filer: prioritize done channel over context cancellation

If data is already available (done channel closed), return it even if
the reader's context is also cancelled. This avoids unnecessary errors
when the download has already completed.

weed/filer/reader_cache.go

@@ -240,13 +240,21 @@ func (s *SingleChunkCacher) readChunkAt(ctx context.Context, buf []byte, offset
 	s.wg.Add(1)
 	defer s.wg.Done()
 
-	// Wait for download to complete, but allow reader cancellation
+	// Wait for download to complete, but allow reader cancellation.
+	// Prioritize checking done first - if data is already available,
+	// return it even if context is also cancelled.
 	select {
 	case <-s.done:
-		// Download completed
-	case <-ctx.Done():
-		// Reader cancelled - download continues for other readers
-		return 0, ctx.Err()
+		// Download already completed, proceed immediately
+	default:
+		// Download not complete, wait for it or context cancellation
+		select {
+		case <-s.done:
+			// Download completed
+		case <-ctx.Done():
+			// Reader cancelled while waiting - download continues for other readers
+			return 0, ctx.Err()
+		}
 	}
 
 	s.Lock()