Merge remote-tracking branch 'upstream/master' into trustless-remote-builder-simple

doc/manual/src/command-ref/opt-common.md

@@ -203,10 +203,9 @@ Most Nix commands accept the following command-line options:
     instead.
 
   - <span id="opt-I">[`-I`](#opt-I)</span> *path*\
-    Add a path to the Nix expression search path. This option may be
-    given multiple times. See the `NIX_PATH` environment variable for
-    information on the semantics of the Nix search path. Paths added
-    through `-I` take precedence over `NIX_PATH`.
+    Add an entry to the [Nix expression search path](@docroot@/command-ref/conf-file.md#conf-nix-path).
+    This option may be given multiple times.
+    Paths added through `-I` take precedence over [`NIX_PATH`](@docroot@/command-ref/env-common.md#env-NIX_PATH).
 
   - <span id="opt-option">[`--option`](#opt-option)</span> *name* *value*\
     Set the Nix configuration option *name* to *value*. This overrides

perl/lib/Nix/Store.xs

@@ -293,7 +293,13 @@ SV * makeFixedOutputPath(int recursive, char * algo, char * hash, char * name)
         try {
             auto h = Hash::parseAny(hash, parseHashType(algo));
             auto method = recursive ? FileIngestionMethod::Recursive : FileIngestionMethod::Flat;
-            auto path = store()->makeFixedOutputPath(method, h, name);
+            auto path = store()->makeFixedOutputPath(name, FixedOutputInfo {
+                .hash = {
+                    .method = method,
+                    .hash = h,
+                },
+                .references = {},
+            });
             XPUSHs(sv_2mortal(newSVpv(store()->printStorePath(path).c_str(), 0)));
         } catch (Error & e) {
             croak("%s", e.what());

src/build-remote/build-remote.cc

@@ -350,8 +350,9 @@ connected:
                     debug("missing output %s", outputName);
                     assert(optResult);
                     auto & result = *optResult;
-                    assert(result.builtOutputs.count(thisOutputId));
-                    auto newRealisation = result.builtOutputs.at(thisOutputId);
+                    auto i = result.builtOutputs.find(outputName);
+                    assert(i != result.builtOutputs.end());
+                    auto & newRealisation = i->second;
                     missingRealisations.insert(newRealisation);
                     missingPaths.insert(newRealisation.outPath);
                 }

src/libcmd/installables.cc

@@ -593,8 +593,8 @@ std::vector<std::pair<ref<Installable>, BuiltPathWithResult>> Installable::build
                 std::visit(overloaded {
                     [&](const DerivedPath::Built & bfd) {
                         std::map<std::string, StorePath> outputs;
-                        for (auto & path : buildResult.builtOutputs)
-                            outputs.emplace(path.first.outputName, path.second.outPath);
+                        for (auto & [outputName, realisation] : buildResult.builtOutputs)
+                            outputs.emplace(outputName, realisation.outPath);
                         res.push_back({aux.installable, {
                             .path = BuiltPath::Built { bfd.drvPath, outputs },
                             .info = aux.info,

src/libexpr/primops.cc

@@ -1293,7 +1293,13 @@ drvName, Bindings * attrs, Value & v)
         auto h = newHashAllowEmpty(*outputHash, parseHashTypeOpt(outputHashAlgo));
 
         auto method = ingestionMethod.value_or(FileIngestionMethod::Flat);
-        auto outPath = state.store->makeFixedOutputPath(method, h, drvName);
+        auto outPath = state.store->makeFixedOutputPath(drvName, FixedOutputInfo {
+            .hash = {
+                .method = method,
+                .hash = h,
+            },
+            .references = {},
+        });
         drv.env["out"] = state.store->printStorePath(outPath);
         drv.outputs.insert_or_assign("out",
             DerivationOutput::CAFixed {
@@ -2103,7 +2109,13 @@ static void addPath(
 
         std::optional<StorePath> expectedStorePath;
         if (expectedHash)
-            expectedStorePath = state.store->makeFixedOutputPath(method, *expectedHash, name);
+            expectedStorePath = state.store->makeFixedOutputPath(name, FixedOutputInfo {
+                .hash = {
+                    .method = method,
+                    .hash = *expectedHash,
+                },
+                .references = {},
+            });
 
         if (!expectedHash || !state.store->isValidPath(*expectedStorePath)) {
             StorePath dstPath = settings.readOnlyMode

src/libexpr/primops/fetchTree.cc

@@ -243,10 +243,15 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
 
     // early exit if pinned and already in the store
     if (expectedHash && expectedHash->type == htSHA256) {
-        auto expectedPath =
-            unpack
-            ? state.store->makeFixedOutputPath(FileIngestionMethod::Recursive, *expectedHash, name, {})
-            : state.store->makeFixedOutputPath(FileIngestionMethod::Flat, *expectedHash, name, {});
+        auto expectedPath = state.store->makeFixedOutputPath(
+            name,
+            FixedOutputInfo {
+                .hash = {
+                    .method = unpack ? FileIngestionMethod::Recursive : FileIngestionMethod::Flat,
+                    .hash = *expectedHash,
+                },
+                .references = {}
+            });
 
         if (state.store->isValidPath(expectedPath)) {
             state.allowAndSetStorePathString(expectedPath, v);

src/libfetchers/fetchers.cc

@@ -210,7 +210,13 @@ StorePath Input::computeStorePath(Store & store) const
     auto narHash = getNarHash();
     if (!narHash)
         throw Error("cannot compute store path for unlocked input '%s'", to_string());
-    return store.makeFixedOutputPath(FileIngestionMethod::Recursive, *narHash, getName());
+    return store.makeFixedOutputPath(getName(), FixedOutputInfo {
+        .hash = {
+            .method = FileIngestionMethod::Recursive,
+            .hash = *narHash,
+        },
+        .references = {},
+    });
 }
 
 std::string Input::getType() const

src/libfetchers/tarball.cc

@@ -71,15 +71,19 @@ DownloadFileResult downloadFile(
         dumpString(res.data, sink);
         auto hash = hashString(htSHA256, res.data);
         ValidPathInfo info {
-            store->makeFixedOutputPath(FileIngestionMethod::Flat, hash, name),
+            *store,
+            name,
+            FixedOutputInfo {
+                .hash = {
+                    .method = FileIngestionMethod::Flat,
+                    .hash = hash,
+                },
+                .references = {},
+            },
             hashString(htSHA256, sink.s),
         };
         info.narSize = sink.s.size();
-        info.ca = FixedOutputHash {
-            .method = FileIngestionMethod::Flat,
-            .hash = hash,
-        };
-        auto source = StringSource(sink.s);
+        auto source = StringSource { sink.s };
         store->addToStore(info, source, NoRepair, NoCheckSigs);
         storePath = std::move(info.path);
     }

src/libstore/binary-cache-store.cc

@@ -306,11 +306,22 @@ StorePath BinaryCacheStore::addToStoreFromDump(Source & dump, std::string_view n
         unsupported("addToStoreFromDump");
     return addToStoreCommon(dump, repair, CheckSigs, [&](HashResult nar) {
         ValidPathInfo info {
-            makeFixedOutputPath(method, nar.first, name, references),
+            *this,
+            name,
+            FixedOutputInfo {
+                .hash = {
+                    .method = method,
+                    .hash = nar.first,
+                },
+                .references = {
+                    .others = references,
+                    // caller is not capable of creating a self-reference, because this is content-addressed without modulus
+                    .self = false,
+                },
+            },
             nar.first,
         };
         info.narSize = nar.second;
-        info.references = references;
         return info;
     })->path;
 }
@@ -414,15 +425,22 @@ StorePath BinaryCacheStore::addToStore(
     });
     return addToStoreCommon(*source, repair, CheckSigs, [&](HashResult nar) {
         ValidPathInfo info {
-            makeFixedOutputPath(method, h, name, references),
+            *this,
+            name,
+            FixedOutputInfo {
+                .hash = {
+                    .method = method,
+                    .hash = h,
+                },
+                .references = {
+                    .others = references,
+                    // caller is not capable of creating a self-reference, because this is content-addressed without modulus
+                    .self = false,
+                },
+            },
             nar.first,
         };
         info.narSize = nar.second;
-        info.references = references;
-        info.ca = FixedOutputHash {
-            .method = method,
-            .hash = h,
-        };
         return info;
     })->path;
 }
@@ -434,7 +452,7 @@ StorePath BinaryCacheStore::addTextToStore(
     RepairFlag repair)
 {
     auto textHash = hashString(htSHA256, s);
-    auto path = makeTextPath(name, textHash, references);
+    auto path = makeTextPath(name, TextInfo { { textHash }, references });
 
     if (!repair && isValidPath(path))
         return path;
@@ -443,10 +461,16 @@ StorePath BinaryCacheStore::addTextToStore(
     dumpString(s, sink);
     StringSource source(sink.s);
     return addToStoreCommon(source, repair, CheckSigs, [&](HashResult nar) {
-        ValidPathInfo info { path, nar.first };
+        ValidPathInfo info {
+            *this,
+            std::string { name },
+            TextInfo {
+                { .hash = textHash },
+                references,
+            },
+            nar.first,
+        };
         info.narSize = nar.second;
-        info.ca = TextHash { textHash };
-        info.references = references;
         return info;
     })->path;
 }

src/libstore/build-result.hh

@@ -83,16 +83,11 @@ struct BuildResult
      */
     bool isNonDeterministic = false;
 
-    /**
-     * The derivation we built or the store path we substituted.
-     */
-    DerivedPath path;
-
     /**
      * For derivations, a mapping from the names of the wanted outputs
      * to actual paths.
      */
-    DrvOutputs builtOutputs;
+    SingleDrvOutputs builtOutputs;
 
     /**
      * The start/stop times of the build (or one of the rounds, if it
@@ -116,4 +111,15 @@ struct BuildResult
     }
 };
 
+/**
+ * A `BuildResult` together with its "primary key".
+ */
+struct KeyedBuildResult : BuildResult
+{
+    /**
+     * The derivation we built or the store path we substituted.
+     */
+    DerivedPath path;
+};
+
 }

src/libstore/build/derivation-goal.cc

@@ -145,8 +145,20 @@ void DerivationGoal::work()
 void DerivationGoal::addWantedOutputs(const OutputsSpec & outputs)
 {
     auto newWanted = wantedOutputs.union_(outputs);
+    switch (needRestart) {
+    case NeedRestartForMoreOutputs::OutputsUnmodifedDontNeed:
         if (!newWanted.isSubsetOf(wantedOutputs))
-        needRestart = true;
+            needRestart = NeedRestartForMoreOutputs::OutputsAddedDoNeed;
+        break;
+    case NeedRestartForMoreOutputs::OutputsAddedDoNeed:
+        /* No need to check whether we added more outputs, because a
+           restart is already queued up. */
+        break;
+    case NeedRestartForMoreOutputs::BuildInProgressWillNotNeed:
+        /* We are already building all outputs, so it doesn't matter if
+           we now want more. */
+        break;
+    };
     wantedOutputs = newWanted;
 }
 
@@ -297,12 +309,29 @@ void DerivationGoal::outputsSubstitutionTried()
         In particular, it may be the case that the hole in the closure is
         an output of the current derivation, which causes a loop if retried.
      */
-    if (nrIncompleteClosure > 0 && nrIncompleteClosure == nrFailed) retrySubstitution = true;
+    {
+        bool substitutionFailed =
+            nrIncompleteClosure > 0 &&
+            nrIncompleteClosure == nrFailed;
+        switch (retrySubstitution) {
+        case RetrySubstitution::NoNeed:
+            if (substitutionFailed)
+                retrySubstitution = RetrySubstitution::YesNeed;
+            break;
+        case RetrySubstitution::YesNeed:
+            // Should not be able to reach this state from here.
+            assert(false);
+            break;
+        case RetrySubstitution::AlreadyRetried:
+            debug("substitution failed again, but we already retried once. Not retrying again.");
+            break;
+        }
+    }
 
     nrFailed = nrNoSubstituters = nrIncompleteClosure = 0;
 
-    if (needRestart) {
-        needRestart = false;
+    if (needRestart == NeedRestartForMoreOutputs::OutputsAddedDoNeed) {
+        needRestart = NeedRestartForMoreOutputs::OutputsUnmodifedDontNeed;
         haveDerivation();
         return;
     }
@@ -330,6 +359,10 @@ void DerivationGoal::outputsSubstitutionTried()
    produced using a substitute.  So we have to build instead. */
 void DerivationGoal::gaveUpOnSubstitution()
 {
+    /* At this point we are building all outputs, so if more are wanted there
+       is no need to restart. */
+    needRestart = NeedRestartForMoreOutputs::BuildInProgressWillNotNeed;
+
     /* The inputs must be built before we can build this goal. */
     inputDrvOutputs.clear();
     if (useDerivation)
@@ -451,8 +484,8 @@ void DerivationGoal::inputsRealised()
         return;
     }
 
-    if (retrySubstitution && !retriedSubstitution) {
-        retriedSubstitution = true;
+    if (retrySubstitution == RetrySubstitution::YesNeed) {
+        retrySubstitution = RetrySubstitution::AlreadyRetried;
         haveDerivation();
         return;
     }
@@ -570,8 +603,6 @@ void DerivationGoal::inputsRealised()
        build hook. */
     state = &DerivationGoal::tryToBuild;
     worker.wakeUp(shared_from_this());
-
-    buildResult = BuildResult { .path = buildResult.path };
 }
 
 void DerivationGoal::started()
@@ -982,7 +1013,7 @@ void DerivationGoal::resolvedFinished()
     auto resolvedDrv = *resolvedDrvGoal->drv;
     auto & resolvedResult = resolvedDrvGoal->buildResult;
 
-    DrvOutputs builtOutputs;
+    SingleDrvOutputs builtOutputs;
 
     if (resolvedResult.success()) {
         auto resolvedHashes = staticOutputHashes(worker.store, resolvedDrv);
@@ -1008,7 +1039,7 @@ void DerivationGoal::resolvedFinished()
                     worker.store.printStorePath(drvPath), wantedOutput);
 
             auto realisation = [&]{
-              auto take1 = get(resolvedResult.builtOutputs, DrvOutput { *resolvedHash, wantedOutput });
+              auto take1 = get(resolvedResult.builtOutputs, wantedOutput);
               if (take1) return *take1;
 
