Add gc policy plugin

Add garbage collection as a background process and policy
configuration for configuring when to run garbage collection.
By default garbage collection will run when deletion occurs
and no more than 20ms out of every second.

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

metadata/snapshot.go

@@ -604,13 +604,14 @@ func validateSnapshot(info *snapshot.Info) error {
 	return nil
 }
 
-func (s *snapshotter) garbageCollect(ctx context.Context) error {
-	logger := log.G(ctx).WithField("snapshotter", s.name)
-	lt1 := time.Now()
+func (s *snapshotter) garbageCollect(ctx context.Context) (d time.Duration, err error) {
 	s.l.Lock()
+	t1 := time.Now()
 	defer func() {
+		if err == nil {
+			d = time.Now().Sub(t1)
+		}
 		s.l.Unlock()
-		logger.WithField("t", time.Now().Sub(lt1)).Debugf("garbage collected")
 	}()
 
 	seen := map[string]struct{}{}
@@ -654,23 +655,26 @@ func (s *snapshotter) garbageCollect(ctx context.Context) error {
 
 		return nil
 	}); err != nil {
-		return err
+		return 0, err
 	}
 
 	roots, err := s.walkTree(ctx, seen)
 	if err != nil {
-		return err
+		return 0, err
 	}
 
-	// TODO: Unlock before prune (once nodes are fully unavailable)
+	// TODO: Unlock before removal (once nodes are fully unavailable).
+	// This could be achieved through doing prune inside the lock
+	// and having a cleanup method which actually performs the
+	// deletions on the snapshotters which support it.
 
 	for _, node := range roots {
 		if err := s.pruneBranch(ctx, node); err != nil {
-			return err
+			return 0, err
 		}
 	}
 
-	return nil
+	return
 }
 
 type treeNode struct {