[glib] g_source_set_ready_time: Move no-op fast-path under the lock

[Simon McVittie <smcv src gnome org>]

commit a4686b8ea18c585666edece7b92147a92fcfb841
Author: Simon McVittie <smcv collabora com>
Date:   Sun Dec 24 15:19:31 2017 +0000

    g_source_set_ready_time: Move no-op fast-path under the lock
    
    If we don't take the lock, then we don't have the necessary
    "happens before" relationships to avoid this situation:
    
    * source->priv->ready_time was equal to ready_time until recently
    * another thread has set source->priv->ready_time to a different value
    * that write hasn't become visible to this thread yet
    * result: we should reset the ready_time, but we don't
    
    Signed-off-by: Simon McVittie <smcv collabora com>
    Bug: https://bugzilla.gnome.org/show_bug.cgi?id=791754
    Bug-Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=884654

 glib/gmain.c |   11 ++++++++---
 1 files changed, 8 insertions(+), 3 deletions(-)
---
diff --git a/glib/gmain.c b/glib/gmain.c
index 1fde50f..c2e539e 100644
--- a/glib/gmain.c
+++ b/glib/gmain.c
@@ -1848,14 +1848,19 @@ g_source_set_ready_time (GSource *source,
   g_return_if_fail (source != NULL);
   g_return_if_fail (source->ref_count > 0);
 
-  if (source->priv->ready_time == ready_time)
-    return;
-
   context = source->context;
 
   if (context)
     LOCK_CONTEXT (context);
 
+  if (source->priv->ready_time == ready_time)
+    {
+      if (context)
+        UNLOCK_CONTEXT (context);
+
+      return;
+    }
+
   source->priv->ready_time = ready_time;
 
   TRACE (GLIB_SOURCE_SET_READY_TIME (source, ready_time));