Improve host fallback behaviour in docker remote

This commit improves the fallback behaviour when resolving and
fetching images with multiple hosts. If an error is encountered
when resolving and fetching images, and more than one host is being
used, we will try the same operation on the next host. The error
from the first host is preserved so that if all hosts fail, we can
display the error from the first host.

fixes #3850

Signed-off-by: Alex Price <aprice@atlassian.com>

remotes/docker/fetcher.go

@@ -95,41 +95,49 @@ func (r dockerFetcher) Fetch(ctx context.Context, desc ocispec.Descriptor) (io.R
 			images.MediaTypeDockerSchema1Manifest,
 			ocispec.MediaTypeImageManifest, ocispec.MediaTypeImageIndex:
 
+			var firstErr error
 			for _, host := range r.hosts {
 				req := r.request(host, http.MethodGet, "manifests", desc.Digest.String())
 
 				rc, err := r.open(ctx, req, desc.MediaType, offset)
 				if err != nil {
-					if errdefs.IsNotFound(err) {
-						continue // try another host
+					// Store the error for referencing later
+					if firstErr == nil {
+						firstErr = err
 					}
-
-					return nil, err
+					continue // try another host
 				}
 
 				return rc, nil
 			}
+
+			return nil, firstErr
 		}
 
 		// Finally use blobs endpoints
+		var firstErr error
 		for _, host := range r.hosts {
 			req := r.request(host, http.MethodGet, "blobs", desc.Digest.String())
 
 			rc, err := r.open(ctx, req, desc.MediaType, offset)
 			if err != nil {
-				if errdefs.IsNotFound(err) {
-					continue // try another host
+				// Store the error for referencing later
+				if firstErr == nil {
+					firstErr = err
 				}
-
-				return nil, err
+				continue // try another host
 			}
 
 			return rc, nil
 		}
 
-		return nil, errors.Wrapf(errdefs.ErrNotFound,
-			"could not fetch content descriptor %v (%v) from remote",
-			desc.Digest, desc.MediaType)
+		if errdefs.IsNotFound(firstErr) {
+			firstErr = errors.Wrapf(errdefs.ErrNotFound,
+				"could not fetch content descriptor %v (%v) from remote",
+				desc.Digest, desc.MediaType)
+		}
+
+		return nil, firstErr
 
 	})
 }