Add function to transactionally replace bs inside BdrvChild.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
---
 include/block/block.h |  2 ++
 block.c               | 31 +++++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/include/block/block.h b/include/block/block.h
index 3477290f9a..740038a892 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -361,6 +361,8 @@ int bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top,
                 Error **errp);
 int bdrv_replace_node(BlockDriverState *from, BlockDriverState *to,
                       Error **errp);
+int bdrv_replace_child_bs(BdrvChild *child, BlockDriverState *new_bs,
+                          Error **errp);
 BlockDriverState *bdrv_insert_node(BlockDriverState *bs, QDict *node_options,
                                    int flags, Error **errp);
 int bdrv_drop_filter(BlockDriverState *bs, Error **errp);
diff --git a/block.c b/block.c
index e97ce0b1c8..b2b66263f9 100644
--- a/block.c
+++ b/block.c
@@ -5048,6 +5048,37 @@ out:
     return ret;
 }
 
+/* Not for empty child */
+int bdrv_replace_child_bs(BdrvChild *child, BlockDriverState *new_bs,
+                          Error **errp)
+{
+    int ret;
+    Transaction *tran = tran_new();
+    g_autoptr(GHashTable) found = NULL;
+    g_autoptr(GSList) refresh_list = NULL;
+    BlockDriverState *old_bs = child->bs;
+
+    bdrv_ref(old_bs);
+    bdrv_drained_begin(old_bs);
+    bdrv_drained_begin(new_bs);
+
+    bdrv_replace_child_tran(child, new_bs, tran);
+
+    found = g_hash_table_new(NULL, NULL);
+    refresh_list = bdrv_topological_dfs(refresh_list, found, old_bs);
+    refresh_list = bdrv_topological_dfs(refresh_list, found, new_bs);
+
+    ret = bdrv_list_refresh_perms(refresh_list, NULL, tran, errp);
+
+    tran_finalize(tran, ret);
+
+    bdrv_drained_end(old_bs);
+    bdrv_drained_end(new_bs);
+    bdrv_unref(old_bs);
+
+    return ret;
+}
+
 static void bdrv_delete(BlockDriverState *bs)
 {
     assert(bdrv_op_blocker_is_empty(bs));
-- 
2.29.2