Prefer digest over tag on pull

If a reference passed to the pull code contains both a tag and a digest,
currently the tag is used instead of the digest in the request to the
registry. This is the wrong behavior. Change it to favor the digest.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

distribution/pull_v2.go

@@ -334,18 +334,18 @@ func (p *v2Puller) pullV2Tag(ctx context.Context, ref reference.Named) (tagUpdat
 		manifest    distribution.Manifest
 		tagOrDigest string // Used for logging/progress only
 	)
-	if tagged, isTagged := ref.(reference.NamedTagged); isTagged {
-		manifest, err = manSvc.Get(ctx, "", distribution.WithTag(tagged.Tag()))
-		if err != nil {
-			return false, allowV1Fallback(err)
-		}
-		tagOrDigest = tagged.Tag()
-	} else if digested, isDigested := ref.(reference.Canonical); isDigested {
+	if digested, isDigested := ref.(reference.Canonical); isDigested {
 		manifest, err = manSvc.Get(ctx, digested.Digest())
 		if err != nil {
 			return false, err
 		}
 		tagOrDigest = digested.Digest().String()
+	} else if tagged, isTagged := ref.(reference.NamedTagged); isTagged {
+		manifest, err = manSvc.Get(ctx, "", distribution.WithTag(tagged.Tag()))
+		if err != nil {
+			return false, allowV1Fallback(err)
+		}
+		tagOrDigest = tagged.Tag()
 	} else {
 		return false, fmt.Errorf("internal error: reference has neither a tag nor a digest: %s", reference.FamiliarString(ref))
 	}