Spawn: Improve handling of startup notification

The ID for startup notification is transmitted to the spawned process via the
DESKTOP_STARTUP_ID environment variable. Before this commit, we set this
variable in the main process. This meant that if we started something "without"
a startup id, then it might get the ID that was used by the last spawn and which
was still saved in our env. Fix this by setting the environment variable only
after fork().

Small anecdote: The above wasn't enough to make Daniel's test case succeed and
at first I couldn't figure out why.

Turns out that rxvt-unicode doesn't unset the DESKTOP_STARTUP_ID environment
variable (I think it should, according to some spec), even though it supports
startup notification. So awesome was already started with DESKTOP_STARTUP_ID set
and thus all spawned processes used this ID.

Fix this by explicitly unsetting DESKTOP_STARTUP_ID if we don't set any new
value (even though this breaks encapsulation; we shouldn't have to care about
this "implementation detail" of libstartup-notification).

Signed-off-by: Uli Schlachter <psychon@znc.in>

spawn.c

@@ -283,7 +283,14 @@ spawn_launchee_timeout(gpointer context)
 static void
 spawn_callback(gpointer user_data)
 {
+    SnLauncherContext *context = (SnLauncherContext *) user_data;
     setsid();
+
+    if (context)
+        sn_launcher_context_setup_child_process(context);
+    else
+        /* Unset in case awesome was already started with this variable set */
+        unsetenv("DESKTOP_STARTUP_ID");
 }
 
 /** Parse a command line.
@@ -400,11 +407,10 @@ luaA_spawn(lua_State *L)
         /* app will have AWESOME_SPAWN_TIMEOUT seconds to complete,
          * or the timeout function will terminate the launch sequence anyway */
         g_timeout_add_seconds(AWESOME_SPAWN_TIMEOUT, spawn_launchee_timeout, context);
-        sn_launcher_context_setup_child_process(context);
     }
 
     retval = g_spawn_async_with_pipes(NULL, argv, NULL, G_SPAWN_SEARCH_PATH,
-                                      spawn_callback, NULL, &pid,
+                                      spawn_callback, context, &pid,
                                       stdin_ptr, stdout_ptr, stderr_ptr, &error);
     g_strfreev(argv);
     if(!retval)