Doing the opposite can make adding the child node to a non-drained node,
as apply_subtree_drain is only done in ->attach() and thus make
assert_bdrv_graph_writable fail.

This can happen for example during a transaction rollback (test 245,
test_io_with_graph_changes):
1. a node is removed from the graph, thus it is undrained
2. then something happens, and we need to roll back the transactions
   through tran_abort()
3. at this point, the current code would first attach the undrained node
   to the graph via QLIST_INSERT_HEAD, and then call ->attach() that
   will take care of restoring the drain with apply_subtree_drain(),
   leaving the node undrained between the two operations.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 block.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/block.c b/block.c
index 29de2b62b5..fb5bc3077a 100644
--- a/block.c
+++ b/block.c
@@ -2879,8 +2879,6 @@ static void bdrv_replace_child_noperm(BdrvChild **childp,
     }
 
     if (new_bs) {
-        assert_bdrv_graph_writable(new_bs);
-        QLIST_INSERT_HEAD(&new_bs->parents, child, next_parent);
 
         /*
          * Detaching the old node may have led to the new node's
@@ -2897,6 +2895,10 @@ static void bdrv_replace_child_noperm(BdrvChild **childp,
         if (child->klass->attach) {
             child->klass->attach(child);
         }
+
+        assert_bdrv_graph_writable(new_bs);
+        QLIST_INSERT_HEAD(&new_bs->parents, child, next_parent);
+
     }
 
     /*
-- 
2.31.1