Get rid of DerivationBuilder::note*Mismatch

It's fine to set these worker flags a little later in the control flow,
since we'll be sure to reach those points in the error cases. And doing
that is much nicer than having these tangled callbacks.

I originally made the callbacks to meticulously recreate the exact
behavior which I didn't quite understand. Now, thanks to cleaning up the
error handling, I do understand what is going on, so I can be confident
that this change is safe to make.

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

@@ -652,16 +652,6 @@ Goal::Co DerivationBuildingGoal::tryToBuild()
                     goal.worker.childTerminated(&goal);
                 }
 
-                void noteHashMismatch() override
-                {
-                    goal.worker.hashMismatch = true;
-                }
-
-                void noteCheckMismatch() override
-                {
-                    goal.worker.checkMismatch = true;
-                }
-
                 void markContentsGood(const StorePath & path) override
                 {
                     goal.worker.markContentsGood(path);
@@ -818,6 +808,26 @@ Goal::Co DerivationBuildingGoal::tryToBuild()
     // N.B. cannot use `std::visit` with co-routine return
     if (auto * ste = std::get_if<0>(&res)) {
         outputLocks.unlock();
+// Allow selecting a subset of enum values
+#  pragma GCC diagnostic push
+#  pragma GCC diagnostic ignored "-Wswitch-enum"
+        switch (ste->status) {
+        case BuildResult::HashMismatch:
+            worker.hashMismatch = true;
+            /* See header, the protocols don't know about `HashMismatch`
+               yet, so change it to `OutputRejected`, which they expect
+               for this case (hash mismatch is a type of output
+               rejection). */
+            ste->status = BuildResult::OutputRejected;
+            break;
+        case BuildResult::NotDeterministic:
+            worker.checkMismatch = true;
+            break;
+        default:
+            /* Other statuses need no adjusting */
+            break;
+        }
+#  pragma GCC diagnostic pop
         co_return doneFailure(std::move(*ste));
     } else if (auto * builtOutputs = std::get_if<1>(&res)) {
         StorePathSet outputPaths;

src/libstore/include/nix/store/build-result.hh

@@ -36,6 +36,10 @@ struct BuildResult
         NotDeterministic,
         ResolvesToAlreadyValid,
         NoSubstituters,
+        /// A certain type of `OutputRejected`. The protocols do not yet
+        /// know about this one, so change it back to `OutputRejected`
+        /// before serialization.
+        HashMismatch,
     } status = MiscFailure;
 
     /**

src/libstore/include/nix/store/build/derivation-builder.hh

@@ -134,9 +134,6 @@ struct DerivationBuilderCallbacks
      */
     virtual void childTerminated() = 0;
 
-    virtual void noteHashMismatch(void) = 0;
-    virtual void noteCheckMismatch(void) = 0;
-
     virtual void markContentsGood(const StorePath & path) = 0;
 };
 

src/libstore/unix/build/derivation-builder.cc

@@ -1659,9 +1659,8 @@ SingleDrvOutputs DerivationBuilderImpl::registerOutputs()
                     if (wanted != got) {
                         /* Throw an error after registering the path as
                            valid. */
-                        miscMethods->noteHashMismatch();
                         delayedException = std::make_exception_ptr(BuildError(
-                            BuildResult::OutputRejected,
+                            BuildResult::HashMismatch,
                             "hash mismatch in fixed-output derivation '%s':\n  specified: %s\n     got:    %s",
                             store.printStorePath(drvPath),
                             wanted.to_string(HashFormat::SRI, true),
@@ -1670,7 +1669,7 @@ SingleDrvOutputs DerivationBuilderImpl::registerOutputs()
                     if (!newInfo0.references.empty()) {
                         auto numViolations = newInfo.references.size();
                         delayedException = std::make_exception_ptr(BuildError(
-                            BuildResult::OutputRejected,
+                            BuildResult::HashMismatch,
                             "fixed-output derivations must not reference store paths: '%s' references %d distinct paths, e.g. '%s'",
                             store.printStorePath(drvPath),
                             numViolations,
@@ -1746,7 +1745,6 @@ SingleDrvOutputs DerivationBuilderImpl::registerOutputs()
                 continue;
             ValidPathInfo oldInfo(*store.queryPathInfo(newInfo.path));
             if (newInfo.narHash != oldInfo.narHash) {
-                miscMethods->noteCheckMismatch();
                 if (settings.runDiffHook || settings.keepFailed) {
                     auto dst = store.toRealPath(finalDestPath + ".check");
                     deletePath(dst);