block: Fix blockdev-snapshot error handling

For blockdev-snapshot, external_snapshot_prepare() accepts an arbitrary
node reference at first and only checks later whether it already has a
backing file. Between those places, other errors can occur.

Therefore checking in external_snapshot_abort() whether state->new_bs
has a backing file is not sufficient to tell whether bdrv_append() was
already completed or not. Trying to undo the bdrv_append() when it
wasn't even executed is wrong.

Introduce a new boolean flag in the state to fix this.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

blockdev.c

@@ -1614,6 +1614,7 @@ typedef struct ExternalSnapshotState {
     BlockDriverState *old_bs;
     BlockDriverState *new_bs;
     AioContext *aio_context;
+    bool overlay_appended;
 } ExternalSnapshotState;
 
 static void external_snapshot_prepare(BlkActionState *common,
@@ -1780,6 +1781,7 @@ static void external_snapshot_prepare(BlkActionState *common,
         error_propagate(errp, local_err);
         return;
     }
+    state->overlay_appended = true;
 }
 
 static void external_snapshot_commit(BlkActionState *common)
@@ -1803,7 +1805,7 @@ static void external_snapshot_abort(BlkActionState *common)
     ExternalSnapshotState *state =
                              DO_UPCAST(ExternalSnapshotState, common, common);
     if (state->new_bs) {
-        if (state->new_bs->backing) {
+        if (state->overlay_appended) {
             bdrv_replace_in_backing_chain(state->new_bs, state->old_bs);
         }
     }