Merge pull request #339 from stevvooe/signature-fetch-prevent-false-sharing

registry/storage: prevent false sharing in signature fetch

registry/storage/signaturestore.go

@@ -36,8 +36,13 @@ func (s *signatureStore) Get(dgst digest.Digest) ([][]byte, error) {
 	}
 
 	var wg sync.WaitGroup
-	signatures := make([][]byte, len(signaturePaths)) // make space for everything
-	errCh := make(chan error, 1)                      // buffered chan so one proceeds
+	type result struct {
+		index     int
+		signature []byte
+		err       error
+	}
+	ch := make(chan result)
+
 	for i, sigPath := range signaturePaths {
 		// Append the link portion
 		sigPath = path.Join(sigPath, "link")
@@ -47,33 +52,42 @@ func (s *signatureStore) Get(dgst digest.Digest) ([][]byte, error) {
 			defer wg.Done()
 			context.GetLogger(s.ctx).
 				Debugf("fetching signature from %q", sigPath)
-			p, err := s.blobStore.linked(sigPath)
-			if err != nil {
+
+			r := result{index: idx}
+			if p, err := s.blobStore.linked(sigPath); err != nil {
 				context.GetLogger(s.ctx).
 					Errorf("error fetching signature from %q: %v", sigPath, err)
-
-				// try to send an error, if it hasn't already been sent.
-				select {
-				case errCh <- err:
-				default:
+				r.err = err
+			} else {
+				r.signature = p
 			}
 
-				return
-			}
-			signatures[idx] = p
+			ch <- r
 		}(i, sigPath)
 	}
+	done := make(chan struct{})
+	go func() {
 		wg.Wait()
+		close(done)
+	}()
 
+	// aggregrate the results
+	signatures := make([][]byte, len(signaturePaths))
+loop:
+	for {
 		select {
-	case err := <-errCh:
-		// just return the first error, similar to single threaded code.
-		return nil, err
-	default:
-		// pass
+		case result := <-ch:
+			signatures[result.index] = result.signature
+			if result.err != nil && err == nil {
+				// only set the first one.
+				err = result.err
+			}
+		case <-done:
+			break loop
+		}
 	}
 
-	return signatures, nil
+	return signatures, err
 }
 
 func (s *signatureStore) Put(dgst digest.Digest, signatures ...[]byte) error {