Merge branch 'master' into valid_deriver_2

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

@@ -1 +1 @@
-
+This section lists advanced topics related to builds and builds performance

doc/manual/src/command-ref/nix-prefetch-url.md

@@ -31,15 +31,18 @@ store already contains a file with the same hash and base name.
 Otherwise, the file is downloaded, and an error is signaled if the
 actual hash of the file does not match the specified hash.
 
-This command prints the hash on standard output. Additionally, if the
+This command prints the hash on standard output.
-option `--print-path` is used, the path of the downloaded file in the
+The hash is printed using base-32 unless `--type md5` is specified,
-Nix store is also printed.
+in which case it's printed using base-16.
+Additionally, if the option `--print-path` is used,
+the path of the downloaded file in the Nix store is also printed.
 
 # Options
 
   - `--type` *hashAlgo*\
-    Use the specified cryptographic hash algorithm, which can be one of
+    Use the specified cryptographic hash algorithm,
-    `md5`, `sha1`, `sha256`, and `sha512`.
+    which can be one of `md5`, `sha1`, `sha256`, and `sha512`.
+    The default is `sha256`.
 
   - `--print-path`\
     Print the store path of the downloaded file on standard output.

doc/manual/src/contributing/testing.md

@@ -7,7 +7,8 @@ under `src/{library_name}/tests` using the
 [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.
+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, or the `GTEST_FILTER` environment variable.
 
 ## Functional tests
 

doc/manual/src/glossary.md

@@ -1,6 +1,7 @@
 # Glossary
 
-  - [derivation]{#gloss-derivation}\
+- [derivation]{#gloss-derivation}
+
   A description of a build task. The result of a derivation is a
   store object. Derivations are typically specified in Nix expressions
   using the [`derivation` primitive](./language/derivations.md). These are
@@ -9,7 +10,8 @@
 
   [derivation]: #gloss-derivation
 
-  - [store derivation]{#gloss-store-derivation}\
+- [store derivation]{#gloss-store-derivation}
+
   A [derivation] represented as a `.drv` file in the [store].
   It has a [store path], like any [store object].
 
@@ -19,14 +21,16 @@
 
   [store derivation]: #gloss-store-derivation
 
-  - [instantiate]{#gloss-instantiate}, instantiation\
+- [instantiate]{#gloss-instantiate}, instantiation
+
   Translate a [derivation] into a [store derivation].
 
   See [`nix-instantiate`](./command-ref/nix-instantiate.md).
 
   [instantiate]: #gloss-instantiate
 
-  - [realise]{#gloss-realise}, realisation\
+- [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].
@@ -37,16 +41,19 @@
 
   [realise]: #gloss-realise
 
-  - [content-addressed derivation]{#gloss-content-addressed-derivation}\
+- [content-addressed derivation]{#gloss-content-addressed-derivation}
+
   A derivation which has the
   [`__contentAddressed`](./language/advanced-attributes.md#adv-attr-__contentAddressed)
   attribute set to `true`.
 
-  - [fixed-output derivation]{#gloss-fixed-output-derivation}\
+- [fixed-output derivation]{#gloss-fixed-output-derivation}
+
   A derivation which includes the
   [`outputHash`](./language/advanced-attributes.md#adv-attr-outputHash) attribute.
 
-  - [store]{#gloss-store}\
+- [store]{#gloss-store}
+
   The location in the file system where store objects live. Typically
   `/nix/store`.
 
@@ -67,17 +74,20 @@
   [store]: #gloss-store
   [local store]: #gloss-local-store
 
-  - [chroot store]{#gloss-chroot-store}\
+- [chroot store]{#gloss-chroot-store}
+
   A [local store] whose canonical path is anything other than `/nix/store`.
 
-  - [binary cache]{#gloss-binary-cache}\
+- [binary cache]{#gloss-binary-cache}
+
   A *binary cache* is a Nix store which uses a different format: its
   metadata and signatures are kept in `.narinfo` files rather than in a
   [Nix database]. This different format simplifies serving store objects
   over the network, but cannot host builds. Examples of binary caches
   include S3 buckets and the [NixOS binary cache](https://cache.nixos.org).
 
-  - [store path]{#gloss-store-path}\
+- [store path]{#gloss-store-path}
+
   The location of a [store object] in the file system, i.e., an
   immediate child of the Nix store directory.
 
@@ -85,38 +95,44 @@
 
   [store path]: #gloss-store-path
 
-  - [file system object]{#gloss-store-object}\
+- [file system object]{#gloss-store-object}
+
   The Nix data model for representing simplified file system data.
 
   See [File System Object](@docroot@/architecture/file-system-object.md) for details.
 
   [file system object]: #gloss-file-system-object
 
-  - [store object]{#gloss-store-object}\
+- [store object]{#gloss-store-object}
+
 
   A store object consists of a [file system object], [reference]s to other store objects, and other metadata.
   It can be referred to by a [store path].
 
   [store object]: #gloss-store-object
 
-  - [input-addressed store object]{#gloss-input-addressed-store-object}\
+- [input-addressed store object]{#gloss-input-addressed-store-object}
+
   A store object produced by building a
   non-[content-addressed](#gloss-content-addressed-derivation),
   non-[fixed-output](#gloss-fixed-output-derivation)
   derivation.
 
-  - [output-addressed store object]{#gloss-output-addressed-store-object}\
+- [output-addressed store object]{#gloss-output-addressed-store-object}
+
   A [store object] whose [store path] is determined by its contents.
   This includes derivations, the outputs of [content-addressed derivations](#gloss-content-addressed-derivation), and the outputs of [fixed-output derivations](#gloss-fixed-output-derivation).
 
-  - [substitute]{#gloss-substitute}\
+- [substitute]{#gloss-substitute}
+
   A substitute is a command invocation stored in the [Nix database] that
   describes how to build a store object, bypassing the normal build
   mechanism (i.e., derivations). Typically, the substitute builds the
   store object by downloading a pre-built version of the store object
   from some server.
 
-  - [substituter]{#gloss-substituter}\
+- [substituter]{#gloss-substituter}
+
   An additional [store]{#gloss-store} from which Nix can obtain store objects instead of building them.
   Often the substituter is a [binary cache](#gloss-binary-cache), but any store can serve as substituter.
 
@@ -124,13 +140,15 @@
 
   [substituter]: #gloss-substituter
 
-  - [purity]{#gloss-purity}\
+- [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
   builder can rely on external inputs such as the network or the
   system time) but the Nix model assumes it.
 
-  - [Nix database]{#gloss-nix-database}\
+- [Nix database]{#gloss-nix-database}
+
   An SQlite database to track [reference]s between [store object]s.
   This is an implementation detail of the [local store].
 
@@ -138,14 +156,16 @@
 
   [Nix database]: #gloss-nix-database
 
-  - [Nix expression]{#gloss-nix-expression}\
+- [Nix expression]{#gloss-nix-expression}
+
   A high-level description of software packages and compositions
   thereof. Deploying software using Nix entails writing Nix
   expressions for your packages. Nix expressions are translated to
   derivations that are stored in the Nix store. These derivations can
   then be built.
 
-  - [reference]{#gloss-reference}\
+- [reference]{#gloss-reference}
+
   A [store object] `O` is said to have a *reference* to a store object `P` if a [store path] to `P` appears in the contents of `O`.
 
   Store objects can refer to both other store objects and themselves.
@@ -154,11 +174,13 @@
 
   [reference]: #gloss-reference
 
-  - [reachable]{#gloss-reachable}\
+- [reachable]{#gloss-reachable}
+
   A store path `Q` is reachable from another store path `P` if `Q`
   is in the *closure* of the *references* relation.
 
-  - [closure]{#gloss-closure}\
+- [closure]{#gloss-closure}
+
   The closure of a store path is the set of store paths that are
   directly or indirectly “reachable” from that store path; that is,
   it’s the closure of the path under the *references* relation. For
@@ -175,15 +197,18 @@
 
   [closure]: #gloss-closure
 
-  - [output path]{#gloss-output-path}\
+- [output path]{#gloss-output-path}
+
   A [store path] produced by a [derivation].
 
   [output path]: #gloss-output-path
 
-  - [deriver]{#gloss-deriver}\
+- [deriver]{#gloss-deriver}
+
   The [store derivation] that produced an [output path].
 
-  - [validity]{#gloss-validity}\
+- [validity]{#gloss-validity}
+
   A store path is valid if all [store object]s in its [closure] can be read from the [store].
 
   For a [local store], this means:
@@ -193,34 +218,41 @@
 
   [validity]: #gloss-validity
 
-  - [user environment]{#gloss-user-env}\
+- [user environment]{#gloss-user-env}
+
   An automatically generated store object that consists of a set of
   symlinks to “active” applications, i.e., other store paths. These
   are generated automatically by
   [`nix-env`](./command-ref/nix-env.md). See *profiles*.
 
-  - [profile]{#gloss-profile}\
+- [profile]{#gloss-profile}
+
   A symlink to the current *user environment* of a user, e.g.,
   `/nix/var/nix/profiles/default`.
 
