Prevent false sharing in signature fetch

The original implementation wrote to different locations in a shared slice.
While this is theoretically okay, we end up thrashing the cpu cache since
multiple slice members may be on the same cache line. So, even though each
thread has its own memory location, there may be contention over the cache
line. This changes the code to aggregate to a slice in a single goroutine.

In reality, this change likely won't have any performance impact. The theory
proposed above hasn't really even been tested. Either way, we can consider it
and possibly go forward.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

docs/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:
-				}
-
-				return
+				r.err = err
+			} else {
+				r.signature = p
 			}
-			signatures[idx] = p
+
+			ch <- r
 		}(i, sigPath)
 	}
-	wg.Wait()
+	done := make(chan struct{})
+	go func() {
+		wg.Wait()
+		close(done)
+	}()
 
-	select {
-	case err := <-errCh:
-		// just return the first error, similar to single threaded code.
-		return nil, err
-	default:
-		// pass
+	// aggregrate the results
+	signatures := make([][]byte, len(signaturePaths))
+loop:
+	for {
+		select {
+		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 {