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

[Thomas Haller <thaller src gnome org>]

commit 8dc9a66b97ec2136c83ad570017c892f24dd12f2
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.
    
    Maybe this is even a bug. For now, at least document this behavior.

 glib/gmain.c | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/glib/gmain.c b/glib/gmain.c
index 2e1ab3a25..c7c47ad6d 100644
--- a/glib/gmain.c
+++ b/glib/gmain.c
@@ -1761,6 +1761,11 @@ 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.
+ *
+ * The @notify function gets invoked during g_source_destroy() if the source
+ * was attached, or during g_source_unref() otherwise.
+ * Calling g_source_set_callback() will also invoke the @notify function of
+ * the previous callback, if any.
  **/
 void
 g_source_set_callback (GSource        *source,