               /* The above `get` should work. But sateful tracking of
@@ -1033,7 +1064,7 @@ void DerivationGoal::resolvedFinished()
                 worker.store.registerDrvOutput(newRealisation);
             }
             outputPaths.insert(realisation.outPath);
-            builtOutputs.emplace(realisation.id, realisation);
+            builtOutputs.emplace(wantedOutput, realisation);
         }
 
         runPostBuildHook(
@@ -1158,7 +1189,7 @@ HookReply DerivationGoal::tryBuildHook()
 }
 
 
-DrvOutputs DerivationGoal::registerOutputs()
+SingleDrvOutputs DerivationGoal::registerOutputs()
 {
     /* When using a build hook, the build hook can register the output
        as valid (by doing `nix-store --import').  If so we don't have
@@ -1320,7 +1351,7 @@ OutputPathMap DerivationGoal::queryDerivationOutputMap()
 }
 
 
-std::pair<bool, DrvOutputs> DerivationGoal::checkPathValidity()
+std::pair<bool, SingleDrvOutputs> DerivationGoal::checkPathValidity()
 {
     if (!drv->type().isPure()) return { false, {} };
 
@@ -1333,7 +1364,7 @@ std::pair<bool, DrvOutputs> DerivationGoal::checkPathValidity()
             return static_cast<StringSet>(names);
         },
     }, wantedOutputs.raw());
-    DrvOutputs validOutputs;
+    SingleDrvOutputs validOutputs;
 
     for (auto & i : queryPartialDerivationOutputMap()) {
         auto initialOutput = get(initialOutputs, i.first);
@@ -1376,7 +1407,7 @@ std::pair<bool, DrvOutputs> DerivationGoal::checkPathValidity()
             }
         }
         if (info.wanted && info.known && info.known->isValid())
-            validOutputs.emplace(drvOutput, Realisation { drvOutput, info.known->path });
+            validOutputs.emplace(i.first, Realisation { drvOutput, info.known->path });
     }
 
     // If we requested all the outputs, we are always fine.
@@ -1400,7 +1431,7 @@ std::pair<bool, DrvOutputs> DerivationGoal::checkPathValidity()
 }
 
 
-DrvOutputs DerivationGoal::assertPathValidity()
+SingleDrvOutputs DerivationGoal::assertPathValidity()
 {
     auto [allValid, validOutputs] = checkPathValidity();
     if (!allValid)
@@ -1411,7 +1442,7 @@ DrvOutputs DerivationGoal::assertPathValidity()
 
 void DerivationGoal::done(
     BuildResult::Status status,
-    DrvOutputs builtOutputs,
+    SingleDrvOutputs builtOutputs,
     std::optional<Error> ex)
 {
     buildResult.status = status;
@@ -1452,12 +1483,28 @@ void DerivationGoal::waiteeDone(GoalPtr waitee, ExitCode result)
 {
     Goal::waiteeDone(waitee, result);
 
-    if (waitee->buildResult.success())
-        if (auto bfd = std::get_if<DerivedPath::Built>(&waitee->buildResult.path))
-            for (auto & [output, realisation] : waitee->buildResult.builtOutputs)
+    if (!useDerivation) return;
+    auto & fullDrv = *dynamic_cast<Derivation *>(drv.get());
+
+    auto * dg = dynamic_cast<DerivationGoal *>(&*waitee);
+    if (!dg) return;
+
+    auto outputs = fullDrv.inputDrvs.find(dg->drvPath);
+    if (outputs == fullDrv.inputDrvs.end()) return;
+
+    for (auto & outputName : outputs->second) {
+        auto buildResult = dg->getBuildResult(DerivedPath::Built {
+            .drvPath = dg->drvPath,
+            .outputs = OutputsSpec::Names { outputName },
+        });
+        if (buildResult.success()) {
+            auto i = buildResult.builtOutputs.find(outputName);
+            if (i != buildResult.builtOutputs.end())
                 inputDrvOutputs.insert_or_assign(
-                    { bfd->drvPath, output.outputName },
-                    realisation.outPath);
+                    { dg->drvPath, outputName },
+                    i->second.outPath);
+        }
+    }
 }
 
 }

src/libstore/build/derivation-goal.hh

@@ -78,22 +78,58 @@ struct DerivationGoal : public Goal
      */
     std::map<std::pair<StorePath, std::string>, StorePath> inputDrvOutputs;
 
+    /**
+     * See `needRestart`; just for that field.
+     */
+    enum struct NeedRestartForMoreOutputs {
+        /**
+         * The goal state machine is progressing based on the current value of
+         * `wantedOutputs. No actions are needed.
+         */
+        OutputsUnmodifedDontNeed,
+        /**
+         * `wantedOutputs` has been extended, but the state machine is
+         * proceeding according to its old value, so we need to restart.
+         */
+        OutputsAddedDoNeed,
+        /**
+         * The goal state machine has progressed to the point of doing a build,
+         * in which case all outputs will be produced, so extensions to
+         * `wantedOutputs` no longer require a restart.
+         */
+        BuildInProgressWillNotNeed,
+    };
+
     /**
      * Whether additional wanted outputs have been added.
      */
-    bool needRestart = false;
+    NeedRestartForMoreOutputs needRestart = NeedRestartForMoreOutputs::OutputsUnmodifedDontNeed;
+
+    /**
+     * See `retrySubstitution`; just for that field.
+     */
+    enum RetrySubstitution {
+        /**
+         * No issues have yet arose, no need to restart.
+         */
+        NoNeed,
+        /**
+         * Something failed and there is an incomplete closure. Let's retry
+         * substituting.
+         */
+        YesNeed,
+        /**
+         * We are current or have already retried substitution, and whether or
+         * not something goes wrong we will not retry again.
+         */
+        AlreadyRetried,
+    };
 
     /**
      * Whether to retry substituting the outputs after building the
      * inputs. This is done in case of an incomplete closure.
      */
-    bool retrySubstitution = false;
-
-    /**
-     * Whether we've retried substitution, in which case we won't try
-     * again.
-     */
-    bool retriedSubstitution = false;
+    RetrySubstitution retrySubstitution = RetrySubstitution::NoNeed;
 
     /**
      * The derivation stored at drvPath.
@@ -217,7 +253,7 @@ struct DerivationGoal : public Goal
      * Check that the derivation outputs all exist and register them
      * as valid.
      */
-    virtual DrvOutputs registerOutputs();
+    virtual SingleDrvOutputs registerOutputs();
 
     /**
      * Open a log file and a pipe to it.
@@ -270,17 +306,17 @@ struct DerivationGoal : public Goal
      * Update 'initialOutputs' to determine the current status of the
      * outputs of the derivation. Also returns a Boolean denoting
      * whether all outputs are valid and non-corrupt, and a
-     * 'DrvOutputs' structure containing the valid and wanted
+     * 'SingleDrvOutputs' structure containing the valid and wanted
      * outputs.
      */
-    std::pair<bool, DrvOutputs> checkPathValidity();
+    std::pair<bool, SingleDrvOutputs> checkPathValidity();
 
     /**
      * Aborts if any output is not valid or corrupt, and otherwise
-     * returns a 'DrvOutputs' structure containing the wanted
+     * returns a 'SingleDrvOutputs' structure containing the wanted
      * outputs.
      */
-    DrvOutputs assertPathValidity();
+    SingleDrvOutputs assertPathValidity();
 
     /**
      * Forcibly kill the child process, if any.
@@ -293,7 +329,7 @@ struct DerivationGoal : public Goal
 
     void done(
         BuildResult::Status status,
-        DrvOutputs builtOutputs = {},
+        SingleDrvOutputs builtOutputs = {},
         std::optional<Error> ex = {});
 
     void waiteeDone(GoalPtr waitee, ExitCode result) override;

src/libstore/build/entry-points.cc

@@ -10,16 +10,8 @@ void Store::buildPaths(const std::vector<DerivedPath> & reqs, BuildMode buildMod
     Worker worker(*this, evalStore ? *evalStore : *this);
 
     Goals goals;
-    for (const auto & br : reqs) {
-        std::visit(overloaded {
-            [&](const DerivedPath::Built & bfd) {
-                goals.insert(worker.makeDerivationGoal(bfd.drvPath, bfd.outputs, buildMode));
-            },
-            [&](const DerivedPath::Opaque & bo) {
-                goals.insert(worker.makePathSubstitutionGoal(bo.path, buildMode == bmRepair ? Repair : NoRepair));
-            },
-        }, br.raw());
-    }
+    for (auto & br : reqs)
+        goals.insert(worker.makeGoal(br, buildMode));
 
     worker.run(goals);
 
@@ -47,7 +39,7 @@ void Store::buildPaths(const std::vector<DerivedPath> & reqs, BuildMode buildMod
     }
 }
 
-std::vector<BuildResult> Store::buildPathsWithResults(
+std::vector<KeyedBuildResult> Store::buildPathsWithResults(
     const std::vector<DerivedPath> & reqs,
     BuildMode buildMode,
     std::shared_ptr<Store> evalStore)
@@ -55,23 +47,23 @@ std::vector<BuildResult> Store::buildPathsWithResults(
     Worker worker(*this, evalStore ? *evalStore : *this);
 
     Goals goals;
-    for (const auto & br : reqs) {
-        std::visit(overloaded {
-            [&](const DerivedPath::Built & bfd) {
-                goals.insert(worker.makeDerivationGoal(bfd.drvPath, bfd.outputs, buildMode));
-            },
-            [&](const DerivedPath::Opaque & bo) {
-                goals.insert(worker.makePathSubstitutionGoal(bo.path, buildMode == bmRepair ? Repair : NoRepair));
-            },
-        }, br.raw());
+    std::vector<std::pair<const DerivedPath &, GoalPtr>> state;
+
+    for (const auto & req : reqs) {
+        auto goal = worker.makeGoal(req, buildMode);
+        goals.insert(goal);
+        state.push_back({req, goal});
     }
 
     worker.run(goals);
 
-    std::vector<BuildResult> results;
+    std::vector<KeyedBuildResult> results;
 
-    for (auto & i : goals)
-        results.push_back(i->buildResult);
+    for (auto & [req, goalPtr] : state)
+        results.emplace_back(KeyedBuildResult {
+            goalPtr->getBuildResult(req),
+            /* .path = */ req,
+        });
 
     return results;
 }
@@ -84,15 +76,14 @@ BuildResult Store::buildDerivation(const StorePath & drvPath, const BasicDerivat
 
     try {
         worker.run(Goals{goal});
-        return goal->buildResult;
+        return goal->getBuildResult(DerivedPath::Built {
+            .drvPath = drvPath,
+            .outputs = OutputsSpec::All {},
+        });
     } catch (Error & e) {
         return BuildResult {
             .status = BuildResult::MiscFailure,
             .errorMsg = e.msg(),
-            .path = DerivedPath::Built {
-                .drvPath = drvPath,
-                .outputs = OutputsSpec::All { },
-            },
         };
     };
 }

src/libstore/build/goal.cc

@@ -11,6 +11,29 @@ bool CompareGoalPtrs::operator() (const GoalPtr & a, const GoalPtr & b) const {
 }
 
 
+BuildResult Goal::getBuildResult(const DerivedPath & req) {
+    BuildResult res { buildResult };
+
+    if (auto pbp = std::get_if<DerivedPath::Built>(&req)) {
+        auto & bp = *pbp;
+
+        /* Because goals are in general shared between derived paths
+           that share the same derivation, we need to filter their
+           results to get back just the results we care about.
+         */
+
+        for (auto it = res.builtOutputs.begin(); it != res.builtOutputs.end();) {
+            if (bp.outputs.contains(it->first))
+                ++it;
+            else
+                it = res.builtOutputs.erase(it);
+        }
+    }
+
+    return res;
+}
+
+
 void addToWeakGoals(WeakGoals & goals, GoalPtr p)
 {
     if (goals.find(p) != goals.end())

src/libstore/build/goal.hh

@@ -81,11 +81,26 @@ struct Goal : public std::enable_shared_from_this<Goal>
      */
     ExitCode exitCode = ecBusy;
 
+protected:
     /**
      * Build result.
      */
     BuildResult buildResult;
 
+public:
+
+    /**
+     * Project a `BuildResult` with just the information that pertains
+     * to the given request.
+     *
+     * In general, goals may be aliased between multiple requests, and
+     * the stored `BuildResult` has information for the union of all
+     * requests. We don't want to leak what the other request are for
+     * sake of both privacy and determinism, and this "safe accessor"
+     * ensures we don't.
+     */
+    BuildResult getBuildResult(const DerivedPath &);
+
     /**
      * Exception containing an error message, if any.
      */
@@ -93,7 +108,6 @@ struct Goal : public std::enable_shared_from_this<Goal>
 
     Goal(Worker & worker, DerivedPath path)
         : worker(worker)
-        , buildResult { .path = std::move(path) }
     { }
 
     virtual ~Goal()

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

@@ -1335,7 +1335,7 @@ struct RestrictedStore : public virtual RestrictedStoreConfig, public virtual Lo
                 result.rethrow();
     }
 
-    std::vector<BuildResult> buildPathsWithResults(
+    std::vector<KeyedBuildResult> buildPathsWithResults(
         const std::vector<DerivedPath> & paths,
         BuildMode buildMode = bmNormal,
         std::shared_ptr<Store> evalStore = nullptr) override
@@ -2174,7 +2174,7 @@ void LocalDerivationGoal::runChild()
 }
 
 
-DrvOutputs LocalDerivationGoal::registerOutputs()
+SingleDrvOutputs LocalDerivationGoal::registerOutputs()
 {
     /* When using a build hook, the build hook can register the output
        as valid (by doing `nix-store --import').  If so we don't have
@@ -2395,27 +2395,26 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
             }
         };
 
