[PATCH 3/4] nbd: make client_close synchronous

[Vladimir Sementsov-Ogievskiy]

client_close doesn't guarantee that client is closed: nbd_trip() keeps
reference to it. Let's wait for nbd_trip to finish.

Without this fix, the following crash is possible:

- export bitmap through unternal Qemu NBD server
- connect a client
- shutdown Qemu

On shutdown nbd_export_close_all is called, but it actually don't wait
for nbd_trip() to finish and to release its references. So, export is
not release, and exported bitmap remains busy, and on try to remove the
bitmap (which is part of bdrv_close()) the assertion fairs:

bdrv_release_dirty_bitmap_locked: Assertion `!bdrv_dirty_bitmap_busy(bitmap)' 
failed

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 nbd/server.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/nbd/server.c b/nbd/server.c
index 20754e9ebc..5e27a8d31a 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -1419,6 +1419,8 @@ static void client_close(NBDClient *client, bool 
negotiated)
     qio_channel_shutdown(client->ioc, QIO_CHANNEL_SHUTDOWN_BOTH,
                          NULL);
 
+    AIO_WAIT_WHILE(client->exp->ctx, client->recv_coroutine);
+
     /* Also tell the client, so that they release their reference.  */
     if (client->close_fn) {
         client->close_fn(client, negotiated);
@@ -2450,6 +2452,7 @@ static coroutine_fn void nbd_trip(void *opaque)
 
     trace_nbd_trip();
     if (client->closing) {
+        client->recv_coroutine = NULL;
         nbd_client_put(client);
         return;
     }
-- 
2.21.0