backends/native: Protect MetaBarrierManagerNative

While barriers will be added from the main thread, the native barrier
manager will sit close to the MetaSeatImpl in its own thread. Add the
necessary locking so that we can pass MetaBarrierImplNative from the
UI thread to the input thread, and ensure the MetaBarrier signals are
still emitted in the UI thread.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1403>

src/backends/native/meta-barrier-native.c

@@ -43,6 +43,7 @@
 struct _MetaBarrierManagerNative
 {
   GHashTable *barriers;
+  GMutex mutex;
 };
 
 typedef enum
@@ -77,6 +78,7 @@ struct _MetaBarrierImplNative
   int                       trigger_serial;
   guint32                   last_event_time;
   MetaBarrierDirection      blocked_dir;
+  GMainContext             *main_context;
 };
 
 G_DEFINE_TYPE (MetaBarrierImplNative,
@@ -340,6 +342,49 @@ typedef struct _MetaBarrierEventData
   float               dy;
 } MetaBarrierEventData;
 
+typedef struct
+{
+  MetaBarrierEvent *event;
+  MetaBarrier *barrier;
+  MetaBarrierState state;
+} MetaBarrierIdleData;
+
+static gboolean
+emit_event_idle (MetaBarrierIdleData *idle_data)
+{
+  if (idle_data->state == META_BARRIER_STATE_HELD)
+    _meta_barrier_emit_hit_signal (idle_data->barrier, idle_data->event);
+  else
+    _meta_barrier_emit_left_signal (idle_data->barrier, idle_data->event);
+
+  meta_barrier_event_unref (idle_data->event);
+
+  return G_SOURCE_REMOVE;
+}
+
+static void
+queue_event (MetaBarrierImplNative *self,
+             MetaBarrierEvent      *event)
+{
+  MetaBarrierIdleData *idle_data;
+  GSource *source;
+
+  idle_data = g_new0 (MetaBarrierIdleData, 1);
+  idle_data->state = self->state;
+  idle_data->barrier = self->barrier;
+  idle_data->event = event;
+
+  source = g_idle_source_new ();
+  g_source_set_priority (source, G_PRIORITY_HIGH);
+  g_source_set_callback (source,
+                         (GSourceFunc) emit_event_idle,
+                         idle_data,
+                         g_free);
+
+  g_source_attach (source, self->main_context);
+  g_source_unref (source);
+}
+
 static void
 emit_barrier_event (MetaBarrierImplNative *self,
                     guint32                time,
@@ -350,7 +395,6 @@ emit_barrier_event (MetaBarrierImplNative *self,
                     float                  dx,
                     float                  dy)
 {
-  MetaBarrier *barrier = self->barrier;
   MetaBarrierEvent *event = g_slice_new0 (MetaBarrierEvent);
   MetaBarrierState old_state = self->state;
 
@@ -389,12 +433,7 @@ emit_barrier_event (MetaBarrierImplNative *self,
 
   self->last_event_time = time;
 
-  if (self->state == META_BARRIER_STATE_HELD)
-    _meta_barrier_emit_hit_signal (barrier, event);
-  else
-    _meta_barrier_emit_left_signal (barrier, event);
-
-  meta_barrier_event_unref (event);
+  queue_event (self, event);
 }
 
 static void
@@ -479,6 +518,8 @@ meta_barrier_manager_native_process (MetaBarrierManagerNative *manager,
   if (!clutter_input_device_get_coords (device, NULL, &prev_pos))
     return;
 
+  g_mutex_lock (&manager->mutex);
+
   prev_x = prev_pos.x;
   prev_y = prev_pos.y;
 
@@ -523,6 +564,8 @@ meta_barrier_manager_native_process (MetaBarrierManagerNative *manager,
   g_hash_table_foreach (manager->barriers,
                         maybe_emit_barrier_event,
                         &barrier_event_data);
+
+  g_mutex_unlock (&manager->mutex);
 }
 
 static gboolean
@@ -549,7 +592,10 @@ _meta_barrier_impl_native_destroy (MetaBarrierImpl *impl)
 {
   MetaBarrierImplNative *self = META_BARRIER_IMPL_NATIVE (impl);
 
+  g_mutex_lock (&self->manager->mutex);
   g_hash_table_remove (self->manager->barriers, self);
+  g_mutex_unlock (&self->manager->mutex);
+  g_main_context_unref (self->main_context);
   self->is_active = FALSE;
 }
 
@@ -565,10 +611,13 @@ meta_barrier_impl_native_new (MetaBarrier *barrier)
 
   self->barrier = barrier;
   self->is_active = TRUE;
+  self->main_context = g_main_context_ref_thread_default ();
 
   manager = meta_seat_native_get_barrier_manager (META_SEAT_NATIVE (seat));
   self->manager = manager;
+  g_mutex_lock (&manager->mutex);
   g_hash_table_add (manager->barriers, self);
+  g_mutex_unlock (&manager->mutex);
 
   return META_BARRIER_IMPL (self);
 }
@@ -596,6 +645,7 @@ meta_barrier_manager_native_new (void)
   manager = g_new0 (MetaBarrierManagerNative, 1);
 
   manager->barriers = g_hash_table_new (NULL, NULL);
+  g_mutex_init (&manager->mutex);
 
   return manager;
 }