-  - [installable]{#gloss-installable}\
+- [installable]{#gloss-installable}
+
   Something that can be realised in the Nix store.
 
   See [installables](./command-ref/new-cli/nix.md#installables) for [`nix` commands](./command-ref/new-cli/nix.md) (experimental) for details.
 
-  - [NAR]{#gloss-nar}\
+- [NAR]{#gloss-nar}
+
   A *N*ix *AR*chive. This is a serialisation of a path in the Nix
   store. It can contain regular files, directories and symbolic
   links.  NARs are generated and unpacked using `nix-store --dump`
   and `nix-store --restore`.
 
-  - [`∅`]{#gloss-emtpy-set}\
+- [`∅`]{#gloss-emtpy-set}
+
   The empty set symbol. In the context of profile history, this denotes a package is not present in a particular version of the profile.
 
-  - [`ε`]{#gloss-epsilon}\
+- [`ε`]{#gloss-epsilon}
+
   The epsilon symbol. In the context of a package, this means the version is empty. More precisely, the derivation does not have a version attribute.
 
-  - [string interpolation]{#gloss-string-interpolation}\
+- [string interpolation]{#gloss-string-interpolation}
+
   Expanding expressions enclosed in `${ }` within a [string], [path], or [attribute name].
 
   See [String interpolation](./language/string-interpolation.md) for details.
@@ -229,7 +261,8 @@
   [path]: ./language/values.md#type-path
   [attribute name]: ./language/values.md#attribute-set
 
-  - [experimental feature]{#gloss-experimental-feature}\
+- [experimental feature]{#gloss-experimental-feature}
+
   Not yet stabilized functionality guarded by named experimental feature flags.
   These flags are enabled or disabled with the [`experimental-features`](./command-ref/conf-file.html#conf-experimental-features) setting.
 

doc/manual/src/protocols/tarball-fetcher.md

@@ -20,8 +20,8 @@ Link: <flakeref>; rel="immutable"
 
 (Note the required `<` and `>` characters around *flakeref*.)
 
-*flakeref* must be a tarball flakeref. It can contain flake attributes
+*flakeref* must be a tarball flakeref. It can contain the tarball flake attributes
-such as `narHash`, `rev` and `revCount`. If `narHash` is included, its
+`narHash`, `rev`, `revCount` and `lastModified`. If `narHash` is included, its
 value must be the NAR hash of the unpacked tarball (as computed via
 `nix hash path`). Nix checks the contents of the returned tarball
 against the `narHash` attribute. The `rev` and `revCount` attributes

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

@@ -23,6 +23,8 @@
 - Introduce a new [`outputOf`](@docroot@/language/builtins.md#builtins-outputOf) builtin.
   It is part of the [`dynamic-derivations`](@docroot@/contributing/experimental-features.md#xp-feature-dynamic-derivations) experimental feature.
 
+- Flake follow paths at depths greater than 2 are now handled correctly, preventing "follows a non-existent input" errors.
+
 - [`nix-store --query`](@docroot@/command-ref/nix-store/query.md) gained a new type of query: `--valid-derivers`. It returns all `.drv` files in the local store that *can be* used to build the output passed in argument.
 This is in contrast to `--deriver`, which returns the single `.drv` file that *was actually* used to build the output passed in argument. In case the output was substituted from a binary cache,
 this `.drv` file may only exist on said binary cache and not locally.

flake.nix

@@ -19,9 +19,11 @@
         then ""
         else "pre${builtins.substring 0 8 (self.lastModifiedDate or self.lastModified or "19700101")}_${self.shortRev or "dirty"}";
 
+      linux32BitSystems = [ "i686-linux" ];
       linux64BitSystems = [ "x86_64-linux" "aarch64-linux" ];
-      linuxSystems = linux64BitSystems ++ [ "i686-linux" ];
+      linuxSystems = linux32BitSystems ++ linux64BitSystems;
-      systems = linuxSystems ++ [ "x86_64-darwin" "aarch64-darwin" ];
+      darwinSystems = [ "x86_64-darwin" "aarch64-darwin" ];
+      systems = linuxSystems ++ darwinSystems;
       
       crossSystems = [ "armv6l-linux" "armv7l-linux" ];
 

src/libexpr/flake/lockfile.cc

@@ -345,7 +345,7 @@ void LockFile::check()
 
     for (auto & [inputPath, input] : inputs) {
         if (auto follows = std::get_if<1>(&input)) {
-            if (!follows->empty() && !get(inputs, *follows))
+            if (!follows->empty() && !findInput(*follows))
                 throw Error("input '%s' follows a non-existent input '%s'",
                     printInputPath(inputPath),
                     printInputPath(*follows));

src/libexpr/primops.cc

@@ -1520,15 +1520,25 @@ static RegisterPrimOp primop_storePath({
 
 static void prim_pathExists(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
+    auto & arg = *args[0];
+
     /* We don’t check the path right now, because we don’t want to
        throw if the path isn’t allowed, but just return false (and we
        can’t just catch the exception here because we still want to
-       throw if something in the evaluation of `*args[0]` tries to
+       throw if something in the evaluation of `arg` tries to
        access an unauthorized path). */
-    auto path = realisePath(state, pos, *args[0], { .checkForPureEval = false });
+    auto path = realisePath(state, pos, arg, { .checkForPureEval = false });
+
+    /* SourcePath doesn't know about trailing slash. */
+    auto mustBeDir = arg.type() == nString && arg.str().ends_with("/");
 
     try {
-        v.mkBool(state.checkSourcePath(path).pathExists());
+        auto checked = state.checkSourcePath(path);
+        auto exists = checked.pathExists();
+        if (exists && mustBeDir) {
+            exists = checked.lstat().type == InputAccessor::tDirectory;
+        }
+        v.mkBool(exists);
     } catch (SysError & e) {
         /* Don't give away info from errors while canonicalising
            ‘path’ in restricted mode. */
@@ -1843,7 +1853,7 @@ static void prim_outputOf(EvalState & state, const PosIdx pos, Value * * args, V
 {
     SingleDerivedPath drvPath = state.coerceToSingleDerivedPath(pos, *args[0], "while evaluating the first argument to builtins.outputOf");
 
-    std::string_view outputName = state.forceStringNoCtx(*args[1], pos, "while evaluating the second argument to builtins.outputOf");
+    OutputNameView outputName = state.forceStringNoCtx(*args[1], pos, "while evaluating the second argument to builtins.outputOf");
 
     state.mkSingleDerivedPathString(
         SingleDerivedPath::Built {

src/libfetchers/tarball.cc

@@ -232,7 +232,7 @@ struct CurlInputScheme : InputScheme
         if (type != inputType()) return {};
 
         // FIXME: some of these only apply to TarballInputScheme.
-        std::set<std::string> allowedNames = {"type", "url", "narHash", "name", "unpack", "rev", "revCount"};
+        std::set<std::string> allowedNames = {"type", "url", "narHash", "name", "unpack", "rev", "revCount", "lastModified"};
         for (auto & [name, value] : attrs)
             if (!allowedNames.count(name))
                 throw Error("unsupported %s input attribute '%s'", *type, name);
@@ -310,6 +310,9 @@ struct TarballInputScheme : CurlInputScheme
             input = immutableInput;
         }
 
+        if (result.lastModified && !input.attrs.contains("lastModified"))
+            input.attrs.insert_or_assign("lastModified", uint64_t(result.lastModified));
+
         return {result.tree.storePath, std::move(input)};
     }
 };

