Merge pull request moby#3066 from jedevc/fix-shared-op-sigsegv-followup

solver: correct cache nil return value in Exec

Author: tonistiigi

solver/jobs.go

@@ -619,8 +619,9 @@ type sharedOp struct {
 	subBuilder *subBuilder
 	err        error
 
-	execRes *execRes
-	execErr error
+	execRes  *execRes
+	execDone bool
+	execErr  error
 
 	cacheRes  []*CacheMap
 	cacheDone bool
@@ -817,10 +818,10 @@ func (s *sharedOp) Exec(ctx context.Context, inputs []Result) (outputs []Result,
 	}
 	flightControlKey := "exec"
 	res, err := s.g.Do(ctx, flightControlKey, func(ctx context.Context) (ret interface{}, retErr error) {
-		if s.execErr != nil {
-			return nil, s.execErr
-		}
-		if s.execRes != nil {
+		if s.execDone {
+			if s.execErr != nil {
+				return nil, s.execErr
+			}
 			return s.execRes, nil
 		}
 		release, err := op.Acquire(ctx)
@@ -857,6 +858,7 @@ func (s *sharedOp) Exec(ctx context.Context, inputs []Result) (outputs []Result,
 			}
 		}
 		if complete {
+			s.execDone = true
 			if res != nil {
 				var subExporters []ExportableCacheKey
 				s.subBuilder.mu.Lock()