-        auto rewriteRefs = [&]() -> std::pair<bool, StorePathSet> {
+        auto rewriteRefs = [&]() -> StoreReferences {
             /* In the CA case, we need the rewritten refs to calculate the
                final path, therefore we look for a *non-rewritten
                self-reference, and use a bool rather try to solve the
                computationally intractable fixed point. */
-            std::pair<bool, StorePathSet> res {
-                false,
-                {},
+            StoreReferences res {
+                .self = false,
             };
             for (auto & r : references) {
                 auto name = r.name();
                 auto origHash = std::string { r.hashPart() };
                 if (r == *scratchPath) {
-                    res.first = true;
+                    res.self = true;
                 } else if (auto outputRewrite = get(outputRewrites, origHash)) {
                     std::string newRef = *outputRewrite;
                     newRef += '-';
                     newRef += name;
-                    res.second.insert(StorePath { newRef });
+                    res.others.insert(StorePath { newRef });
                 } else {
-                    res.second.insert(r);
+                    res.others.insert(r);
                 }
             }
             return res;
@@ -2448,18 +2447,22 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
                 break;
             }
             auto got = caSink.finish().first;
-            auto refs = rewriteRefs();
-
-            auto finalPath = worker.store.makeFixedOutputPath(
-                    outputHash.method,
-                    got,
+            ValidPathInfo newInfo0 {
+                worker.store,
                 outputPathName(drv->name, outputName),
-                    refs.second,
-                    refs.first);
-            if (*scratchPath != finalPath) {
+                FixedOutputInfo {
+                    .hash = {
+                        .method = outputHash.method,
+                        .hash = got,
+                    },
+                    .references = rewriteRefs(),
+                },
+                Hash::dummy,
+            };
+            if (*scratchPath != newInfo0.path) {
                 // Also rewrite the output path
                 auto source = sinkToSource([&](Sink & nextSink) {
-                    RewritingSink rsink2(oldHashPart, std::string(finalPath.hashPart()), nextSink);
+                    RewritingSink rsink2(oldHashPart, std::string(newInfo0.path.hashPart()), nextSink);
                     dumpPath(actualPath, rsink2);
                     rsink2.flush();
                 });
@@ -2470,19 +2473,8 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
             }
 
             HashResult narHashAndSize = hashPath(htSHA256, actualPath);
-            ValidPathInfo newInfo0 {
-                finalPath,
-                narHashAndSize.first,
-            };
-
+            newInfo0.narHash = narHashAndSize.first;
             newInfo0.narSize = narHashAndSize.second;
-            newInfo0.ca = FixedOutputHash {
-                .method = outputHash.method,
-                .hash = got,
-            };
-            newInfo0.references = refs.second;
-            if (refs.first)
-                newInfo0.references.insert(newInfo0.path);
 
             assert(newInfo0.ca);
             return newInfo0;
@@ -2504,8 +2496,8 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
                 ValidPathInfo newInfo0 { requiredFinalPath, narHashAndSize.first };
                 newInfo0.narSize = narHashAndSize.second;
                 auto refs = rewriteRefs();
-                newInfo0.references = refs.second;
-                if (refs.first)
+                newInfo0.references = std::move(refs.others);
+                if (refs.self)
                     newInfo0.references.insert(newInfo0.path);
                 return newInfo0;
             },
@@ -2519,7 +2511,7 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
                 /* Check wanted hash */
                 const Hash & wanted = dof.hash.hash;
                 assert(newInfo0.ca);
-                auto got = getContentAddressHash(*newInfo0.ca);
+                auto got = newInfo0.ca->getHash();
                 if (wanted != got) {
                     /* Throw an error after registering the path as
                        valid. */
@@ -2691,7 +2683,7 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
        means it's safe to link the derivation to the output hash. We must do
        that for floating CA derivations, which otherwise couldn't be cached,
        but it's fine to do in all cases. */
-    DrvOutputs builtOutputs;
+    SingleDrvOutputs builtOutputs;
 
     for (auto & [outputName, newInfo] : infos) {
         auto oldinfo = get(initialOutputs, outputName);
@@ -2710,7 +2702,7 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
             worker.store.registerDrvOutput(thisRealisation);
         }
         if (wantedOutputs.contains(outputName))
-            builtOutputs.emplace(thisRealisation.id, thisRealisation);
+            builtOutputs.emplace(outputName, thisRealisation);
     }
 
     return builtOutputs;

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

@@ -237,7 +237,7 @@ struct LocalDerivationGoal : public DerivationGoal
      * Check that the derivation outputs all exist and register them
      * as valid.
      */
-    DrvOutputs registerOutputs() override;
+    SingleDrvOutputs registerOutputs() override;
 
     void signRealisation(Realisation &) override;
 

src/libstore/build/substitution-goal.cc

@@ -95,7 +95,9 @@ void PathSubstitutionGoal::tryNext()
     subs.pop_front();
 
     if (ca) {
-        subPath = sub->makeFixedOutputPathFromCA(storePath.name(), *ca);
+        subPath = sub->makeFixedOutputPathFromCA(
+            std::string { storePath.name() },
+            ContentAddressWithReferences::withoutRefs(*ca));
         if (sub->storeDir == worker.store.storeDir)
             assert(subPath == storePath);
     } else if (sub->storeDir != worker.store.storeDir) {

src/libstore/build/worker.cc

@@ -92,6 +92,7 @@ std::shared_ptr<PathSubstitutionGoal> Worker::makePathSubstitutionGoal(const Sto
     return goal;
 }
 
+
 std::shared_ptr<DrvOutputSubstitutionGoal> Worker::makeDrvOutputSubstitutionGoal(const DrvOutput& id, RepairFlag repair, std::optional<ContentAddress> ca)
 {
     std::weak_ptr<DrvOutputSubstitutionGoal> & goal_weak = drvOutputSubstitutionGoals[id];
@@ -104,6 +105,20 @@ std::shared_ptr<DrvOutputSubstitutionGoal> Worker::makeDrvOutputSubstitutionGoal
     return goal;
 }
 
+
+GoalPtr Worker::makeGoal(const DerivedPath & req, BuildMode buildMode)
+{
+    return std::visit(overloaded {
+        [&](const DerivedPath::Built & bfd) -> GoalPtr {
+            return makeDerivationGoal(bfd.drvPath, bfd.outputs, buildMode);
+        },
+        [&](const DerivedPath::Opaque & bo) -> GoalPtr {
+            return makePathSubstitutionGoal(bo.path, buildMode == bmRepair ? Repair : NoRepair);
+        },
+    }, req.raw());
+}
+
+
 template<typename K, typename G>
 static void removeGoal(std::shared_ptr<G> goal, std::map<K, std::weak_ptr<G>> & goalMap)
 {

src/libstore/build/worker.hh

@@ -181,7 +181,7 @@ public:
      */
 
     /**
-     * derivation goal
+     * @ref DerivationGoal "derivation goal"
      */
 private:
     std::shared_ptr<DerivationGoal> makeDerivationGoalCommon(
@@ -196,11 +196,19 @@ public:
         const OutputsSpec & wantedOutputs, BuildMode buildMode = bmNormal);
 
     /**
-     * substitution goal
+     * @ref SubstitutionGoal "substitution goal"
      */
     std::shared_ptr<PathSubstitutionGoal> makePathSubstitutionGoal(const StorePath & storePath, RepairFlag repair = NoRepair, std::optional<ContentAddress> ca = std::nullopt);
     std::shared_ptr<DrvOutputSubstitutionGoal> makeDrvOutputSubstitutionGoal(const DrvOutput & id, RepairFlag repair = NoRepair, std::optional<ContentAddress> ca = std::nullopt);
 
+    /**
+     * Make a goal corresponding to the `DerivedPath`.
+     *
+     * It will be a `DerivationGoal` for a `DerivedPath::Built` or
+     * a `SubstitutionGoal` for a `DerivedPath::Opaque`.
+     */
+    GoalPtr makeGoal(const DerivedPath & req, BuildMode buildMode = bmNormal);
+
     /**
      * Remove a dead goal.
      */

src/libstore/content-address.cc

@@ -9,7 +9,7 @@ std::string FixedOutputHash::printMethodAlgo() const
     return makeFileIngestionPrefix(method) + printHashType(hash.type);
 }
 
-std::string makeFileIngestionPrefix(const FileIngestionMethod m)
+std::string makeFileIngestionPrefix(FileIngestionMethod m)
 {
     switch (m) {
     case FileIngestionMethod::Flat:
@@ -21,39 +21,35 @@ std::string makeFileIngestionPrefix(const FileIngestionMethod m)
     }
 }
 
-std::string makeFixedOutputCA(FileIngestionMethod method, const Hash & hash)
-{
-    return "fixed:"
-        + makeFileIngestionPrefix(method)
-        + hash.to_string(Base32, true);
-}
-
-std::string renderContentAddress(ContentAddress ca)
+std::string ContentAddress::render() const
 {
     return std::visit(overloaded {
-        [](TextHash & th) {
-            return "text:" + th.hash.to_string(Base32, true);
+        [](const TextHash & th) {
+            return "text:"
+                + th.hash.to_string(Base32, true);
         },
-        [](FixedOutputHash & fsh) {
-            return makeFixedOutputCA(fsh.method, fsh.hash);
+        [](const FixedOutputHash & fsh) {
+            return "fixed:"
+                + makeFileIngestionPrefix(fsh.method)
+                + fsh.hash.to_string(Base32, true);
         }
-    }, ca);
+    }, raw);
 }
 
-std::string renderContentAddressMethod(ContentAddressMethod cam)
+std::string ContentAddressMethod::render() const
 {
     return std::visit(overloaded {
-        [](TextHashMethod & th) {
+        [](const TextHashMethod & th) {
             return std::string{"text:"} + printHashType(htSHA256);
         },
-        [](FixedOutputHashMethod & fshm) {
+        [](const FixedOutputHashMethod & fshm) {
             return "fixed:" + makeFileIngestionPrefix(fshm.fileIngestionMethod) + printHashType(fshm.hashType);
         }
-    }, cam);
+    }, raw);
 }
 
-/*
-  Parses content address strings up to the hash.
+/**
+ * Parses content address strings up to the hash.
  */
 static ContentAddressMethod parseContentAddressMethodPrefix(std::string_view & rest)
 {
@@ -97,7 +93,7 @@ static ContentAddressMethod parseContentAddressMethodPrefix(std::string_view & r
         throw UsageError("content address prefix '%s' is unrecognized. Recogonized prefixes are 'text' or 'fixed'", prefix);
 }
 
-ContentAddress parseContentAddress(std::string_view rawCa) {
+ContentAddress ContentAddress::parse(std::string_view rawCa) {
     auto rest = rawCa;
 
     ContentAddressMethod caMethod = parseContentAddressMethodPrefix(rest);
@@ -115,10 +111,10 @@ ContentAddress parseContentAddress(std::string_view rawCa) {
                     .hash = Hash::parseNonSRIUnprefixed(rest, std::move(fohMethod.hashType)),
                 });
             },
-        }, caMethod);
+        }, caMethod.raw);
 }
 
-ContentAddressMethod parseContentAddressMethod(std::string_view caMethod)
+ContentAddressMethod ContentAddressMethod::parse(std::string_view caMethod)
 {
     std::string asPrefix = std::string{caMethod} + ":";
     // parseContentAddressMethodPrefix takes its argument by reference
@@ -126,26 +122,55 @@ ContentAddressMethod parseContentAddressMethod(std::string_view caMethod)
     return parseContentAddressMethodPrefix(asPrefixView);
 }
 
-std::optional<ContentAddress> parseContentAddressOpt(std::string_view rawCaOpt)
+std::optional<ContentAddress> ContentAddress::parseOpt(std::string_view rawCaOpt)
 {
-    return rawCaOpt == "" ? std::optional<ContentAddress>() : parseContentAddress(rawCaOpt);
+    return rawCaOpt == ""
+        ? std::nullopt
+        : std::optional { ContentAddress::parse(rawCaOpt) };
 };
 
 std::string renderContentAddress(std::optional<ContentAddress> ca)
 {
-    return ca ? renderContentAddress(*ca) : "";
+    return ca ? ca->render() : "";
 }
 
-Hash getContentAddressHash(const ContentAddress & ca)
+const Hash & ContentAddress::getHash() const
 {
     return std::visit(overloaded {
-        [](const TextHash & th) {
+        [](const TextHash & th) -> auto & {
             return th.hash;
         },
-        [](const FixedOutputHash & fsh) {
+        [](const FixedOutputHash & fsh) -> auto & {
             return fsh.hash;
-        }
-    }, ca);
+        },
+    }, raw);
+}
+
+bool StoreReferences::empty() const
+{
+    return !self && others.empty();
+}
+
+size_t StoreReferences::size() const
+{
+    return (self ? 1 : 0) + others.size();
+}
+
+ContentAddressWithReferences ContentAddressWithReferences::withoutRefs(const ContentAddress & ca) {
+    return std::visit(overloaded {
+        [&](const TextHash & h) -> ContentAddressWithReferences {
+            return TextInfo {
+                .hash = h,
+                .references = {},
+            };
+        },
+        [&](const FixedOutputHash & h) -> ContentAddressWithReferences {
+            return FixedOutputInfo {
+                .hash = h,
+                .references = {},
+            };
+        },
+    }, ca.raw);
 }
 
 }

src/libstore/content-address.hh

@@ -3,12 +3,30 @@
 
 #include <variant>
 #include "hash.hh"
+#include "path.hh"
 #include "comparator.hh"
 
 namespace nix {
 
+/*
+ * Content addressing method
+ */
+
+/* We only have one way to hash text with references, so this is a single-value
+   type, mainly useful with std::variant.
+*/
+
 /**
- * An enumeration of the ways we can serialize file system objects.
+ * The single way we can serialize "text" file system objects.
+ *
+ * Somewhat obscure, used by \ref Derivation derivations and
+ * `builtins.toFile` currently.
+ */
+struct TextHashMethod : std::monostate { };
+
+/**
+ * An enumeration of the main ways we can serialize file system
+ * objects.
  */
 enum struct FileIngestionMethod : uint8_t {
     /**
@@ -22,6 +40,53 @@ enum struct FileIngestionMethod : uint8_t {
     Recursive = true
 };
 
