mount: fix racing conditions

prevent wrong reading when the SingleChunkCacher is started, but not finished yet

weed/filer/reader_cache.go

@@ -19,6 +19,7 @@ type ReaderCache struct {
 
 type SingleChunkCacher struct {
 	sync.RWMutex
+	cond          *sync.Cond
 	parent        *ReaderCache
 	chunkFileId   string
 	data          []byte
@@ -140,6 +141,7 @@ func newSingleChunkCacher(parent *ReaderCache, fileId string, cipherKey []byte,
 		chunkSize:   chunkSize,
 		shouldCache: shouldCache,
 	}
+	t.cond = sync.NewCond(t)
 	return t
 }
 
@@ -168,6 +170,7 @@ func (s *SingleChunkCacher) startCaching() {
 	if s.shouldCache {
 		s.parent.chunkCache.SetChunk(s.chunkFileId, s.data)
 	}
+	s.cond.Broadcast()
 
 	return
 }
@@ -183,6 +186,10 @@ func (s *SingleChunkCacher) readChunkAt(buf []byte, offset int64) (int, error) {
 	s.RLock()
 	defer s.RUnlock()
 
+	for s.completedTime.IsZero() {
+		s.cond.Wait()
+	}
+
 	if s.err != nil {
 		return 0, s.err
 	}