Adding function to resolve tag to digest in daemon

Signed-off-by: Nishant Totla <nishanttotla@gmail.com>

daemon/cluster/executor/backend.go

@@ -10,6 +10,7 @@ import (
 	"github.com/docker/docker/api/types/filters"
 	"github.com/docker/docker/api/types/network"
 	clustertypes "github.com/docker/docker/daemon/cluster/provider"
+	"github.com/docker/docker/reference"
 	"github.com/docker/libnetwork"
 	"github.com/docker/libnetwork/cluster"
 	networktypes "github.com/docker/libnetwork/types"
@@ -44,4 +45,5 @@ type Backend interface {
 	UnsubscribeFromEvents(listener chan interface{})
 	UpdateAttachment(string, string, string, *network.NetworkingConfig) error
 	WaitForDetachment(context.Context, string, string, string, string) error
+	ResolveTagToDigest(context.Context, reference.NamedTagged, *types.AuthConfig) (string, error)
 }

daemon/image_pull.go

@@ -104,3 +104,41 @@ func (daemon *Daemon) pullImageWithReference(ctx context.Context, ref reference.
 	<-writesDone
 	return err
 }
+
+func (daemon *Daemon) ResolveTagToDigest(ctx context.Context, ref reference.NamedTagged, authConfig *types.AuthConfig) (string, error) {
+	// get repository info
+	repoInfo, err := daemon.RegistryService.ResolveRepository(ref)
+	if err != nil {
+		return "", err
+	}
+	// makes sure name is not empty or `scratch`
+	if err := distribution.ValidateRepoName(repoInfo.Name()); err != nil {
+		return "", err
+	}
+
+	// get endpoints
+	endpoints, err := daemon.RegistryService.LookupPullEndpoints(repoInfo.Hostname())
+	if err != nil {
+		return "", err
+	}
+
+	// retrieve repository
+	// TODO(nishanttotla): More sophisticated selection of endpoint
+	repo, confirmedV2, err := distribution.NewV2Repository(ctx, repoInfo, endpoints[0], nil, authConfig, "pull")
+
+	if err != nil {
+		return "", err
+	}
+	digest := ""
+
+	// only retrieve digest if the repo is v2
+	if confirmedV2 {
+		dscrptr, err := repo.Tags(ctx).Get(ctx, ref.Tag())
+		if err != nil {
+			return "", err
+		} else {
+			digest = dscrptr.Digest.String()
+		}
+	}
+	return digest, nil
+}