+/**
+ * Compute the prefix to the hash algorithm which indicates how the
+ * files were ingested.
+ */
+std::string makeFileIngestionPrefix(FileIngestionMethod m);
+
+struct FixedOutputHashMethod {
+    FileIngestionMethod fileIngestionMethod;
+    HashType hashType;
+
+    GENERATE_CMP(FixedOutputHashMethod, me->fileIngestionMethod, me->hashType);
+};
+
+/**
+ * An enumeration of all the ways we can serialize file system objects.
+ *
+ * Just the type of a content address. Combine with the hash itself, and
+ * we have a `ContentAddress` as defined below. Combine that, in turn,
+ * with info on references, and we have `ContentAddressWithReferences`,
+ * as defined further below.
+ */
+struct ContentAddressMethod
+{
+    typedef std::variant<
+        TextHashMethod,
+        FixedOutputHashMethod
+    > Raw;
+
+    Raw raw;
+
+    GENERATE_CMP(ContentAddressMethod, me->raw);
+
+    /* The moral equivalent of `using Raw::Raw;` */
+    ContentAddressMethod(auto &&... arg)
+        : raw(std::forward<decltype(arg)>(arg)...)
+    { }
+
+    static ContentAddressMethod parse(std::string_view rawCaMethod);
+
+    std::string render() const;
+};
+
+
+/*
+ * Mini content address
+ */
+
 /**
  * Somewhat obscure, used by \ref Derivation derivations and
  * `builtins.toFile` currently.
@@ -36,7 +101,7 @@ struct TextHash {
 };
 
 /**
- * For path computed by makeFixedOutputPath.
+ * Used by most store objects that are content-addressed.
  */
 struct FixedOutputHash {
     /**
@@ -65,41 +130,96 @@ struct FixedOutputHash {
  * - ‘fixed:<r?>:<ht>:<h>’: For paths computed by
  *   Store::makeFixedOutputPath() / Store::addToStore().
  */
-typedef std::variant<
+struct ContentAddress
+{
+    typedef std::variant<
         TextHash,
         FixedOutputHash
-> ContentAddress;
+    > Raw;
 
-/**
- * Compute the prefix to the hash algorithm which indicates how the
- * files were ingested.
- */
-std::string makeFileIngestionPrefix(const FileIngestionMethod m);
+    Raw raw;
 
-/**
+    GENERATE_CMP(ContentAddress, me->raw);
+
+    /* The moral equivalent of `using Raw::Raw;` */
+    ContentAddress(auto &&... arg)
+        : raw(std::forward<decltype(arg)>(arg)...)
+    { }
+
+    /**
      * Compute the content-addressability assertion (ValidPathInfo::ca) for
      * paths created by Store::makeFixedOutputPath() / Store::addToStore().
      */
-std::string makeFixedOutputCA(FileIngestionMethod method, const Hash & hash);
+    std::string render() const;
 
-std::string renderContentAddress(ContentAddress ca);
+    static ContentAddress parse(std::string_view rawCa);
+
+    static std::optional<ContentAddress> parseOpt(std::string_view rawCaOpt);
+
+    const Hash & getHash() const;
+};
 
 std::string renderContentAddress(std::optional<ContentAddress> ca);
 
-ContentAddress parseContentAddress(std::string_view rawCa);
-
-std::optional<ContentAddress> parseContentAddressOpt(std::string_view rawCaOpt);
-
-Hash getContentAddressHash(const ContentAddress & ca);
 
 /*
-  We only have one way to hash text with references, so this is single-value
-  type is only useful in std::variant.
-*/
-struct TextHashMethod { };
-struct FixedOutputHashMethod {
-  FileIngestionMethod fileIngestionMethod;
-  HashType hashType;
+ * Full content address
+ *
+ * See the schema for store paths in store-api.cc
+ */
+
+/**
+ * A set of references to other store objects.
+ *
+ * References to other store objects are tracked with store paths, self
+ * references however are tracked with a boolean.
+ */
+struct StoreReferences {
+    /**
+     * References to other store objects
+     */
+    StorePathSet others;
+
+    /**
+     * Reference to this store object
+     */
+    bool self = false;
+
+    /**
+     * @return true iff no references, i.e. others is empty and self is
+     * false.
+     */
+    bool empty() const;
+
+    /**
+     * Returns the numbers of references, i.e. the size of others + 1
+     * iff self is true.
+     */
+    size_t size() const;
+
+    GENERATE_CMP(StoreReferences, me->self, me->others);
+};
+
+// This matches the additional info that we need for makeTextPath
+struct TextInfo {
+    TextHash hash;
+    /**
+     * References to other store objects only; self references
+     * disallowed
+     */
+    StorePathSet references;
+
+    GENERATE_CMP(TextInfo, me->hash, me->references);
+};
+
+struct FixedOutputInfo {
+    FixedOutputHash hash;
+    /**
+     * References to other store objects or this one.
+     */
+    StoreReferences references;
+
+    GENERATE_CMP(FixedOutputInfo, me->hash, me->references);
 };
 
 /**
@@ -107,13 +227,27 @@ struct FixedOutputHashMethod {
  *
  * A ContentAddress without a Hash.
  */
-typedef std::variant<
-    TextHashMethod,
-    FixedOutputHashMethod
-  > ContentAddressMethod;
+struct ContentAddressWithReferences
+{
+    typedef std::variant<
+        TextInfo,
+        FixedOutputInfo
+    > Raw;
 
-ContentAddressMethod parseContentAddressMethod(std::string_view rawCaMethod);
+    Raw raw;
 
-std::string renderContentAddressMethod(ContentAddressMethod caMethod);
+    GENERATE_CMP(ContentAddressWithReferences, me->raw);
+
+    /* The moral equivalent of `using Raw::Raw;` */
+    ContentAddressWithReferences(auto &&... arg)
+        : raw(std::forward<decltype(arg)>(arg)...)
+    { }
+
+    /**
+     * Create a ContentAddressWithReferences from a mere ContentAddress, by
+     * assuming no references in all cases.
+     */
+    static ContentAddressWithReferences withoutRefs(const ContentAddress &);
+};
 
 }

src/libstore/daemon.cc

@@ -401,21 +401,21 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
             logger->startWork();
             auto pathInfo = [&]() {
                 // NB: FramedSource must be out of scope before logger->stopWork();
-                ContentAddressMethod contentAddressMethod = parseContentAddressMethod(camStr);
+                ContentAddressMethod contentAddressMethod = ContentAddressMethod::parse(camStr);
                 FramedSource source(from);
                 // TODO this is essentially RemoteStore::addCAToStore. Move it up to Store.
                 return std::visit(overloaded {
-                    [&](TextHashMethod &) {
+                    [&](const TextHashMethod &) {
                         // We could stream this by changing Store
                         std::string contents = source.drain();
                         auto path = store->addTextToStore(name, contents, refs, repair);
                         return store->queryPathInfo(path);
                     },
-                    [&](FixedOutputHashMethod & fohm) {
+                    [&](const FixedOutputHashMethod & fohm) {
                         auto path = store->addToStoreFromDump(source, name, fohm.fileIngestionMethod, fohm.hashType, repair, refs);
                         return store->queryPathInfo(path);
                     },
-                }, contentAddressMethod);
+                }, contentAddressMethod.raw);
             }();
             logger->stopWork();
 
@@ -637,7 +637,10 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
             to << res.timesBuilt << res.isNonDeterministic << res.startTime << res.stopTime;
         }
         if (GET_PROTOCOL_MINOR(clientVersion) >= 28) {
-            worker_proto::write(*store, to, res.builtOutputs);
+            DrvOutputs builtOutputs;
+            for (auto & [output, realisation] : res.builtOutputs)
+                builtOutputs.insert_or_assign(realisation.id, realisation);
+            worker_proto::write(*store, to, builtOutputs);
         }
         break;
     }
@@ -880,7 +883,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         info.references = worker_proto::read(*store, from, Phantom<StorePathSet> {});
         from >> info.registrationTime >> info.narSize >> info.ultimate;
         info.sigs = readStrings<StringSet>(from);
-        info.ca = parseContentAddressOpt(readString(from));
+        info.ca = ContentAddress::parseOpt(readString(from));
         from >> repair >> dontCheckSigs;
         if (!trusted && dontCheckSigs)
             dontCheckSigs = false;

src/libstore/derivations.cc

@@ -36,8 +36,8 @@ 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
 {
     return store.makeFixedOutputPath(
-        hash.method, hash.hash,
-        outputPathName(drvName, outputName));
+        outputPathName(drvName, outputName),
+        { hash, {} });
 }
 
 
@@ -942,7 +942,7 @@ void Derivation::checkInvariants(Store & store, const StorePath & drvPath) const
                 envHasRightPath(doia.path, i.first);
             },
             [&](const DerivationOutput::CAFixed & dof) {
-                StorePath path = store.makeFixedOutputPath(dof.hash.method, dof.hash.hash, drvName);
+                StorePath path = store.makeFixedOutputPath(drvName, { dof.hash, {} });
                 envHasRightPath(path, i.first);
             },
             [&](const DerivationOutput::CAFloating &) {

src/libstore/globals.cc

@@ -22,6 +22,9 @@
 #include <dlfcn.h>
 #endif
 
+#include "config-impl.hh"
+
+
 namespace nix {
 
 
@@ -192,18 +195,18 @@ NLOHMANN_JSON_SERIALIZE_ENUM(SandboxMode, {
     {SandboxMode::smDisabled, false},
 });
 
-template<> void BaseSetting<SandboxMode>::set(const std::string & str, bool append)
+template<> SandboxMode BaseSetting<SandboxMode>::parse(const std::string & str) const
 {
-    if (str == "true") value = smEnabled;
-    else if (str == "relaxed") value = smRelaxed;
-    else if (str == "false") value = smDisabled;
+    if (str == "true") return smEnabled;
+    else if (str == "relaxed") return smRelaxed;
+    else if (str == "false") return smDisabled;
     else throw UsageError("option '%s' has invalid value '%s'", name, str);
 }
 
-template<> bool BaseSetting<SandboxMode>::isAppendable()
+template<> struct BaseSetting<SandboxMode>::trait
 {
-    return false;
-}
+    static constexpr bool appendable = false;
+};
 
 template<> std::string BaseSetting<SandboxMode>::to_string() const
 {
@@ -235,23 +238,23 @@ template<> void BaseSetting<SandboxMode>::convertToArg(Args & args, const std::s
     });
 }
 
-void MaxBuildJobsSetting::set(const std::string & str, bool append)
+unsigned int MaxBuildJobsSetting::parse(const std::string & str) const
 {
-    if (str == "auto") value = std::max(1U, std::thread::hardware_concurrency());
+    if (str == "auto") return std::max(1U, std::thread::hardware_concurrency());
     else {
         if (auto n = string2Int<decltype(value)>(str))
-            value = *n;
+            return *n;
         else
             throw UsageError("configuration setting '%s' should be 'auto' or an integer", name);
     }
 }
 
 
-void PluginFilesSetting::set(const std::string & str, bool append)
+Paths PluginFilesSetting::parse(const std::string & str) const
 {
     if (pluginsLoaded)
         throw UsageError("plugin-files set after plugins were loaded, you may need to move the flag before the subcommand");
-    BaseSetting<Paths>::set(str, append);
+    return BaseSetting<Paths>::parse(str);
 }
 
 

src/libstore/globals.hh

@@ -26,7 +26,7 @@ struct MaxBuildJobsSetting : public BaseSetting<unsigned int>
         options->addSetting(this);
     }
 
-    void set(const std::string & str, bool append = false) override;
+    unsigned int parse(const std::string & str) const override;
 };
 
 struct PluginFilesSetting : public BaseSetting<Paths>
@@ -43,7 +43,7 @@ struct PluginFilesSetting : public BaseSetting<Paths>
         options->addSetting(this);
     }
 
-    void set(const std::string & str, bool append = false) override;
+    Paths parse(const std::string & str) const override;
 };
 
 const uint32_t maxIdsPerBuild =

src/libstore/legacy-ssh-store.cc

@@ -156,7 +156,7 @@ struct LegacySSHStore : public virtual LegacySSHStoreConfig, public virtual Stor
                     throw Error("NAR hash is now mandatory");
                 info->narHash = Hash::parseAnyPrefixed(s);
             }
-            info->ca = parseContentAddressOpt(readString(conn->from));
+            info->ca = ContentAddress::parseOpt(readString(conn->from));
             info->sigs = readStrings<StringSet>(conn->from);
 
             auto s = readString(conn->from);
@@ -287,19 +287,18 @@ public:
 
         conn->to.flush();
 
-        BuildResult status {
-            .path = DerivedPath::Built {
-                .drvPath = drvPath,
-                .outputs = OutputsSpec::All { },
-            },
-        };
+        BuildResult status;
         status.status = (BuildResult::Status) readInt(conn->from);
         conn->from >> status.errorMsg;
 
         if (GET_PROTOCOL_MINOR(conn->remoteVersion) >= 3)
             conn->from >> status.timesBuilt >> status.isNonDeterministic >> status.startTime >> status.stopTime;
         if (GET_PROTOCOL_MINOR(conn->remoteVersion) >= 6) {
-            status.builtOutputs = worker_proto::read(*this, conn->from, Phantom<DrvOutputs> {});
+            auto builtOutputs = worker_proto::read(*this, conn->from, Phantom<DrvOutputs> {});
+            for (auto && [output, realisation] : builtOutputs)
+                status.builtOutputs.insert_or_assign(
+                    std::move(output.outputName),
+                    std::move(realisation));
         }
         return status;
     }
@@ -330,7 +329,7 @@ public:
 
         conn->to.flush();
 
-        BuildResult result { .path = DerivedPath::Opaque { StorePath::dummy } };
+        BuildResult result;
         result.status = (BuildResult::Status) readInt(conn->from);
 
         if (!result.success()) {

src/libstore/local-store.cc

@@ -710,6 +710,7 @@ void canonicalisePathMetaData(const Path & path,
     canonicalisePathMetaData(path, uidRange, inodesSeen);
 }
 
