[glib/wip/jtojnar/gsrc-dstr-example] Modernize g_source_is_destroyed example

[Jan Tojnar <jtojnar src gnome org>]

commit 71b3deb901d1a0eb427317bf380ff4ae3332bf6d
Author: Jan Tojnar <jtojnar gmail com>
Date:   Mon Jan 4 14:19:22 2021 +0100

    Modernize g_source_is_destroyed example
    
    gtk_threads_{leave,enter} API is deprecated and a narrower critical section,
    guarding just idle_id manipulation, is better anyway.
    
    Fixes: https://gitlab.gnome.org/GNOME/glib/-/issues/2279

 glib/gmain.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)
---
diff --git a/glib/gmain.c b/glib/gmain.c
index 88140c865..b350de60f 100644
--- a/glib/gmain.c
+++ b/glib/gmain.c
@@ -3116,10 +3116,10 @@ g_main_current_source (void)
  * {
  *   SomeWidget *self = data;
  *    
- *   GDK_THREADS_ENTER ();
+ *   g_mutex_lock (&self->idle_id_mutex);
  *   // do stuff with self
  *   self->idle_id = 0;
- *   GDK_THREADS_LEAVE ();
+ *   g_mutex_unlock (&self->idle_id_mutex);
  *    
  *   return G_SOURCE_REMOVE;
  * }
@@ -3127,9 +3127,19 @@ g_main_current_source (void)
  * static void 
  * some_widget_do_stuff_later (SomeWidget *self)
  * {
+ *   g_mutex_lock (&self->idle_id_mutex);
  *   self->idle_id = g_idle_add (idle_callback, self);
+ *   g_mutex_unlock (&self->idle_id_mutex);
  * }
  *  
+ * static void
+ * some_widget_init (SomeWidget *self)
+ * {
+ *   g_mutex_init (&self->idle_id_mutex);
+ *
+ *   // ...
+ * }
+ *
  * static void 
  * some_widget_finalize (GObject *object)
  * {
@@ -3138,6 +3148,8 @@ g_main_current_source (void)
  *   if (self->idle_id)
  *     g_source_remove (self->idle_id);
  *    
+ *   g_mutex_clear (&self->idle_id_mutex);
+ *
  *   G_OBJECT_CLASS (parent_class)->finalize (object);
  * }
  * ]|
@@ -3154,12 +3166,12 @@ g_main_current_source (void)
  * {
  *   SomeWidget *self = data;
  *   
- *   GDK_THREADS_ENTER ();
+ *   g_mutex_lock (&self->idle_id_mutex);
  *   if (!g_source_is_destroyed (g_main_current_source ()))
  *     {
  *       // do stuff with self
  *     }
- *   GDK_THREADS_LEAVE ();
+ *   g_mutex_unlock (&self->idle_id_mutex);
  *   
  *   return FALSE;
  * }