
+  [*personality*](https://man7.org/linux/man-pages/man2/personality.2.html)
+  for the development shell in the same way as the actual build of the
+  derivation. This makes shells for `i686-linux` derivations work
+  correctly on `x86_64-linux`.
+
+* You can now disable the global flake registry by setting the `flake-registry`
+  configuration option to an empty string. The same can be achieved at runtime with
+  `--flake-registry ""`.

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

@@ -1,23 +1,10 @@
 # Release X.Y (202?-??-??)
 
-* The `repeat` and `enforce-determinism` options have been removed
+* A new function `builtins.readFileType` is available. It is similar to
-  since they had been broken under many circumstances for a long time.
+  `builtins.readDir` but acts on a single file or directory.
 
-* You can now use [flake references] in the [old command line interface], e.g.
+* The `builtins.readDir` function has been optimized when encountering not-yet-known
-
+  file types from POSIX's `readdir`. In such cases the type of each file is/was
-   [flake references]: ../command-ref/new-cli/nix3-flake.md#flake-references
+  discovered by making multiple syscalls. This change makes these operations
-   [old command line interface]: ../command-ref/main-commands.md
+  lazy such that these lookups will only be performed if the attribute is used.
-
+  This optimization affects a minority of filesystems and operating systems.
-  ```
-  # nix-build flake:nixpkgs -A hello
-  # nix-build -I nixpkgs=flake:github:NixOS/nixpkgs/nixos-22.05 \
-      '<nixpkgs>' -A hello
-  # NIX_PATH=nixpkgs=flake:nixpkgs nix-build '<nixpkgs>' -A hello
-  ```
-
-* Instead of "antiquotation", the more common term [string interpolation](../language/string-interpolation.md) is now used consistently.
-  Historical release notes were not changed.
-
-* Error traces have been reworked to provide detailed explanations and more
-  accurate error locations. A short excerpt of the trace is now shown by
-  default when an error occurs.

flake.nix

@@ -82,7 +82,9 @@
         });
 
         configureFlags =
-          lib.optionals stdenv.isLinux [
+          [
+            "CXXFLAGS=-I${lib.getDev rapidcheck}/extras/gtest/include"
+          ] ++ lib.optionals stdenv.isLinux [
             "--with-boost=${boost}/lib"
             "--with-sandbox-shell=${sh}/bin/busybox"
           ]
@@ -116,6 +118,7 @@
             boost
             lowdown-nix
             gtest
+            rapidcheck
           ]
           ++ lib.optionals stdenv.isLinux [libseccomp]
           ++ lib.optional (stdenv.isLinux || stdenv.isDarwin) libsodium
@@ -532,6 +535,12 @@
               mkdir $out
             '';
 
+        tests.nixpkgsLibTests =
+          nixpkgs.lib.genAttrs systems (system:
+            import (nixpkgs + "/lib/tests/release.nix")
+              { pkgs = nixpkgsFor.${system}; }
+          );
+
         metrics.nixpkgs = import "${nixpkgs-regression}/pkgs/top-level/metrics.nix" {
           pkgs = nixpkgsFor.x86_64-linux;
           nixpkgs = nixpkgs-regression;
@@ -562,6 +571,7 @@
         binaryTarball = self.hydraJobs.binaryTarball.${system};
         perlBindings = self.hydraJobs.perlBindings.${system};
         installTests = self.hydraJobs.installTests.${system};
+        nixpkgsLibTests = self.hydraJobs.tests.nixpkgsLibTests.${system};
       } // (nixpkgs.lib.optionalAttrs (builtins.elem system linux64BitSystems)) {
         dockerImage = self.hydraJobs.dockerImage.${system};
       });

scripts/install-systemd-multi-user.sh

@@ -24,12 +24,17 @@ $1
 EOF
 }
 
+escape_systemd_env() {
+    temp_var="${1//\'/\\\'}"
+    echo "${temp_var//\%/%%}"
+}
+
 # Gather all non-empty proxy environment variables into a string
 create_systemd_proxy_env() {
     vars="http_proxy https_proxy ftp_proxy no_proxy HTTP_PROXY HTTPS_PROXY FTP_PROXY NO_PROXY"
     for v in $vars; do
         if [ "x${!v:-}" != "x" ]; then
-            echo "Environment=${v}=${!v}"
+            echo "Environment=${v}=$(escape_systemd_env ${!v})"
         fi
     done
 }

src/libcmd/repl.cc

@@ -397,7 +397,7 @@ StringSet NixRepl::completePrefix(const std::string & prefix)
             Expr * e = parseString(expr);
             Value v;
             e->eval(*state, *env, v);
-            state->forceAttrs(v, noPos, "nevermind, it is ignored anyway");
+            state->forceAttrs(v, noPos, "while evaluating an attrset for the purpose of completion (this error should not be displayed; file an issue?)");
 
             for (auto & i : *v.attrs) {
                 std::string_view name = state->symbols[i.name];

src/libexpr/eval.cc

@@ -11,7 +11,9 @@
 
 #include <algorithm>
 #include <chrono>
+#include <iostream>
 #include <cstring>
+#include <optional>
 #include <unistd.h>
 #include <sys/time.h>
 #include <sys/resource.h>
@@ -1927,7 +1929,9 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
             /* skip canonization of first path, which would only be not
             canonized in the first place if it's coming from a ./${foo} type
             path */
-            auto part = state.coerceToString(i_pos, vTmp, context, false, firstType == nString, !first, "while evaluating a path segment");
+            auto part = state.coerceToString(i_pos, vTmp, context,
+                                             "while evaluating a path segment",
+                                             false, firstType == nString, !first);
             sSize += part->size();
             s.emplace_back(std::move(part));
         }
@@ -2123,15 +2127,16 @@ std::optional<std::string> EvalState::tryAttrsToString(const PosIdx pos, Value &
     if (i != v.attrs->end()) {
         Value v1;
         callFunction(*i->value, v, v1, pos);
-        return coerceToString(pos, v1, context, coerceMore, copyToStore,
+        return coerceToString(pos, v1, context,
-                "while evaluating the result of the `toString` attribute").toOwned();
+                "while evaluating the result of the `__toString` attribute",
+                coerceMore, copyToStore).toOwned();
     }
 
     return {};
 }
 
-BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet & context,
+BackedStringView EvalState::coerceToString(const PosIdx pos, Value &v, PathSet &context,
-    bool coerceMore, bool copyToStore, bool canonicalizePath, std::string_view errorCtx)
+    std::string_view errorCtx, bool coerceMore, bool copyToStore, bool canonicalizePath)
 {
     forceValue(v, pos);
 
@@ -2154,13 +2159,23 @@ BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet
         if (maybeString)
             return std::move(*maybeString);
         auto i = v.attrs->find(sOutPath);
-        if (i == v.attrs->end())
+        if (i == v.attrs->end()) {
-            error("cannot coerce a set to a string", showType(v)).withTrace(pos, errorCtx).debugThrow<TypeError>();
+            error("cannot coerce %1% to a string", showType(v))
-        return coerceToString(pos, *i->value, context, coerceMore, copyToStore, canonicalizePath, errorCtx);
+                .withTrace(pos, errorCtx)
+                .debugThrow<TypeError>();
+        }
+        return coerceToString(pos, *i->value, context, errorCtx,
+                              coerceMore, copyToStore, canonicalizePath);
     }
 
