fs: synchronized meta file writing

fix https://github.com/chrislusf/seaweedfs/issues/1175

weed/shell/command_fs_meta_save.go

@@ -74,6 +74,19 @@ func (c *commandFsMetaSave) Do(args []string, commandEnv *CommandEnv, writer io.
 	}
 	defer dst.Close()
 
+	var wg sync.WaitGroup
+	wg.Add(1)
+	outputChan := make(chan []byte, 1024)
+	go func() {
+		sizeBuf := make([]byte, 4)
+		for b := range outputChan {
+			util.Uint32toBytes(sizeBuf, uint32(len(b)))
+			dst.Write(sizeBuf)
+			dst.Write(b)
+		}
+		wg.Done()
+	}()
+
 	var dirCount, fileCount uint64
 
 	err = doTraverseBFS(ctx, writer, commandEnv.getFilerClient(filerServer, filerPort), filer2.FullPath(path), func(parentPath filer2.FullPath, entry *filer_pb.Entry) {
@@ -89,11 +102,7 @@ func (c *commandFsMetaSave) Do(args []string, commandEnv *CommandEnv, writer io.
 			return
 		}
 
-		sizeBuf := make([]byte, 4)
-		util.Uint32toBytes(sizeBuf, uint32(len(bytes)))
-
-		dst.Write(sizeBuf)
-		dst.Write(bytes)
+		outputChan <- bytes
 
 		if entry.IsDirectory {
 			atomic.AddUint64(&dirCount, 1)
@@ -107,9 +116,13 @@ func (c *commandFsMetaSave) Do(args []string, commandEnv *CommandEnv, writer io.
 
 	})
 
+	close(outputChan)
+
+	wg.Wait()
+
 	if err == nil {
-		fmt.Fprintf(writer, "\ntotal %d directories, %d files", dirCount, fileCount)
-		fmt.Fprintf(writer, "\nmeta data for http://%s:%d%s is saved to %s\n", filerServer, filerPort, path, fileName)
+		fmt.Fprintf(writer, "total %d directories, %d files\n", dirCount, fileCount)
+		fmt.Fprintf(writer, "meta data for http://%s:%d%s is saved to %s\n", filerServer, filerPort, path, fileName)
 	}
 
 	return err