[#1284] writecache: Allow to seal writecache async

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

pkg/local_object_storage/shard/control.go

@@ -178,6 +178,7 @@ func (s *Shard) Init(ctx context.Context) error {
 	if !m.NoMetabase() {
 		s.rb.Start(ctx, s.blobStor, s.metaBase, s.log)
 	}
+	s.writecacheSealCancel.Store(dummyCancel)
 	return nil
 }
 
@@ -350,6 +351,8 @@ func (s *Shard) Close() error {
 	}
 
 	if s.hasWriteCache() {
+		prev := s.writecacheSealCancel.Swap(notInitializedCancel)
+		prev.cancel() // no need to wait: writecache.Seal and writecache.Close lock the same mutex
 		components = append(components, s.writeCache)
 	}
 
@@ -428,6 +431,9 @@ func (s *Shard) lockExclusive() func() {
 		cancelGC := val.(context.CancelFunc)
 		cancelGC()
 	}
+	if c := s.writecacheSealCancel.Load(); c != nil {
+		c.cancel()
+	}
 	s.m.Lock()
 	s.setModeRequested.Store(false)
 	return s.m.Unlock