Don't copy Protobuf messages via assignment

This isn't safe, as the Protobuf struct can contain fields that are
protected by locks. It's only safe to create new messages by copying
fields individually, or by using proto.Merge().

Author: EdSchouten

pkg/scheduler/in_memory_build_queue.go

@@ -2508,19 +2508,24 @@ func (t *task) complete(bq *InMemoryBuildQueue, executeResponse *remoteexecution
 					// Already running too many background tasks.
 					backgroundInitialSizeClassLearner.Abandoned()
 				} else {
-					backgroundAction := *t.desiredState.Action
-					backgroundAction.DoNotCache = true
-					backgroundAction.Timeout = durationpb.New(backgroundTimeout)
 					backgroundTask := &task{
 						operations:              map[*invocation]*operation{},
 						actionDigest:            t.actionDigest,
-						desiredState:            t.desiredState,
 						targetID:                t.targetID,
 						expectedDuration:        backgroundExpectedDuration,
 						initialSizeClassLearner: backgroundInitialSizeClassLearner,
 						stageChangeWakeup:       make(chan struct{}),
 					}
-					backgroundTask.desiredState.Action = &backgroundAction
+
+					// Set do_not_cache to ensure that
+					// background learning doesn't
+					// overwrite the ActionResult obtained
+					// by the foreground task.
+					proto.Merge(&backgroundTask.desiredState, &t.desiredState)
+					backgroundAction := backgroundTask.desiredState.Action
+					backgroundAction.DoNotCache = true
+					backgroundAction.Timeout = durationpb.New(backgroundTimeout)
+
 					backgroundTask.newOperation(bq, pq.backgroundLearningOperationPriority, backgroundInvocation, true)
 					backgroundTask.schedule(bq)
 				}