docker rmi -f works with stopped containers + revamped error messages

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)

server/server.go

@@ -1952,6 +1952,7 @@ func (srv *Server) DeleteImage(name string, imgs *engine.Table, first, force, no
 	var (
 		repoName, tag string
 		tags          = []string{}
+		tagDeleted    bool
 	)
 
 	repoName, tag = utils.ParseRepositoryTag(name)
@@ -2002,7 +2003,7 @@ func (srv *Server) DeleteImage(name string, imgs *engine.Table, first, force, no
 
 	//Untag the current image
 	for _, tag := range tags {
-		tagDeleted, err := srv.daemon.Repositories().Delete(repoName, tag)
+		tagDeleted, err = srv.daemon.Repositories().Delete(repoName, tag)
 		if err != nil {
 			return err
 		}
@@ -2016,7 +2017,7 @@ func (srv *Server) DeleteImage(name string, imgs *engine.Table, first, force, no
 	tags = srv.daemon.Repositories().ByID()[img.ID]
 	if (len(tags) <= 1 && repoName == "") || len(tags) == 0 {
 		if len(byParents[img.ID]) == 0 {
-			if err := srv.canDeleteImage(img.ID); err != nil {
+			if err := srv.canDeleteImage(img.ID, force, tagDeleted); err != nil {
 				return err
 			}
 			if err := srv.daemon.Repositories().DeleteAll(img.ID); err != nil {
@@ -2059,7 +2060,11 @@ func (srv *Server) ImageDelete(job *engine.Job) engine.Status {
 	return engine.StatusOK
 }
 
-func (srv *Server) canDeleteImage(imgID string) error {
+func (srv *Server) canDeleteImage(imgID string, force, untagged bool) error {
+	var message string
+	if untagged {
+		message = " (but image was untagged)"
+	}
 	for _, container := range srv.daemon.List() {
 		parent, err := srv.daemon.Repositories().LookupImage(container.Image)
 		if err != nil {
@@ -2068,7 +2073,14 @@ func (srv *Server) canDeleteImage(imgID string) error {
 
 		if err := parent.WalkHistory(func(p *image.Image) error {
 			if imgID == p.ID {
-				return fmt.Errorf("Conflict, cannot delete %s because the container %s is using it", utils.TruncateID(imgID), utils.TruncateID(container.ID))
+				if container.State.IsRunning() {
+					if force {
+						return fmt.Errorf("Conflict, cannot force delete %s because the running container %s is using it%s, stop it and retry", utils.TruncateID(imgID), utils.TruncateID(container.ID), message)
+					}
+					return fmt.Errorf("Conflict, cannot delete %s because the running container %s is using it%s, stop it and use -f to force", utils.TruncateID(imgID), utils.TruncateID(container.ID), message)
+				} else if !force {
+					return fmt.Errorf("Conflict, cannot delete %s because the container %s is using it%s, use -f to force", utils.TruncateID(imgID), utils.TruncateID(container.ID), message)
+				}
 			}
 			return nil
 		}); err != nil {