block: bdrv_append(): don't consume reference

We have too much comments for this feature. It seems better just don't
do it. Most of real users (tests don't count) have to create additional
reference.

Drop also comment in external_snapshot_prepare:
 - bdrv_append doesn't "remove" old bs in common sense, it sounds
   strange
 - the fact that bdrv_append can fail is obvious from the context
 - the fact that we must rollback all changes in transaction abort is
   known (it's the direct role of abort)

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20210428151804.439460-5-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 3 insertions, 22 deletions

diff --git a/block.c b/block.c
index c5b887cec1..1e7e8907e4 100644
--- a/block.c
+++ b/block.c
@@ -3213,11 +3213,6 @@ static BlockDriverState *bdrv_append_temp_snapshot(BlockDriverState *bs,
         goto out;
     }
 
-    /* bdrv_append() consumes a strong reference to bs_snapshot
-     * (i.e. it will call bdrv_unref() on it) even on error, so in
-     * order to be able to return one, we have to increase
-     * bs_snapshot's refcount here */
-    bdrv_ref(bs_snapshot);
     ret = bdrv_append(bs_snapshot, bs, errp);
     if (ret < 0) {
         bs_snapshot = NULL;
@@ -4679,36 +4674,22 @@ int bdrv_replace_node(BlockDriverState *from, BlockDriverState *to,
  * bs_new must not be attached to a BlockBackend.
  *
  * This function does not create any image files.
- *
- * bdrv_append() takes ownership of a bs_new reference and unrefs it because
- * that's what the callers commonly need. bs_new will be referenced by the old
- * parents of bs_top after bdrv_append() returns. If the caller needs to keep a
- * reference of its own, it must call bdrv_ref().
  */
 int bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top,
                 Error **errp)
 {
     int ret = bdrv_set_backing_hd(bs_new, bs_top, errp);
     if (ret < 0) {
-        goto out;
+        return ret;
     }
 
     ret = bdrv_replace_node(bs_top, bs_new, errp);
     if (ret < 0) {
         bdrv_set_backing_hd(bs_new, NULL, &error_abort);
-        goto out;
+        return ret;
     }
 
-    ret = 0;
-
-out:
-    /*
-     * bs_new is now referenced by its new parents, we don't need the
-     * additional reference any more.
-     */
-    bdrv_unref(bs_new);
-
-    return ret;
+    return 0;
 }
 
 static void bdrv_delete(BlockDriverState *bs)