block/backup: add 'never' policy to bitmap sync mode

This adds a "never" policy for bitmap synchronization. Regardless of if
the job succeeds or fails, we never update the bitmap. This can be used
to perform differential backups, or simply to avoid the job modifying a
bitmap.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190709232550.10724-7-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

block/backup.c

@@ -274,8 +274,11 @@ static void backup_cleanup_sync_bitmap(BackupBlockJob *job, int ret)
     BdrvDirtyBitmap *bm;
     BlockDriverState *bs = blk_bs(job->common.blk);
 
-    if (ret < 0) {
-        /* Merge the successor back into the parent, delete nothing. */
+    if (ret < 0 || job->bitmap_mode == BITMAP_SYNC_MODE_NEVER) {
+        /*
+         * Failure, or we don't want to synchronize the bitmap.
+         * Merge the successor back into the parent, delete nothing.
+         */
         bm = bdrv_reclaim_dirty_bitmap(bs, job->sync_bitmap, NULL);
         assert(bm);
     } else {

qapi/block-core.json

@@ -1146,10 +1146,13 @@
 # @on-success: The bitmap is only synced when the operation is successful.
 #              This is the behavior always used for 'INCREMENTAL' backups.
 #
+# @never: The bitmap is never synchronized with the operation, and is
+#         treated solely as a read-only manifest of blocks to copy.
+#
 # Since: 4.2
 ##
 { 'enum': 'BitmapSyncMode',
-  'data': ['on-success'] }
+  'data': ['on-success', 'never'] }
 
 ##
 # @MirrorCopyMode: