kms: Add API to add a GSource that'll be invoked in the impl context

The MetaKmsImpl implementation may need to add a GSource that should be
invoked in the right context; e.g. a idle callback, timeout etc. It
cannot just add it itself, since it's the responsibility of MetaKms to
determine what is the impl context and what is the main context, so add
API to MetaKms to ensure the callback is invoked correctly.

It's the responsibility of the caller to eventually remove and destroy
the GSource.

https://gitlab.gnome.org/GNOME/mutter/issues/548
https://gitlab.gnome.org/GNOME/mutter/merge_requests/525

src/backends/native/meta-kms-private.h

@@ -41,6 +41,10 @@ gboolean meta_kms_run_impl_task_sync (MetaKms              *kms,
                                       gpointer              user_data,
                                       GError              **error);
 
+GSource * meta_kms_add_source_in_impl (MetaKms     *kms,
+                                       GSourceFunc  func,
+                                       gpointer     user_data);
+
 gboolean meta_kms_in_impl_task (MetaKms *kms);
 
 #define meta_assert_in_kms_impl(kms) \

src/backends/native/meta-kms.c

@@ -34,6 +34,12 @@ typedef struct _MetaKmsCallbackData
   GDestroyNotify user_data_destroy;
 } MetaKmsCallbackData;
 
+typedef struct _MetaKmsSimpleImplSource
+{
+  GSource source;
+  MetaKms *kms;
+} MetaKmsSimpleImplSource;
+
 struct _MetaKms
 {
   GObject parent;
@@ -117,6 +123,48 @@ meta_kms_run_impl_task_sync (MetaKms              *kms,
   return ret;
 }
 
+static gboolean
+simple_impl_source_dispatch (GSource     *source,
+                             GSourceFunc  callback,
+                             gpointer     user_data)
+{
+  MetaKmsSimpleImplSource *simple_impl_source =
+    (MetaKmsSimpleImplSource *) source;
+  MetaKms *kms = simple_impl_source->kms;
+  gboolean ret;
+
+  kms->in_impl_task = TRUE;
+  ret = callback (user_data);
+  kms->in_impl_task = FALSE;
+
+  return ret;
+}
+
+static GSourceFuncs simple_impl_source_funcs = {
+  .dispatch = simple_impl_source_dispatch,
+};
+
+GSource *
+meta_kms_add_source_in_impl (MetaKms     *kms,
+                             GSourceFunc  func,
+                             gpointer     user_data)
+{
+  GSource *source;
+  MetaKmsSimpleImplSource *simple_impl_source;
+
+  meta_assert_in_kms_impl (kms);
+
+  source = g_source_new (&simple_impl_source_funcs,
+                         sizeof (MetaKmsSimpleImplSource));
+  simple_impl_source = (MetaKmsSimpleImplSource *) source;
+  simple_impl_source->kms = kms;
+
+  g_source_set_callback (source, func, user_data, NULL);
+  g_source_attach (source, g_main_context_get_thread_default ());
+
+  return source;
+}
+
 gboolean
 meta_kms_in_impl_task (MetaKms *kms)
 {