[PATCH] block/snapshot: Clarify goto fallback behavior

[Max Reitz <mreitz@redhat.com>]

In the bdrv_snapshot_goto() fallback code, we work with a pointer to
either bs->file or bs->backing.  We close that child, close the node
(with .bdrv_close()), apply the snapshot on the child node, and then
re-open the node (with .bdrv_open()).

In order for .bdrv_open() to attach the same child node that we had
before, we pass "file={child-node}" or "backing={child-node}" to it.
Therefore, when .bdrv_open() has returned success, we can assume that
bs->file or bs->backing (respectively) points to our original child
again.  This is verified by an assertion.

All of this is not immediately clear from a quick glance at the code,
so add a comment to the assertion what it is for, and why it is valid.
It certainly confused Coverity.

Reported-by: Coverity (CID 1452774)
Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block/snapshot.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/block/snapshot.c b/block/snapshot.c
index e8ae9a28c1..cce5e35b35 100644
--- a/block/snapshot.c
+++ b/block/snapshot.c
@@ -275,13 +275,16 @@ int bdrv_snapshot_goto(BlockDriverState *bs,
         qobject_unref(file_options);
         g_free(subqdict_prefix);
 
+        /* Force .bdrv_open() below to re-attach fallback_bs on *fallback_ptr */
         qdict_put_str(options, (*fallback_ptr)->name,
                       bdrv_get_node_name(fallback_bs));
 
+        /* Now close bs, apply the snapshot on fallback_bs, and re-open bs */
         if (drv->bdrv_close) {
             drv->bdrv_close(bs);
         }
 
+        /* .bdrv_open() will re-attach it */
         bdrv_unref_child(bs, *fallback_ptr);
         *fallback_ptr = NULL;
 
@@ -296,7 +299,16 @@ int bdrv_snapshot_goto(BlockDriverState *bs,
             return ret < 0 ? ret : open_ret;
         }
 
-        assert(fallback_bs == (*fallback_ptr)->bs);
+        /*
+         * fallback_ptr is &bs->file or &bs->backing.  *fallback_ptr
+         * was closed above and set to NULL, but the .bdrv_open() call
+         * has opened it again, because we set the respective option
+         * (with the qdict_put_str() call above).
+         * Assert that .bdrv_open() has attached some child on
+         * *fallback_ptr, and that it has attached the one we wanted
+         * it to (i.e., fallback_bs).
+         */
+        assert(*fallback_ptr && fallback_bs == (*fallback_ptr)->bs);
         bdrv_unref(fallback_bs);
         return ret;
     }
-- 
2.31.1