-    if (v.type() == nExternal)
+    if (v.type() == nExternal) {
-        return v.external->coerceToString(positions[pos], context, coerceMore, copyToStore, errorCtx);
+        try {
+            return v.external->coerceToString(positions[pos], context, coerceMore, copyToStore);
+        } catch (Error & e) {
+            e.addTrace(nullptr, errorCtx);
+            throw;
+        }
+    }
 
     if (coerceMore) {
         /* Note that `false' is represented as an empty string for
@@ -2175,8 +2190,9 @@ BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet
             std::string result;
             for (auto [n, v2] : enumerate(v.listItems())) {
                 try {
-                    result += *coerceToString(noPos, *v2, context, coerceMore, copyToStore, canonicalizePath,
+                    result += *coerceToString(noPos, *v2, context,
-                            "while evaluating one element of the list");
+                            "while evaluating one element of the list",
+                            coerceMore, copyToStore, canonicalizePath);
                 } catch (Error & e) {
                     e.addTrace(positions[pos], errorCtx);
                     throw;
@@ -2190,7 +2206,9 @@ BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet
         }
     }
 
-    error("cannot coerce %1% to a string", showType(v)).withTrace(pos, errorCtx).debugThrow<TypeError>();
+    error("cannot coerce %1% to a string", showType(v))
+        .withTrace(pos, errorCtx)
+        .debugThrow<TypeError>();
 }
 
 
@@ -2220,7 +2238,7 @@ StorePath EvalState::copyPathToStore(PathSet & context, const Path & path)
 
 Path EvalState::coerceToPath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx)
 {
-    auto path = coerceToString(pos, v, context, false, false, true, errorCtx).toOwned();
+    auto path = coerceToString(pos, v, context, errorCtx, false, false, true).toOwned();
     if (path == "" || path[0] != '/')
         error("string '%1%' doesn't represent an absolute path", path).withTrace(pos, errorCtx).debugThrow<EvalError>();
     return path;
@@ -2229,7 +2247,7 @@ Path EvalState::coerceToPath(const PosIdx pos, Value & v, PathSet & context, std
 
 StorePath EvalState::coerceToStorePath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx)
 {
-    auto path = coerceToString(pos, v, context, false, false, true, errorCtx).toOwned();
+    auto path = coerceToString(pos, v, context, errorCtx, false, false, true).toOwned();
     if (auto storePath = store->maybeParseStorePath(path))
         return *storePath;
     error("path '%1%' is not in the Nix store", path).withTrace(pos, errorCtx).debugThrow<EvalError>();
@@ -2433,13 +2451,11 @@ void EvalState::printStats()
 }
 
 
-std::string ExternalValueBase::coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore, std::string_view errorCtx) const
+std::string ExternalValueBase::coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore) const
 {
-    auto e = TypeError({
+    throw TypeError({
         .msg = hintfmt("cannot coerce %1% to a string", showType())
     });
-    e.addTrace(pos, errorCtx);
-    throw e;
 }
 
 

src/libexpr/eval.hh

@@ -203,6 +203,9 @@ public:
         throw std::move(error);
     }
 
+    // This is dangerous, but gets in line with the idea that error creation and
+    // throwing should not allocate on the stack of hot functions.
+    // as long as errors are immediately thrown, it works.
     ErrorBuilder * errorBuilder;
 
     template<typename... Args>
@@ -375,9 +378,9 @@ public:
        booleans and lists to a string.  If `copyToStore' is set,
        referenced paths are copied to the Nix store as a side effect. */
     BackedStringView coerceToString(const PosIdx pos, Value & v, PathSet & context,
+        std::string_view errorCtx,
         bool coerceMore = false, bool copyToStore = true,
-        bool canonicalizePath = true,
+        bool canonicalizePath = true);
-        std::string_view errorCtx = "");
 
     StorePath copyPathToStore(PathSet & context, const Path & path);
 

src/libexpr/flake/flake.cc

@@ -264,7 +264,7 @@ static Flake getFlake(
                 PathSet emptyContext = {};
                 flake.config.settings.emplace(
                     state.symbols[setting.name],
-                    state.coerceToString(setting.pos, *setting.value, emptyContext, false, true, true, "") .toOwned());
+                    state.coerceToString(setting.pos, *setting.value, emptyContext, "", false, true, true) .toOwned());
             }
             else if (setting.value->type() == nInt)
                 flake.config.settings.emplace(

src/libexpr/primops.cc

@@ -350,26 +350,22 @@ void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v)
     auto elems = args[0]->listElems();
     auto count = args[0]->listSize();
     if (count == 0)
-        state.debugThrowLastTrace(EvalError({
+        state.error("at least one argument to 'exec' required").atPos(pos).debugThrow<EvalError>();
-            .msg = hintfmt("at least one argument to 'exec' required"),
-            .errPos = state.positions[pos]
-        }));
     PathSet context;
-    auto program = state.coerceToString(pos, *elems[0], context, false, false,
+    auto program = state.coerceToString(pos, *elems[0], context,
-           "while evaluating the first element of the argument passed to builtins.exec").toOwned();
+            "while evaluating the first element of the argument passed to builtins.exec",
+            false, false).toOwned();
     Strings commandArgs;
     for (unsigned int i = 1; i < args[0]->listSize(); ++i) {
-        commandArgs.push_back(state.coerceToString(pos, *elems[i], context, false, false,
+        commandArgs.push_back(
-                    "while evaluating an element of the argument passed to builtins.exec").toOwned());
+                state.coerceToString(pos, *elems[i], context,
+                        "while evaluating an element of the argument passed to builtins.exec",
+                        false, false).toOwned());
     }
     try {
         auto _ = state.realiseContext(context); // FIXME: Handle CA derivations
     } catch (InvalidPathError & e) {
-        state.debugThrowLastTrace(EvalError({
+        state.error("cannot execute '%1%', since path '%2%' is not valid", program, e.path).atPos(pos).debugThrow<EvalError>();
-            .msg = hintfmt("cannot execute '%1%', since path '%2%' is not valid",
-                program, e.path),
-            .errPos = state.positions[pos]
-        }));
     }
 
     auto output = runProgram(program, true, commandArgs);
@@ -598,6 +594,7 @@ struct CompareValues
                     state.error("cannot compare %s with %s; values of that type are incomparable", showType(*v1), showType(*v2)).debugThrow<EvalError>();
             }
         } catch (Error & e) {
+            if (!errorCtx.empty())
                 e.addTrace(nullptr, errorCtx);
             throw;
         }
@@ -620,15 +617,7 @@ static Bindings::iterator getAttr(
 {
     Bindings::iterator value = attrSet->find(attrSym);
     if (value == attrSet->end()) {
-        throw TypeError({
+        state.error("attribute '%s' missing", state.symbols[attrSym]).withTrace(noPos, errorCtx).debugThrow<TypeError>();
-            .msg = hintfmt("attribute '%s' missing %s", state.symbols[attrSym], normaltxt(errorCtx)),
-            .errPos = state.positions[attrSet->pos],
-        });
-        // TODO XXX
-        // Adding another trace for the function name to make it clear
-        // which call received wrong arguments.
-        //e.addTrace(state.positions[pos], hintfmt("while invoking '%s'", funcName));
-        //state.debugThrowLastTrace(e);
     }
     return value;
 }
@@ -801,8 +790,10 @@ static void prim_addErrorContext(EvalState & state, const PosIdx pos, Value * *
         v = *args[1];
     } catch (Error & e) {
         PathSet context;
-        e.addTrace(nullptr, state.coerceToString(pos, *args[0], context,
+        auto message = state.coerceToString(pos, *args[0], context,
-                "while evaluating the error message passed to builtins.addErrorContext").toOwned());
+                "while evaluating the error message passed to builtins.addErrorContext",
+                false, false).toOwned();
+        e.addTrace(nullptr, message, true);
         throw;
     }
 }
@@ -1006,6 +997,7 @@ static void prim_second(EvalState & state, const PosIdx pos, Value * * args, Val
  * Derivations
  *************************************************************/
 
+static void derivationStrictInternal(EvalState & state, const std::string & name, Bindings * attrs, Value & v);
 
