Try next mirror in case of non-404 errors, too

Signed-off-by: Sebastian Hasler <sebastian.hasler@gmx.net>
(cherry picked from commit c0e2f4b)
Signed-off-by: Wei Fu <fuweid89@gmail.com>

remotes/docker/resolver.go

@@ -229,10 +229,10 @@ func (r *dockerResolver) Resolve(ctx context.Context, ref string) (string, ocisp
 	}
 
 	var (
-		lastErr error
-		paths   [][]string
-		dgst    = refspec.Digest()
-		caps    = HostCapabilityPull
+		firstErr error
+		paths    [][]string
+		dgst     = refspec.Digest()
+		caps     = HostCapabilityPull
 	)
 
 	if dgst != "" {
@@ -283,8 +283,8 @@ func (r *dockerResolver) Resolve(ctx context.Context, ref string) (string, ocisp
 					err = errors.Wrapf(err, "pull access denied, repository does not exist or may require authorization")
 				}
 				// Store the error for referencing later
-				if lastErr == nil {
-					lastErr = err
+				if firstErr == nil {
+					firstErr = err
 				}
 				log.G(ctx).WithError(err).Info("trying next host")
 				continue // try another host
@@ -296,7 +296,14 @@ func (r *dockerResolver) Resolve(ctx context.Context, ref string) (string, ocisp
 					log.G(ctx).Info("trying next host - response was http.StatusNotFound")
 					continue
 				}
-				return "", ocispec.Descriptor{}, errors.Errorf("unexpected status code %v: %v", u, resp.Status)
+				if resp.StatusCode > 399 {
+					// Set firstErr when encountering the first non-404 status code.
+					if firstErr == nil {
+						firstErr = errors.Errorf("pulling from host %s failed with status code %v: %v", host.Host, u, resp.Status)
+					}
+					continue // try another host
+				}
+				return "", ocispec.Descriptor{}, errors.Errorf("pulling from host %s failed with unexpected status code %v: %v", host.Host, u, resp.Status)
 			}
 			size := resp.ContentLength
 			contentType := getManifestMediaType(resp)
@@ -359,8 +366,8 @@ func (r *dockerResolver) Resolve(ctx context.Context, ref string) (string, ocisp
 			}
 			// Prevent resolving to excessively large manifests
 			if size > MaxManifestSize {
-				if lastErr == nil {
-					lastErr = errors.Wrapf(errdefs.ErrNotFound, "rejecting %d byte manifest for %s", size, ref)
+				if firstErr == nil {
+					firstErr = errors.Wrapf(errdefs.ErrNotFound, "rejecting %d byte manifest for %s", size, ref)
 				}
 				continue
 			}
@@ -376,11 +383,15 @@ func (r *dockerResolver) Resolve(ctx context.Context, ref string) (string, ocisp
 		}
 	}
 
-	if lastErr == nil {
-		lastErr = errors.Wrap(errdefs.ErrNotFound, ref)
+	// If above loop terminates without return, then there was an error.
+	// "firstErr" contains the first non-404 error. That is, "firstErr == nil"
+	// means that either no registries were given or each registry returned 404.
+
+	if firstErr == nil {
+		firstErr = errors.Wrap(errdefs.ErrNotFound, ref)
 	}
 
-	return "", ocispec.Descriptor{}, lastErr
+	return "", ocispec.Descriptor{}, firstErr
 }
 
 func (r *dockerResolver) Fetcher(ctx context.Context, ref string) (remotes.Fetcher, error) {