fix 8926: rmi dangling is unsafe when pulling

Signed-off-by: Ma Shimiao <mashimiao.fnst@cn.fujitsu.com>
Signed-off-by: Tibor Vass <tibor@docker.com>

docs/session.go

@@ -23,6 +23,7 @@ import (
 	"github.com/docker/docker/cliconfig"
 	"github.com/docker/docker/pkg/httputils"
 	"github.com/docker/docker/pkg/ioutils"
+	"github.com/docker/docker/pkg/stringid"
 	"github.com/docker/docker/pkg/tarsum"
 )
 
@@ -35,6 +36,7 @@ type Session struct {
 	client        *http.Client
 	// TODO(tiborvass): remove authConfig
 	authConfig *cliconfig.AuthConfig
+	id         string
 }
 
 type authTransport struct {
@@ -158,6 +160,7 @@ func NewSession(client *http.Client, authConfig *cliconfig.AuthConfig, endpoint
 		authConfig:    authConfig,
 		client:        client,
 		indexEndpoint: endpoint,
+		id:            stringid.GenerateRandomID(),
 	}
 
 	var alwaysSetBasicAuth bool
@@ -188,6 +191,11 @@ func NewSession(client *http.Client, authConfig *cliconfig.AuthConfig, endpoint
 	return r, nil
 }
 
+// ID returns this registry session's ID.
+func (r *Session) ID() string {
+	return r.id
+}
+
 // Retrieve the history of a given image from the Registry.
 // Return a list of the parent's json (requested image included)
 func (r *Session) GetRemoteHistory(imgID, registry string) ([]string, error) {