+
 void LocalStore::registerDrvOutput(const Realisation & info, CheckSigsFlag checkSigs)
 {
     experimentalFeatureSettings.require(Xp::CaDerivations);
@@ -888,7 +889,7 @@ std::shared_ptr<const ValidPathInfo> LocalStore::queryPathInfoInternal(State & s
     if (s) info->sigs = tokenizeString<StringSet>(s, " ");
 
     s = (const char *) sqlite3_column_text(state.stmts->QueryPathInfo, 7);
-    if (s) info->ca = parseContentAddressOpt(s);
+    if (s) info->ca = ContentAddress::parseOpt(s);
 
     /* Get the references. */
     auto useQueryReferences(state.stmts->QueryReferences.use()(info->id));
@@ -1221,7 +1222,7 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
                     printStorePath(info.path), info.narSize, hashResult.second);
 
             if (info.ca) {
-                if (auto foHash = std::get_if<FixedOutputHash>(&*info.ca)) {
+                if (auto foHash = std::get_if<FixedOutputHash>(&info.ca->raw)) {
                     auto actualFoHash = hashCAPath(
                         foHash->method,
                         foHash->hash.type,
@@ -1234,7 +1235,7 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
                             actualFoHash.hash.to_string(Base32, true));
                     }
                 }
-                if (auto textHash = std::get_if<TextHash>(&*info.ca)) {
+                if (auto textHash = std::get_if<TextHash>(&info.ca->raw)) {
                     auto actualTextHash = hashString(htSHA256, readFile(realPath));
                     if (textHash->hash != actualTextHash) {
                         throw Error("ca hash mismatch importing path '%s';\n  specified: %s\n  got:       %s",
@@ -1320,7 +1321,19 @@ StorePath LocalStore::addToStoreFromDump(Source & source0, std::string_view name
 
     auto [hash, size] = hashSink->finish();
 
-    auto dstPath = makeFixedOutputPath(method, hash, name, references);
+    ContentAddressWithReferences desc = FixedOutputInfo {
+        .hash = {
+            .method = method,
+            .hash = hash,
+        },
+        .references = {
+            .others = references,
+            // caller is not capable of creating a self-reference, because this is content-addressed without modulus
+            .self = false,
+        },
+    };
+
+    auto dstPath = makeFixedOutputPathFromCA(name, desc);
 
     addTempRoot(dstPath);
 
@@ -1364,10 +1377,13 @@ StorePath LocalStore::addToStoreFromDump(Source & source0, std::string_view name
 
             optimisePath(realPath, repair);
 
-            ValidPathInfo info { dstPath, narHash.first };
+            ValidPathInfo info {
+                *this,
+                name,
+                std::move(desc),
+                narHash.first
+            };
             info.narSize = narHash.second;
-            info.references = references;
-            info.ca = FixedOutputHash { .method = method, .hash = hash };
             registerValidPath(info);
         }
 
@@ -1384,7 +1400,10 @@ StorePath LocalStore::addTextToStore(
     const StorePathSet & references, RepairFlag repair)
 {
     auto hash = hashString(htSHA256, s);
-    auto dstPath = makeTextPath(name, hash, references);
+    auto dstPath = makeTextPath(name, TextInfo {
+        { .hash = hash },
+        references,
+    });
 
     addTempRoot(dstPath);
 

src/libstore/make-content-addressed.cc

@@ -27,18 +27,17 @@ std::map<StorePath, StorePath> makeContentAddressed(
 
         StringMap rewrites;
 
-        StorePathSet references;
-        bool hasSelfReference = false;
+        StoreReferences refs;
         for (auto & ref : oldInfo->references) {
             if (ref == path)
-                hasSelfReference = true;
+                refs.self = true;
             else {
                 auto i = remappings.find(ref);
                 auto replacement = i != remappings.end() ? i->second : ref;
                 // FIXME: warn about unremapped paths?
                 if (replacement != ref)
                     rewrites.insert_or_assign(srcStore.printStorePath(ref), srcStore.printStorePath(replacement));
-                references.insert(std::move(replacement));
+                refs.others.insert(std::move(replacement));
             }
         }
 
@@ -49,24 +48,28 @@ std::map<StorePath, StorePath> makeContentAddressed(
 
         auto narModuloHash = hashModuloSink.finish().first;
 
-        auto dstPath = dstStore.makeFixedOutputPath(
-            FileIngestionMethod::Recursive, narModuloHash, path.name(), references, hasSelfReference);
+        ValidPathInfo info {
+            dstStore,
+            path.name(),
+            FixedOutputInfo {
+                .hash = {
+                    .method = FileIngestionMethod::Recursive,
+                    .hash = narModuloHash,
+                },
+                .references = std::move(refs),
+            },
+            Hash::dummy,
+        };
 
-        printInfo("rewriting '%s' to '%s'", pathS, srcStore.printStorePath(dstPath));
+        printInfo("rewriting '%s' to '%s'", pathS, dstStore.printStorePath(info.path));
 
         StringSink sink2;
-        RewritingSink rsink2(oldHashPart, std::string(dstPath.hashPart()), sink2);
+        RewritingSink rsink2(oldHashPart, std::string(info.path.hashPart()), sink2);
         rsink2(sink.s);
         rsink2.flush();
 
-        ValidPathInfo info { dstPath, hashString(htSHA256, sink2.s) };
-        info.references = std::move(references);
-        if (hasSelfReference) info.references.insert(info.path);
+        info.narHash = hashString(htSHA256, sink2.s);
         info.narSize = sink.s.size();
-        info.ca = FixedOutputHash {
-            .method = FileIngestionMethod::Recursive,
-            .hash = narModuloHash,
-        };
 
         StringSource source(sink2.s);
         dstStore.addToStore(info, source);

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

@@ -273,7 +273,7 @@ public:
                 narInfo->deriver = StorePath(queryNAR.getStr(9));
             for (auto & sig : tokenizeString<Strings>(queryNAR.getStr(10), " "))
                 narInfo->sigs.insert(sig);
-            narInfo->ca = parseContentAddressOpt(queryNAR.getStr(11));
+            narInfo->ca = ContentAddress::parseOpt(queryNAR.getStr(11));
 
             return {oValid, narInfo};
         });

src/libstore/nar-info.cc

@@ -74,7 +74,7 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
         else if (name == "CA") {
             if (ca) throw corrupt();
             // FIXME: allow blank ca or require skipping field?
-            ca = parseContentAddressOpt(value);
+            ca = ContentAddress::parseOpt(value);
         }
 
         pos = eol + 1;

src/libstore/nar-info.hh

@@ -17,6 +17,9 @@ struct NarInfo : ValidPathInfo
     uint64_t fileSize = 0;
 
     NarInfo() = delete;
+    NarInfo(const Store & store, std::string && name, ContentAddressWithReferences && ca, Hash narHash)
+        : ValidPathInfo(store, std::move(name), std::move(ca), narHash)
+    { }
     NarInfo(StorePath && path, Hash narHash) : ValidPathInfo(std::move(path), narHash) { }
     NarInfo(const ValidPathInfo & info) : ValidPathInfo(info) { }
     NarInfo(const Store & store, const std::string & s, const std::string & whence);

src/libstore/path-info.cc

@@ -21,25 +21,45 @@ void ValidPathInfo::sign(const Store & store, const SecretKey & secretKey)
     sigs.insert(secretKey.signDetached(fingerprint(store)));
 }
 
-
-bool ValidPathInfo::isContentAddressed(const Store & store) const
+std::optional<ContentAddressWithReferences> ValidPathInfo::contentAddressWithReferences() const
 {
-    if (! ca) return false;
+    if (! ca)
+        return std::nullopt;
 
-    auto caPath = std::visit(overloaded {
-        [&](const TextHash & th) {
-            return store.makeTextPath(path.name(), th.hash, references);
+    return std::visit(overloaded {
+        [&](const TextHash & th) -> ContentAddressWithReferences {
+            assert(references.count(path) == 0);
+            return TextInfo {
+                .hash = th,
+                .references = references,
+            };
         },
-        [&](const FixedOutputHash & fsh) {
+        [&](const FixedOutputHash & foh) -> ContentAddressWithReferences {
             auto refs = references;
             bool hasSelfReference = false;
             if (refs.count(path)) {
                 hasSelfReference = true;
                 refs.erase(path);
             }
-            return store.makeFixedOutputPath(fsh.method, fsh.hash, path.name(), refs, hasSelfReference);
-        }
-    }, *ca);
+            return FixedOutputInfo {
+                .hash = foh,
+                .references = {
+                    .others = std::move(refs),
+                    .self = hasSelfReference,
+                },
+            };
+        },
+    }, ca->raw);
+}
+
+bool ValidPathInfo::isContentAddressed(const Store & store) const
+{
+    auto fullCaOpt = contentAddressWithReferences();
+
+    if (! fullCaOpt)
+        return false;
+
+    auto caPath = store.makeFixedOutputPathFromCA(path.name(), *fullCaOpt);
 
     bool res = caPath == path;
 
@@ -77,6 +97,29 @@ Strings ValidPathInfo::shortRefs() const
 }
 
 
+ValidPathInfo::ValidPathInfo(
+    const Store & store,
+    std::string_view name,
+    ContentAddressWithReferences && ca,
+    Hash narHash)
+      : path(store.makeFixedOutputPathFromCA(name, ca))
+      , narHash(narHash)
+{
+    std::visit(overloaded {
+        [this](TextInfo && ti) {
+            this->references = std::move(ti.references);
+            this->ca = std::move((TextHash &&) ti);
+        },
+        [this](FixedOutputInfo && foi) {
+            this->references = std::move(foi.references.others);
+            if (foi.references.self)
+                this->references.insert(path);
+            this->ca = std::move((FixedOutputHash &&) foi);
+        },
+    }, std::move(ca).raw);
+}
+
+
 ValidPathInfo ValidPathInfo::read(Source & source, const Store & store, unsigned int format)
 {
     return read(source, store, format, store.parseStorePath(readString(source)));
@@ -93,7 +136,7 @@ ValidPathInfo ValidPathInfo::read(Source & source, const Store & store, unsigned
     if (format >= 16) {
         source >> info.ultimate;
         info.sigs = readStrings<StringSet>(source);
-        info.ca = parseContentAddressOpt(readString(source));
+        info.ca = ContentAddress::parseOpt(readString(source));
     }
     return info;
 }

src/libstore/path-info.hh

@@ -92,6 +92,13 @@ struct ValidPathInfo
 
     void sign(const Store & store, const SecretKey & secretKey);
 
+	/**
+	 * @return The `ContentAddressWithReferences` that determines the
+	 * store path for a content-addressed store object, `std::nullopt`
+	 * for an input-addressed store object.
+	 */
+    std::optional<ContentAddressWithReferences> contentAddressWithReferences() const;
+
     /**
      * @return true iff the path is verifiably content-addressed.
      */
@@ -118,6 +125,9 @@ struct ValidPathInfo
     ValidPathInfo(StorePath && path, Hash narHash) : path(std::move(path)), narHash(narHash) { };
     ValidPathInfo(const StorePath & path, Hash narHash) : path(path), narHash(narHash) { };
 
+    ValidPathInfo(const Store & store,
+        std::string_view name, ContentAddressWithReferences && ca, Hash narHash);
+
     virtual ~ValidPathInfo() { }
 
     static ValidPathInfo read(Source & source, const Store & store, unsigned int format);

src/libstore/realisation.hh

@@ -13,9 +13,25 @@ namespace nix {
 
 class Store;
 
+/**
+ * A general `Realisation` key.
+ *
+ * This is similar to a `DerivedPath::Opaque`, but the derivation is
+ * identified by its "hash modulo" instead of by its store path.
+ */
 struct DrvOutput {
-    // The hash modulo of the derivation
+    /**
+     * The hash modulo of the derivation.
+     *
+     * Computed from the derivation itself for most types of
+     * derivations, but computed from the (fixed) content address of the
+     * output for fixed-output derivations.
+     */
     Hash drvHash;
+
+    /**
+     * The name of the output.
+     */
     std::string outputName;
 
     std::string to_string() const;
@@ -60,6 +76,21 @@ struct Realisation {
     GENERATE_CMP(Realisation, me->id, me->outPath);
 };
 
+/**
+ * Collection type for a single derivation's outputs' `Realisation`s.
+ *
+ * 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;
+
+/**
+ * Collection type for multiple derivations' outputs' `Realisation`s.
+ *
+ * `DrvOutput` is used because in general the derivations are not all
+ * the same, so we need to identify firstly which derivation, and
+ * secondly which output of that derivation.
+ */
 typedef std::map<DrvOutput, Realisation> DrvOutputs;
 
 struct OpaquePath {

src/libstore/remote-store.cc

@@ -78,7 +78,7 @@ void write(const Store & store, Sink & out, const std::optional<TrustedFlag> & o
 
 ContentAddress read(const Store & store, Source & from, Phantom<ContentAddress> _)
 {
-    return parseContentAddress(readString(from));
+    return ContentAddress::parse(readString(from));
 }
 
 void write(const Store & store, Sink & out, const ContentAddress & ca)
@@ -125,10 +125,26 @@ void write(const Store & store, Sink & out, const DrvOutput & drvOutput)
 }
 
