block: Introduce BdrvChildRole.update_filename

There is no good reason for bdrv_drop_intermediate() to know the active
layer above the subchain it is operating on - even more so, because
the assumption that there is a single active layer above it is not
generally true.

In order to prepare removal of the active parameter, use a BdrvChildRole
callback to update the backing file string in the overlay image instead
of directly calling bdrv_change_backing_file().

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/include/block/block_int.h b/include/block/block_int.h
index 79366b94b5..7e8a206239 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -544,6 +544,12 @@ struct BdrvChildRole {
 
     void (*attach)(BdrvChild *child);
     void (*detach)(BdrvChild *child);
+
+    /* Notifies the parent that the filename of its child has changed (e.g.
+     * because the direct child was removed from the backing chain), so that it
+     * can update its reference. */
+    int (*update_filename)(BdrvChild *child, BlockDriverState *new_base,
+                           const char *filename, Error **errp);
 };
 
 extern const BdrvChildRole child_file;