[PATCH wayland] client: ensure thread safety for wl_display_roundtrip_queue()

[Ucan, Emre (ADITG/SW1)]

We have to block other threads from reading events. Otherwise other threads
may assign done event to the default queue, if they call
wl_display_read_events before setting the queue for the callback proxy.
This would cause a deadlock in wl_display_dispatch_queue,
because the default queue is not dispatched.

We can call wl_display_prepare_read_queue API which blocks on success
other threads from reading events and call wl_display_cancel read after
the queue is set to the proxy. This new implementation ensures thread
safety for the API.

Signed-off-by: Emre Ucan <eucan at de.adit-jv.com>
---
 src/wayland-client.c |   16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/src/wayland-client.c b/src/wayland-client.c
index ef12bfc..00d8cda 100644
--- a/src/wayland-client.c
+++ b/src/wayland-client.c
@@ -1075,12 +1075,26 @@ wl_display_roundtrip_queue(struct wl_display *display, struct wl_event_queue *qu
 	struct wl_callback *callback;
 	int done, ret = 0;
 
+	/*We have to block other threads from reading events. Otherwise other threads
+	 * may assign done event to the default queue, if they call wl_display_read_events
+	 * before setting the queue for the callback proxy. This would cause a deadlock
+	 * in wl_display_dispatch_queue, because the default queue is not dispatched.*/
+	while (ret >= 0 && wl_display_prepare_read_queue(display, queue))
+		ret = wl_display_dispatch_queue_pending(display, queue);
+
+	if (ret < 0)
+		return ret;
+
 	done = 0;
 	callback = wl_display_sync(display);
-	if (callback == NULL)
+	if (callback == NULL) {
+		wl_display_cancel_read(display);
 		return -1;
+	}
+
 	wl_proxy_set_queue((struct wl_proxy *) callback, queue);
 	wl_callback_add_listener(callback, &sync_listener, &done);
+	wl_display_cancel_read(display);
 	while (!done && ret >= 0)
 		ret = wl_display_dispatch_queue(display, queue);
 
-- 
1.7.9.5