 
-BuildResult read(const Store & store, Source & from, Phantom<BuildResult> _)
+KeyedBuildResult read(const Store & store, Source & from, Phantom<KeyedBuildResult> _)
 {
     auto path = worker_proto::read(store, from, Phantom<DerivedPath> {});
-    BuildResult res { .path = path };
+    auto br = worker_proto::read(store, from, Phantom<BuildResult> {});
+    return KeyedBuildResult {
+        std::move(br),
+        /* .path = */ std::move(path),
+    };
+}
+
+void write(const Store & store, Sink & to, const KeyedBuildResult & res)
+{
+    worker_proto::write(store, to, res.path);
+    worker_proto::write(store, to, static_cast<const BuildResult &>(res));
+}
+
+
+BuildResult read(const Store & store, Source & from, Phantom<BuildResult> _)
+{
+    BuildResult res;
     res.status = (BuildResult::Status) readInt(from);
     from
         >> res.errorMsg
@@ -136,13 +152,16 @@ BuildResult read(const Store & store, Source & from, Phantom<BuildResult> _)
         >> res.isNonDeterministic
         >> res.startTime
         >> res.stopTime;
-    res.builtOutputs = worker_proto::read(store, from, Phantom<DrvOutputs> {});
+    auto builtOutputs = worker_proto::read(store, from, Phantom<DrvOutputs> {});
+    for (auto && [output, realisation] : builtOutputs)
+        res.builtOutputs.insert_or_assign(
+            std::move(output.outputName),
+            std::move(realisation));
     return res;
 }
 
 void write(const Store & store, Sink & to, const BuildResult & res)
 {
-    worker_proto::write(store, to, res.path);
     to
         << res.status
         << res.errorMsg
@@ -150,7 +169,10 @@ void write(const Store & store, Sink & to, const BuildResult & res)
         << res.isNonDeterministic
         << res.startTime
         << res.stopTime;
-    worker_proto::write(store, to, res.builtOutputs);
+    DrvOutputs builtOutputs;
+    for (auto & [output, realisation] : res.builtOutputs)
+        builtOutputs.insert_or_assign(realisation.id, realisation);
+    worker_proto::write(store, to, builtOutputs);
 }
 
 
@@ -168,7 +190,7 @@ void write(const Store & store, Sink & out, const std::optional<StorePath> & sto
 
 std::optional<ContentAddress> read(const Store & store, Source & from, Phantom<std::optional<ContentAddress>> _)
 {
-    return parseContentAddressOpt(readString(from));
+    return ContentAddress::parseOpt(readString(from));
 }
 
 void write(const Store & store, Sink & out, const std::optional<ContentAddress> & caOpt)
@@ -586,7 +608,7 @@ ref<const ValidPathInfo> RemoteStore::addCAToStore(
         conn->to
             << wopAddToStore
             << name
-            << renderContentAddressMethod(caMethod);
+            << caMethod.render();
         worker_proto::write(*this, conn->to, references);
         conn->to << repair;
 
@@ -644,7 +666,7 @@ ref<const ValidPathInfo> RemoteStore::addCAToStore(
                 }
 
             }
-        }, caMethod);
+        }, caMethod.raw);
         auto path = parseStorePath(readString(conn->from));
         // Release our connection to prevent a deadlock in queryPathInfo().
         conn_.reset();
@@ -865,7 +887,7 @@ void RemoteStore::buildPaths(const std::vector<DerivedPath> & drvPaths, BuildMod
     readInt(conn->from);
 }
 
-std::vector<BuildResult> RemoteStore::buildPathsWithResults(
+std::vector<KeyedBuildResult> RemoteStore::buildPathsWithResults(
     const std::vector<DerivedPath> & paths,
     BuildMode buildMode,
     std::shared_ptr<Store> evalStore)
@@ -880,7 +902,7 @@ std::vector<BuildResult> RemoteStore::buildPathsWithResults(
         writeDerivedPaths(*this, conn, paths);
         conn->to << buildMode;
         conn.processStderr();
-        return worker_proto::read(*this, conn->from, Phantom<std::vector<BuildResult>> {});
+        return worker_proto::read(*this, conn->from, Phantom<std::vector<KeyedBuildResult>> {});
     } else {
         // Avoid deadlock.
         conn_.reset();
@@ -889,21 +911,25 @@ std::vector<BuildResult> RemoteStore::buildPathsWithResults(
         // fails, but meh.
         buildPaths(paths, buildMode, evalStore);
 
-        std::vector<BuildResult> results;
+        std::vector<KeyedBuildResult> results;
 
         for (auto & path : paths) {
             std::visit(
                 overloaded {
                     [&](const DerivedPath::Opaque & bo) {
-                        results.push_back(BuildResult {
+                        results.push_back(KeyedBuildResult {
+                            {
                                 .status = BuildResult::Substituted,
-                            .path = bo,
+                            },
+                            /* .path = */ bo,
                         });
                     },
                     [&](const DerivedPath::Built & bfd) {
-                        BuildResult res {
-                            .status = BuildResult::Built,
-                            .path = bfd,
+                        KeyedBuildResult res {
+                            {
+                                .status = BuildResult::Built
+                            },
+                            /* .path = */ bfd,
                         };
 
                         OutputPathMap outputs;
@@ -922,10 +948,10 @@ std::vector<BuildResult> RemoteStore::buildPathsWithResults(
                                     queryRealisation(outputId);
                                 if (!realisation)
                                     throw MissingRealisation(outputId);
-                                res.builtOutputs.emplace(realisation->id, *realisation);
+                                res.builtOutputs.emplace(output, *realisation);
                             } else {
                                 res.builtOutputs.emplace(
-                                    outputId,
+                                    output,
                                     Realisation {
                                         .id = outputId,
                                         .outPath = outputPath,
@@ -952,12 +978,7 @@ BuildResult RemoteStore::buildDerivation(const StorePath & drvPath, const BasicD
     writeDerivation(conn->to, *this, drv);
     conn->to << buildMode;
     conn.processStderr();
-    BuildResult res {
-        .path = DerivedPath::Built {
-            .drvPath = drvPath,
-            .outputs = OutputsSpec::All { },
-        },
-    };
+    BuildResult res;
     res.status = (BuildResult::Status) readInt(conn->from);
     conn->from >> res.errorMsg;
     if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 29) {
@@ -965,7 +986,10 @@ BuildResult RemoteStore::buildDerivation(const StorePath & drvPath, const BasicD
     }
     if (GET_PROTOCOL_MINOR(conn->daemonVersion) >= 28) {
         auto builtOutputs = worker_proto::read(*this, conn->from, Phantom<DrvOutputs> {});
-        res.builtOutputs = builtOutputs;
+        for (auto && [output, realisation] : builtOutputs)
+            res.builtOutputs.insert_or_assign(
+                std::move(output.outputName),
+                std::move(realisation));
     }
     return res;
 }

src/libstore/remote-store.hh

@@ -114,7 +114,7 @@ public:
 
     void buildPaths(const std::vector<DerivedPath> & paths, BuildMode buildMode, std::shared_ptr<Store> evalStore) override;
 
-    std::vector<BuildResult> buildPathsWithResults(
+    std::vector<KeyedBuildResult> buildPathsWithResults(
         const std::vector<DerivedPath> & paths,
         BuildMode buildMode,
         std::shared_ptr<Store> evalStore) override;

src/libstore/store-api.cc

@@ -7,6 +7,7 @@
 #include "nar-info-disk-cache.hh"
 #include "thread-pool.hh"
 #include "url.hh"
+#include "references.hh"
 #include "archive.hh"
 #include "callback.hh"
 #include "remote-store.hh"
@@ -98,10 +99,12 @@ StorePath Store::followLinksToStorePath(std::string_view path) const
        silly, but it's done that way for compatibility).  <id> is the
        name of the output (usually, "out").
 
-   <h2> = base-16 representation of a SHA-256 hash of:
+   <h2> = base-16 representation of a SHA-256 hash of <s2>
+
+   <s2> =
      if <type> = "text:...":
        the string written to the resulting store path
-     if <type> = "source":
+     if <type> = "source:...":
        the serialisation of the path from which this store path is
        copied, as returned by hashPath()
      if <type> = "output:<id>":
@@ -162,63 +165,63 @@ StorePath Store::makeOutputPath(std::string_view id,
 }
 
 
+/* Stuff the references (if any) into the type.  This is a bit
+   hacky, but we can't put them in, say, <s2> (per the grammar above)
+   since that would be ambiguous. */
 static std::string makeType(
     const Store & store,
     std::string && type,
-    const StorePathSet & references,
-    bool hasSelfReference = false)
+    const StoreReferences & references)
 {
-    for (auto & i : references) {
+    for (auto & i : references.others) {
         type += ":";
         type += store.printStorePath(i);
     }
-    if (hasSelfReference) type += ":self";
+    if (references.self) type += ":self";
     return std::move(type);
 }
 
 
-StorePath Store::makeFixedOutputPath(
-    FileIngestionMethod method,
-    const Hash & hash,
-    std::string_view name,
-    const StorePathSet & references,
-    bool hasSelfReference) const
+StorePath Store::makeFixedOutputPath(std::string_view name, const FixedOutputInfo & info) const
 {
-    if (hash.type == htSHA256 && method == FileIngestionMethod::Recursive) {
-        return makeStorePath(makeType(*this, "source", references, hasSelfReference), hash, name);
+    if (info.hash.hash.type == htSHA256 && info.hash.method == FileIngestionMethod::Recursive) {
+        return makeStorePath(makeType(*this, "source", info.references), info.hash.hash, name);
     } else {
-        assert(references.empty());
+        assert(info.references.size() == 0);
         return makeStorePath("output:out",
             hashString(htSHA256,
                 "fixed:out:"
-                + makeFileIngestionPrefix(method)
-                + hash.to_string(Base16, true) + ":"),
+                + makeFileIngestionPrefix(info.hash.method)
+                + info.hash.hash.to_string(Base16, true) + ":"),
             name);
     }
 }
 
-StorePath Store::makeFixedOutputPathFromCA(std::string_view name, ContentAddress ca,
-    const StorePathSet & references, bool hasSelfReference) const
+
+StorePath Store::makeTextPath(std::string_view name, const TextInfo & info) const
+{
+    assert(info.hash.hash.type == htSHA256);
+    return makeStorePath(
+        makeType(*this, "text", StoreReferences {
+            .others = info.references,
+            .self = false,
+        }),
+        info.hash.hash,
+        name);
+}
+
+
+StorePath Store::makeFixedOutputPathFromCA(std::string_view name, const ContentAddressWithReferences & ca) const
 {
     // New template
     return std::visit(overloaded {
-        [&](const TextHash & th) {
-            return makeTextPath(name, th.hash, references);
+        [&](const TextInfo & ti) {
+            return makeTextPath(name, ti);
         },
-        [&](const FixedOutputHash & fsh) {
-            return makeFixedOutputPath(fsh.method, fsh.hash, name, references, hasSelfReference);
+        [&](const FixedOutputInfo & foi) {
+            return makeFixedOutputPath(name, foi);
         }
-    }, ca);
-}
-
-StorePath Store::makeTextPath(std::string_view name, const Hash & hash,
-    const StorePathSet & references) const
-{
-    assert(hash.type == htSHA256);
-    /* Stuff the references (if any) into the type.  This is a bit
-       hacky, but we can't put them in `s' since that would be
-       ambiguous. */
-    return makeStorePath(makeType(*this, "text", references), hash, name);
+    }, ca.raw);
 }
 
 
@@ -228,7 +231,14 @@ std::pair<StorePath, Hash> Store::computeStorePathForPath(std::string_view name,
     Hash h = method == FileIngestionMethod::Recursive
         ? hashPath(hashAlgo, srcPath, filter).first
         : hashFile(hashAlgo, srcPath);
-    return std::make_pair(makeFixedOutputPath(method, h, name), h);
+    FixedOutputInfo caInfo {
+        .hash = {
+            .method = method,
+            .hash = h,
+        },
+        .references = {},
+    };
+    return std::make_pair(makeFixedOutputPath(name, caInfo), h);
 }
 
 
@@ -237,7 +247,10 @@ StorePath Store::computeStorePathForText(
     std::string_view s,
     const StorePathSet & references) const
 {
-    return makeTextPath(name, hashString(htSHA256, s), references);
+    return makeTextPath(name, TextInfo {
+        { .hash = hashString(htSHA256, s) },
+        references,
+    });
 }
 
 
@@ -425,11 +438,18 @@ ValidPathInfo Store::addToStoreSlow(std::string_view name, const Path & srcPath,
         throw Error("hash mismatch for '%s'", srcPath);
 
     ValidPathInfo info {
-        makeFixedOutputPath(method, hash, name),
+        *this,
+        name,
+        FixedOutputInfo {
+            .hash = {
+                .method = method,
+                .hash = hash,
+            },
+            .references = {},
+        },
         narHash,
     };
     info.narSize = narSize;
-    info.ca = FixedOutputHash { .method = method, .hash = hash };
 
     if (!isValidPath(info.path)) {
         auto source = sinkToSource([&](Sink & scratchpadSink) {
@@ -521,7 +541,9 @@ void Store::querySubstitutablePathInfos(const StorePathCAMap & paths, Substituta
 
             // Recompute store path so that we can use a different store root.
             if (path.second) {
-                subPath = makeFixedOutputPathFromCA(path.first.name(), *path.second);
+                subPath = makeFixedOutputPathFromCA(
+                    path.first.name(),
+                    ContentAddressWithReferences::withoutRefs(*path.second));
                 if (sub->storeDir == storeDir)
                     assert(subPath == path.first);
                 if (subPath != path.first)
@@ -538,10 +560,11 @@ void Store::querySubstitutablePathInfos(const StorePathCAMap & paths, Substituta
                 auto narInfo = std::dynamic_pointer_cast<const NarInfo>(
                     std::shared_ptr<const ValidPathInfo>(info));
                 infos.insert_or_assign(path.first, SubstitutablePathInfo{
-                    info->deriver,
-                    info->references,
-                    narInfo ? narInfo->fileSize : 0,
-                    info->narSize});
+                    .deriver = info->deriver,
+                    .references = info->references,
+                    .downloadSize = narInfo ? narInfo->fileSize : 0,
+                    .narSize = info->narSize,
+                });
             } catch (InvalidPath &) {
             } catch (SubstituterDisabled &) {
             } catch (Error & e) {
@@ -1025,7 +1048,9 @@ void copyStorePath(
     // recompute store path on the chance dstStore does it differently
     if (info->ca && info->references.empty()) {
         auto info2 = make_ref<ValidPathInfo>(*info);
-        info2->path = dstStore.makeFixedOutputPathFromCA(info->path.name(), *info->ca);
+        info2->path = dstStore.makeFixedOutputPathFromCA(
+            info->path.name(),
+            info->contentAddressWithReferences().value());
         if (dstStore.storeDir == srcStore.storeDir)
             assert(info->path == info2->path);
         info = info2;
@@ -1137,7 +1162,9 @@ std::map<StorePath, StorePath> copyPaths(
         auto storePathForSrc = currentPathInfo.path;
         auto storePathForDst = storePathForSrc;
         if (currentPathInfo.ca && currentPathInfo.references.empty()) {
-            storePathForDst = dstStore.makeFixedOutputPathFromCA(storePathForSrc.name(), *currentPathInfo.ca);
+            storePathForDst = dstStore.makeFixedOutputPathFromCA(
+                currentPathInfo.path.name(),
+                currentPathInfo.contentAddressWithReferences().value());
             if (dstStore.storeDir == srcStore.storeDir)
                 assert(storePathForDst == storePathForSrc);
             if (storePathForDst != storePathForSrc)

src/libstore/store-api.hh

@@ -92,6 +92,7 @@ enum BuildMode { bmNormal, bmRepair, bmCheck };
 enum TrustedFlag : bool { NotTrusted = false, Trusted = true };
 
 struct BuildResult;
+struct KeyedBuildResult;
 
 
 typedef std::map<StorePath, std::optional<ContentAddress>> StorePathCAMap;
@@ -268,17 +269,11 @@ public:
     StorePath makeOutputPath(std::string_view id,
         const Hash & hash, std::string_view name) const;
 
