volume: sync volume file right before compaction

fix https://github.com/chrislusf/seaweedfs/issues/1237

weed/storage/backend/backend.go

@@ -19,6 +19,7 @@ type BackendStorageFile interface {
 	io.Closer
 	GetStat() (datSize int64, modTime time.Time, err error)
 	Name() string
+	Sync() error
 }
 
 type BackendStorage interface {

weed/storage/backend/disk_file.go

@@ -48,3 +48,7 @@ func (df *DiskFile) GetStat() (datSize int64, modTime time.Time, err error) {
 func (df *DiskFile) Name() string {
 	return df.fullFilePath
 }
+
+func (df *DiskFile) Sync() error {
+	return df.File.Sync()
+}

weed/storage/backend/memory_map/memory_map_backend.go

@@ -58,3 +58,7 @@ func (mmf *MemoryMappedFile) GetStat() (datSize int64, modTime time.Time, err er
 func (mmf *MemoryMappedFile) Name() string {
 	return mmf.mm.File.Name()
 }
+
+func (mm *MemoryMappedFile) Sync() error {
+	return nil
+}

weed/storage/backend/s3_backend/s3_backend.go

@@ -179,3 +179,7 @@ func (s3backendStorageFile S3BackendStorageFile) GetStat() (datSize int64, modTi
 func (s3backendStorageFile S3BackendStorageFile) Name() string {
 	return s3backendStorageFile.key
 }
+
+func (s3backendStorageFile S3BackendStorageFile) Sync() error {
+	return nil
+}