FS: sync abortMultipart cleanup and bg append (#3388)

backgroundAppend type's abort method should wait for appendParts to finish
writing ongoing appending of parts in the background before cleaning up
the part files.

cmd/fs-v1-background-append.go

@@ -115,13 +115,14 @@ func (b *backgroundAppend) complete(disk StorageAPI, bucket, object, uploadID st
 // Called after complete-multipart-upload or abort-multipart-upload so that the appendParts go-routine is not left dangling.
 func (b *backgroundAppend) abort(uploadID string) {
 	b.Lock()
-	defer b.Unlock()
 	info, ok := b.infoMap[uploadID]
 	if !ok {
+		b.Unlock()
 		return
 	}
 	delete(b.infoMap, uploadID)
-	close(info.abortCh)
+	b.Unlock()
+	info.abortCh <- struct{}{}
 }
 
 // This is run as a go-routine that appends the parts in the background.

cmd/fs-v1-multipart.go

@@ -637,11 +637,12 @@ func (fs fsObjects) CompleteMultipartUpload(bucket string, object string, upload
 // the directory at '.minio.sys/multipart/bucket/object/uploadID' holding
 // all the upload parts.
 func (fs fsObjects) abortMultipartUpload(bucket, object, uploadID string) error {
+	// Signal appendParts routine to stop waiting for new parts to arrive.
+	fs.bgAppend.abort(uploadID)
 	// Cleanup all uploaded parts.
 	if err := cleanupUploadedParts(bucket, object, uploadID, fs.storage); err != nil {
 		return err
 	}
-	fs.bgAppend.abort(uploadID)
 	// remove entry from uploads.json with quorum
 	if err := fs.removeUploadID(bucket, object, uploadID); err != nil {
 		return toObjectErr(err, bucket, object)