-    StorePath makeFixedOutputPath(FileIngestionMethod method,
-        const Hash & hash, std::string_view name,
-        const StorePathSet & references = {},
-        bool hasSelfReference = false) const;
+    StorePath makeFixedOutputPath(std::string_view name, const FixedOutputInfo & info) const;
 
-    StorePath makeTextPath(std::string_view name, const Hash & hash,
-        const StorePathSet & references = {}) const;
+    StorePath makeTextPath(std::string_view name, const TextInfo & info) const;
 
-    StorePath makeFixedOutputPathFromCA(std::string_view name, ContentAddress ca,
-        const StorePathSet & references = {},
-        bool hasSelfReference = false) const;
+    StorePath makeFixedOutputPathFromCA(std::string_view name, const ContentAddressWithReferences & ca) const;
 
     /**
      * Preparatory part of addToStore().
@@ -575,7 +570,7 @@ public:
      * case of a build/substitution error, this function won't throw an
      * exception, but return a BuildResult containing an error message.
      */
-    virtual std::vector<BuildResult> buildPathsWithResults(
+    virtual std::vector<KeyedBuildResult> buildPathsWithResults(
         const std::vector<DerivedPath> & paths,
         BuildMode buildMode = bmNormal,
         std::shared_ptr<Store> evalStore = nullptr);

src/libstore/worker-protocol.hh

@@ -103,6 +103,7 @@ MAKE_WORKER_PROTO(, DerivedPath);
 MAKE_WORKER_PROTO(, Realisation);
 MAKE_WORKER_PROTO(, DrvOutput);
 MAKE_WORKER_PROTO(, BuildResult);
+MAKE_WORKER_PROTO(, KeyedBuildResult);
 MAKE_WORKER_PROTO(, std::optional<TrustedFlag>);
 
 MAKE_WORKER_PROTO(template<typename T>, std::vector<T>);

src/libutil/config-impl.hh

@@ -0,0 +1,71 @@
+#pragma once
+/**
+ * @file
+ *
+ * Template implementations (as opposed to mere declarations).
+ *
+ * One only needs to include this when one is declaring a
+ * `BaseClass<CustomType>` setting, or as derived class of such an
+ * instantiation.
+ */
+
+#include "config.hh"
+
+namespace nix {
+
+template<> struct BaseSetting<Strings>::trait
+{
+    static constexpr bool appendable = true;
+};
+template<> struct BaseSetting<StringSet>::trait
+{
+    static constexpr bool appendable = true;
+};
+template<> struct BaseSetting<StringMap>::trait
+{
+    static constexpr bool appendable = true;
+};
+template<> struct BaseSetting<std::set<ExperimentalFeature>>::trait
+{
+    static constexpr bool appendable = true;
+};
+
+template<typename T>
+struct BaseSetting<T>::trait
+{
+    static constexpr bool appendable = false;
+};
+
+template<typename T>
+bool BaseSetting<T>::isAppendable()
+{
+    return trait::appendable;
+}
+
+template<> void BaseSetting<Strings>::appendOrSet(Strings && newValue, bool append);
+template<> void BaseSetting<StringSet>::appendOrSet(StringSet && newValue, bool append);
+template<> void BaseSetting<StringMap>::appendOrSet(StringMap && newValue, bool append);
+template<> void BaseSetting<std::set<ExperimentalFeature>>::appendOrSet(std::set<ExperimentalFeature> && newValue, bool append);
+
+template<typename T>
+void BaseSetting<T>::appendOrSet(T && newValue, bool append)
+{
+    static_assert(!trait::appendable, "using default `appendOrSet` implementation with an appendable type");
+    assert(!append);
+    value = std::move(newValue);
+}
+
+template<typename T>
+void BaseSetting<T>::set(const std::string & str, bool append)
+{
+    if (experimentalFeatureSettings.isEnabled(experimentalFeature))
+        appendOrSet(parse(str), append);
+    else {
+        assert(experimentalFeature);
+        warn("Ignoring setting '%s' because experimental feature '%s' is not enabled",
+            name,
+            showExperimentalFeature(*experimentalFeature));
+    }
+}
+
+}

src/libutil/config.cc

@@ -3,6 +3,8 @@
 #include "abstract-setting-to-json.hh"
 #include "experimental-features.hh"
 
+#include "config-impl.hh"
+
 #include <nlohmann/json.hpp>
 
 namespace nix {
@@ -80,6 +82,8 @@ void Config::getSettings(std::map<std::string, SettingInfo> & res, bool overridd
 void AbstractConfig::applyConfig(const std::string & contents, const std::string & path) {
     unsigned int pos = 0;
 
+    std::vector<std::pair<std::string, std::string>> parsedContents;
+
     while (pos < contents.size()) {
         std::string line;
         while (pos < contents.size() && contents[pos] != '\n')
@@ -125,8 +129,21 @@ void AbstractConfig::applyConfig(const std::string & contents, const std::string
         auto i = tokens.begin();
         advance(i, 2);
 
-        set(name, concatStringsSep(" ", Strings(i, tokens.end()))); // FIXME: slow
+        parsedContents.push_back({
+            name,
+            concatStringsSep(" ", Strings(i, tokens.end())),
+        });
     };
+
+    // First apply experimental-feature related settings
+    for (auto & [name, value] : parsedContents)
+        if (name == "experimental-features" || name == "extra-experimental-features")
+            set(name, value);
+
+    // Then apply other settings
+    for (auto & [name, value] : parsedContents)
+        if (name != "experimental-features" && name != "extra-experimental-features")
+            set(name, value);
 }
 
 void AbstractConfig::applyConfigFile(const Path & path)
@@ -202,12 +219,6 @@ void AbstractSetting::convertToArg(Args & args, const std::string & category)
 {
 }
 
-template<typename T>
-bool BaseSetting<T>::isAppendable()
-{
-    return false;
-}
-
 template<typename T>
 void BaseSetting<T>::convertToArg(Args & args, const std::string & category)
 {
@@ -231,9 +242,9 @@ void BaseSetting<T>::convertToArg(Args & args, const std::string & category)
         });
 }
 
-template<> void BaseSetting<std::string>::set(const std::string & str, bool append)
+template<> std::string BaseSetting<std::string>::parse(const std::string & str) const
 {
-    value = str;
+    return str;
 }
 
 template<> std::string BaseSetting<std::string>::to_string() const
@@ -242,11 +253,11 @@ template<> std::string BaseSetting<std::string>::to_string() const
 }
 
 template<typename T>
-void BaseSetting<T>::set(const std::string & str, bool append)
+T BaseSetting<T>::parse(const std::string & str) const
 {
     static_assert(std::is_integral<T>::value, "Integer required.");
     if (auto n = string2Int<T>(str))
-        value = *n;
+        return *n;
     else
         throw UsageError("setting '%s' has invalid value '%s'", name, str);
 }
@@ -258,12 +269,12 @@ std::string BaseSetting<T>::to_string() const
     return std::to_string(value);
 }
 
-template<> void BaseSetting<bool>::set(const std::string & str, bool append)
+template<> bool BaseSetting<bool>::parse(const std::string & str) const
 {
     if (str == "true" || str == "yes" || str == "1")
-        value = true;
+        return true;
     else if (str == "false" || str == "no" || str == "0")
-        value = false;
+        return false;
     else
         throw UsageError("Boolean setting '%s' has invalid value '%s'", name, str);
 }
@@ -291,16 +302,15 @@ template<> void BaseSetting<bool>::convertToArg(Args & args, const std::string &
     });
 }
 
-template<> void BaseSetting<Strings>::set(const std::string & str, bool append)
+template<> Strings BaseSetting<Strings>::parse(const std::string & str) const
 {
-    auto ss = tokenizeString<Strings>(str);
-    if (!append) value.clear();
-    for (auto & s : ss) value.push_back(std::move(s));
+    return tokenizeString<Strings>(str);
 }
 
-template<> bool BaseSetting<Strings>::isAppendable()
+template<> void BaseSetting<Strings>::appendOrSet(Strings && newValue, bool append)
 {
-    return true;
+    if (!append) value.clear();
+    for (auto && s : std::move(newValue)) value.push_back(std::move(s));
 }
 
 template<> std::string BaseSetting<Strings>::to_string() const
@@ -308,16 +318,16 @@ template<> std::string BaseSetting<Strings>::to_string() const
     return concatStringsSep(" ", value);
 }
 
-template<> void BaseSetting<StringSet>::set(const std::string & str, bool append)
+template<> StringSet BaseSetting<StringSet>::parse(const std::string & str) const
 {
-    if (!append) value.clear();
-    for (auto & s : tokenizeString<StringSet>(str))
-        value.insert(s);
+    return tokenizeString<StringSet>(str);
 }
 
-template<> bool BaseSetting<StringSet>::isAppendable()
+template<> void BaseSetting<StringSet>::appendOrSet(StringSet && newValue, bool append)
 {
-    return true;
+    if (!append) value.clear();
+    for (auto && s : std::move(newValue))
+        value.insert(s);
 }
 
 template<> std::string BaseSetting<StringSet>::to_string() const
@@ -325,21 +335,24 @@ template<> std::string BaseSetting<StringSet>::to_string() const
     return concatStringsSep(" ", value);
 }
 
-template<> void BaseSetting<std::set<ExperimentalFeature>>::set(const std::string & str, bool append)
+template<> std::set<ExperimentalFeature> BaseSetting<std::set<ExperimentalFeature>>::parse(const std::string & str) const
 {
-    if (!append) value.clear();
+    std::set<ExperimentalFeature> res;
     for (auto & s : tokenizeString<StringSet>(str)) {
         auto thisXpFeature = parseExperimentalFeature(s);
         if (thisXpFeature)
-            value.insert(thisXpFeature.value());
+            res.insert(thisXpFeature.value());
         else
             warn("unknown experimental feature '%s'", s);
     }
+    return res;
 }
 
-template<> bool BaseSetting<std::set<ExperimentalFeature>>::isAppendable()
+template<> void BaseSetting<std::set<ExperimentalFeature>>::appendOrSet(std::set<ExperimentalFeature> && newValue, bool append)
 {
-    return true;
+    if (!append) value.clear();
+    for (auto && s : std::move(newValue))
+        value.insert(s);
 }
 
 template<> std::string BaseSetting<std::set<ExperimentalFeature>>::to_string() const
@@ -350,20 +363,23 @@ template<> std::string BaseSetting<std::set<ExperimentalFeature>>::to_string() c
     return concatStringsSep(" ", stringifiedXpFeatures);
 }
 
-template<> void BaseSetting<StringMap>::set(const std::string & str, bool append)
+template<> StringMap BaseSetting<StringMap>::parse(const std::string & str) const
 {
-    if (!append) value.clear();
+    StringMap res;
     for (auto & s : tokenizeString<Strings>(str)) {
         auto eq = s.find_first_of('=');
         if (std::string::npos != eq)
-            value.emplace(std::string(s, 0, eq), std::string(s, eq + 1));
+            res.emplace(std::string(s, 0, eq), std::string(s, eq + 1));
         // else ignored
     }
+    return res;
 }
 
-template<> bool BaseSetting<StringMap>::isAppendable()
+template<> void BaseSetting<StringMap>::appendOrSet(StringMap && newValue, bool append)
 {
-    return true;
+    if (!append) value.clear();
+    for (auto && [k, v] : std::move(newValue))
+        value.emplace(std::move(k), std::move(v));
 }
 
 template<> std::string BaseSetting<StringMap>::to_string() const
@@ -387,15 +403,15 @@ template class BaseSetting<StringSet>;
 template class BaseSetting<StringMap>;
 template class BaseSetting<std::set<ExperimentalFeature>>;
 
-void PathSetting::set(const std::string & str, bool append)
+Path PathSetting::parse(const std::string & str) const
 {
     if (str == "") {
         if (allowEmpty)
-            value = "";
+            return "";
         else
             throw UsageError("setting '%s' cannot be empty", name);
     } else
-        value = canonPath(str);
+        return canonPath(str);
 }
 
 bool GlobalConfig::set(const std::string & name, const std::string & value)

src/libutil/config.hh

@@ -215,8 +215,11 @@ protected:
 
     virtual void set(const std::string & value, bool append = false) = 0;
 
-    virtual bool isAppendable()
-    { return false; }
+    /**
+     * Whether the type is appendable; i.e. whether the `append`
+     * parameter to `set()` is allowed to be `true`.
+     */
+    virtual bool isAppendable() = 0;
 
     virtual std::string to_string() const = 0;
 
@@ -241,6 +244,23 @@ protected:
     const T defaultValue;
     const bool documentDefault;
 
