bdrv_change_backing_file() can result in yields.  Therefore, @base may
no longer be the the backing_bs() of s->bottom afterwards.

Just swap the order of the two calls to fix this.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190703172813.6868-4-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block/stream.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/stream.c b/block/stream.c
index b27e61625d..6ac1e7bec4 100644
--- a/block/stream.c
+++ b/block/stream.c
@@ -78,8 +78,8 @@ static int stream_prepare(Job *job)
                 base_fmt = base->drv->format_name;
             }
         }
-        ret = bdrv_change_backing_file(bs, base_id, base_fmt);
         bdrv_set_backing_hd(bs, base, &local_err);
+        ret = bdrv_change_backing_file(bs, base_id, base_fmt);
         if (local_err) {
             error_report_err(local_err);
             return -EPERM;
-- 
2.21.0