throttle-groups: drain before detaching ThrottleState

I/O requests hang after stop/cont commands at least since QEMU 2.10.0
with -drive iops=100:

  (guest)$ dd if=/dev/zero of=/dev/vdb oflag=direct count=1000
  (qemu) stop
  (qemu) cont
  ...I/O is stuck...

This happens because blk_set_aio_context() detaches the ThrottleState
while requests may still be in flight:

  if (tgm->throttle_state) {
      throttle_group_detach_aio_context(tgm);
      throttle_group_attach_aio_context(tgm, new_context);
  }

This patch encloses the detach/attach calls in a drained region so no
I/O request is left hanging.  Also add assertions so we don't make the
same mistake again in the future.
Reported-by: NYongxue Hong <yhong@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NAlberto Garcia <berto@igalia.com>
Message-id: 20171110151934.16883-1-stefanha@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block/block-backend.c

@@ -1752,8 +1752,10 @@ void blk_set_aio_context(BlockBackend *blk, AioContext *new_context)
 
     if (bs) {
         if (tgm->throttle_state) {
+            bdrv_drained_begin(bs);
             throttle_group_detach_aio_context(tgm);
             throttle_group_attach_aio_context(tgm, new_context);
+            bdrv_drained_end(bs);
         }
         bdrv_set_aio_context(bs, new_context);
     }

block/throttle-groups.c

@@ -594,6 +594,12 @@ void throttle_group_attach_aio_context(ThrottleGroupMember *tgm,
 void throttle_group_detach_aio_context(ThrottleGroupMember *tgm)
 {
     ThrottleTimers *tt = &tgm->throttle_timers;
+
+    /* Requests must have been drained */
+    assert(tgm->pending_reqs[0] == 0 && tgm->pending_reqs[1] == 0);
+    assert(qemu_co_queue_empty(&tgm->throttled_reqs[0]));
+    assert(qemu_co_queue_empty(&tgm->throttled_reqs[1]));
+
     throttle_timers_detach_aio_context(tt);
     tgm->aio_context = NULL;
 }