remotes: support cross-repo-push

With distribution source label in content store, select the longest
common prefix components as condidate mount blob source and try to push
with mount blob.

Fix #2964

Signed-off-by: Wei Fu <fuweid89@gmail.com>

remotes/docker/handler.go

@@ -110,3 +110,45 @@ func appendDistributionSourceLabel(originLabel, repo string) string {
 func distributionSourceLabelKey(source string) string {
 	return fmt.Sprintf("%s.%s", labelDistributionSource, source)
 }
+
+// selectRepositoryMountCandidate will select the repo which has longest
+// common prefix components as the candidate.
+func selectRepositoryMountCandidate(refspec reference.Spec, sources map[string]string) string {
+	u, err := url.Parse("dummy://" + refspec.Locator)
+	if err != nil {
+		// NOTE: basically, it won't be error here
+		return ""
+	}
+
+	source, target := u.Hostname(), strings.TrimPrefix(u.Path, "/")
+	repoLabel, ok := sources[distributionSourceLabelKey(source)]
+	if !ok || repoLabel == "" {
+		return ""
+	}
+
+	n, match := 0, ""
+	components := strings.Split(target, "/")
+	for _, repo := range strings.Split(repoLabel, ",") {
+		// the target repo is not a candidate
+		if repo == target {
+			continue
+		}
+
+		if l := commonPrefixComponents(components, repo); l >= n {
+			n, match = l, repo
+		}
+	}
+	return match
+}
+
+func commonPrefixComponents(components []string, target string) int {
+	targetComponents := strings.Split(target, "/")
+
+	i := 0
+	for ; i < len(components) && i < len(targetComponents); i++ {
+		if components[i] != targetComponents[i] {
+			break
+		}
+	}
+	return i
+}