[mutter] launcher: Use $XDG_SESSION_ID if available

[Marge Bot <marge-bot src gnome org>]

commit 8ebdae809222af3c1ea99a0296a59bdaaa6d977b
Author: Sebastian Wick <sebastian wick redhat com>
Date:   Mon Jan 24 17:31:32 2022 +0100

    launcher: Use $XDG_SESSION_ID if available
    
    In some cases mutter is started in the user scope from a TTY (for
    example using toolbox). Using sd_pid_get_session fails because it's not
    in the session scope so it falls back to the primary session
    (sd_uid_get_display). We want to start mutter on the TTY we started
    mutter on however. Instead of relying on the scope to figure out the
    correct session we first look at $XDG_SESSION_ID which is set by
    systemd_pam.so.
    
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2254>

 src/backends/native/meta-launcher.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)
---
diff --git a/src/backends/native/meta-launcher.c b/src/backends/native/meta-launcher.c
index 2d2f14dcf5..e968baeed8 100644
--- a/src/backends/native/meta-launcher.c
+++ b/src/backends/native/meta-launcher.c
@@ -72,10 +72,29 @@ find_systemd_session (gchar **session_id,
   g_auto (GStrv) sessions = NULL;
   int n_sessions;
   int saved_errno;
+  const char *xdg_session_id = NULL;
 
   g_assert (session_id != NULL);
   g_assert (error == NULL || *error == NULL);
 
+  xdg_session_id = g_getenv ("XDG_SESSION_ID");
+  if (xdg_session_id)
+    {
+      saved_errno = sd_session_is_active (xdg_session_id);
+      if (saved_errno < 0)
+        {
+          g_set_error (error,
+                       G_IO_ERROR,
+                       G_IO_ERROR_NOT_FOUND,
+                       "Failed to get status of XDG_SESSION_ID session (%s)",
+                       g_strerror (-saved_errno));
+          return FALSE;
+        }
+
+      *session_id = g_strdup (xdg_session_id);
+      return TRUE;
+    }
+
   /* if we are in a logind session, we can trust that value, so use it. This
    * happens for example when you run mutter directly from a VT but when
    * systemd starts us we will not be in a logind session. */