Make `KeyedBuildResult`, `BuildResult` like before, and fix bug another way

In https://github.com/NixOS/nix/pull/6311#discussion_r834863823, I
realized since derivation goals' wanted outputs can "grow" due to
overlapping dependencies (See `DerivationGoal::addWantedOutputs`, called
by `Worker::makeDerivationGoalCommon`), the previous bug fix had an
unfortunate side effect of causing more pointless rebuilds.

In paticular, we have this situation:

1. Goal made from `DerivedPath::Built { foo, {a} }`.

2. Goal gives on on substituting, starts building.

3. Goal made from `DerivedPath::Built { foo, {b} }`, in fact is just
   modified original goal.

4. Though the goal had gotten as far as building, so all outputs were
   going to be produced, `addWantedOutputs` no longer knows that and so
   the goal is flagged to be restarted.

This might sound far-fetched with input-addressed drvs, where we usually
basically have all our goals "planned out" before we start doing
anything, but with CA derivation goals and especially RFC 92, where *drv
resolution* means goals are created after some building is completed, it
is more likely to happen.

So the first thing to do was restore the clearing of `wantedOutputs` we
used to do, and then filter the outputs in `buildPathsWithResults` to
only get the ones we care about.

But fix also has its own side effect in that the `DerivedPath` in the
`BuildResult` in `DerivationGoal` cannot be trusted; it is merely the
*first* `DerivedPath` for which this goal was originally created.

To remedy this, I made `BuildResult` be like it was before, and instead
made `KeyedBuildResult` be a subclass wit the path. Only
`buildPathsWithResults` returns `KeyedBuildResult`s, everything else
just becomes like it was before, where the "key" is unambiguous from
context.

I think separating the "primary key" field(s) from the other fields is
good practical in general anyways. (I would like to do the same thing
for `ValidPathInfo`.) Among other things, it allows constructions like
`std::map<Key, ThingWithKey>` where doesn't contain duplicate keys and
just precludes the possibility of those duplicate keys being out of
sync.

We might leverage the above someday to overload `buildPathsWithResults`
to take a *set* of return a *map* per the above.

-----

Unfortunately, we need to avoid C++20 strictness on designated
initializers.

(BTW
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2287r1.html
this offers some new syntax for this use-case. Hopefully this will be
adopted and we can eventually use it.)

No having that yet, maybe it would be better to not make
`KeyedBuildResult` a subclass to just avoid this.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

11 files changed, 130 insertions, 51 deletions

diff --git a/src/libstore/build-result.hh b/src/libstore/build-result.hh
index 27d1a1b6c..e07296eab 100644
--- a/src/libstore/build-result.hh
+++ b/src/libstore/build-result.hh
@@ -84,11 +84,6 @@ 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.
      */
@@ -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;
+};
+
 }
diff --git a/src/libstore/build/derivation-goal.cc b/src/libstore/build/derivation-goal.cc
index 26faf8c8e..606f9fd48 100644
--- a/src/libstore/build/derivation-goal.cc
+++ b/src/libstore/build/derivation-goal.cc
@@ -330,6 +330,10 @@ void DerivationGoal::outputsSubstitutionTried()
    produced using a substitute.  So we have to build instead. */
 void DerivationGoal::gaveUpOnSubstitution()
 {
+    /* Make sure checkPathValidity() from now on checks all
+       outputs. */
+    wantedOutputs = OutputsSpec::All { };
+
     /* The inputs must be built before we can build this goal. */
     inputDrvOutputs.clear();
     if (useDerivation)
@@ -570,8 +574,6 @@ void DerivationGoal::inputsRealised()
        build hook. */
     state = &DerivationGoal::tryToBuild;
     worker.wakeUp(shared_from_this());
-
-    buildResult = BuildResult { .path = buildResult.path };
 }
 
 void DerivationGoal::started()
@@ -1452,12 +1454,28 @@ 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()) {
+            for (auto & [output, realisation] : buildResult.builtOutputs) {
                 inputDrvOutputs.insert_or_assign(
-                    { bfd->drvPath, output.outputName },
+                    { dg->drvPath, output.outputName },
                     realisation.outPath);
+            }
+        }
+    }
 }
 
