volume: add a note file to avoid incomplete volume files

fix https://github.com/chrislusf/seaweedfs/issues/1567

weed/server/volume_grpc_copy.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"math"
 	"os"
 	"time"
@@ -60,6 +61,8 @@ func (vs *VolumeServer) VolumeCopy(ctx context.Context, req *volume_server_pb.Vo
 
 		volumeFileName = storage.VolumeFileName(location.Directory, volFileInfoResp.Collection, int(req.VolumeId))
 
+		ioutil.WriteFile(volumeFileName+".note", []byte(fmt.Sprintf("copying from %s", req.SourceDataNode)), 0755)
+
 		// println("source:", volFileInfoResp.String())
 		if err := vs.doCopyFile(client, false, req.Collection, req.VolumeId, volFileInfoResp.CompactionRevision, volFileInfoResp.DatFileSize, volumeFileName, ".dat", false, true); err != nil {
 			return err
@@ -73,6 +76,8 @@ func (vs *VolumeServer) VolumeCopy(ctx context.Context, req *volume_server_pb.Vo
 			return err
 		}
 
+		os.Remove(volumeFileName+".note")
+
 		return nil
 	})