When a drained node changes its AioContext, we need to move its
aio_disable_external() to the new context, too.

Without this fix, drain_end will try to reenable the new context, which
has never been disabled, so an assertion failure is triggered.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
 block.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/block.c b/block.c
index 17bc1d3dca..aefb5701f5 100644
--- a/block.c
+++ b/block.c
@@ -5227,6 +5227,9 @@ void bdrv_detach_aio_context(BlockDriverState *bs)
         bdrv_detach_aio_context(child->bs);
     }
 
+    if (bs->quiesce_counter) {
+        aio_enable_external(bs->aio_context);
+    }
     bs->aio_context = NULL;
 }
 
@@ -5240,6 +5243,10 @@ void bdrv_attach_aio_context(BlockDriverState *bs,
         return;
     }
 
+    if (bs->quiesce_counter) {
+        aio_disable_external(new_context);
+    }
+
     bs->aio_context = new_context;
 
     QLIST_FOREACH(child, &bs->children, next) {
-- 
2.20.1