commit: Implement bdrv_commit_top.bdrv_co_get_block_status

In some cases, bdrv_co_get_block_status() is called recursively for the
whole backing chain. The automatically inserted bdrv_commit_top filter
driver must not stop the recursion, so implement a callback that simply
forwards the request to bs->backing.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

block/commit.c

@@ -232,6 +232,17 @@ static int coroutine_fn bdrv_commit_top_preadv(BlockDriverState *bs,
     return bdrv_co_preadv(bs->backing, offset, bytes, qiov, flags);
 }
 
+static int64_t coroutine_fn bdrv_commit_top_get_block_status(
+    BlockDriverState *bs, int64_t sector_num, int nb_sectors, int *pnum,
+    BlockDriverState **file)
+{
+    *pnum = nb_sectors;
+    *file = bs->backing->bs;
+    return BDRV_BLOCK_RAW | BDRV_BLOCK_OFFSET_VALID | BDRV_BLOCK_DATA |
+           (sector_num << BDRV_SECTOR_BITS);
+}
+
+
 static void bdrv_commit_top_close(BlockDriverState *bs)
 {
 }
@@ -248,10 +259,11 @@ static void bdrv_commit_top_child_perm(BlockDriverState *bs, BdrvChild *c,
 /* Dummy node that provides consistent read to its users without requiring it
  * from its backing file and that allows writes on the backing file chain. */
 static BlockDriver bdrv_commit_top = {
-    .format_name        = "commit_top",
-    .bdrv_co_preadv     = bdrv_commit_top_preadv,
-    .bdrv_close         = bdrv_commit_top_close,
-    .bdrv_child_perm    = bdrv_commit_top_child_perm,
+    .format_name                = "commit_top",
+    .bdrv_co_preadv             = bdrv_commit_top_preadv,
+    .bdrv_co_get_block_status   = bdrv_commit_top_get_block_status,
+    .bdrv_close                 = bdrv_commit_top_close,
+    .bdrv_child_perm            = bdrv_commit_top_child_perm,
 };
 
 void commit_start(const char *job_id, BlockDriverState *bs,