[gdm] gdm-{wayland, x}-session: omit GNOME_SHELL_SESSION_MODE from environment

[Ray Strode <halfline src gnome org>]

commit 9bd3c5f04efc0da7de9fc27237244e22942ef720
Author: Xiaoguang Wang <xwang suse com>
Date:   Mon Aug 7 14:26:08 2017 +0800

    gdm-{wayland,x}-session: omit GNOME_SHELL_SESSION_MODE from environment
    
    After a user logs out, the systemd --user session may still be running
    (if, say, the user session was left around in the closing state,
    because of pulseaudio).
    
    If that happens, new user sessions will inherit the environment of
    previous sessions.  Some of these environment variables are already
    intentionally pruned, DISPLAY, XAUTHORITY, WAYLAND_SOCKET, etc.  One
    environment variable that isn't pruned, but should be, is
    GNOME_SHELL_SESSION_MODE.
    
    If it lingers, users could end up in classic mode even though they
    picked the non-classic mode from the gear menu.
    
    This commit adds GNOME_SHELL_SESSION_MODE to the list of environment
    variables to prune.
    
    (minor changes to original patch and commit message from Ray Strode)
    
    https://bugzilla.gnome.org/show_bug.cgi?id=785918

 daemon/gdm-wayland-session.c |    1 +
 daemon/gdm-x-session.c       |    2 ++
 2 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/daemon/gdm-wayland-session.c b/daemon/gdm-wayland-session.c
index efdb34e..742eb26 100644
--- a/daemon/gdm-wayland-session.c
+++ b/daemon/gdm-wayland-session.c
@@ -324,6 +324,7 @@ spawn_session (State        *state,
         g_subprocess_launcher_unsetenv (launcher, "XAUTHORITY");
         g_subprocess_launcher_unsetenv (launcher, "WAYLAND_DISPLAY");
         g_subprocess_launcher_unsetenv (launcher, "WAYLAND_SOCKET");
+        g_subprocess_launcher_unsetenv (launcher, "GNOME_SHELL_SESSION_MODE");
 
         subprocess = g_subprocess_launcher_spawnv (launcher,
                                                    (const char * const *) argv,
diff --git a/daemon/gdm-x-session.c b/daemon/gdm-x-session.c
index b919e6e..3ac4861 100644
--- a/daemon/gdm-x-session.c
+++ b/daemon/gdm-x-session.c
@@ -643,6 +643,8 @@ spawn_session (State        *state,
                 g_subprocess_launcher_setenv (launcher, "WINDOWPATH", vt, TRUE);
         }
 
+        g_subprocess_launcher_unsetenv (launcher, "GNOME_SHELL_SESSION_MODE");
+
         if (run_script) {
                 subprocess = g_subprocess_launcher_spawn (launcher,
                                                           &error,