remotes/docker: Add MountedFrom and Exists push status

This makes it possible to check whether content didn't actually need to
be pushed to the remote registry and was cross-repo mounted or already
existed.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

remotes/docker/pusher.go

@@ -23,6 +23,7 @@ import (
 	"io"
 	"net/http"
 	"net/url"
+	"path"
 	"strings"
 	"sync"
 	"time"
@@ -137,6 +138,9 @@ func (p dockerPusher) push(ctx context.Context, desc ocispec.Descriptor, ref str
 			if exists {
 				p.tracker.SetStatus(ref, Status{
 					Committed: true,
+					PushStatus: PushStatus{
+						Exists: true,
+					},
 					Status: content.Status{
 						Ref:    ref,
 						Total:  desc.Size,
@@ -164,6 +168,7 @@ func (p dockerPusher) push(ctx context.Context, desc ocispec.Descriptor, ref str
 		// Start upload request
 		req = p.request(host, http.MethodPost, "blobs", "uploads/")
 
+		mountedFrom := ""
 		var resp *http.Response
 		if fromRepo := selectRepositoryMountCandidate(p.refspec, desc.Annotations); fromRepo != "" {
 			preq := requestWithMountFrom(req, desc.Digest.String(), fromRepo)
@@ -180,11 +185,14 @@ func (p dockerPusher) push(ctx context.Context, desc ocispec.Descriptor, ref str
 				return nil, err
 			}
 
-			if resp.StatusCode == http.StatusUnauthorized {
+			switch resp.StatusCode {
+			case http.StatusUnauthorized:
 				log.G(ctx).Debugf("failed to mount from repository %s", fromRepo)
 
 				resp.Body.Close()
 				resp = nil
+			case http.StatusCreated:
+				mountedFrom = path.Join(p.refspec.Locator, fromRepo)
 			}
 		}
 
@@ -204,6 +212,9 @@ func (p dockerPusher) push(ctx context.Context, desc ocispec.Descriptor, ref str
 		case http.StatusCreated:
 			p.tracker.SetStatus(ref, Status{
 				Committed: true,
+				PushStatus: PushStatus{
+					MountedFrom: mountedFrom,
+				},
 				Status: content.Status{
 					Ref:    ref,
 					Total:  desc.Size,