src/libstore/build/create-derivation-and-realise-goal.cc

@@ -0,0 +1,157 @@
+#include "create-derivation-and-realise-goal.hh"
+#include "worker.hh"
+
+namespace nix {
+
+CreateDerivationAndRealiseGoal::CreateDerivationAndRealiseGoal(ref<SingleDerivedPath> drvReq,
+    const OutputsSpec & wantedOutputs, Worker & worker, BuildMode buildMode)
+    : Goal(worker, DerivedPath::Built { .drvPath = drvReq, .outputs = wantedOutputs })
+    , drvReq(drvReq)
+    , wantedOutputs(wantedOutputs)
+    , buildMode(buildMode)
+{
+    state = &CreateDerivationAndRealiseGoal::getDerivation;
+    name = fmt(
+        "outer obtaining drv from '%s' and then building outputs %s",
+        drvReq->to_string(worker.store),
+        std::visit(overloaded {
+            [&](const OutputsSpec::All) -> std::string {
+                return "* (all of them)";
+            },
+            [&](const OutputsSpec::Names os) {
+                return concatStringsSep(", ", quoteStrings(os));
+            },
+        }, wantedOutputs.raw));
+    trace("created outer");
+
+    worker.updateProgress();
+}
+
+
+CreateDerivationAndRealiseGoal::~CreateDerivationAndRealiseGoal()
+{
+}
+
+
+static StorePath pathPartOfReq(const SingleDerivedPath & req)
+{
+    return std::visit(overloaded {
+        [&](const SingleDerivedPath::Opaque & bo) {
+            return bo.path;
+        },
+        [&](const SingleDerivedPath::Built & bfd) {
+            return pathPartOfReq(*bfd.drvPath);
+        },
+    }, req.raw());
+}
+
+
+std::string CreateDerivationAndRealiseGoal::key()
+{
+    /* Ensure that derivations get built in order of their name,
+       i.e. a derivation named "aardvark" always comes before "baboon". And
+       substitution goals and inner derivation goals always happen before
+       derivation goals (due to "b$"). */
+    return "c$" + std::string(pathPartOfReq(*drvReq).name()) + "$" + drvReq->to_string(worker.store);
+}
+
+
+void CreateDerivationAndRealiseGoal::timedOut(Error && ex)
+{
+}
+
+
+void CreateDerivationAndRealiseGoal::work()
+{
+    (this->*state)();
+}
+
+
+void CreateDerivationAndRealiseGoal::addWantedOutputs(const OutputsSpec & outputs)
+{
+    /* If we already want all outputs, there is nothing to do. */
+    auto newWanted = wantedOutputs.union_(outputs);
+    bool needRestart = !newWanted.isSubsetOf(wantedOutputs);
+    wantedOutputs = newWanted;
+
+    if (!needRestart) return;
+
+    if (!optDrvPath)
+        // haven't started steps where the outputs matter yet
+        return;
+    worker.makeDerivationGoal(*optDrvPath, outputs, buildMode);
+}
+
+
+void CreateDerivationAndRealiseGoal::getDerivation()
+{
+    trace("outer init");
+
+    /* The first thing to do is to make sure that the derivation
+       exists.  If it doesn't, it may be created through a
+       substitute. */
+    if (auto optDrvPath = [this]() -> std::optional<StorePath> {
+        if (buildMode != bmNormal) return std::nullopt;
+
+        auto drvPath = StorePath::dummy;
+        try {
+            drvPath = resolveDerivedPath(worker.store, *drvReq);
+        } catch (MissingRealisation) {
+            return std::nullopt;
+        }
+        return worker.evalStore.isValidPath(drvPath) || worker.store.isValidPath(drvPath)
+            ? std::optional { drvPath }
+            : std::nullopt;
+    }()) {
+        trace(fmt("already have drv '%s' for '%s', can go straight to building",
+            worker.store.printStorePath(*optDrvPath),
+            drvReq->to_string(worker.store)));
+
+        loadAndBuildDerivation();
+    } else {
+        trace("need to obtain drv we want to build");
+
+        addWaitee(worker.makeGoal(DerivedPath::fromSingle(*drvReq)));
+
+        state = &CreateDerivationAndRealiseGoal::loadAndBuildDerivation;
+        if (waitees.empty()) work();
+    }
+}
+
+
+void CreateDerivationAndRealiseGoal::loadAndBuildDerivation()
+{
+    trace("outer load and build derivation");
+
+    if (nrFailed != 0) {
+        amDone(ecFailed, Error("cannot build missing derivation '%s'", drvReq->to_string(worker.store)));
+        return;
+    }
+
+    StorePath drvPath = resolveDerivedPath(worker.store, *drvReq);
+    /* Build this step! */
+    concreteDrvGoal = worker.makeDerivationGoal(drvPath, wantedOutputs, buildMode);
+    addWaitee(upcast_goal(concreteDrvGoal));
+    state = &CreateDerivationAndRealiseGoal::buildDone;
+    optDrvPath = std::move(drvPath);
+    if (waitees.empty()) work();
+}
+
+
+void CreateDerivationAndRealiseGoal::buildDone()
+{
+    trace("outer build done");
+
+    buildResult = upcast_goal(concreteDrvGoal)->getBuildResult(DerivedPath::Built {
+        .drvPath = drvReq,
+        .outputs = wantedOutputs,
+    });
+
+    if (buildResult.success())
+        amDone(ecSuccess);
+    else
+        amDone(ecFailed, Error("building '%s' failed", drvReq->to_string(worker.store)));
+}
+
+
+}

src/libstore/build/create-derivation-and-realise-goal.hh

@@ -0,0 +1,96 @@
+#pragma once
+
+#include "parsed-derivations.hh"
+#include "lock.hh"
+#include "store-api.hh"
+#include "pathlocks.hh"
+#include "goal.hh"
+
+namespace nix {
+
+struct DerivationGoal;
+
+/**
+ * This goal type is essentially the serial composition (like function
+ * composition) of a goal for getting a derivation, and then a
+ * `DerivationGoal` using the newly-obtained derivation.
+ *
+ * In the (currently experimental) general inductive case of derivations
+ * that are themselves build outputs, that first goal will be *another*
+ * `CreateDerivationAndRealiseGoal`. In the (much more common) base-case
+ * where the derivation has no provence and is just referred to by
+ * (content-addressed) store path, that first goal is a
+ * `SubstitutionGoal`.
+ *
+ * If we already have the derivation (e.g. if the evalutator has created
+ * the derivation locally and then instructured the store to build it),
+ * we can skip the first goal entirely as a small optimization.
+ */
+struct CreateDerivationAndRealiseGoal : public Goal
+{
+    /**
+     * How to obtain a store path of the derivation to build.
+     */
+    ref<SingleDerivedPath> drvReq;
+
+    /**
+     * The path of the derivation, once obtained.
+     **/
+    std::optional<StorePath> optDrvPath;
+
+    /**
+     * The goal for the corresponding concrete derivation.
+     **/
+    std::shared_ptr<DerivationGoal> concreteDrvGoal;
+
+    /**
+     * The specific outputs that we need to build.
+     */
+    OutputsSpec wantedOutputs;
+
+    typedef void (CreateDerivationAndRealiseGoal::*GoalState)();
+    GoalState state;
+
+    /**
+     * The final output paths of the build.
+     *
+     * - For input-addressed derivations, always the precomputed paths
+     *
+     * - For content-addressed derivations, calcuated from whatever the
+     *   hash ends up being. (Note that fixed outputs derivations that
+     *   produce the "wrong" output still install that data under its
+     *   true content-address.)
+     */
+    OutputPathMap finalOutputs;
+
+    BuildMode buildMode;
+
+    CreateDerivationAndRealiseGoal(ref<SingleDerivedPath> drvReq,
+        const OutputsSpec & wantedOutputs, Worker & worker,
+        BuildMode buildMode = bmNormal);
+    virtual ~CreateDerivationAndRealiseGoal();
+
+    void timedOut(Error && ex) override;
+
+    std::string key() override;
+
+    void work() override;
+
+    /**
+     * Add wanted outputs to an already existing derivation goal.
+     */
+    void addWantedOutputs(const OutputsSpec & outputs);
+
+    /**
+     * The states.
+     */
+    void getDerivation();
+    void loadAndBuildDerivation();
+    void buildDone();
+
+    JobCategory jobCategory() const override {
+        return JobCategory::Administration;
+    };
+};
+
+}

src/libstore/build/derivation-goal.cc

@@ -71,7 +71,7 @@ DerivationGoal::DerivationGoal(const StorePath & drvPath,
     , wantedOutputs(wantedOutputs)
     , buildMode(buildMode)
 {
-    state = &DerivationGoal::getDerivation;
+    state = &DerivationGoal::loadDerivation;
     name = fmt(
         "building of '%s' from .drv file",
         DerivedPath::Built { makeConstantStorePathRef(drvPath), wantedOutputs }.to_string(worker.store));
@@ -164,24 +164,6 @@ void DerivationGoal::addWantedOutputs(const OutputsSpec & outputs)
 }
 
 
-void DerivationGoal::getDerivation()
-{
-    trace("init");
-
-    /* The first thing to do is to make sure that the derivation
-       exists.  If it doesn't, it may be created through a
-       substitute. */
-    if (buildMode == bmNormal && worker.evalStore.isValidPath(drvPath)) {
-        loadDerivation();
-        return;
-    }
-
-    addWaitee(upcast_goal(worker.makePathSubstitutionGoal(drvPath)));
-
-    state = &DerivationGoal::loadDerivation;
-}
-
-
 void DerivationGoal::loadDerivation()
 {
     trace("loading derivation");
@@ -380,7 +362,12 @@ void DerivationGoal::gaveUpOnSubstitution()
                         worker.store.printStorePath(i.first));
             }
 
-            addWaitee(worker.makeDerivationGoal(i.first, i.second, buildMode == bmRepair ? bmRepair : bmNormal));
+            addWaitee(worker.makeGoal(
+                DerivedPath::Built {
+                    .drvPath = makeConstantStorePathRef(i.first),
+                    .outputs = i.second,
+                },
+                buildMode == bmRepair ? bmRepair : bmNormal));
         }
 