 /* Construct (as a unobservable side effect) a Nix derivation
    expression that performs the derivation described by the argument
@@ -1016,32 +1008,68 @@ static void prim_second(EvalState & state, const PosIdx pos, Value * * args, Val
    derivation. */
 static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    using nlohmann::json;
     state.forceAttrs(*args[0], pos, "while evaluating the argument passed to builtins.derivationStrict");
 
+    Bindings * attrs = args[0]->attrs;
+
     /* Figure out the name first (for stack backtraces). */
-    Bindings::iterator attr = getAttr(state, state.sName, args[0]->attrs, "in the attrset passed as argument to builtins.derivationStrict");
+    Bindings::iterator nameAttr = getAttr(state, state.sName, attrs, "in the attrset passed as argument to builtins.derivationStrict");
 
     std::string drvName;
-    const auto posDrvName = attr->pos;
     try {
-        drvName = state.forceStringNoCtx(*attr->value, pos, "while evaluating the `name` attribute passed to builtins.derivationStrict");
+        drvName = state.forceStringNoCtx(*nameAttr->value, pos, "while evaluating the `name` attribute passed to builtins.derivationStrict");
     } catch (Error & e) {
-        e.addTrace(state.positions[posDrvName], "while evaluating the derivation attribute 'name'");
+        e.addTrace(state.positions[nameAttr->pos], "while evaluating the derivation attribute 'name'");
         throw;
     }
 
+    try {
+        derivationStrictInternal(state, drvName, attrs, v);
+    } catch (Error & e) {
+        Pos pos = state.positions[nameAttr->pos];
+        /*
+         * Here we make two abuses of the error system
+         *
+         * 1. We print the location as a string to avoid a code snippet being
+         * printed. While the location of the name attribute is a good hint, the
+         * exact code there is irrelevant.
+         *
+         * 2. We mark this trace as a frame trace, meaning that we stop printing
+         * less important traces from now on. In particular, this prevents the
+         * display of the automatic "while calling builtins.derivationStrict"
+         * trace, which is of little use for the public we target here.
+         *
+         * Please keep in mind that error reporting is done on a best-effort
+         * basis in nix. There is no accurate location for a derivation, as it
+         * often results from the composition of several functions
+         * (derivationStrict, derivation, mkDerivation, mkPythonModule, etc.)
+         */
+        e.addTrace(nullptr, hintfmt(
+                "while evaluating derivation '%s'\n"
+                "  whose name attribute is located at %s",
+                drvName, pos), true);
+        throw;
+    }
+}
+
+static void derivationStrictInternal(EvalState & state, const std::string &
+drvName, Bindings * attrs, Value & v)
+{
     /* Check whether attributes should be passed as a JSON file. */
+    using nlohmann::json;
     std::optional<json> jsonObject;
-    attr = args[0]->attrs->find(state.sStructuredAttrs);
+    auto attr = attrs->find(state.sStructuredAttrs);
-    if (attr != args[0]->attrs->end() && state.forceBool(*attr->value, pos, "while evaluating the `__structuredAttrs` attribute passed to builtins.derivationStrict"))
+    if (attr != attrs->end() &&
+        state.forceBool(*attr->value, noPos,
+                        "while evaluating the `__structuredAttrs` "
+                        "attribute passed to builtins.derivationStrict"))
         jsonObject = json::object();
 
     /* Check whether null attributes should be ignored. */
     bool ignoreNulls = false;
-    attr = args[0]->attrs->find(state.sIgnoreNulls);
+    attr = attrs->find(state.sIgnoreNulls);
-    if (attr != args[0]->attrs->end())
+    if (attr != attrs->end())
-        ignoreNulls = state.forceBool(*attr->value, pos, "while evaluating the `__ignoreNulls` attribute passed to builtins.derivationStrict");
+        ignoreNulls = state.forceBool(*attr->value, noPos, "while evaluating the `__ignoreNulls` attribute " "passed to builtins.derivationStrict");
 
     /* Build the derivation expression by processing the attributes. */
     Derivation drv;
@@ -1058,7 +1086,7 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
     StringSet outputs;
     outputs.insert("out");
 
-    for (auto & i : args[0]->attrs->lexicographicOrder(state.symbols)) {
+    for (auto & i : attrs->lexicographicOrder(state.symbols)) {
         if (i->name == state.sIgnoreNulls) continue;
         const std::string & key = state.symbols[i->name];
         vomit("processing attribute '%1%'", key);
@@ -1069,7 +1097,7 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
             else
                 state.debugThrowLastTrace(EvalError({
                     .msg = hintfmt("invalid value '%s' for 'outputHashMode' attribute", s),
-                    .errPos = state.positions[posDrvName]
+                    .errPos = state.positions[noPos]
                 }));
         };
 
@@ -1079,7 +1107,7 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
                 if (outputs.find(j) != outputs.end())
                     state.debugThrowLastTrace(EvalError({
                         .msg = hintfmt("duplicate derivation output '%1%'", j),
-                        .errPos = state.positions[posDrvName]
+                        .errPos = state.positions[noPos]
                     }));
                 /* !!! Check whether j is a valid attribute
                    name. */
@@ -1089,34 +1117,35 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
                 if (j == "drv")
                     state.debugThrowLastTrace(EvalError({
                         .msg = hintfmt("invalid derivation output name 'drv'" ),
-                        .errPos = state.positions[posDrvName]
+                        .errPos = state.positions[noPos]
                     }));
                 outputs.insert(j);
             }
             if (outputs.empty())
                 state.debugThrowLastTrace(EvalError({
                     .msg = hintfmt("derivation cannot have an empty set of outputs"),
-                    .errPos = state.positions[posDrvName]
+                    .errPos = state.positions[noPos]
                 }));
         };
 
         try {
+            // This try-catch block adds context for most errors.
+            // Use this empty error context to signify that we defer to it.
+            const std::string_view context_below("");
 
             if (ignoreNulls) {
-                state.forceValue(*i->value, pos);
+                state.forceValue(*i->value, noPos);
                 if (i->value->type() == nNull) continue;
             }
 
             if (i->name == state.sContentAddressed) {
-                contentAddressed = state.forceBool(*i->value, pos,
+                contentAddressed = state.forceBool(*i->value, noPos, context_below);
-                        "while evaluating the `__contentAddressed` attribute passed to builtins.derivationStrict");
                 if (contentAddressed)
                     settings.requireExperimentalFeature(Xp::CaDerivations);
             }
 
             else if (i->name == state.sImpure) {
-                isImpure = state.forceBool(*i->value, pos,
+                isImpure = state.forceBool(*i->value, noPos, context_below);
-                        "while evaluating the 'impure' attribute passed to builtins.derivationStrict");
                 if (isImpure)
                     settings.requireExperimentalFeature(Xp::ImpureDerivations);
             }
@@ -1124,11 +1153,11 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
             /* The `args' attribute is special: it supplies the
                command-line arguments to the builder. */
             else if (i->name == state.sArgs) {
-                state.forceList(*i->value, pos,
+                state.forceList(*i->value, noPos, context_below);
-                        "while evaluating the `args` attribute passed to builtins.derivationStrict");
                 for (auto elem : i->value->listItems()) {
-                    auto s = state.coerceToString(posDrvName, *elem, context, true,
+                    auto s = state.coerceToString(noPos, *elem, context,
-                            "while evaluating an element of the `args` argument passed to builtins.derivationStrict").toOwned();
+                            "while evaluating an element of the argument list",
+                            true).toOwned();
                     drv.args.push_back(s);
                 }
             }
@@ -1141,29 +1170,29 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
 
                     if (i->name == state.sStructuredAttrs) continue;
 
