[Qemu-block] [PATCH 1/2] block: change parent backing link when *tqe_prev == NULL

[Jeff Cody]

In change_parent_backing_link(), we only inserted the new
BlockDriverState entry into the device_list if the tqe_prev pointer was
NULL.   However, we must also allow insertion when the BDS pointed
to by the tqe_prev pointer is NULL as well.

This fixes a bug with external snapshots, and live active layer commits.

After a live snapshot occurs, the active layer and the base layer both
have a non-NULL tqe_prev field in the device_list, although the base
node's tqe_prev field points to a NULL entry.

Once the active commit is finished, bdrv_replace_in_backing_chain() is
called to set the base node as the new active node, and remove the
node that was the prior active layer from the device_list.

If we only check against the tqe_prev pointer field and not the entity
it is pointing to, then we fail to insert base image into the device
list.  The previous active layer is still removed from the device_list,
leaving an empty device_list queue.

With an empty device_list queue, odd behavior occurs - such as not
allowing any more live snapshots.

This commit fixes this issue, by checking for a NULL tqe_prev entity
in the devices_list.

Signed-off-by: Jeff Cody <address@hidden>
---
 block.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block.c b/block.c
index 5709d3d..0b8526b 100644
--- a/block.c
+++ b/block.c
@@ -2272,7 +2272,7 @@ static void change_parent_backing_link(BlockDriverState 
*from,
     }
     if (from->blk) {
         blk_set_bs(from->blk, to);
-        if (!to->device_list.tqe_prev) {
+        if (!to->device_list.tqe_prev || !*to->device_list.tqe_prev) {
             QTAILQ_INSERT_BEFORE(from, to, device_list);
         }
         QTAILQ_REMOVE(&bdrv_states, from, device_list);
-- 
1.9.3