     /* Copy the input sources from the eval store to the build
@@ -452,7 +439,12 @@ void DerivationGoal::repairClosure()
         if (drvPath2 == outputsToDrv.end())
             addWaitee(upcast_goal(worker.makePathSubstitutionGoal(i, Repair)));
         else
-            addWaitee(worker.makeDerivationGoal(drvPath2->second, OutputsSpec::All(), bmRepair));
+            addWaitee(worker.makeGoal(
+                DerivedPath::Built {
+                    .drvPath = makeConstantStorePathRef(drvPath2->second),
+                    .outputs = OutputsSpec::All { },
+                },
+                bmRepair));
     }
 
     if (waitees.empty()) {
@@ -1483,7 +1475,7 @@ void DerivationGoal::waiteeDone(GoalPtr waitee, ExitCode result)
     if (!useDerivation) return;
     auto & fullDrv = *dynamic_cast<Derivation *>(drv.get());
 
-    auto * dg = dynamic_cast<DerivationGoal *>(&*waitee);
+    auto * dg = tryGetConcreteDrvGoal(waitee);
     if (!dg) return;
 
     auto outputs = fullDrv.inputDrvs.find(dg->drvPath);

src/libstore/build/derivation-goal.hh

@@ -50,6 +50,13 @@ struct InitialOutput {
     std::optional<InitialOutputStatus> known;
 };
 
+/**
+ * A goal for building some or all of the outputs of a derivation.
+ *
+ * The derivation must already be present, either in the store in a drv
+ * or in memory. If the derivation itself needs to be gotten first, a
+ * `CreateDerivationAndRealiseGoal` goal must be used instead.
+ */
 struct DerivationGoal : public Goal
 {
     /**
@@ -66,8 +73,7 @@ struct DerivationGoal : public Goal
     std::shared_ptr<DerivationGoal> resolvedDrvGoal;
 
     /**
-     * The specific outputs that we need to build.  Empty means all of
+     * The specific outputs that we need to build.
-     * them.
      */
     OutputsSpec wantedOutputs;
 
@@ -229,7 +235,6 @@ struct DerivationGoal : public Goal
     /**
      * The states.
      */
-    void getDerivation();
     void loadDerivation();
     void haveDerivation();
     void outputsSubstitutionTried();
@@ -334,7 +339,9 @@ struct DerivationGoal : public Goal
 
     StorePathSet exportReferences(const StorePathSet & storePaths);
 
-    JobCategory jobCategory() override { return JobCategory::Build; };
+    JobCategory jobCategory() const override {
+        return JobCategory::Build;
+    };
 };
 
 MakeError(NotDeterministic, BuildError);

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

@@ -73,7 +73,9 @@ public:
     void work() override;
     void handleEOF(int fd) override;
 
-    JobCategory jobCategory() override { return JobCategory::Substitution; };
+    JobCategory jobCategory() const override {
+        return JobCategory::Substitution;
+    };
 };
 
 }

src/libstore/build/entry-points.cc

@@ -1,5 +1,6 @@
 #include "worker.hh"
 #include "substitution-goal.hh"
+#include "create-derivation-and-realise-goal.hh"
 #include "derivation-goal.hh"
 #include "local-store.hh"
 
@@ -15,7 +16,7 @@ void Store::buildPaths(const std::vector<DerivedPath> & reqs, BuildMode buildMod
 
     worker.run(goals);
 
-    StorePathSet failed;
+    StringSet failed;
     std::optional<Error> ex;
     for (auto & i : goals) {
         if (i->ex) {
@@ -25,8 +26,10 @@ void Store::buildPaths(const std::vector<DerivedPath> & reqs, BuildMode buildMod
                 ex = std::move(i->ex);
         }
         if (i->exitCode != Goal::ecSuccess) {
-            if (auto i2 = dynamic_cast<DerivationGoal *>(i.get())) failed.insert(i2->drvPath);
+            if (auto i2 = dynamic_cast<CreateDerivationAndRealiseGoal *>(i.get()))
-            else if (auto i2 = dynamic_cast<PathSubstitutionGoal *>(i.get())) failed.insert(i2->storePath);
+                failed.insert(i2->drvReq->to_string(*this));
+            else if (auto i2 = dynamic_cast<PathSubstitutionGoal *>(i.get()))
+                failed.insert(printStorePath(i2->storePath));
         }
     }
 
@@ -35,7 +38,7 @@ void Store::buildPaths(const std::vector<DerivedPath> & reqs, BuildMode buildMod
         throw std::move(*ex);
     } else if (!failed.empty()) {
         if (ex) logError(ex->info());
-        throw Error(worker.failingExitStatus(), "build of %s failed", showPaths(failed));
+        throw Error(worker.failingExitStatus(), "build of %s failed", concatStringsSep(", ", quoteStrings(failed)));
     }
 }
 
@@ -124,8 +127,11 @@ void Store::repairPath(const StorePath & path)
         auto info = queryPathInfo(path);
         if (info->deriver && isValidPath(*info->deriver)) {
             goals.clear();
+            goals.insert(worker.makeGoal(DerivedPath::Built {
+                .drvPath = makeConstantStorePathRef(*info->deriver),
                 // FIXME: Should just build the specific output we need.
-            goals.insert(worker.makeDerivationGoal(*info->deriver, OutputsSpec::All { }, bmRepair));
+                .outputs = OutputsSpec::All { },
+            }, bmRepair));
             worker.run(goals);
         } else
             throw Error(worker.failingExitStatus(), "cannot repair path '%s'", printStorePath(path));

src/libstore/build/goal.cc

@@ -11,7 +11,7 @@ bool CompareGoalPtrs::operator() (const GoalPtr & a, const GoalPtr & b) const {
 }
 
 
-BuildResult Goal::getBuildResult(const DerivedPath & req) {
+BuildResult Goal::getBuildResult(const DerivedPath & req) const {
     BuildResult res { buildResult };
 
     if (auto pbp = std::get_if<DerivedPath::Built>(&req)) {

src/libstore/build/goal.hh

@@ -41,8 +41,24 @@ typedef std::map<StorePath, WeakGoalPtr> WeakGoalMap;
  * of each category in parallel.
  */
 enum struct JobCategory {
+    /**
+     * A build of a derivation; it will use CPU and disk resources.
+     */
     Build,
+    /**
+     * A substitution an arbitrary store object; it will use network resources.
+     */
     Substitution,
+    /**
+     * A goal that does no "real" work by itself, and just exists to depend on
+     * other goals which *do* do real work. These goals therefore are not
+     * limited.
+     *
+     * These goals cannot infinitely create themselves, so there is no risk of
+     * a "fork bomb" type situation (which would be a problem even though the
+     * goal do no real work) either.
+     */
+    Administration,
 };
 
 struct Goal : public std::enable_shared_from_this<Goal>
@@ -110,7 +126,7 @@ public:
      * sake of both privacy and determinism, and this "safe accessor"
      * ensures we don't.
      */
-    BuildResult getBuildResult(const DerivedPath &);
+    BuildResult getBuildResult(const DerivedPath &) const;
 
     /**
      * Exception containing an error message, if any.
@@ -144,7 +160,7 @@ public:
 
     void trace(std::string_view s);
 
-    std::string getName()
+    std::string getName() const
     {
         return name;
     }
@@ -166,7 +182,7 @@ public:
      * @brief Hint for the scheduler, which concurrency limit applies.
      * @see JobCategory
      */
-    virtual JobCategory jobCategory() = 0;
+    virtual JobCategory jobCategory() const = 0;
 };
 
 void addToWeakGoals(WeakGoals & goals, GoalPtr p);

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

@@ -2955,7 +2955,7 @@ bool LocalDerivationGoal::isReadDesc(int fd)
 }
 
 
-StorePath LocalDerivationGoal::makeFallbackPath(std::string_view outputName)
+StorePath LocalDerivationGoal::makeFallbackPath(OutputNameView outputName)
 {
     return worker.store.makeStorePath(
         "rewrite:" + std::string(drvPath.to_string()) + ":name:" + std::string(outputName),

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

@@ -297,7 +297,7 @@ struct LocalDerivationGoal : public DerivationGoal
      * @todo Add option to randomize, so we can audit whether our
      * rewrites caught everything
      */
-    StorePath makeFallbackPath(std::string_view outputName);
+    StorePath makeFallbackPath(OutputNameView outputName);
 };
 
 }

src/libstore/build/substitution-goal.hh

@@ -117,7 +117,9 @@ public:
     /* Called by destructor, can't be overridden */
     void cleanup() override final;
 
-    JobCategory jobCategory() override { return JobCategory::Substitution; };
+    JobCategory jobCategory() const override {
+        return JobCategory::Substitution;
+    };
 };
 
 }

src/libstore/build/worker.cc

@@ -2,6 +2,7 @@
 #include "worker.hh"
 #include "substitution-goal.hh"
 #include "drv-output-substitution-goal.hh"
+#include "create-derivation-and-realise-goal.hh"
 #include "local-derivation-goal.hh"
 #include "hook-instance.hh"
 