-                    (*jsonObject)[key] = printValueAsJSON(state, true, *i->value, pos, context);
+                    (*jsonObject)[key] = printValueAsJSON(state, true, *i->value, noPos, context);
 
                     if (i->name == state.sBuilder)
-                        drv.builder = state.forceString(*i->value, context, posDrvName, "while evaluating the `builder` attribute passed to builtins.derivationStrict");
+                        drv.builder = state.forceString(*i->value, context, noPos, context_below);
                     else if (i->name == state.sSystem)
-                        drv.platform = state.forceStringNoCtx(*i->value, posDrvName, "while evaluating the `system` attribute passed to builtins.derivationStrict");
+                        drv.platform = state.forceStringNoCtx(*i->value, noPos, context_below);
                     else if (i->name == state.sOutputHash)
-                        outputHash = state.forceStringNoCtx(*i->value, posDrvName, "while evaluating the `outputHash` attribute passed to builtins.derivationStrict");
+                        outputHash = state.forceStringNoCtx(*i->value, noPos, context_below);
                     else if (i->name == state.sOutputHashAlgo)
-                        outputHashAlgo = state.forceStringNoCtx(*i->value, posDrvName, "while evaluating the `outputHashAlgo` attribute passed to builtins.derivationStrict");
+                        outputHashAlgo = state.forceStringNoCtx(*i->value, noPos, context_below);
                     else if (i->name == state.sOutputHashMode)
-                        handleHashMode(state.forceStringNoCtx(*i->value, posDrvName, "while evaluating the `outputHashMode` attribute passed to builtins.derivationStrict"));
+                        handleHashMode(state.forceStringNoCtx(*i->value, noPos, context_below));
                     else if (i->name == state.sOutputs) {
                         /* Require ‘outputs’ to be a list of strings. */
-                        state.forceList(*i->value, posDrvName, "while evaluating the `outputs` attribute passed to builtins.derivationStrict");
+                        state.forceList(*i->value, noPos, context_below);
                         Strings ss;
                         for (auto elem : i->value->listItems())
-                            ss.emplace_back(state.forceStringNoCtx(*elem, posDrvName, "while evaluating an element of the `outputs` attribute passed to builtins.derivationStrict"));
+                            ss.emplace_back(state.forceStringNoCtx(*elem, noPos, context_below));
                         handleOutputs(ss);
                     }
 
                 } else {
-                    auto s = state.coerceToString(i->pos, *i->value, context, true, "while evaluating an attribute passed to builtins.derivationStrict").toOwned();
+                    auto s = state.coerceToString(noPos, *i->value, context, context_below, true).toOwned();
                     drv.env.emplace(key, s);
                     if (i->name == state.sBuilder) drv.builder = std::move(s);
                     else if (i->name == state.sSystem) drv.platform = std::move(s);
@@ -1177,8 +1206,8 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
             }
 
         } catch (Error & e) {
-            e.addTrace(nullptr,
+            e.addTrace(state.positions[i->pos],
-                hintfmt("while evaluating the attribute '%1%' of the derivation '%2%'", key, drvName),
+                hintfmt("while evaluating attribute '%1%' of derivation '%2%'", key, drvName),
                 true);
             throw;
         }
@@ -1223,20 +1252,20 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
     if (drv.builder == "")
         state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("required attribute 'builder' missing"),
-            .errPos = state.positions[posDrvName]
+            .errPos = state.positions[noPos]
         }));
 
     if (drv.platform == "")
         state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("required attribute 'system' missing"),
-            .errPos = state.positions[posDrvName]
+            .errPos = state.positions[noPos]
         }));
 
     /* Check whether the derivation name is valid. */
     if (isDerivation(drvName))
         state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("derivation names are not allowed to end in '%s'", drvExtension),
-            .errPos = state.positions[posDrvName]
+            .errPos = state.positions[noPos]
         }));
 
     if (outputHash) {
@@ -1247,7 +1276,7 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
         if (outputs.size() != 1 || *(outputs.begin()) != "out")
             state.debugThrowLastTrace(Error({
                 .msg = hintfmt("multiple outputs are not supported in fixed-output derivations"),
-                .errPos = state.positions[posDrvName]
+                .errPos = state.positions[noPos]
             }));
 
         auto h = newHashAllowEmpty(*outputHash, parseHashTypeOpt(outputHashAlgo));
@@ -1274,7 +1303,7 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
         if (contentAddressed && isImpure)
             throw EvalError({
                 .msg = hintfmt("derivation cannot be both content-addressed and impure"),
-                .errPos = state.positions[posDrvName]
+                .errPos = state.positions[noPos]
             });
 
         auto ht = parseHashTypeOpt(outputHashAlgo).value_or(htSHA256);
@@ -1318,7 +1347,7 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
                 if (!h)
                     throw AssertionError({
                         .msg = hintfmt("derivation produced no hash for output '%s'", i),
-                        .errPos = state.positions[posDrvName],
+                        .errPos = state.positions[noPos],
                     });
                 auto outPath = state.store->makeOutputPath(i, *h, drvName);
                 drv.env[i] = state.store->printStorePath(outPath);
@@ -1351,11 +1380,12 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
         drvHashes.lock()->insert_or_assign(drvPath, h);
     }
 
-    auto attrs = state.buildBindings(1 + drv.outputs.size());
+    auto result = state.buildBindings(1 + drv.outputs.size());
-    attrs.alloc(state.sDrvPath).mkString(drvPathS, {"=" + drvPathS});
+    result.alloc(state.sDrvPath).mkString(drvPathS, {"=" + drvPathS});
     for (auto & i : drv.outputs)
-        mkOutputString(state, attrs, drvPath, drv, i);
+        mkOutputString(state, result, drvPath, drv, i);
-    v.mkAttrs(attrs);
+
+    v.mkAttrs(result);
 }
 
 static RegisterPrimOp primop_derivationStrict(RegisterPrimOp::Info {
@@ -1498,7 +1528,9 @@ static RegisterPrimOp primop_pathExists({
 static void prim_baseNameOf(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    v.mkString(baseNameOf(*state.coerceToString(pos, *args[0], context, false, false, "while evaluating the first argument passed to builtins.baseNameOf")), context);
+    v.mkString(baseNameOf(*state.coerceToString(pos, *args[0], context,
+            "while evaluating the first argument passed to builtins.baseNameOf",
+            false, false)), context);
 }
 
 static RegisterPrimOp primop_baseNameOf({
@@ -1518,7 +1550,9 @@ static RegisterPrimOp primop_baseNameOf({
 static void prim_dirOf(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    auto path = state.coerceToString(pos, *args[0], context, false, false, "while evaluating the first argument passed to builtins.dirOf");
+    auto path = state.coerceToString(pos, *args[0], context,
+            "while evaluating the first argument passed to builtins.dirOf",
+            false, false);
     auto dir = dirOf(*path);
     if (args[0]->type() == nPath) v.mkPath(dir); else v.mkString(dir, context);
 }
@@ -1584,8 +1618,9 @@ static void prim_findFile(EvalState & state, const PosIdx pos, Value * * args, V
         i = getAttr(state, state.sPath, v2->attrs, "in an element of the __nixPath");
 
         PathSet context;
-        auto path = state.coerceToString(pos, *i->value, context, false, false,
+        auto path = state.coerceToString(pos, *i->value, context,
-                "while evaluating the `path` attribute of an element of the list passed to builtins.findFile").toOwned();
+                "while evaluating the `path` attribute of an element of the list passed to builtins.findFile",
+                false, false).toOwned();
 
         try {
             auto rewrites = state.realiseContext(context);
@@ -1638,23 +1673,73 @@ static RegisterPrimOp primop_hashFile({
     .fun = prim_hashFile,
 });
 
