Fix #9052

It's hard to reproduce this race condition going forward (e.g. it
depends on the precise order that goals are scheduled, but
https://github.com/roberth/nix-9052 current reproduces it, and this
fixes that.

src/libstore/build/derivation-creation-and-realisation-goal.cc

@@ -51,16 +51,23 @@ void DerivationCreationAndRealisationGoal::addWantedOutputs(const OutputsSpec &
 {
     /* If we already want all outputs, there is nothing to do. */
     auto newWanted = wantedOutputs.union_(outputs);
-    bool needRestart = !newWanted.isSubsetOf(wantedOutputs);
+    bool moreWanted = !newWanted.isSubsetOf(wantedOutputs);
     wantedOutputs = newWanted;
 
-    if (!needRestart)
+    if (!moreWanted)
         return;
 
     if (!optDrvPath)
         // haven't started steps where the outputs matter yet
         return;
-    worker.makeDerivationGoal(*optDrvPath, outputs, buildMode);
+    auto g = worker.makeDerivationGoal(*optDrvPath, outputs, buildMode);
+
+    if (!(upcast_goal(g)->exitCode == ecSuccess || upcast_goal(g)->exitCode == ecFailed || upcast_goal(g)->exitCode == ecNoSubstituters || upcast_goal(g)->exitCode == ecIncompleteClosure)) {
+        /* The original concrete goal has already finished and been
+           destroyed, so reset this variable and double-suspend */
+        concreteDrvGoal = g;
+        needsRestart = true;
+    }
 }
 
 Goal::Co DerivationCreationAndRealisationGoal::init()
@@ -108,8 +115,12 @@ Goal::Co DerivationCreationAndRealisationGoal::init()
         g->preserveException = true;
     }
     optDrvPath = std::move(drvPath);
-    addWaitee(upcast_goal(concreteDrvGoal));
-    co_await Suspend{};
+
+    do {
+        needsRestart = false;
+        addWaitee(upcast_goal(concreteDrvGoal));
+        co_await Suspend{};
+    } while (needsRestart);
 
     trace("outer build done");
 

src/libstore/build/derivation-creation-and-realisation-goal.hh

@@ -47,6 +47,11 @@ struct DerivationCreationAndRealisationGoal : public Goal
      */
     OutputsSpec wantedOutputs;
 
+    /**
+     * Inner goal got recreated, need to wait again potentially.
+     */
+    bool needsRestart = false;
+
     /**
      * The final output paths of the build.
      *