Fix a race we hit during conformance tests

The imagebuilder.Executor.imageMap map is written in one goroutine which
waits on StageExecutors, and is read from the goroutines that run the
StageExecutors, so we need to synchronize access to the map.

Signed-off-by: Nalin Dahyabhai <[email protected]>

imagebuildah/executor.go

@@ -578,7 +578,9 @@ func (b *Executor) Build(ctx context.Context, stages imagebuilder.Stages) (image
 		// If this is an intermediate stage, make a note of the ID, so
 		// that we can look it up later.
 		if r.Index < len(stages)-1 && r.ImageID != "" {
+			b.stagesLock.Lock()
 			b.imageMap[stage.Name] = r.ImageID
+			b.stagesLock.Unlock()
 			// We're not populating the cache with intermediate
 			// images, so add this one to the list of images that
 			// we'll remove later.

imagebuildah/stage_executor.go

@@ -645,9 +645,11 @@ func (s *StageExecutor) prepare(ctx context.Context, from string, initializeIBCo
 
 	// Check and see if the image is a pseudonym for the end result of a
 	// previous stage, named by an AS clause in the Dockerfile.
+	s.executor.stagesLock.Lock()
 	if asImageFound, ok := s.executor.imageMap[from]; ok {
 		builderOptions.FromImage = asImageFound
 	}
+	s.executor.stagesLock.Unlock()
 	builder, err = buildah.NewBuilder(ctx, s.executor.store, builderOptions)
 	if err != nil {
 		return nil, errors.Wrapf(err, "error creating build container")
@@ -773,9 +775,11 @@ func (s *StageExecutor) Execute(ctx context.Context, base string) (imgID string,
 	if isStage, err := s.executor.waitForStage(ctx, base, s.stages[:s.index]); isStage && err != nil {
 		return "", nil, err
 	}
+	s.executor.stagesLock.Lock()
 	if stageImage, isPreviousStage := s.executor.imageMap[base]; isPreviousStage {
 		base = stageImage
 	}
+	s.executor.stagesLock.Unlock()
 
 	// Create the (first) working container for this stage.  Reinitializing
 	// the imagebuilder configuration may alter the list of steps we have,