+
+/* Stringize a directory entry enum. Used by `readFileType' and `readDir'. */
+static const char * dirEntTypeToString(unsigned char dtType)
+{
+    /* Enum DT_(DIR|LNK|REG|UNKNOWN) */
+    switch(dtType) {
+        case DT_REG: return "regular";   break;
+        case DT_DIR: return "directory"; break;
+        case DT_LNK: return "symlink";   break;
+        default:     return "unknown";   break;
+    }
+    return "unknown";  /* Unreachable */
+}
+
+
+static void prim_readFileType(EvalState & state, const PosIdx pos, Value * * args, Value & v)
+{
+    auto path = realisePath(state, pos, *args[0]);
+    /* Retrieve the directory entry type and stringize it. */
+    v.mkString(dirEntTypeToString(getFileType(path)));
+}
+
+static RegisterPrimOp primop_readFileType({
+    .name = "__readFileType",
+    .args = {"p"},
+    .doc = R"(
+      Determine the directory entry type of a filesystem node, being
+      one of "directory", "regular", "symlink", or "unknown".
+    )",
+    .fun = prim_readFileType,
+});
+
 /* Read a directory (without . or ..) */
 static void prim_readDir(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     auto path = realisePath(state, pos, *args[0]);
 
+    // Retrieve directory entries for all nodes in a directory.
+    // This is similar to `getFileType` but is optimized to reduce system calls
+    // on many systems.
     DirEntries entries = readDirectory(path);
 
     auto attrs = state.buildBindings(entries.size());
 
+    // If we hit unknown directory entry types we may need to fallback to
+    // using `getFileType` on some systems.
+    // In order to reduce system calls we make each lookup lazy by using
+    // `builtins.readFileType` application.
+    Value * readFileType = nullptr;
+
     for (auto & ent : entries) {
-        if (ent.type == DT_UNKNOWN)
+        auto & attr = attrs.alloc(ent.name);
-            ent.type = getFileType(path + "/" + ent.name);
+        if (ent.type == DT_UNKNOWN) {
-        attrs.alloc(ent.name).mkString(
+            // Some filesystems or operating systems may not be able to return
-            ent.type == DT_REG ? "regular" :
+            // detailed node info quickly in this case we produce a thunk to
-            ent.type == DT_DIR ? "directory" :
+            // query the file type lazily.
-            ent.type == DT_LNK ? "symlink" :
+            auto epath = state.allocValue();
-            "unknown");
+            Path path2 = path + "/" + ent.name;
+            epath->mkString(path2);
+            if (!readFileType)
+                readFileType = &state.getBuiltin("readFileType");
+            attr.mkApp(readFileType, epath);
+        } else {
+            // This branch of the conditional is much more likely.
+            // Here we just stringize the directory entry type.
+            attr.mkString(dirEntTypeToString(ent.type));
+        }
     }
 
     v.mkAttrs(attrs);
@@ -2634,14 +2719,9 @@ static void prim_zipAttrsWith(EvalState & state, const PosIdx pos, Value * * arg
 
     for (unsigned int n = 0; n < listSize; ++n) {
         Value * vElem = listElems[n];
-        try {
         state.forceAttrs(*vElem, noPos, "while evaluating a value of the list passed as second argument to builtins.zipAttrsWith");
         for (auto & attr : *vElem->attrs)
             attrsSeen[attr.name].first++;
-        } catch (TypeError & e) {
-            e.addTrace(state.positions[pos], hintfmt("while invoking '%s'", "zipAttrsWith"));
-            state.debugThrowLastTrace(e);
-        }
     }
 
     auto attrs = state.buildBindings(attrsSeen.size());
@@ -3025,13 +3105,13 @@ static void prim_genList(EvalState & state, const PosIdx pos, Value * * args, Va
     auto len = state.forceInt(*args[1], pos, "while evaluating the second argument passed to builtins.genList");
 
     if (len < 0)
-        state.debugThrowLastTrace(EvalError({
+        state.error("cannot create list of size %1%", len).debugThrow<EvalError>();
-            .msg = hintfmt("cannot create list of size %1%", len),
+
-            .errPos = state.positions[pos]
+    // More strict than striclty (!) necessary, but acceptable
-        }));
+    // as evaluating map without accessing any values makes little sense.
+    state.forceFunction(*args[0], noPos, "while evaluating the first argument passed to builtins.genList");
 
     state.mkList(v, len);
-
     for (unsigned int n = 0; n < (unsigned int) len; ++n) {
         auto arg = state.allocValue();
         arg->mkInt(n);
@@ -3079,6 +3159,8 @@ static void prim_sort(EvalState & state, const PosIdx pos, Value * * args, Value
     auto comparator = [&](Value * a, Value * b) {
         /* Optimization: if the comparator is lessThan, bypass
            callFunction. */
+        /* TODO: (layus) this is absurd. An optimisation like this
+           should be outside the lambda creation */
         if (args[0]->isPrimOp() && args[0]->primOp->fun == prim_lessThan)
             return CompareValues(state, noPos, "while evaluating the ordering function passed to builtins.sort")(a, b);
 
@@ -3239,12 +3321,7 @@ static void prim_concatMap(EvalState & state, const PosIdx pos, Value * * args,
     for (unsigned int n = 0; n < nrLists; ++n) {
         Value * vElem = args[1]->listElems()[n];
         state.callFunction(*args[0], *vElem, lists[n], pos);
-        try {
         state.forceList(lists[n], lists[n].determinePos(args[0]->determinePos(pos)), "while evaluating the return value of the function passed to buitlins.concatMap");
-        } catch (TypeError &e) {
-            e.addTrace(state.positions[pos], hintfmt("while invoking '%s'", "concatMap"));
-            state.debugThrowLastTrace(e);
-        }
         len += lists[n].listSize();
     }
 
@@ -3424,7 +3501,7 @@ static void prim_lessThan(EvalState & state, const PosIdx pos, Value * * args, V
     state.forceValue(*args[0], pos);
     state.forceValue(*args[1], pos);
     // pos is exact here, no need for a message.
-    CompareValues comp(state, pos, "");
+    CompareValues comp(state, noPos, "");
     v.mkBool(comp(args[0], args[1]));
 }
 
@@ -3451,7 +3528,9 @@ static RegisterPrimOp primop_lessThan({
 static void prim_toString(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    auto s = state.coerceToString(pos, *args[0], context, true, false, "while evaluating the first argument passed to builtins.toString");
+    auto s = state.coerceToString(pos, *args[0], context,
+            "while evaluating the first argument passed to builtins.toString",
+            true, false);
     v.mkString(*s, context);
 }
 
@@ -3803,21 +3882,18 @@ static void prim_replaceStrings(EvalState & state, const PosIdx pos, Value * * a
     state.forceList(*args[0], pos, "while evaluating the first argument passed to builtins.replaceStrings");
     state.forceList(*args[1], pos, "while evaluating the second argument passed to builtins.replaceStrings");
     if (args[0]->listSize() != args[1]->listSize())
-        state.debugThrowLastTrace(EvalError({
+        state.error("'from' and 'to' arguments passed to builtins.replaceStrings have different lengths").atPos(pos).debugThrow<EvalError>();
-            .msg = hintfmt("'from' and 'to' arguments to 'replaceStrings' have different lengths"),
-            .errPos = state.positions[pos]
-        }));
 
     std::vector<std::string> from;
     from.reserve(args[0]->listSize());
     for (auto elem : args[0]->listItems())
-        from.emplace_back(state.forceString(*elem, pos, "while evaluating one of the strings to replace in builtins.replaceStrings"));
+        from.emplace_back(state.forceString(*elem, pos, "while evaluating one of the strings to replace passed to builtins.replaceStrings"));
 
