[PATCH v2 1/2] block/backup-top: fix failure path

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

We can't access top after call bdrv_backup_top_drop, as it is already
freed at this time.

Also, no needs to unref target child by hand, it will be unrefed on
bdrv_close() automatically.

So, just do bdrv_backup_top_drop if append succeed and one bdrv_unref
otherwise.

Note, that in !appended case bdrv_unref(top) moved into drained section
on source. It doesn't really matter, but just for code simplicity.

Fixes: 7df7868b96404
Cc: qemu-stable@nongnu.org # v4.2.0
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
---
 block/backup-top.c | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/block/backup-top.c b/block/backup-top.c
index b8d863ff08..cdfec782e2 100644
--- a/block/backup-top.c
+++ b/block/backup-top.c
@@ -190,6 +190,7 @@ BlockDriverState *bdrv_backup_top_append(BlockDriverState *source,
     BlockDriverState *top = bdrv_new_open_driver(&bdrv_backup_top_filter,
                                                  filter_node_name,
                                                  BDRV_O_RDWR, errp);
+    bool appended = false;
 
     if (!top) {
         return NULL;
@@ -212,8 +213,9 @@ BlockDriverState *bdrv_backup_top_append(BlockDriverState *source,
     bdrv_append(top, source, &local_err);
     if (local_err) {
         error_prepend(&local_err, "Cannot append backup-top filter: ");
-        goto append_failed;
+        goto fail;
     }
+    appended = true;
 
     /*
      * bdrv_append() finished successfully, now we can require permissions
@@ -224,14 +226,14 @@ BlockDriverState *bdrv_backup_top_append(BlockDriverState *source,
     if (local_err) {
         error_prepend(&local_err,
                       "Cannot set permissions for backup-top filter: ");
-        goto failed_after_append;
+        goto fail;
     }
 
     state->bcs = block_copy_state_new(top->backing, state->target,
                                       cluster_size, write_flags, &local_err);
     if (local_err) {
         error_prepend(&local_err, "Cannot create block-copy-state: ");
-        goto failed_after_append;
+        goto fail;
     }
     *bcs = state->bcs;
 
@@ -239,14 +241,15 @@ BlockDriverState *bdrv_backup_top_append(BlockDriverState *source,
 
     return top;
 
-failed_after_append:
-    state->active = false;
-    bdrv_backup_top_drop(top);
+fail:
+    if (appended) {
+        state->active = false;
+        bdrv_backup_top_drop(top);
+    } else {
+        bdrv_unref(top);
+    }
 
-append_failed:
     bdrv_drained_end(source);
-    bdrv_unref_child(top, state->target);
-    bdrv_unref(top);
     error_propagate(errp, local_err);
 
     return NULL;
-- 
2.21.0