@@ -41,6 +42,24 @@ Worker::~Worker()
 }
 
 
+std::shared_ptr<CreateDerivationAndRealiseGoal> Worker::makeCreateDerivationAndRealiseGoal(
+    ref<SingleDerivedPath> drvReq,
+    const OutputsSpec & wantedOutputs,
+    BuildMode buildMode)
+{
+    std::weak_ptr<CreateDerivationAndRealiseGoal> & goal_weak = outerDerivationGoals.ensureSlot(*drvReq).value;
+    std::shared_ptr<CreateDerivationAndRealiseGoal> goal = goal_weak.lock();
+    if (!goal) {
+        goal = std::make_shared<CreateDerivationAndRealiseGoal>(drvReq, wantedOutputs, *this, buildMode);
+        goal_weak = goal;
+        wakeUp(goal);
+    } else {
+        goal->addWantedOutputs(wantedOutputs);
+    }
+    return goal;
+}
+
+
 std::shared_ptr<DerivationGoal> Worker::makeDerivationGoalCommon(
     const StorePath & drvPath,
     const OutputsSpec & wantedOutputs,
@@ -111,10 +130,7 @@ GoalPtr Worker::makeGoal(const DerivedPath & req, BuildMode buildMode)
 {
     return std::visit(overloaded {
         [&](const DerivedPath::Built & bfd) -> GoalPtr {
-            if (auto bop = std::get_if<DerivedPath::Opaque>(&*bfd.drvPath))
+            return makeCreateDerivationAndRealiseGoal(bfd.drvPath, bfd.outputs, buildMode);
-                return makeDerivationGoal(bop->path, bfd.outputs, buildMode);
-            else
-                throw UnimplementedError("Building dynamic derivations in one shot is not yet implemented.");
         },
         [&](const DerivedPath::Opaque & bo) -> GoalPtr {
             return makePathSubstitutionGoal(bo.path, buildMode == bmRepair ? Repair : NoRepair);
@@ -123,24 +139,46 @@ GoalPtr Worker::makeGoal(const DerivedPath & req, BuildMode buildMode)
 }
 
 
+template<typename K, typename V, typename F>
+static void cullMap(std::map<K, V> & goalMap, F f)
+{
+    for (auto i = goalMap.begin(); i != goalMap.end();)
+        if (!f(i->second))
+            i = goalMap.erase(i);
+        else ++i;
+}
+
+
 template<typename K, typename G>
 static void removeGoal(std::shared_ptr<G> goal, std::map<K, std::weak_ptr<G>> & goalMap)
 {
     /* !!! inefficient */
-    for (auto i = goalMap.begin();
+    cullMap(goalMap, [&](const std::weak_ptr<G> & gp) -> bool {
-         i != goalMap.end(); )
+        return gp.lock() != goal;
-        if (i->second.lock() == goal) {
+    });
-            auto j = i; ++j;
+}
-            goalMap.erase(i);
+
-            i = j;
+template<typename K>
-        }
+static void removeGoal(std::shared_ptr<CreateDerivationAndRealiseGoal> goal, std::map<K, DerivedPathMap<std::weak_ptr<CreateDerivationAndRealiseGoal>>::ChildNode> & goalMap);
-        else ++i;
+
+template<typename K>
+static void removeGoal(std::shared_ptr<CreateDerivationAndRealiseGoal> goal, std::map<K, DerivedPathMap<std::weak_ptr<CreateDerivationAndRealiseGoal>>::ChildNode> & goalMap)
+{
+    /* !!! inefficient */
+    cullMap(goalMap, [&](DerivedPathMap<std::weak_ptr<CreateDerivationAndRealiseGoal>>::ChildNode & node) -> bool {
+        if (node.value.lock() == goal)
+            node.value.reset();
+        removeGoal(goal, node.childMap);
+        return !node.value.expired() || !node.childMap.empty();
+    });
 }
 
 
 void Worker::removeGoal(GoalPtr goal)
 {
-    if (auto drvGoal = std::dynamic_pointer_cast<DerivationGoal>(goal))
+    if (auto drvGoal = std::dynamic_pointer_cast<CreateDerivationAndRealiseGoal>(goal))
+        nix::removeGoal(drvGoal, outerDerivationGoals.map);
+    else if (auto drvGoal = std::dynamic_pointer_cast<DerivationGoal>(goal))
         nix::removeGoal(drvGoal, derivationGoals);
     else if (auto subGoal = std::dynamic_pointer_cast<PathSubstitutionGoal>(goal))
         nix::removeGoal(subGoal, substitutionGoals);
@@ -198,8 +236,19 @@ void Worker::childStarted(GoalPtr goal, const std::set<int> & fds,
     child.respectTimeouts = respectTimeouts;
     children.emplace_back(child);
     if (inBuildSlot) {
-        if (goal->jobCategory() == JobCategory::Substitution) nrSubstitutions++;
+        switch (goal->jobCategory()) {
-        else nrLocalBuilds++;
+        case JobCategory::Substitution:
+            nrSubstitutions++;
+            break;
+        case JobCategory::Build:
+            nrLocalBuilds++;
+            break;
+        case JobCategory::Administration:
+            /* Intentionally not limited, see docs */
+            break;
+        default:
+            abort();
+        }
     }
 }
 
@@ -211,12 +260,20 @@ void Worker::childTerminated(Goal * goal, bool wakeSleepers)
     if (i == children.end()) return;
 
     if (i->inBuildSlot) {
-        if (goal->jobCategory() == JobCategory::Substitution) {
+        switch (goal->jobCategory()) {
+        case JobCategory::Substitution:
             assert(nrSubstitutions > 0);
             nrSubstitutions--;
-        } else {
+            break;
+        case JobCategory::Build:
             assert(nrLocalBuilds > 0);
             nrLocalBuilds--;
+            break;
+        case JobCategory::Administration:
+            /* Intentionally not limited, see docs */
+            break;
+        default:
+            abort();
         }
     }
 
@@ -267,9 +324,9 @@ void Worker::run(const Goals & _topGoals)
 
     for (auto & i : _topGoals) {
         topGoals.insert(i);
-        if (auto goal = dynamic_cast<DerivationGoal *>(i.get())) {
+        if (auto goal = dynamic_cast<CreateDerivationAndRealiseGoal *>(i.get())) {
             topPaths.push_back(DerivedPath::Built {
-                .drvPath = makeConstantStorePathRef(goal->drvPath),
+                .drvPath = goal->drvReq,
                 .outputs = goal->wantedOutputs,
             });
         } else if (auto goal = dynamic_cast<PathSubstitutionGoal *>(i.get())) {
@@ -522,11 +579,26 @@ void Worker::markContentsGood(const StorePath & path)
 }
 
 
-GoalPtr upcast_goal(std::shared_ptr<PathSubstitutionGoal> subGoal) {
+GoalPtr upcast_goal(std::shared_ptr<PathSubstitutionGoal> subGoal)
-    return subGoal;
+{
-}
-GoalPtr upcast_goal(std::shared_ptr<DrvOutputSubstitutionGoal> subGoal) {
     return subGoal;
 }
 
+GoalPtr upcast_goal(std::shared_ptr<DrvOutputSubstitutionGoal> subGoal)
+{
+    return subGoal;
+}
+
+GoalPtr upcast_goal(std::shared_ptr<DerivationGoal> subGoal)
+{
+    return subGoal;
+}
+
+const DerivationGoal * tryGetConcreteDrvGoal(GoalPtr waitee)
+{
+    auto * odg = dynamic_cast<CreateDerivationAndRealiseGoal *>(&*waitee);
+    if (!odg) return nullptr;
+    return &*odg->concreteDrvGoal;
+}
+
 }

src/libstore/build/worker.hh

@@ -4,6 +4,7 @@
 #include "types.hh"
 #include "lock.hh"
 #include "store-api.hh"
+#include "derived-path-map.hh"
 #include "goal.hh"
 #include "realisation.hh"
 
@@ -13,6 +14,7 @@
 namespace nix {
 
 /* Forward definition. */
+struct CreateDerivationAndRealiseGoal;
 struct DerivationGoal;
 struct PathSubstitutionGoal;
 class DrvOutputSubstitutionGoal;
@@ -31,9 +33,23 @@ class DrvOutputSubstitutionGoal;
  */
 GoalPtr upcast_goal(std::shared_ptr<PathSubstitutionGoal> subGoal);
 GoalPtr upcast_goal(std::shared_ptr<DrvOutputSubstitutionGoal> subGoal);
+GoalPtr upcast_goal(std::shared_ptr<DerivationGoal> subGoal);
 
 typedef std::chrono::time_point<std::chrono::steady_clock> steady_time_point;
 
+/**
+ * The current implementation of impure derivations has
+ * `DerivationGoal`s accumulate realisations from their waitees.
+ * Unfortunately, `DerivationGoal`s don't directly depend on other
+ * goals, but instead depend on `CreateDerivationAndRealiseGoal`s.
+ *
+ * We try not to share any of the details of any goal type with any
+ * other, for sake of modularity and quicker rebuilds. This means we
+ * cannot "just" downcast and fish out the field. So as an escape hatch,
+ * we have made the function, written in `worker.cc` where all the goal
+ * types are visible, and use it instead.
+ */
+const DerivationGoal * tryGetConcreteDrvGoal(GoalPtr waitee);
 