     std::vector<std::pair<std::string, PathSet>> to;
     to.reserve(args[1]->listSize());
     for (auto elem : args[1]->listItems()) {
         PathSet ctx;
-        auto s = state.forceString(*elem, ctx, pos, "while evaluating one of the replacement strings of builtins.replaceStrings");
+        auto s = state.forceString(*elem, ctx, pos, "while evaluating one of the replacement strings passed to builtins.replaceStrings");
         to.emplace_back(s, std::move(ctx));
     }
 

src/libexpr/primops/context.cc

@@ -83,15 +83,13 @@ static void prim_getContext(EvalState & state, const PosIdx pos, Value * * args,
     state.forceString(*args[0], context, pos, "while evaluating the argument passed to builtins.getContext");
     auto contextInfos = std::map<StorePath, ContextInfo>();
     for (const auto & p : context) {
-        Path drv;
-        std::string output;
         NixStringContextElem ctx = NixStringContextElem::parse(*state.store, p);
         std::visit(overloaded {
             [&](NixStringContextElem::DrvDeep & d) {
                 contextInfos[d.drvPath].allOutputs = true;
             },
             [&](NixStringContextElem::Built & b) {
-                contextInfos[b.drvPath].outputs.emplace_back(std::move(output));
+                contextInfos[b.drvPath].outputs.emplace_back(std::move(b.output));
             },
             [&](NixStringContextElem::Opaque & o) {
                 contextInfos[o.path].path = true;

src/libexpr/primops/fetchMercurial.cc

@@ -22,7 +22,9 @@ static void prim_fetchMercurial(EvalState & state, const PosIdx pos, Value * * a
         for (auto & attr : *args[0]->attrs) {
             std::string_view n(state.symbols[attr.name]);
             if (n == "url")
-                url = state.coerceToString(attr.pos, *attr.value, context, false, false, "while evaluating the `url` attribute passed to builtins.fetchMercurial").toOwned();
+                url = state.coerceToString(attr.pos, *attr.value, context,
+                        "while evaluating the `url` attribute passed to builtins.fetchMercurial",
+                        false, false).toOwned();
             else if (n == "rev") {
                 // Ugly: unlike fetchGit, here the "rev" attribute can
                 // be both a revision or a branch/tag name.
@@ -48,7 +50,9 @@ static void prim_fetchMercurial(EvalState & state, const PosIdx pos, Value * * a
             });
 
     } else
-        url = state.coerceToString(pos, *args[0], context, false, false, "while evaluating the first argument passed to builtins.fetchMercurial").toOwned();
+        url = state.coerceToString(pos, *args[0], context,
+                "while evaluating the first argument passed to builtins.fetchMercurial",
+                false, false).toOwned();
 
     // FIXME: git externals probably can be used to bypass the URI
     // whitelist. Ah well.

src/libexpr/primops/fetchTree.cc

@@ -125,7 +125,7 @@ static void fetchTree(
             if (attr.name == state.sType) continue;
             state.forceValue(*attr.value, attr.pos);
             if (attr.value->type() == nPath || attr.value->type() == nString) {
-                auto s = state.coerceToString(attr.pos, *attr.value, context, false, false, "").toOwned();
+                auto s = state.coerceToString(attr.pos, *attr.value, context, "", false, false).toOwned();
                 attrs.emplace(state.symbols[attr.name],
                     state.symbols[attr.name] == "url"
                     ? type == "git"
@@ -151,7 +151,9 @@ static void fetchTree(
 
         input = fetchers::Input::fromAttrs(std::move(attrs));
     } else {
-        auto url = state.coerceToString(pos, *args[0], context, false, false, "while evaluating the first argument passed to the fetcher").toOwned();
+        auto url = state.coerceToString(pos, *args[0], context,
+                "while evaluating the first argument passed to the fetcher",
+                false, false).toOwned();
 
         if (type == "git") {
             fetchers::Attrs attrs;
@@ -218,6 +220,9 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
     } else
         url = state.forceStringNoCtx(*args[0], pos, "while evaluating the url we should fetch");
 
+    if (who == "fetchTarball")
+        url = evalSettings.resolvePseudoUrl(*url);
+
     state.checkURI(*url);
 
     if (name == "")

src/libexpr/value.hh

@@ -89,7 +89,7 @@ class ExternalValueBase
     /* Coerce the value to a string. Defaults to uncoercable, i.e. throws an
      * error.
      */
-    virtual std::string coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore, std::string_view errorCtx) const;
+    virtual std::string coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore) const;
 
     /* Compare to another value of the same type. Defaults to uncomparable,
      * i.e. always false.

src/libstore/derivations.hh

@@ -13,6 +13,7 @@
 
 namespace nix {
 
+class Store;
 
 /* Abstract syntax of derivations. */
 

src/libstore/misc.cc

@@ -319,7 +319,7 @@ OutputPathMap resolveDerivedPath(Store & store, const DerivedPath::Built & bfd,
         [&](const OutputsSpec::Names & names) {
             return static_cast<std::set<std::string>>(names);
         },
-    }, bfd.outputs);
+    }, bfd.outputs.raw());
     for (auto & output : outputNames) {
         auto outputHash = get(outputHashes, output);
         if (!outputHash)

src/libstore/outputs-spec.cc

@@ -1,8 +1,10 @@
-#include "util.hh"
-#include "outputs-spec.hh"
-#include "nlohmann/json.hpp"
-
 #include <regex>
+#include <nlohmann/json.hpp>
+
+#include "util.hh"
+#include "regex-combinators.hh"
+#include "outputs-spec.hh"
+#include "path-regex.hh"
 
 namespace nix {
 
@@ -18,10 +20,14 @@ bool OutputsSpec::contains(const std::string & outputName) const
     }, raw());
 }
 
+static std::string outputSpecRegexStr =
+    regex::either(
+        regex::group(R"(\*)"),
+        regex::group(regex::list(nameRegexStr)));
 
 std::optional<OutputsSpec> OutputsSpec::parseOpt(std::string_view s)
 {
-    static std::regex regex(R"((\*)|([a-z]+(,[a-z]+)*))");
+    static std::regex regex(std::string { outputSpecRegexStr });
 
     std::smatch match;
     std::string s2 { s }; // until some improves std::regex
@@ -42,7 +48,7 @@ OutputsSpec OutputsSpec::parse(std::string_view s)
 {
     std::optional spec = parseOpt(s);
     if (!spec)
-        throw Error("Invalid outputs specifier: '%s'", s);
+        throw Error("invalid outputs specifier '%s'", s);
     return *spec;
 }
 
@@ -65,7 +71,7 @@ std::pair<std::string_view, ExtendedOutputsSpec> ExtendedOutputsSpec::parse(std:
 {
     std::optional spec = parseOpt(s);
     if (!spec)
-        throw Error("Invalid extended outputs specifier: '%s'", s);
+        throw Error("invalid extended outputs specifier '%s'", s);
     return *spec;
 }
 
@@ -163,7 +169,7 @@ void adl_serializer<OutputsSpec>::to_json(json & json, OutputsSpec t) {
         [&](const OutputsSpec::Names & names) {
             json = names;
         },
-    }, t);
+    }, t.raw());
 }
 
 
@@ -183,7 +189,7 @@ void adl_serializer<ExtendedOutputsSpec>::to_json(json & json, ExtendedOutputsSp
         [&](const ExtendedOutputsSpec::Explicit & e) {
             adl_serializer<OutputsSpec>::to_json(json, e);
         },
-    }, t);
+    }, t.raw());
 }
 