 }
diff --git a/src/libstore/build/entry-points.cc b/src/libstore/build/entry-points.cc
index 7d725b881..74eae0692 100644
--- a/src/libstore/build/entry-points.cc
+++ b/src/libstore/build/entry-points.cc
@@ -39,7 +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)
@@ -47,15 +47,23 @@ std::vector<BuildResult> Store::buildPathsWithResults(
     Worker worker(*this, evalStore ? *evalStore : *this);
 
     Goals goals;
-    for (const auto & br : reqs)
-        goals.insert(worker.makeGoal(br, buildMode));
+    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;
 }
@@ -68,15 +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 { },
-            },
         };
     };
 }
diff --git a/src/libstore/build/goal.cc b/src/libstore/build/goal.cc
index d59b94797..13b2e509a 100644
--- a/src/libstore/build/goal.cc
+++ b/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.outputName))
+                ++it;
+            else
+                it = res.builtOutputs.erase(it);
+        }
+    }
+
+    return res;
+}
+
+
 void addToWeakGoals(WeakGoals & goals, GoalPtr p)
 {
     if (goals.find(p) != goals.end())
diff --git a/src/libstore/build/goal.hh b/src/libstore/build/goal.hh
index f4bf6f38b..c0e12a2ed 100644
--- a/src/libstore/build/goal.hh
+++ b/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()
diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index 58d6901d3..af937f6b1 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/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
diff --git a/src/libstore/legacy-ssh-store.cc b/src/libstore/legacy-ssh-store.cc
index d2ddbbe5f..7b40b27e0 100644
--- a/src/libstore/legacy-ssh-store.cc
+++ b/src/libstore/legacy-ssh-store.cc
@@ -287,12 +287,7 @@ 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;
 
@@ -330,7 +325,7 @@ public:
 
         conn->to.flush();
 
-        BuildResult result { .path = DerivedPath::Opaque { StorePath::dummy } };
+        BuildResult result;
         result.status = (BuildResult::Status) readInt(conn->from);
 
         if (!result.success()) {
diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index b862902d1..734e6f27f 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -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
@@ -142,7 +158,6 @@ BuildResult read(const Store & store, Source & from, Phantom<BuildResult> _)
 
 void write(const Store & store, Sink & to, const BuildResult & res)
 {
-    worker_proto::write(store, to, res.path);
     to
         << res.status
         << res.errorMsg
@@ -865,7 +880,7 @@ 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 +895,7 @@ 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 +904,25 @@ 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 {
-                            .status = BuildResult::Substituted,
-                            .path = bo,
+                        results.push_back(KeyedBuildResult {
+                            {
+                                .status = BuildResult::Substituted,
+                            },
+                            /* .path = */ bo,
                         });
                     },
                     [&](const DerivedPath::Built & bfd) {
-                        BuildResult res {
-                            .status = BuildResult::Built,
-                            .path = bfd,
+                        KeyedBuildResult res {
+                            {
+                                .status = BuildResult::Built
+                            },
+                            /* .path = */ bfd,
                         };
 
                         OutputPathMap outputs;
@@ -952,12 +971,7 @@ 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) {
diff --git a/src/libstore/remote-store.hh b/src/libstore/remote-store.hh
index 1c45f543e..a30466647 100644
--- a/src/libstore/remote-store.hh
+++ b/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;
diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index 5bee272bf..9d6880de9 100644
--- a/src/libstore/store-api.hh
+++ b/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;
@@ -575,7 +576,7 @@ 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);
diff --git a/src/libstore/worker-protocol.hh b/src/libstore/worker-protocol.hh
index c7a6f8688..34b2fc17b 100644
--- a/src/libstore/worker-protocol.hh
+++ b/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>);