From dfca67899ea33da08d3aa9e84c1b4487991adad0 Mon Sep 17 00:00:00 2001
From: Ray Strode <rstrode@redhat.com>
Date: Thu, 7 Oct 2021 15:34:27 -0400
Subject: [PATCH 4/4] local-display-factory: Don't crash if Xorg and Wayland
 are both unavailable

At the moment if Wayland doesn't work, the login screen will fall back
to Xorg, and if Xorg doesn't work the login screen will fall back to
Wayland.

But if the fall back choice is disabled explicitly, GDM will just crash.

This commit fixes the crash.

Closes: https://gitlab.gnome.org/GNOME/gdm/-/issues/739
---
 daemon/gdm-local-display-factory.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/daemon/gdm-local-display-factory.c b/daemon/gdm-local-display-factory.c
index eba38671..120847f9 100644
--- a/daemon/gdm-local-display-factory.c
+++ b/daemon/gdm-local-display-factory.c
@@ -651,62 +651,67 @@ ensure_display_for_seat (GdmLocalDisplayFactory *factory,
         gdm_settings_direct_get_boolean (GDM_KEY_XORG_ENABLE, &xorg_enabled);
 
         preferred_display_server = get_preferred_display_server (factory);
 
         if (g_strcmp0 (preferred_display_server, "none") == 0) {
                g_debug ("GdmLocalDisplayFactory: Preferred display server is none, so not creating display");
                return;
         }
 
         ret = sd_seat_can_graphical (seat_id);
 
         if (ret < 0) {
                 g_critical ("Failed to query CanGraphical information for seat %s", seat_id);
                 return;
         }
 
         if (ret == 0) {
                 g_debug ("GdmLocalDisplayFactory: System doesn't currently support graphics");
                 seat_supports_graphics = FALSE;
         } else {
                 g_debug ("GdmLocalDisplayFactory: System supports graphics");
                 seat_supports_graphics = TRUE;
         }
 
         if (g_strcmp0 (seat_id, "seat0") == 0) {
                 is_seat0 = TRUE;
 
                 falling_back = factory->num_failures > 0;
                 session_types = gdm_local_display_factory_get_session_types (factory, falling_back);
 
-                g_debug ("GdmLocalDisplayFactory: New displays on seat0 will use %s%s",
-                         session_types[0], falling_back? " fallback" : "");
+                if (session_types == NULL) {
+                        g_debug ("GdmLocalDisplayFactory: Both Wayland and Xorg are unavailable");
+                        seat_supports_graphics = FALSE;
+                } else {
+                        g_debug ("GdmLocalDisplayFactory: New displays on seat0 will use %s%s",
+                                 session_types[0], falling_back? " fallback" : "");
+                }
         } else {
                 is_seat0 = FALSE;
 
                 g_debug ("GdmLocalDisplayFactory: New displays on seat %s will use X11 fallback", seat_id);
                 /* Force legacy X11 for all auxiliary seats */
                 seat_supports_graphics = TRUE;
                 session_types = g_strdupv ((char **) legacy_session_types);
         }
 
         /* For seat0, we have a fallback logic to still try starting it after
          * SEAT0_GRAPHICS_CHECK_TIMEOUT seconds. i.e. we simply continue even if
          * CanGraphical is unset.
          * This is ugly, but it means we'll come up eventually in some
          * scenarios where no master device is present.
          * Note that we'll force an X11 fallback even though there might be
          * cases where an wayland capable device is present and simply not marked as
          * master-of-seat. In these cases, this should likely be fixed in the
          * udev rules.
          *
          * At the moment, systemd always sets CanGraphical for non-seat0 seats.
          * This is because non-seat0 seats are defined by having master-of-seat
          * set. This means we can avoid the fallback check for non-seat0 seats,
          * which simplifies the code.
          */
         if (is_seat0) {
                 if (!seat_supports_graphics) {
                         if (!factory->seat0_graphics_check_timed_out) {
                                 if (factory->seat0_graphics_check_timeout_id == 0) {
                                         g_debug ("GdmLocalDisplayFactory: seat0 doesn't yet support graphics.  Waiting %d seconds to try again.", SEAT0_GRAPHICS_CHECK_TIMEOUT);
                                         factory->seat0_graphics_check_timeout_id = g_timeout_add_seconds (SEAT0_GRAPHICS_CHECK_TIMEOUT,
-- 
2.34.1