Add synchronous image delete

Synchronous image delete provides an option image delete to wait
until the next garbage collection deletes after an image is removed
before returning success to the caller.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

services/images/service.go

@@ -1,6 +1,8 @@
 package images
 
 import (
+	gocontext "context"
+
 	"github.com/boltdb/bolt"
 	eventstypes "github.com/containerd/containerd/api/events"
 	imagesapi "github.com/containerd/containerd/api/services/images/v1"
@@ -22,26 +24,38 @@ func init() {
 		ID:   "images",
 		Requires: []plugin.Type{
 			plugin.MetadataPlugin,
+			plugin.GCPlugin,
 		},
 		InitFn: func(ic *plugin.InitContext) (interface{}, error) {
 			m, err := ic.Get(plugin.MetadataPlugin)
 			if err != nil {
 				return nil, err
 			}
-			return NewService(m.(*metadata.DB), ic.Events), nil
+			g, err := ic.Get(plugin.GCPlugin)
+			if err != nil {
+				return nil, err
+			}
+
+			return NewService(m.(*metadata.DB), g.(gcScheduler), ic.Events), nil
 		},
 	})
 }
 
+type gcScheduler interface {
+	ScheduleAndWait(gocontext.Context) (metadata.GCStats, error)
+}
+
 type service struct {
 	db        *metadata.DB
+	gc        gcScheduler
 	publisher events.Publisher
 }
 
 // NewService returns the GRPC image server
-func NewService(db *metadata.DB, publisher events.Publisher) imagesapi.ImagesServer {
+func NewService(db *metadata.DB, gc gcScheduler, publisher events.Publisher) imagesapi.ImagesServer {
 	return &service{
 		db:        db,
+		gc:        gc,
 		publisher: publisher,
 	}
 }
@@ -162,6 +176,12 @@ func (s *service) Delete(ctx context.Context, req *imagesapi.DeleteImageRequest)
 		return nil, err
 	}
 
+	if req.Sync {
+		if _, err := s.gc.ScheduleAndWait(ctx); err != nil {
+			return nil, err
+		}
+	}
+
 	return &ptypes.Empty{}, nil
 }