 }

src/libstore/path-regex.hh

@@ -0,0 +1,7 @@
+#pragma once
+
+namespace nix {
+
+static constexpr std::string_view nameRegexStr = R"([0-9a-zA-Z\+\-\._\?=]+)";
+
+}

src/libstore/path.cc

@@ -8,8 +8,10 @@ static void checkName(std::string_view path, std::string_view name)
 {
     if (name.empty())
         throw BadStorePath("store path '%s' has an empty name", path);
-    if (name.size() > 211)
+    if (name.size() > StorePath::MaxPathLen)
-        throw BadStorePath("store path '%s' has a name longer than 211 characters", path);
+        throw BadStorePath("store path '%s' has a name longer than '%d characters",
+            StorePath::MaxPathLen, path);
+    // See nameRegexStr for the definition
     for (auto c : name)
         if (!((c >= '0' && c <= '9')
                 || (c >= 'a' && c <= 'z')

src/libstore/path.hh

@@ -6,7 +6,6 @@
 
 namespace nix {
 
-class Store;
 struct Hash;
 
 class StorePath
@@ -18,6 +17,8 @@ public:
     /* Size of the hash part of store paths, in base-32 characters. */
     constexpr static size_t HashLen = 32; // i.e. 160 bits
 
+    constexpr static size_t MaxPathLen = 211;
+
     StorePath() = delete;
 
     StorePath(std::string_view baseName);

src/libstore/realisation.hh

@@ -10,6 +10,8 @@
 
 namespace nix {
 
+class Store;
+
 struct DrvOutput {
     // The hash modulo of the derivation
     Hash drvHash;

src/libstore/tests/libstoretests.hh

@@ -0,0 +1,23 @@
+#include <gtest/gtest.h>
+#include <gmock/gmock.h>
+
+#include "store-api.hh"
+
+namespace nix {
+
+class LibStoreTest : public ::testing::Test {
+    public:
+        static void SetUpTestSuite() {
+            initLibStore();
+        }
+
+    protected:
+        LibStoreTest()
+            : store(openStore("dummy://"))
+        { }
+
+        ref<Store> store;
+};
+
+
+} /* namespace nix */

src/libstore/tests/local.mk

@@ -12,4 +12,4 @@ libstore-tests_CXXFLAGS += -I src/libstore -I src/libutil
 
 libstore-tests_LIBS = libstore libutil
 
-libstore-tests_LDFLAGS := $(GTEST_LIBS)
+libstore-tests_LDFLAGS := -lrapidcheck $(GTEST_LIBS)

src/libstore/tests/outputs-spec.cc

@@ -40,6 +40,20 @@ TEST(OutputsSpec, names_out) {
     ASSERT_EQ(expected.to_string(), str);
 }
 
+TEST(OutputsSpec, names_underscore) {
+    std::string_view str = "a_b";
+    OutputsSpec expected = OutputsSpec::Names { "a_b" };
+    ASSERT_EQ(OutputsSpec::parse(str), expected);
+    ASSERT_EQ(expected.to_string(), str);
+}
+
+TEST(OutputsSpec, names_numberic) {
+    std::string_view str = "01";
+    OutputsSpec expected = OutputsSpec::Names { "01" };
+    ASSERT_EQ(OutputsSpec::parse(str), expected);
+    ASSERT_EQ(expected.to_string(), str);
+}
+
 TEST(OutputsSpec, names_out_bin) {
     OutputsSpec expected = OutputsSpec::Names { "out", "bin" };
     ASSERT_EQ(OutputsSpec::parse("out,bin"), expected);

src/libstore/tests/path.cc

@@ -0,0 +1,144 @@
+#include <regex>
+
+#include <nlohmann/json.hpp>
+#include <gtest/gtest.h>
+#include <rapidcheck/gtest.h>
+
+#include "path-regex.hh"
+#include "store-api.hh"
+
+#include "libstoretests.hh"
+
+namespace nix {
+
+#define STORE_DIR "/nix/store/"
+#define HASH_PART "g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q"
+
+class StorePathTest : public LibStoreTest
+{
+};
+
+static std::regex nameRegex { std::string { nameRegexStr } };
+
+#define TEST_DONT_PARSE(NAME, STR)                           \
+    TEST_F(StorePathTest, bad_ ## NAME) {                    \
+        std::string_view str =                               \
+            STORE_DIR HASH_PART "-" STR;                     \
+        ASSERT_THROW(                                        \
+            store->parseStorePath(str),                      \
+            BadStorePath);                                   \
+        std::string name { STR };                            \
+        EXPECT_FALSE(std::regex_match(name, nameRegex));     \
+    }
+
+TEST_DONT_PARSE(empty, "")
+TEST_DONT_PARSE(garbage, "&*()")
+TEST_DONT_PARSE(double_star, "**")
+TEST_DONT_PARSE(star_first, "*,foo")
+TEST_DONT_PARSE(star_second, "foo,*")
+TEST_DONT_PARSE(bang, "foo!o")
+
+#undef TEST_DONT_PARSE
+
+#define TEST_DO_PARSE(NAME, STR)                             \
+    TEST_F(StorePathTest, good_ ## NAME) {                   \
+        std::string_view str =                               \
+            STORE_DIR HASH_PART "-" STR;                     \
+        auto p = store->parseStorePath(str);                 \
+        std::string name { p.name() };                       \
+        EXPECT_TRUE(std::regex_match(name, nameRegex));      \
+    }
+
+// 0-9 a-z A-Z + - . _ ? =
+
+TEST_DO_PARSE(numbers, "02345")
+TEST_DO_PARSE(lower_case, "foo")
+TEST_DO_PARSE(upper_case, "FOO")
+TEST_DO_PARSE(plus, "foo+bar")
+TEST_DO_PARSE(dash, "foo-dev")
+TEST_DO_PARSE(underscore, "foo_bar")
+TEST_DO_PARSE(period, "foo.txt")
+TEST_DO_PARSE(question_mark, "foo?why")
+TEST_DO_PARSE(equals_sign, "foo=foo")
+
+#undef TEST_DO_PARSE
+
+// For rapidcheck
+void showValue(const StorePath & p, std::ostream & os) {
+    os << p.to_string();
+}
+
+}
+
+namespace rc {
+using namespace nix;
+
+template<>
+struct Arbitrary<StorePath> {
+    static Gen<StorePath> arbitrary();
+};
+
+Gen<StorePath> Arbitrary<StorePath>::arbitrary()
+{
+    auto len = *gen::inRange<size_t>(1, StorePath::MaxPathLen);
+
+    std::string pre { HASH_PART "-" };
+    pre.reserve(pre.size() + len);
+
+    for (size_t c = 0; c < len; ++c) {
+        switch (auto i = *gen::inRange<uint8_t>(0, 10 + 2 * 26 + 6)) {
+            case 0 ... 9:
+                pre += '0' + i;
+            case 10 ... 35:
+                pre += 'A' + (i - 10);
+                break;
+            case 36 ... 61:
+                pre += 'a' + (i - 36);
+                break;
+            case 62:
+                pre += '+';
+                break;
+            case 63:
+                pre += '-';
+                break;
+            case 64:
+                pre += '.';
+                break;
+            case 65:
+                pre += '_';
+                break;
+            case 66:
+                pre += '?';
+                break;
+            case 67:
+                pre += '=';
+                break;
+            default:
+                assert(false);
+        }
+    }
+
+    return gen::just(StorePath { pre });
+}
+
+} // namespace rc
+
+namespace nix {
+
+RC_GTEST_FIXTURE_PROP(
+    StorePathTest,
+    prop_regex_accept,
+    (const StorePath & p))
+{
+    RC_ASSERT(std::regex_match(std::string { p.name() }, nameRegex));
+}
+
+RC_GTEST_FIXTURE_PROP(
+    StorePathTest,
+    prop_round_rip,
+    (const StorePath & p))
+{
+    RC_ASSERT(p == store->parseStorePath(store->printStorePath(p)));
+}
+
+}

