[gnome-session] gsm-util: avoid overwriting a env variable with itself

[Ray Strode <halfline src gnome org>]

commit 771dc6c12c051048fdb386703813081954e56492
Author: Giovanni Campagna <gcampagn cs stanford edu>
Date:   Wed Nov 4 09:36:44 2015 -0800

    gsm-util: avoid overwriting a env variable with itself
    
    This avoids a potentially thread-safety issue in our use of
    gsm_util_setenv() to push locale variables to DBus.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=756324

 gnome-session/gsm-util.c |   14 ++++++++++----
 1 files changed, 10 insertions(+), 4 deletions(-)
---
diff --git a/gnome-session/gsm-util.c b/gnome-session/gsm-util.c
index b7d2cda..e7ceec4 100644
--- a/gnome-session/gsm-util.c
+++ b/gnome-session/gsm-util.c
@@ -497,10 +497,16 @@ gsm_util_setenv (const char *variable,
 {
         GError *bus_error;
 
-        if (!value)
-                g_unsetenv (variable);
-        else
-                g_setenv (variable, value, TRUE);
+        /* Note: we're intentionally comparing pointers here:
+           The goal is to avoid an un-threadsafe env variable update
+           when this API is used in maybe_push_env_var() in main.c
+        */
+        if (g_getenv (variable) != value) {
+                if (!value)
+                        g_unsetenv (variable);
+                else
+                        g_setenv (variable, value, TRUE);
+        }
 
         bus_error = NULL;