[glib/th/gdbus-doc-unsubscribe-idle] main: document notify function gets called during g_source_destroy()

[Thomas Haller <thaller src gnome org>]

commit 3b4ff77bcdb4fc3bc79ccc955605c686fc243052
Author: Thomas Haller <thaller redhat com>
Date:   Sun Apr 18 21:01:19 2021 +0200

    main: document notify function gets called during g_source_destroy()
    
    This seems non-obvious to me. Document it.
    
    It also seems important to know, because it means that the data pointer
    might already be destroyed, before the source is unreferenced for good.
    For example, if the data pointer keeps a reference to the GSource,
    and it might seem sensible to call:
    
        g_source_destroy(data->source);
        g_source_unref(data->source); /* <<< data is already destroyed? */
    
    This leads to a crash, if the source was attached to a context.

 glib/gmain.c | 7 +++++++
 1 file changed, 7 insertions(+)
---
diff --git a/glib/gmain.c b/glib/gmain.c
index 2e1ab3a25..4fe587a67 100644
--- a/glib/gmain.c
+++ b/glib/gmain.c
@@ -1370,6 +1370,10 @@ g_source_destroy_internal (GSource      *source,
  *
  * This function is safe to call from any thread, regardless of which thread
  * the #GMainContext is running in.
+ *
+ * If the source is currently attached to a #GMainContext, destroying it
+ * will effectively unset the callback similar to calling g_source_set_callback().
+ * This can mean, that the data's #GDestroyNotify gets called right away.
  */
 void
 g_source_destroy (GSource *source)
@@ -1761,6 +1765,9 @@ g_source_set_callback_indirect (GSource              *source,
  * It is safe to call this function multiple times on a source which has already
  * been attached to a context. The changes will take effect for the next time
  * the source is dispatched after this call returns.
+ *
+ * Note that g_source_destroy() for a currently attached source has the effect
+ * of also unsetting the callback.
  **/
 void
 g_source_set_callback (GSource        *source,