+    /**
+     * Parse the string into a `T`.
+     *
+     * Used by `set()`.
+     */
+    virtual T parse(const std::string & str) const;
+
+    /**
+     * Append or overwrite `value` with `newValue`.
+     *
+     * Some types to do not support appending in which case `append`
+     * should never be passed. The default handles this case.
+     *
+     * @param append Whether to append or overwrite.
+     */
+    virtual void appendOrSet(T && newValue, bool append);
+
 public:
 
     BaseSetting(const T & def,
@@ -268,9 +288,25 @@ public:
     template<typename U>
     void setDefault(const U & v) { if (!overridden) value = v; }
 
-    void set(const std::string & str, bool append = false) override;
+    /**
+     * Require any experimental feature the setting depends on
+     *
+     * Uses `parse()` to get the value from `str`, and `appendOrSet()`
+     * to set it.
+     */
+    void set(const std::string & str, bool append = false) override final;
 
-    bool isAppendable() override;
+    /**
+     * C++ trick; This is template-specialized to compile-time indicate whether
+     * the type is appendable.
+     */
+    struct trait;
+
+    /**
+     * Always defined based on the C++ magic
+     * with `trait` above.
+     */
+    bool isAppendable() override final;
 
     virtual void override(const T & v)
     {
@@ -336,7 +372,7 @@ public:
         options->addSetting(this);
     }
 
-    void set(const std::string & str, bool append = false) override;
+    Path parse(const std::string & str) const override;
 
     Path operator +(const char * p) const { return value + p; }
 

src/libutil/tests/config.cc

@@ -82,6 +82,7 @@ namespace nix {
             TestSetting() : AbstractSetting("test", "test", {}) {}
             void set(const std::string & value, bool append) override {}
             std::string to_string() const override { return {}; }
+            bool isAppendable() override { return false; }
         };
 
         Config config;
@@ -90,6 +91,7 @@ namespace nix {
         ASSERT_FALSE(config.set("test", "value"));
         config.addSetting(&setting);
         ASSERT_TRUE(config.set("test", "value"));
+        ASSERT_FALSE(config.set("extra-test", "value"));
     }
 
     TEST(Config, withInitialValue) {

src/nix-store/nix-store.cc

@@ -204,10 +204,10 @@ static void opAddFixed(Strings opFlags, Strings opArgs)
 /* Hack to support caching in `nix-prefetch-url'. */
 static void opPrintFixedPath(Strings opFlags, Strings opArgs)
 {
-    auto recursive = FileIngestionMethod::Flat;
+    auto method = FileIngestionMethod::Flat;
 
     for (auto i : opFlags)
-        if (i == "--recursive") recursive = FileIngestionMethod::Recursive;
+        if (i == "--recursive") method = FileIngestionMethod::Recursive;
         else throw UsageError("unknown flag '%1%'", i);
 
     if (opArgs.size() != 3)
@@ -218,7 +218,13 @@ static void opPrintFixedPath(Strings opFlags, Strings opArgs)
     std::string hash = *i++;
     std::string name = *i++;
 
-    cout << fmt("%s\n", store->printStorePath(store->makeFixedOutputPath(recursive, Hash::parseAny(hash, hashAlgo), name)));
+    cout << fmt("%s\n", store->printStorePath(store->makeFixedOutputPath(name, FixedOutputInfo {
+        .hash = {
+            .method = method,
+            .hash = Hash::parseAny(hash, hashAlgo),
+        },
+        .references = {},
+    })));
 }
 
 
@@ -935,7 +941,10 @@ static void opServe(Strings opFlags, Strings opArgs)
                 if (GET_PROTOCOL_MINOR(clientVersion) >= 3)
                     out << status.timesBuilt << status.isNonDeterministic << status.startTime << status.stopTime;
                 if (GET_PROTOCOL_MINOR(clientVersion) >= 6) {
-                    worker_proto::write(*store, out, status.builtOutputs);
+                    DrvOutputs builtOutputs;
+                    for (auto & [output, realisation] : status.builtOutputs)
+                        builtOutputs.insert_or_assign(realisation.id, realisation);
+                    worker_proto::write(*store, out, builtOutputs);
                 }
 
                 break;
@@ -964,7 +973,7 @@ static void opServe(Strings opFlags, Strings opArgs)
                 info.references = worker_proto::read(*store, in, Phantom<StorePathSet> {});
                 in >> info.registrationTime >> info.narSize >> info.ultimate;
                 info.sigs = readStrings<StringSet>(in);
-                info.ca = parseContentAddressOpt(readString(in));
+                info.ca = ContentAddress::parseOpt(readString(in));
 
                 if (info.narSize == 0)
                     throw Error("narInfo is too old and missing the narSize field");

src/nix/add-to-store.cc

@@ -42,14 +42,18 @@ struct CmdAddToStore : MixDryRun, StoreCommand
         }
 
         ValidPathInfo info {
-            store->makeFixedOutputPath(ingestionMethod, hash, *namePart),
+            *store,
+            std::move(*namePart),
+            FixedOutputInfo {
+                .hash = {
+                    .method = std::move(ingestionMethod),
+                    .hash = std::move(hash),
+                },
+                .references = {},
+            },
             narHash,
         };
         info.narSize = sink.s.size();
-        info.ca = std::optional { FixedOutputHash {
-            .method = ingestionMethod,
-            .hash = hash,
-        } };
 
         if (!dryRun) {
             auto source = StringSource(sink.s);

src/nix/daemon.cc

@@ -274,11 +274,11 @@ static std::pair<TrustedFlag, std::string> authPeer(const PeerInfo & peer)
  * Run a server. The loop opens a socket and accepts new connections from that
  * socket.
  *
- * @param trustClientOpt If present, force trusting or not trusted the client.
- * Otherwise, decide based on the authentication settings and user credentials
- * (from the unix domain socket).
+ * @param forceTrustClientOpt If present, force trusting or not trusted
+ * the client. Otherwise, decide based on the authentication settings
+ * and user credentials (from the unix domain socket).
  */
-static void daemonLoop(std::optional<TrustedFlag> trustClientOpt)
+static void daemonLoop(std::optional<TrustedFlag> forceTrustClientOpt)
 {
     if (chdir("/") == -1)
         throw SysError("cannot change current directory");
@@ -325,8 +325,8 @@ static void daemonLoop(std::optional<TrustedFlag> trustClientOpt)
             TrustedFlag trusted;
             std::string user;
 
-            if (trustClientOpt)
-                trusted = *trustClientOpt;
+            if (forceTrustClientOpt)
+                trusted = *forceTrustClientOpt;
             else {
                 peer = getPeerInfo(remote.get());
                 auto [_trusted, _user] = authPeer(peer);
@@ -436,8 +436,11 @@ static void processStdioConnection(ref<Store> store, TrustedFlag trustClient)
 /**
  * Entry point shared between the new CLI `nix daemon` and old CLI
  * `nix-daemon`.
+ *
+ * @param forceTrustClientOpt See `daemonLoop()` and the parameter with
+ * the same name over there for details.
  */
-static void runDaemon(bool stdio, std::optional<TrustedFlag> trustClientOpt)
+static void runDaemon(bool stdio, std::optional<TrustedFlag> forceTrustClientOpt)
 {
     if (stdio) {
         auto store = openUncachedStore();
@@ -445,15 +448,15 @@ static void runDaemon(bool stdio, std::optional<TrustedFlag> trustClientOpt)
         // If --force-untrusted is passed, we cannot forward the connection and
         // must process it ourselves (before delegating to the next store) to
         // force untrusting the client.
-        if (auto remoteStore = store.dynamic_pointer_cast<RemoteStore>(); remoteStore && (!trustClientOpt || *trustClientOpt != NotTrusted))
+        if (auto remoteStore = store.dynamic_pointer_cast<RemoteStore>(); remoteStore && (!forceTrustClientOpt || *forceTrustClientOpt != NotTrusted))
             forwardStdioConnection(*remoteStore);
         else
             // `Trusted` is passed in the auto (no override case) because we
             // cannot see who is on the other side of a plain pipe. Limiting
             // access to those is explicitly not `nix-daemon`'s responsibility.
-            processStdioConnection(store, trustClientOpt.value_or(Trusted));
+            processStdioConnection(store, forceTrustClientOpt.value_or(Trusted));
     } else
-        daemonLoop(trustClientOpt);
+        daemonLoop(forceTrustClientOpt);
 }
 
 static int main_nix_daemon(int argc, char * * argv)

src/nix/prefetch.cc

@@ -67,7 +67,13 @@ std::tuple<StorePath, Hash> prefetchFile(
        the store. */
     if (expectedHash) {
         hashType = expectedHash->type;
-        storePath = store->makeFixedOutputPath(ingestionMethod, *expectedHash, *name);
+        storePath = store->makeFixedOutputPath(*name, FixedOutputInfo {
+            .hash = {
+                .method = ingestionMethod,
+                .hash = *expectedHash,
+            },
+            .references = {},
+        });
         if (store->isValidPath(*storePath))
             hash = expectedHash;
         else
@@ -118,7 +124,7 @@ std::tuple<StorePath, Hash> prefetchFile(
         auto info = store->addToStoreSlow(*name, tmpFile, ingestionMethod, hashType, expectedHash);
         storePath = info.path;
         assert(info.ca);
-        hash = getContentAddressHash(*info.ca);
+        hash = info.ca->getHash();
     }
 
     return {storePath.value(), hash.value()};

src/nix/profile.cc

@@ -200,12 +200,22 @@ struct ProfileManifest
         auto narHash = hashString(htSHA256, sink.s);
 
         ValidPathInfo info {
-            store->makeFixedOutputPath(FileIngestionMethod::Recursive, narHash, "profile", references),
+            *store,
+            "profile",
+            FixedOutputInfo {
+                .hash = {
+                    .method = FileIngestionMethod::Recursive,
+                    .hash = narHash,
+                },
+                .references = {
+                    .others = std::move(references),
+                    // profiles never refer to themselves
+                    .self = false,
+                },
+            },
             narHash,
         };
-        info.references = std::move(references);
         info.narSize = sink.s.size();
-        info.ca = FixedOutputHash { .method = FileIngestionMethod::Recursive, .hash = info.narHash };
 
         StringSource source(sink.s);
         store->addToStore(info, source);

tests/experimental-features.sh

@@ -23,20 +23,64 @@ source common.sh
 # # Medium case, the configuration effects --help
 # grep_both_ways store gc --help
 
-expect 1 nix --experimental-features 'nix-command' show-config --flake-registry 'https://no'
-nix --experimental-features 'nix-command flakes' show-config --flake-registry 'https://no'
+# Test settings that are gated on experimental features; the setting is ignored
+# with a warning if the experimental feature is not enabled. The order of the
+# `setting = value` lines in the configuration should not matter.
+
+# 'flakes' experimental-feature is disabled before, ignore and warn
+NIX_CONFIG='
+  experimental-features = nix-command
+  accept-flake-config = true
+' nix show-config accept-flake-config 1>$TEST_ROOT/stdout 2>$TEST_ROOT/stderr
+grepQuiet "false" $TEST_ROOT/stdout
+grepQuiet "Ignoring setting 'accept-flake-config' because experimental feature 'flakes' is not enabled" $TEST_ROOT/stderr
+
+# 'flakes' experimental-feature is disabled after, ignore and warn
+NIX_CONFIG='
+  accept-flake-config = true
+  experimental-features = nix-command
+' nix show-config accept-flake-config 1>$TEST_ROOT/stdout 2>$TEST_ROOT/stderr
+grepQuiet "false" $TEST_ROOT/stdout
+grepQuiet "Ignoring setting 'accept-flake-config' because experimental feature 'flakes' is not enabled" $TEST_ROOT/stderr
+
+# 'flakes' experimental-feature is enabled before, process
+NIX_CONFIG='
+  experimental-features = nix-command flakes
+  accept-flake-config = true
+' nix show-config accept-flake-config 1>$TEST_ROOT/stdout 2>$TEST_ROOT/stderr
+grepQuiet "true" $TEST_ROOT/stdout
+grepQuietInverse "Ignoring setting 'accept-flake-config'" $TEST_ROOT/stderr
+
+# 'flakes' experimental-feature is enabled after, process
+NIX_CONFIG='
+  accept-flake-config = true
+  experimental-features = nix-command flakes
+' nix show-config accept-flake-config 1>$TEST_ROOT/stdout 2>$TEST_ROOT/stderr
+grepQuiet "true" $TEST_ROOT/stdout
+grepQuietInverse "Ignoring setting 'accept-flake-config'" $TEST_ROOT/stderr
+
+function exit_code_both_ways {
+    expect 1 nix --experimental-features 'nix-command' "$@" 1>/dev/null
+    nix --experimental-features 'nix-command flakes' "$@" 1>/dev/null
+
+    # Also, the order should not matter
+    expect 1 nix "$@" --experimental-features 'nix-command' 1>/dev/null
+    nix "$@" --experimental-features 'nix-command flakes' 1>/dev/null
+}
+
+exit_code_both_ways show-config --flake-registry 'https://no'
 
 # Double check these are stable
-nix --experimental-features '' --help
-nix --experimental-features '' doctor --help
-nix --experimental-features '' repl --help
-nix --experimental-features '' upgrade-nix --help
+nix --experimental-features '' --help 1>/dev/null
+nix --experimental-features '' doctor --help 1>/dev/null
+nix --experimental-features '' repl --help 1>/dev/null
+nix --experimental-features '' upgrade-nix --help 1>/dev/null
 
 # These 3 arguments are currently given to all commands, which is wrong (as not
 # all care). To deal with fixing later, we simply make them require the
 # nix-command experimental features --- it so happens that the commands we wish
 # stabilizing to do not need them anyways.
 for arg in '--print-build-logs' '--offline' '--refresh'; do
-    nix --experimental-features 'nix-command' "$arg" --help
-    ! nix --experimental-features '' "$arg" --help
+    nix --experimental-features 'nix-command' "$arg" --help 1>/dev/null
+    expect 1 nix --experimental-features '' "$arg" --help 1>/dev/null
 done