gdk/wayland: use the multi-thread safe wayland API

This is required for proper integration with any other library/application that
may perform wayland API calls and poll() the wayland fd from multiple threads.
Using wl_display_dispatch{_queue}() is thread-safe if not mixed with custom
poll() usage, which GSource/GMainContext does.

Essentially, the problem is that multiple threads polling and reading
the same fd is extremely racy.  Use the wayland provided API for allowing
concurrent access to the wayland display fd.

See the wayland man pages for wl_display_prepare_read(),
wl_display_cancel_read() and wl_display_read_events() for more details.

https://bugzilla.gnome.org/show_bug.cgi?id=763852

1 files changed, 44 insertions, 12 deletions

diff --git a/gdk/wayland/gdkeventsource.c b/gdk/wayland/gdkeventsource.c
index f04281cb9a..a239cab046 100644
--- a/gdk/wayland/gdkeventsource.c
+++ b/gdk/wayland/gdkeventsource.c
@@ -28,6 +28,7 @@ typedef struct _GdkWaylandEventSource {
   GPollFD pfd;
   uint32_t mask;
   GdkDisplay *display;
+  gboolean reading;
 } GdkWaylandEventSource;
 
 static gboolean
@@ -50,12 +51,20 @@ gdk_event_source_prepare (GSource *base,
   if (_gdk_event_queue_find_first (source->display) != NULL)
     return TRUE;
 
+  /* wl_display_prepare_read() needs to be balanced with either
+   * wl_display_read_events() or wl_display_cancel_read()
+   * (in gdk_event_source_check() */
+  if (source->reading)
+    return FALSE;
+
+  /* if prepare_read() returns non-zero, there are events to be dispatched */
+  if (wl_display_prepare_read (display->wl_display) != 0)
+    return TRUE;
+  source->reading = TRUE;
+
   if (wl_display_flush (display->wl_display) < 0)
     g_error ("Error flushing display: %s", g_strerror (errno));
 
-  if (wl_display_dispatch_pending (display->wl_display) < 0)
-    g_error ("Error dispatching display: %s", g_strerror (errno));
-
   return FALSE;
 }
 
@@ -63,9 +72,29 @@ static gboolean
 gdk_event_source_check (GSource *base)
 {
   GdkWaylandEventSource *source = (GdkWaylandEventSource *) base;
+  GdkWaylandDisplay *display_wayland = (GdkWaylandDisplay *) source->display;
 
   if (source->display->event_pause_count > 0)
-    return _gdk_event_queue_find_first (source->display) != NULL;
+    {
+      if (source->reading)
+        wl_display_cancel_read (display_wayland->wl_display);
+      source->reading = FALSE;
+
+      return _gdk_event_queue_find_first (source->display) != NULL;
+    }
+
+  /* read the events from the wayland fd into their respective queues if we have data */
+  if (source->reading)
+    {
+      if (source->pfd.revents & G_IO_IN)
+        {
+          if (wl_display_read_events (display_wayland->wl_display) < 0)
+            g_error ("Error reading events from display: %s", g_strerror (errno));
+        }
+      else
+        wl_display_cancel_read (display_wayland->wl_display);
+      source->reading = FALSE;
+    }
 
   return _gdk_event_queue_find_first (source->display) != NULL ||
     source->pfd.revents;
@@ -97,8 +126,14 @@ gdk_event_source_dispatch (GSource     *base,
 }
 
 static void
-gdk_event_source_finalize (GSource *source)
+gdk_event_source_finalize (GSource *base)
 {
+  GdkWaylandEventSource *source = (GdkWaylandEventSource *) base;
+  GdkWaylandDisplay *display = (GdkWaylandDisplay *) source->display;
+
+  if (source->reading)
+    wl_display_cancel_read (display->wl_display);
+  source->reading = FALSE;
 }
 
 static GSourceFuncs wl_glib_source_funcs = {
@@ -157,14 +192,11 @@ _gdk_wayland_display_queue_events (GdkDisplay *display)
   display_wayland = GDK_WAYLAND_DISPLAY (display);
   source = (GdkWaylandEventSource *) display_wayland->event_source;
 
-  if (source->pfd.revents & G_IO_IN)
+  if (wl_display_dispatch_pending (display_wayland->wl_display) < 0)
     {
-      if (wl_display_dispatch (display_wayland->wl_display) < 0)
-        {
-          g_warning ("Error %d (%s) dispatching to Wayland display.",
-                     errno, g_strerror (errno));
-          exit (1);
-        }
+      g_warning ("Error %d (%s) dispatching to Wayland display.",
+                 errno, g_strerror (errno));
+      exit (1);
     }
 
   if (source->pfd.revents & (G_IO_ERR | G_IO_HUP))