Make bdrv_child_cb_attach() call bdrv_backing_attach() for children with
a COW role (and drop the reverse call from bdrv_backing_attach()), so it
can be used for any child (with a proper role set).

Because so far no child has a proper role set, we need a temporary new
callback for child_backing.attach that ensures bdrv_backing_attach() is
called for all COW children that do not have their role set yet.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
 block.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/block.c b/block.c
index aa54d40e4f..3cf1293a7b 100644
--- a/block.c
+++ b/block.c
@@ -942,9 +942,16 @@ static void bdrv_child_cb_drained_end(BdrvChild *child,
     bdrv_drained_end_no_poll(bs, drained_end_counter);
 }
 
+static void bdrv_backing_attach(BdrvChild *c);
+
 static void bdrv_child_cb_attach(BdrvChild *child)
 {
     BlockDriverState *bs = child->opaque;
+
+    if (child->role & BDRV_CHILD_COW) {
+        bdrv_backing_attach(child);
+    }
+
     bdrv_apply_subtree_drain(child, bs);
 }
 
@@ -1178,7 +1185,14 @@ static void bdrv_backing_attach(BdrvChild *c)
                     parent->backing_blocker);
     bdrv_op_unblock(backing_hd, BLOCK_OP_TYPE_BACKUP_TARGET,
                     parent->backing_blocker);
+}
 
+/* XXX: Will be removed along with child_backing */
+static void bdrv_child_cb_attach_backing(BdrvChild *c)
+{
+    if (!(c->role & BDRV_CHILD_COW)) {
+        bdrv_backing_attach(c);
+    }
     bdrv_child_cb_attach(c);
 }
 
@@ -1237,7 +1251,7 @@ static int bdrv_backing_update_filename(BdrvChild *c, BlockDriverState *base,
 const BdrvChildClass child_backing = {
     .parent_is_bds   = true,
     .get_parent_desc = bdrv_child_get_parent_desc,
-    .attach          = bdrv_backing_attach,
+    .attach          = bdrv_child_cb_attach_backing,
     .detach          = bdrv_backing_detach,
     .inherit_options = bdrv_backing_options,
     .drained_begin   = bdrv_child_cb_drained_begin,
-- 
2.24.1