 /**
  * A mapping used to remember for each child process to what goal it
@@ -102,6 +118,9 @@ private:
      * Maps used to prevent multiple instantiations of a goal for the
      * same derivation / path.
      */
+
+    DerivedPathMap<std::weak_ptr<CreateDerivationAndRealiseGoal>> outerDerivationGoals;
+
     std::map<StorePath, std::weak_ptr<DerivationGoal>> derivationGoals;
     std::map<StorePath, std::weak_ptr<PathSubstitutionGoal>> substitutionGoals;
     std::map<DrvOutput, std::weak_ptr<DrvOutputSubstitutionGoal>> drvOutputSubstitutionGoals;
@@ -189,6 +208,9 @@ public:
      * @ref DerivationGoal "derivation goal"
      */
 private:
+    std::shared_ptr<CreateDerivationAndRealiseGoal> makeCreateDerivationAndRealiseGoal(
+        ref<SingleDerivedPath> drvPath,
+        const OutputsSpec & wantedOutputs, BuildMode buildMode = bmNormal);
     std::shared_ptr<DerivationGoal> makeDerivationGoalCommon(
         const StorePath & drvPath, const OutputsSpec & wantedOutputs,
         std::function<std::shared_ptr<DerivationGoal>()> mkDrvGoal);

src/libstore/derivations.cc

@@ -12,7 +12,7 @@
 
 namespace nix {
 
-std::optional<StorePath> DerivationOutput::path(const Store & store, std::string_view drvName, std::string_view outputName) const
+std::optional<StorePath> DerivationOutput::path(const Store & store, std::string_view drvName, OutputNameView outputName) const
 {
     return std::visit(overloaded {
         [](const DerivationOutput::InputAddressed & doi) -> std::optional<StorePath> {
@@ -36,7 +36,7 @@ std::optional<StorePath> DerivationOutput::path(const Store & store, std::string
 }
 
 
-StorePath DerivationOutput::CAFixed::path(const Store & store, std::string_view drvName, std::string_view outputName) const
+StorePath DerivationOutput::CAFixed::path(const Store & store, std::string_view drvName, OutputNameView outputName) const
 {
     return store.makeFixedOutputPathFromCA(
         outputPathName(drvName, outputName),
@@ -466,7 +466,7 @@ bool isDerivation(std::string_view fileName)
 }
 
 
-std::string outputPathName(std::string_view drvName, std::string_view outputName) {
+std::string outputPathName(std::string_view drvName, OutputNameView outputName) {
     std::string res { drvName };
     if (outputName != "out") {
         res += "-";
@@ -810,7 +810,7 @@ void writeDerivation(Sink & out, const Store & store, const BasicDerivation & dr
 }
 
 
-std::string hashPlaceholder(const std::string_view outputName)
+std::string hashPlaceholder(const OutputNameView outputName)
 {
     // FIXME: memoize?
     return "/" + hashString(htSHA256, concatStrings("nix-output:", outputName)).to_string(Base32, false);
@@ -963,7 +963,7 @@ void Derivation::checkInvariants(Store & store, const StorePath & drvPath) const
 const Hash impureOutputHash = hashString(htSHA256, "impure");
 
 nlohmann::json DerivationOutput::toJSON(
-    const Store & store, std::string_view drvName, std::string_view outputName) const
+    const Store & store, std::string_view drvName, OutputNameView outputName) const
 {
     nlohmann::json res = nlohmann::json::object();
     std::visit(overloaded {
@@ -990,7 +990,7 @@ nlohmann::json DerivationOutput::toJSON(
 
 
 DerivationOutput DerivationOutput::fromJSON(
-    const Store & store, std::string_view drvName, std::string_view outputName,
+    const Store & store, std::string_view drvName, OutputNameView outputName,
     const nlohmann::json & _json,
     const ExperimentalFeatureSettings & xpSettings)
 {

src/libstore/derivations.hh

@@ -55,7 +55,7 @@ struct DerivationOutput
          * @param drvName The name of the derivation this is an output of, without the `.drv`.
          * @param outputName The name of this output.
          */
-        StorePath path(const Store & store, std::string_view drvName, std::string_view outputName) const;
+        StorePath path(const Store & store, std::string_view drvName, OutputNameView outputName) const;
 
         GENERATE_CMP(CAFixed, me->ca);
     };
@@ -132,19 +132,19 @@ struct DerivationOutput
      * the safer interface provided by
      * BasicDerivation::outputsAndOptPaths
      */
-    std::optional<StorePath> path(const Store & store, std::string_view drvName, std::string_view outputName) const;
+    std::optional<StorePath> path(const Store & store, std::string_view drvName, OutputNameView outputName) const;
 
     nlohmann::json toJSON(
         const Store & store,
         std::string_view drvName,
-        std::string_view outputName) const;
+        OutputNameView outputName) const;
     /**
      * @param xpSettings Stop-gap to avoid globals during unit tests.
      */
     static DerivationOutput fromJSON(
         const Store & store,
         std::string_view drvName,
-        std::string_view outputName,
+        OutputNameView outputName,
         const nlohmann::json & json,
         const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
 };
@@ -405,7 +405,7 @@ bool isDerivation(std::string_view fileName);
  * This is usually <drv-name>-<output-name>, but is just <drv-name> when
  * the output name is "out".
  */
-std::string outputPathName(std::string_view drvName, std::string_view outputName);
+std::string outputPathName(std::string_view drvName, OutputNameView outputName);
 
 
 /**
@@ -499,7 +499,7 @@ void writeDerivation(Sink & out, const Store & store, const BasicDerivation & dr
  * own outputs without needing to use the hash of a derivation in
  * itself, making the hash near-impossible to calculate.
  */
-std::string hashPlaceholder(const std::string_view outputName);
+std::string hashPlaceholder(const OutputNameView outputName);
 
 extern const Hash impureOutputHash;
 

src/libstore/derived-path-map.cc

@@ -0,0 +1,33 @@
+#include "derived-path-map.hh"
+
+namespace nix {
+
+template<typename V>
+typename DerivedPathMap<V>::ChildNode & DerivedPathMap<V>::ensureSlot(const SingleDerivedPath & k)
+{
+    std::function<ChildNode &(const SingleDerivedPath & )> initIter;
+    initIter = [&](const auto & k) -> auto & {
+        return std::visit(overloaded {
+            [&](const SingleDerivedPath::Opaque & bo) -> auto & {
+                // will not overwrite if already there
+                return map[bo.path];
+            },
+            [&](const SingleDerivedPath::Built & bfd) -> auto & {
+                auto & n = initIter(*bfd.drvPath);
+                return n.childMap[bfd.output];
+            },
+        }, k.raw());
+    };
+    return initIter(k);
+}
+
+}
+
+// instantiations
+
+#include "create-derivation-and-realise-goal.hh"
+namespace nix {
+
+template struct DerivedPathMap<std::weak_ptr<CreateDerivationAndRealiseGoal>>;
+
+}

src/libstore/derived-path-map.hh

@@ -0,0 +1,73 @@
+#pragma once
+
+#include "types.hh"
+#include "derived-path.hh"
+
+namespace nix {
+
+/**
+ * A simple Trie, of sorts. Conceptually a map of `SingleDerivedPath` to
+ * values.
+ *
+ * Concretely, an n-ary tree, as described below. A
+ * `SingleDerivedPath::Opaque` maps to the value of an immediate child
+ * of the root node. A `SingleDerivedPath::Built` maps to a deeper child
+ * node: the `SingleDerivedPath::Built::drvPath` is first mapped to a a
+ * child node (inductively), and then the
+ * `SingleDerivedPath::Built::output` is used to look up that child's
+ * child via its map. In this manner, every `SingleDerivedPath` is
+ * mapped to a child node.
+ *
+ * @param V A type to instantiate for each output. It should probably
+ * should be an "optional" type so not every interior node has to have a
+ * value. For example, the scheduler uses
+ * `DerivedPathMap<std::weak_ptr<CreateDerivationAndRealiseGoal>>` to
+ * remember which goals correspond to which outputs. `* const Something`
+ * or `std::optional<Something>` would also be good choices for
+ * "optional" types.
+ */
+template<typename V>
+struct DerivedPathMap {
+    /**
+     * A child node (non-root node).
+     */
+    struct ChildNode {
+        /**
+         * Value of this child node.
+         *
+         * @see DerivedPathMap for what `V` should be.
+         */
+        V value;
+
+        /**
+         * The map type for the root node.
+         */
+        using Map = std::map<OutputName, ChildNode>;
+
+        /**
+         * The map of the root node.
+         */
+        Map childMap;
+    };
+
+    /**
+     * The map type for the root node.
+     */
+    using Map = std::map<StorePath, ChildNode>;
+
+    /**
+     * The map of root node.
+     */
+    Map map;
+
+    /**
+     * Find the node for `k`, creating it if needed.
+     *
+     * The node is referred to as a "slot" on the assumption that `V` is
+     * some sort of optional type, so the given key can be set or unset
+     * by changing this node.
+     */
+    ChildNode & ensureSlot(const SingleDerivedPath & k);
+};
+
+}

src/libstore/derived-path.cc

@@ -167,7 +167,7 @@ void drvRequireExperiment(
 
 SingleDerivedPath::Built SingleDerivedPath::Built::parse(
     const Store & store, ref<SingleDerivedPath> drv,
-    std::string_view output,
+    OutputNameView output,
     const ExperimentalFeatureSettings & xpSettings)
 {
     drvRequireExperiment(*drv, xpSettings);
@@ -179,7 +179,7 @@ SingleDerivedPath::Built SingleDerivedPath::Built::parse(
 
