[Qemu-block] [PATCH] block: fix external snapshot abort permission error

[Jeff Cody]

In external_snapshot_abort(), we try to undo what was done in
external_snapshot_prepare() calling bdrv_replace_node() to swap the
nodes back.  However, we receive a permissions error as writers are
blocked on the old node, which is now the new node backing file.

An easy fix (initially suggested by Kevin Wolf) is to call
bdrv_set_backing_hd() on the new node, to set the backing node to NULL.

Signed-off-by: Jeff Cody <address@hidden>
---
 blockdev.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/blockdev.c b/blockdev.c
index 892d768..6472548 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -1803,7 +1803,11 @@ static void external_snapshot_abort(BlkActionState 
*common)
                              DO_UPCAST(ExternalSnapshotState, common, common);
     if (state->new_bs) {
         if (state->overlay_appended) {
+            bdrv_ref(state->old_bs);   /* we can't let bdrv_set_backind_hd()
+                                          close state->old_bs; we need it */
+            bdrv_set_backing_hd(state->new_bs, NULL, &error_abort);
             bdrv_replace_node(state->new_bs, state->old_bs, &error_abort);
+            bdrv_unref(state->old_bs); /* bdrv_replace_node() ref'ed old_bs */
         }
     }
 }
-- 
2.9.3