[v4,08/45] block/snapshot: stress that we fallback to primary child

Commit Message

Vladimir Sementsov-Ogievskiy March 29, 2022, 8:40 p.m. UTC

Actually what we chose is a primary child. Let's stress it in the code.

We are going to drop indirect pointer logic here in future. Actually
this commit simplifies the future work: we drop use of indirection in
the assertion now.

Signed-off-by: Vladimir Sementsov-Ogievskiy <[e-mail]>
---
 block/snapshot.c | 30 ++++++++++--------------------
 1 file changed, 10 insertions(+), 20 deletions(-)

Patch

diff --git a/block/snapshot.c b/block/snapshot.c
index ccacda8bd5..12fa0e3904 100644
--- a/block/snapshot.c
+++ b/block/snapshot.c
@@ -158,21 +158,14 @@  bool bdrv_snapshot_find_by_id_and_name(BlockDriverState *bs,
 static BdrvChild **bdrv_snapshot_fallback_ptr(BlockDriverState *bs)
 {
     BdrvChild **fallback;
-    BdrvChild *child;
+    BdrvChild *child = bdrv_primary_child(bs);
 
-    /*
-     * The only BdrvChild pointers that are safe to modify (and which
-     * we can thus return a reference to) are bs->file and
-     * bs->backing.
-     */
-    fallback = &bs->file;
-    if (!*fallback && bs->drv && bs->drv->is_filter) {
-        fallback = &bs->backing;
-    }
-
-    if (!*fallback) {
+    /* We allow fallback only to primary child */
+    if (!child) {
         return NULL;
     }
+    fallback = (child == bs->file ? &bs->file : &bs->backing);
+    assert(*fallback == child);
 
     /*
      * Check that there are no other children that would need to be
@@ -300,15 +293,12 @@  int bdrv_snapshot_goto(BlockDriverState *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).
+         * fallback was a primary child. It 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 BDS as primary child.
          */
-        assert(*fallback_ptr && fallback_bs == (*fallback_ptr)->bs);
+        assert(bdrv_primary_bs(bs) == fallback_bs);
         bdrv_unref(fallback_bs);
         return ret;
     }