 DerivedPath::Built DerivedPath::Built::parse(
     const Store & store, ref<SingleDerivedPath> drv,
-    std::string_view outputsS,
+    OutputNameView outputsS,
     const ExperimentalFeatureSettings & xpSettings)
 {
     drvRequireExperiment(*drv, xpSettings);

src/libstore/derived-path.hh

@@ -42,7 +42,7 @@ struct SingleDerivedPath;
  */
 struct SingleDerivedPathBuilt {
     ref<SingleDerivedPath> drvPath;
-    std::string output;
+    OutputName output;
 
     /**
      * Get the store path this is ultimately derived from (by realising
@@ -71,7 +71,7 @@ struct SingleDerivedPathBuilt {
      */
     static SingleDerivedPathBuilt parse(
         const Store & store, ref<SingleDerivedPath> drvPath,
-        std::string_view outputs,
+        OutputNameView outputs,
         const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
     nlohmann::json toJSON(Store & store) const;
 

src/libstore/downstream-placeholder.cc

@@ -11,7 +11,7 @@ std::string DownstreamPlaceholder::render() const
 
 DownstreamPlaceholder DownstreamPlaceholder::unknownCaOutput(
     const StorePath & drvPath,
-    std::string_view outputName,
+    OutputNameView outputName,
     const ExperimentalFeatureSettings & xpSettings)
 {
     xpSettings.require(Xp::CaDerivations);
@@ -25,7 +25,7 @@ DownstreamPlaceholder DownstreamPlaceholder::unknownCaOutput(
 
 DownstreamPlaceholder DownstreamPlaceholder::unknownDerivation(
     const DownstreamPlaceholder & placeholder,
-    std::string_view outputName,
+    OutputNameView outputName,
     const ExperimentalFeatureSettings & xpSettings)
 {
     xpSettings.require(Xp::DynamicDerivations);

src/libstore/downstream-placeholder.hh

@@ -58,7 +58,7 @@ public:
      */
     static DownstreamPlaceholder unknownCaOutput(
         const StorePath & drvPath,
-        std::string_view outputName,
+        OutputNameView outputName,
         const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
 
     /**
@@ -72,7 +72,7 @@ public:
      */
     static DownstreamPlaceholder unknownDerivation(
         const DownstreamPlaceholder & drvPlaceholder,
-        std::string_view outputName,
+        OutputNameView outputName,
         const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
 
     /**

src/libstore/misc.cc

@@ -399,8 +399,7 @@ StorePath resolveDerivedPath(Store & store, const SingleDerivedPath & req, Store
                     store.printStorePath(drvPath), bfd.output);
             auto & optPath = outputPaths.at(bfd.output);
             if (!optPath)
-                throw Error("'%s' does not yet map to a known concrete store path",
+                throw MissingRealisation(bfd.drvPath->to_string(store), bfd.output);
-                    bfd.to_string(store));
             return *optPath;
         },
     }, req.raw());

src/libstore/outputs-spec.hh

@@ -13,24 +13,36 @@
 
 namespace nix {
 
+/**
+ * An (owned) output name. Just a type alias used to make code more
+ * readible.
+ */
+typedef std::string OutputName;
+
+/**
+ * A borrowed output name. Just a type alias used to make code more
+ * readible.
+ */
+typedef std::string_view OutputNameView;
+
 struct OutputsSpec {
     /**
      * A non-empty set of outputs, specified by name
      */
-    struct Names : std::set<std::string> {
+    struct Names : std::set<OutputName> {
-        using std::set<std::string>::set;
+        using std::set<OutputName>::set;
 
         /* These need to be "inherited manually" */
 
-        Names(const std::set<std::string> & s)
+        Names(const std::set<OutputName> & s)
-            : std::set<std::string>(s)
+            : std::set<OutputName>(s)
         { assert(!empty()); }
 
         /**
          * Needs to be "inherited manually"
          */
-        Names(std::set<std::string> && s)
+        Names(std::set<OutputName> && s)
-            : std::set<std::string>(s)
+            : std::set<OutputName>(s)
         { assert(!empty()); }
 
         /* This set should always be non-empty, so we delete this
@@ -57,7 +69,7 @@ struct OutputsSpec {
      */
     OutputsSpec() = delete;
 
-    bool contains(const std::string & output) const;
+    bool contains(const OutputName & output) const;
 
     /**
      * Create a new OutputsSpec which is the union of this and that.

src/libstore/realisation.hh

@@ -34,7 +34,7 @@ struct DrvOutput {
     /**
      * The name of the output.
      */
-    std::string outputName;
+    OutputName outputName;
 
     std::string to_string() const;
 
@@ -84,7 +84,7 @@ struct Realisation {
  * Since these are the outputs of a single derivation, we know the
  * output names are unique so we can use them as the map key.
  */
-typedef std::map<std::string, Realisation> SingleDrvOutputs;
+typedef std::map<OutputName, Realisation> SingleDrvOutputs;
 
 /**
  * Collection type for multiple derivations' outputs' `Realisation`s.
@@ -146,7 +146,7 @@ public:
     MissingRealisation(DrvOutput & outputId)
         : MissingRealisation(outputId.outputName, outputId.strHash())
     {}
-    MissingRealisation(std::string_view drv, std::string outputName)
+    MissingRealisation(std::string_view drv, OutputName outputName)
         : Error( "cannot operate on output '%s' of the "
                 "unbuilt derivation '%s'",
                 outputName,

src/libutil/error.cc

@@ -14,6 +14,11 @@ void BaseError::addTrace(std::shared_ptr<AbstractPos> && e, hintformat hint, boo
     err.traces.push_front(Trace { .pos = std::move(e), .hint = hint, .frame = frame });
 }
 
+void throwExceptionSelfCheck(){
+    // This is meant to be caught in initLibUtil()
+    throw SysError("C++ exception handling is broken. This would appear to be a problem with the way Nix was compiled and/or linked and/or loaded.");
+}
+
 // c++ std::exception descendants must have a 'const char* what()' function.
 // This stringifies the error and caches it for use by what(), or similarly by msg().
 const std::string & BaseError::calcWhat() const

src/libutil/error.hh

@@ -214,4 +214,8 @@ public:
     }
 };
 
+/** Throw an exception for the purpose of checking that exception handling works; see 'initLibUtil()'.
+ */
+void throwExceptionSelfCheck();
+
 }

src/libutil/util.cc

@@ -48,6 +48,23 @@ extern char * * environ __attribute__((weak));
 namespace nix {
 
 void initLibUtil() {
+    // Check that exception handling works. Exception handling has been observed
+    // not to work on darwin when the linker flags aren't quite right.
+    // In this case we don't want to expose the user to some unrelated uncaught
+    // exception, but rather tell them exactly that exception handling is
+    // broken.
+    // When exception handling fails, the message tends to be printed by the
+    // C++ runtime, followed by an abort.
+    // For example on macOS we might see an error such as
+    // libc++abi: terminating with uncaught exception of type nix::SysError: error: C++ exception handling is broken. This would appear to be a problem with the way Nix was compiled and/or linked and/or loaded.
+    bool caught = false;
+    try {
+        throwExceptionSelfCheck();
+    } catch (nix::Error _e) {
+        caught = true;
+    }
+    // This is not actually the main point of this check, but let's make sure anyway:
+    assert(caught);
 }
 
 std::optional<std::string> getEnv(const std::string & key)

src/nix/daemon.cc

@@ -500,6 +500,45 @@ static RegisterLegacyCommand r_nix_daemon("nix-daemon", main_nix_daemon);
 
 struct CmdDaemon : StoreCommand
 {
+    bool stdio = false;
+    std::optional<TrustedFlag> isTrustedOpt = std::nullopt;
+
+    CmdDaemon()
+    {
+        addFlag({
+            .longName = "stdio",
+            .description = "Attach to standard I/O, instead of trying to bind to a UNIX socket.",
+            .handler = {&stdio, true},
+        });
+
+        addFlag({
+            .longName = "force-trusted",
+            .description = "Force the daemon to trust connecting clients.",
+            .handler = {[&]() {
+                isTrustedOpt = Trusted;
+            }},
+            .experimentalFeature = Xp::DaemonTrustOverride,
+        });
+
+        addFlag({
+            .longName = "force-untrusted",
+            .description = "Force the daemon to not trust connecting clients. The connection will be processed by the receiving daemon before forwarding commands.",
+            .handler = {[&]() {
+                isTrustedOpt = NotTrusted;
+            }},
+            .experimentalFeature = Xp::DaemonTrustOverride,
+        });
+
+        addFlag({
+            .longName = "default-trust",
+            .description = "Use Nix's default trust.",
+            .handler = {[&]() {
+                isTrustedOpt = std::nullopt;
+            }},
+            .experimentalFeature = Xp::DaemonTrustOverride,
+        });
+    }
+
     std::string description() override
     {
         return "daemon to perform store operations on behalf of non-root clients";
@@ -516,7 +555,7 @@ struct CmdDaemon : StoreCommand
 
     void run(ref<Store> store) override
     {
-        runDaemon(false, std::nullopt);
+        runDaemon(stdio, isTrustedOpt);
     }
 };
 

src/nix/daemon.md

@@ -1,20 +1,44 @@
 R""(
 
-# Example
+# Examples
 
-* Run the daemon in the foreground:
+* Run the daemon:
 
