test-bdrv-drain: Don't call bdrv_graph_wrlock() in coroutine context

AIO callbacks are effectively coroutine_mixed_fn. If AIO requests don't
return immediately, their callback is called from the request coroutine.
This means that in AIO callbacks, we can't call no_coroutine_fns such as
bdrv_graph_wrlock(). Unfortunately test-bdrv-drain does so.

Change the test to use a BH to drop out of coroutine context, and add
coroutine_mixed_fn and no_coroutine_fn markers to clarify the context
each function runs in.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20230929145157.45443-2-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

tests/unit/test-bdrv-drain.c

@@ -1168,7 +1168,7 @@ struct detach_by_parent_data {
 };
 static struct detach_by_parent_data detach_by_parent_data;
 
-static void detach_indirect_bh(void *opaque)
+static void no_coroutine_fn detach_indirect_bh(void *opaque)
 {
     struct detach_by_parent_data *data = opaque;
 
@@ -1184,14 +1184,15 @@ static void detach_indirect_bh(void *opaque)
     bdrv_graph_wrunlock();
 }
 
-static void detach_by_parent_aio_cb(void *opaque, int ret)
+static void coroutine_mixed_fn detach_by_parent_aio_cb(void *opaque, int ret)
 {
     struct detach_by_parent_data *data = &detach_by_parent_data;
 
     g_assert_cmpint(ret, ==, 0);
     if (data->by_parent_cb) {
         bdrv_inc_in_flight(data->child_b->bs);
-        detach_indirect_bh(data);
+        aio_bh_schedule_oneshot(qemu_get_current_aio_context(),
+                                detach_indirect_bh, &detach_by_parent_data);
     }
 }