src/libutil/monitor-fd.hh

@@ -25,11 +25,17 @@ public:
                 /* Wait indefinitely until a POLLHUP occurs. */
                 struct pollfd fds[1];
                 fds[0].fd = fd;
-              /* This shouldn't be necessary, but macOS doesn't seem to
+                /* Polling for no specific events (i.e. just waiting
-                 like a zeroed out events field.
+                   for an error/hangup) doesn't work on macOS
-                 See rdar://37537852.
+                   anymore. So wait for read events and ignore
-              */
+                   them. */
-              fds[0].events = POLLHUP;
+                fds[0].events =
+                    #ifdef __APPLE__
+                    POLLRDNORM
+                    #else
+                    0
+                    #endif
+                    ;
                 auto count = poll(fds, 1, -1);
                 if (count == -1) abort(); // can't happen
                 /* This shouldn't happen, but can on macOS due to a bug.
@@ -40,10 +46,15 @@ public:
                    too harmful.
                 */
                 if (count == 0) continue;
-              assert(fds[0].revents & POLLHUP);
+                if (fds[0].revents & POLLHUP) {
                     triggerInterrupt();
                     break;
                 }
+                /* This will only happen on macOS. We sleep a bit to
+                   avoid waking up too often if the client is sending
+                   input. */
+                sleep(1);
+            }
         });
     };
 

src/libutil/regex-combinators.hh

@@ -0,0 +1,30 @@
+#pragma once
+
+#include <string_view>
+
+namespace nix::regex {
+
+// TODO use constexpr string building like
+// https://github.com/akrzemi1/static_string/blob/master/include/ak_toolkit/static_string.hpp
+
+static inline std::string either(std::string_view a, std::string_view b)
+{
+    return std::string { a } + "|" + b;
+}
+
+static inline std::string group(std::string_view a)
+{
+    return std::string { "(" } + a + ")";
+}
+
+static inline std::string many(std::string_view a)
+{
+    return std::string { "(?:" } + a + ")*";
+}
+
+static inline std::string list(std::string_view a)
+{
+    return std::string { a } + many(group("," + a));
+}
+
+}

src/nix/flake.cc

@@ -655,6 +655,19 @@ struct CmdFlakeCheck : FlakeCommand
                             }
                         }
 
+                        else if (
+                            name == "lib"
+                            || name == "darwinConfigurations"
+                            || name == "darwinModules"
+                            || name == "flakeModule"
+                            || name == "flakeModules"
+                            || name == "herculesCI"
+                            || name == "homeConfigurations"
+                            || name == "nixopsConfigurations"
+                            )
+                            // Known but unchecked community attribute
+                            ;
+
                         else
                             warn("unknown flake output '%s'", name);
 

tests/build.sh

@@ -42,20 +42,21 @@ nix build -f multiple-outputs.nix --json 'a^*' --no-link | jq --exit-status '
 nix build -f multiple-outputs.nix --json e --no-link | jq --exit-status '
   (.[0] |
     (.drvPath | match(".*multiple-outputs-e.drv")) and
-    (.outputs | keys == ["a", "b"]))
+    (.outputs | keys == ["a_a", "b"]))
 '
 
 # But not when it's overriden.
-nix build -f multiple-outputs.nix --json e^a --no-link | jq --exit-status '
+nix build -f multiple-outputs.nix --json e^a_a --no-link
+nix build -f multiple-outputs.nix --json e^a_a --no-link | jq --exit-status '
   (.[0] |
     (.drvPath | match(".*multiple-outputs-e.drv")) and
-    (.outputs | keys == ["a"]))
+    (.outputs | keys == ["a_a"]))
 '
 
 nix build -f multiple-outputs.nix --json 'e^*' --no-link | jq --exit-status '
   (.[0] |
     (.drvPath | match(".*multiple-outputs-e.drv")) and
-    (.outputs | keys == ["a", "b", "c"]))
+    (.outputs | keys == ["a_a", "b", "c"]))
 '
 
 # Test building from raw store path to drv not expression.
@@ -104,7 +105,7 @@ nix build "$drv^*" --no-link --json | jq --exit-status '
 nix build --impure -f multiple-outputs.nix --json e --no-link | jq --exit-status '
   (.[0] |
     (.drvPath | match(".*multiple-outputs-e.drv")) and
-    (.outputs | keys == ["a", "b"]))
+    (.outputs | keys == ["a_a", "b"]))
 '
 
 testNormalization () {

tests/lang/eval-okay-context-introspection.exp

@@ -1 +1 @@
-true
+[ true true true true true true ]

tests/lang/eval-okay-context-introspection.nix

@@ -18,7 +18,24 @@ let
     };
   };
 
-  legit-context = builtins.getContext "${path}${drv.outPath}${drv.foo.outPath}${drv.drvPath}";
+  combo-path = "${path}${drv.outPath}${drv.foo.outPath}${drv.drvPath}";
+  legit-context = builtins.getContext combo-path;
 
-  constructed-context = builtins.getContext (builtins.appendContext "" desired-context);
+  reconstructed-path = builtins.appendContext
-in legit-context == constructed-context
+    (builtins.unsafeDiscardStringContext combo-path)
+    desired-context;
+
+  # Eta rule for strings with context.
+  etaRule = str:
+    str == builtins.appendContext
+      (builtins.unsafeDiscardStringContext str)
+      (builtins.getContext str);
+
+in [
+  (legit-context == desired-context)
+  (reconstructed-path == combo-path)
+  (etaRule "foo")
+  (etaRule drv.drvPath)
+  (etaRule drv.foo.outPath)
+  (etaRule (builtins.unsafeDiscardOutputDependency drv.drvPath))
+]

tests/lang/eval-okay-readDir.exp

@@ -1 +1 @@
-{ bar = "regular"; foo = "directory"; }
+{ bar = "regular"; foo = "directory"; ldir = "symlink"; linked = "symlink"; }

tests/lang/eval-okay-readFileType.exp

@@ -0,0 +1 @@
+{ bar = "regular"; foo = "directory"; ldir = "symlink"; linked = "symlink"; }

tests/lang/eval-okay-readFileType.nix

@@ -0,0 +1,6 @@
+{
+  bar    = builtins.readFileType ./readDir/bar;
+  foo    = builtins.readFileType ./readDir/foo;
+  linked = builtins.readFileType ./readDir/linked;
+  ldir   = builtins.readFileType ./readDir/ldir;
+}

tests/lang/readDir/ldir

@@ -0,0 +1 @@
+foo

tests/lang/readDir/linked

@@ -0,0 +1 @@
+foo/git-hates-directories

tests/multiple-outputs.nix

@@ -91,9 +91,9 @@ rec {
 
   e = mkDerivation {
     name = "multiple-outputs-e";
-    outputs = [ "a" "b" "c" ];
+    outputs = [ "a_a" "b" "c" ];
-    meta.outputsToInstall = [ "a" "b" ];
+    meta.outputsToInstall = [ "a_a" "b" ];
-    buildCommand = "mkdir $a $b $c";
+    buildCommand = "mkdir $a_a $b $c";
   };
 
   independent = mkDerivation {
@@ -117,4 +117,14 @@ rec {
       '';
   };
 
+  invalid-output-name-1 = mkDerivation {
+    name = "invalid-output-name-1";
+    outputs = [ "out/"];
+  };
+
+  invalid-output-name-2 = mkDerivation {
+    name = "invalid-output-name-2";
+    outputs = [ "x" "foo$"];
+  };
+
 }

tests/multiple-outputs.sh

@@ -83,3 +83,6 @@ nix-store --gc --keep-derivations --keep-outputs
 nix-store --gc --print-roots
 rm -rf $NIX_STORE_DIR/.links
 rmdir $NIX_STORE_DIR
+
+nix build -f multiple-outputs.nix invalid-output-name-1 2>&1 | grep 'contains illegal character'
+nix build -f multiple-outputs.nix invalid-output-name-2 2>&1 | grep 'contains illegal character'