   ```console
   # nix daemon
   ```
 
+* Run the daemon and listen on standard I/O instead of binding to a UNIX socket:
+
+  ```console
+  # nix daemon --stdio
+  ```
+
+* Run the daemon and force all connections to be trusted:
+
+  ```console
+  # nix daemon --force-trusted
+  ```
+
+* Run the daemon and force all connections to be untrusted:
+
+  ```console
+  # nix daemon --force-untrusted
+  ```
+
+* Run the daemon, listen on standard I/O, and force all connections to use Nix's default trust:
+
+  ```console
+  # nix daemon --stdio --default-trust
+  ```
+
 # Description
 
 This command runs the Nix daemon, which is a required component in
 multi-user Nix installations. It runs build tasks and other
 operations on the Nix store on behalf of non-root users. Usually you
 don't run the daemon directly; instead it's managed by a service
-management framework such as `systemd`.
+management framework such as `systemd` on Linux, or `launchctl` on Darwin.
 
 Note that this daemon does not fork into the background.
 

tests/common/vars-and-functions.sh.in

@@ -195,7 +195,7 @@ expect() {
     shift
     "$@" && res=0 || res="$?"
     if [[ $res -ne $expected ]]; then
-        echo "Expected '$expected' but got '$res' while running '${*@Q}'" >&2
+        echo "Expected exit code '$expected' but got '$res' from command ${*@Q}" >&2
         return 1
     fi
     return 0
@@ -209,7 +209,7 @@ expectStderr() {
     shift
     "$@" 2>&1 && res=0 || res="$?"
     if [[ $res -ne $expected ]]; then
-        echo "Expected '$expected' but got '$res' while running '${*@Q}'" >&2
+        echo "Expected exit code '$expected' but got '$res' from command ${*@Q}" >&2
         return 1
     fi
     return 0

tests/dyn-drv/build-built-drv.sh

@@ -18,4 +18,6 @@ clearStore
 
 drvDep=$(nix-instantiate ./text-hashed-output.nix -A producingDrv)
 
-expectStderr 1 nix build "${drvDep}^out^out" --no-link | grepQuiet "Building dynamic derivations in one shot is not yet implemented"
+out2=$(nix build "${drvDep}^out^out" --no-link)
+
+test $out1 == $out2

tests/flakes/follow-paths.sh

@@ -146,5 +146,87 @@ EOF
 
 git -C $flakeFollowsA add flake.nix
 
-nix flake lock $flakeFollowsA 2>&1 | grep "warning: input 'B' has an override for a non-existent input 'invalid'"
+nix flake lock "$flakeFollowsA" 2>&1 | grep "warning: input 'B' has an override for a non-existent input 'invalid'"
-nix flake lock $flakeFollowsA 2>&1 | grep "warning: input 'B' has an override for a non-existent input 'invalid2'"
+nix flake lock "$flakeFollowsA" 2>&1 | grep "warning: input 'B' has an override for a non-existent input 'invalid2'"
+
+# Now test follow path overloading
+# This tests a lockfile checking regression https://github.com/NixOS/nix/pull/8819
+#
+# We construct the following graph, where p->q means p has input q.
+# A double edge means that the edge gets overridden using `follows`.
+#
+#      A
+#     / \
+#    /   \
+#   v     v
+#   B ==> C   --- follows declared in A
+#    \\  /
+#     \\/     --- follows declared in B
+#      v
+#      D
+#
+# The message was
+#    error: input 'B/D' follows a non-existent input 'B/C/D'
+# 
+# Note that for `B` to resolve its follow for `D`, it needs `C/D`, for which it needs to resolve the follow on `C` first.
+flakeFollowsOverloadA="$TEST_ROOT/follows/overload/flakeA"
+flakeFollowsOverloadB="$TEST_ROOT/follows/overload/flakeA/flakeB"
+flakeFollowsOverloadC="$TEST_ROOT/follows/overload/flakeA/flakeB/flakeC"
+flakeFollowsOverloadD="$TEST_ROOT/follows/overload/flakeA/flakeB/flakeC/flakeD"
+
+# Test following path flakerefs.
+createGitRepo "$flakeFollowsOverloadA"
+mkdir -p "$flakeFollowsOverloadB"
+mkdir -p "$flakeFollowsOverloadC"
+mkdir -p "$flakeFollowsOverloadD"
+
+cat > "$flakeFollowsOverloadD/flake.nix" <<EOF
+{
+    description = "Flake D";
+    inputs = {};
+    outputs = { ... }: {};
+}
+EOF
+
+cat > "$flakeFollowsOverloadC/flake.nix" <<EOF
+{
+    description = "Flake C";
+    inputs.D.url = "path:./flakeD";
+    outputs = { ... }: {};
+}
+EOF
+
+cat > "$flakeFollowsOverloadB/flake.nix" <<EOF
+{
+    description = "Flake B";
+    inputs = {
+        C = {
+            url = "path:./flakeC";
+        };
+        D.follows = "C/D";
+    };
+    outputs = { ... }: {};
+}
+EOF
+
+# input B/D should be able to be found...
+cat > "$flakeFollowsOverloadA/flake.nix" <<EOF
+{
+    description = "Flake A";
+    inputs = {
+        B = {
+            url = "path:./flakeB";
+            inputs.C.follows = "C";
+        };
+        C.url = "path:./flakeB/flakeC";
+    };
+    outputs = { ... }: {};
+}
+EOF
+
+git -C "$flakeFollowsOverloadA" add flake.nix flakeB/flake.nix \
+  flakeB/flakeC/flake.nix flakeB/flakeC/flakeD/flake.nix
+
+nix flake metadata "$flakeFollowsOverloadA"
+nix flake update "$flakeFollowsOverloadA"
+nix flake lock "$flakeFollowsOverloadA"

tests/lang/eval-okay-pathexists.nix

@@ -1,4 +1,7 @@
-builtins.pathExists (builtins.toPath ./lib.nix)
+builtins.pathExists (./lib.nix)
+&& builtins.pathExists (builtins.toPath ./lib.nix)
+&& builtins.pathExists (builtins.toString ./lib.nix)
+&& !builtins.pathExists (builtins.toString ./lib.nix + "/")
 && builtins.pathExists (builtins.toPath (builtins.toString ./lib.nix))
 && !builtins.pathExists (builtins.toPath (builtins.toString ./bla.nix))
 && builtins.pathExists ./lib.nix

tests/tarball.sh

@@ -9,6 +9,7 @@ rm -rf $tarroot
 mkdir -p $tarroot
 cp dependencies.nix $tarroot/default.nix
 cp config.nix dependencies.builder*.sh $tarroot/
+touch -d '@1000000000' $tarroot $tarroot/*
 
 hash=$(nix hash path $tarroot)
 
@@ -36,6 +37,8 @@ test_tarball() {
     nix-build  -o $TEST_ROOT/result -E "import (fetchTree { type = \"tarball\"; url = file:///does-not-exist/must-remain-unused/$tarball; narHash = \"$hash\"; })"
     expectStderr 102 nix-build  -o $TEST_ROOT/result -E "import (fetchTree { type = \"tarball\"; url = file://$tarball; narHash = \"sha256-xdKv2pq/IiwLSnBBJXW8hNowI4MrdZfW+SYqDQs7Tzc=\"; })" | grep 'NAR hash mismatch in input'
 
+    [[ $(nix eval --impure --expr "(fetchTree file://$tarball).lastModified") = 1000000000 ]]
+
     nix-instantiate --strict --eval -E "!((import (fetchTree { type = \"tarball\"; url = file://$tarball; narHash = \"$hash\"; })) ? submodules)" >&2
     nix-instantiate --strict --eval -E "!((import (fetchTree { type = \"tarball\"; url = file://$tarball; narHash = \"$hash\"; })) ? submodules)" 2>&1 | grep 'true'