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-add-never-policy-to-bitmap-sync-mode.patch

@@ -0,0 +1,59 @@
+From 98ed0f915cf3335768ed84ee5dfa54f4e99aaf00 Mon Sep 17 00:00:00 2001
+From: John Snow <jsnow@redhat.com>
+Date: Mon, 29 Jul 2019 16:35:53 -0400
+Subject: [PATCH] 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       | 7 +++++--
+ qapi/block-core.json | 5 ++++-
+ 2 files changed, 9 insertions(+), 3 deletions(-)
+
+diff --git a/block/backup.c b/block/backup.c
+index e37eda80cd..84a56337ac 100644
+--- a/block/backup.c
++++ b/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 {
+diff --git a/qapi/block-core.json b/qapi/block-core.json
+index 97baff3a8c..48a0bfab63 100644
+--- a/qapi/block-core.json
++++ b/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:
+-- 
+2.27.0
+