gl/wayland: use multi-threaded safe event wayland API

Multiple threads may be accessing the wayland fd at the same time which
requires the use of special wayland API to deal with to ensure nobody
will steal reads and cause a stall for anyone else.

gst-libs/gst/gl/wayland/wayland_event_source.c

@@ -51,19 +51,30 @@ static const struct wl_callback_listener sync_listener = {
   sync_callback
 };
 
+/* only thread safe iff called on the same thread @queue is being dispatched on */
 gint
 gst_gl_wl_display_roundtrip_queue (struct wl_display *display,
     struct wl_event_queue *queue)
 {
-  struct wl_callback *callback = wl_display_sync (display);
+  struct wl_callback *callback;
   gboolean done = FALSE;
   gint ret = 0;
 
-  if (callback == NULL)
+  if (queue) {
+    /* creating a wl_proxy and setting the queue is racy with the dispatching
+     * of the default queue */
+    while (wl_display_prepare_read_queue (display, queue) != 0) {
+      if ((ret = wl_display_dispatch_queue_pending (display, queue)) < 0)
+        return ret;
+    }
+  }
+  if (!(callback = wl_display_sync (display))) {
     return -1;
+  }
   wl_callback_add_listener (callback, &sync_listener, &done);
   if (queue) {
     wl_proxy_set_queue ((struct wl_proxy *) callback, queue);
+    wl_display_cancel_read (display);
     while (!done && ret >= 0)
       ret = wl_display_dispatch_queue (display, queue);
   } else {
@@ -84,6 +95,7 @@ typedef struct _WaylandEventSource
   uint32_t mask;
   struct wl_display *display;
   struct wl_event_queue *queue;
+  gboolean reading;
 } WaylandEventSource;
 
 static gboolean
@@ -93,10 +105,29 @@ wayland_event_source_prepare (GSource * base, gint * timeout)
 
   *timeout = -1;
 
-  /* We have to add/remove the GPollFD if we want to update our
+  /* we may be called multiple times for prepare */
-   * poll event mask dynamically.  Instead, let's just flush all
+  if (source->reading)
-   * writes on idle */
+    wl_display_cancel_read (source->display);
-  wl_display_flush (source->display);
+
+  if (source->queue) {
+    while (wl_display_prepare_read_queue (source->display, source->queue) != 0) {
+      if (wl_display_dispatch_queue_pending (source->display,
+              source->queue) < 0) {
+        g_critical ("Failed to dispatch pending events\n");
+      }
+    }
+  } else {
+    while (wl_display_prepare_read (source->display) != 0) {
+      if (wl_display_dispatch_pending (source->display) < 0) {
+        g_critical ("Failed to dispatch pending events\n");
+      }
+    }
+  }
+  source->reading = TRUE;
+
+  /* FIXME: this may return EAGAIN if the fd is full */
+  if (wl_display_flush (source->display) < 0)
+    g_critical ("Failed to flush Wayland connection\n");
 
   return FALSE;
 }
@@ -109,6 +140,13 @@ wayland_event_source_check (GSource * base)
 
   retval = source->pfd.revents;
 
+  if (source->pfd.revents & G_IO_IN) {
+    wl_display_read_events (source->display);
+  } else {
+    wl_display_cancel_read (source->display);
+  }
+  source->reading = FALSE;
+
   return retval;
 }
 
@@ -118,13 +156,11 @@ wayland_event_source_dispatch (GSource * base,
 {
   WaylandEventSource *source = (WaylandEventSource *) base;
 
-  if (source->pfd.revents) {
+  if (source->queue)
-    if (source->queue)
+    wl_display_dispatch_queue_pending (source->display, source->queue);
-      wl_display_dispatch_queue_pending (source->display, source->queue);
+  else
-    else
+    wl_display_dispatch_pending (source->display);
-      wl_display_dispatch_pending (source->display);
+  source->pfd.revents = 0;
-    source->pfd.revents = 0;
-  }
 
   if (callback)
     callback (data);
@@ -132,11 +168,22 @@ wayland_event_source_dispatch (GSource * base,
   return TRUE;
 }
 
+static void
+wayland_event_source_finalize (GSource * base)
+{
+  WaylandEventSource *source = (WaylandEventSource *) base;
+
+  if (source->reading) {
+    wl_display_cancel_read (source->display);
+  }
+  source->reading = FALSE;
+}
+
 static GSourceFuncs wayland_event_source_funcs = {
   wayland_event_source_prepare,
   wayland_event_source_check,
   wayland_event_source_dispatch,
-  NULL
+  wayland_event_source_finalize
 };
 
 GSource *