[gdm/wip/benzea/user-switching-logind-tty] gdm-session-worker: Drop login_vt assuming it is GDM_INITIAL_VT

[Benjamin Berg <bberg src gnome org>]

commit ac85b5a8190f24c7bd9bc3406a3761faa2210a04
Author: Benjamin Berg <bberg redhat com>
Date:   Wed Sep 25 14:51:40 2019 +0200

    gdm-session-worker: Drop login_vt assuming it is GDM_INITIAL_VT
    
    When a session ends, its "session worker" is closed. Since
    3e8220921bb608afd06ed677104fd2244b901a28 (3.33.4), we uninitialise PAM
    when this happens. As part of this procedure, we jump back to the login
    screen, if the screen being killed is not itself the login screen.
    
    This has broken fast user switching. It goes like this - this
    explanation is a bit complicated, bear with us:
    
    We want to jump back to the login screen when a normal user session
    ends, so that people can log in again. We do not want to do this when a
    login screen itself ends. When session workers start up, they query for
    the *currently active VT* and save this in `login_vt`. Then later on, we
    check if our session ID is the same as `login_vt`, and jump to
    `login_vt` if they are different - this means that it was a user session
    not a login session. Querying the currently active VT is fine for the
    first greeter, but when initiating a user switch it's wrong as this
    gives the user VT.
    
    GDM greeters are killed once they have spawned a session. They are
    associated with a logind session, and therefore a PAM session. There are
    some actions performed when unregistering PAM sessions, including the
    previously mentioned VT jump. Before
    3e8220921bb608afd06ed677104fd2244b901a28 we only uninitialised PAM when
    the session itself exited so the bug was masked, but now (since this
    commit), if the login screen's *worker* exits first - as happens in the
    normal case when GDM kills it - we also do this uninitialisation. Since
    we falsely recorded the login screen as the first user's VT, this means
    that checking `login_vt != session_vt` returns `TRUE` and we jump back
    to the previous user's session immediately after logging into the new
    session: fast user switching is broken.
    
    Since the work on shutting down the GDM session has been finished, we
    can assume that the login_vt is always on GDM_INITIAL_VT (see
    example c71bc5d6c3bc2ec448b5c72ce9a811d9c0c7905e
    "local-display-factory: Remove initial VT is in use check" and
    39fb4ff64e6a0653e70a3bfab31da47b49227d59 "manager: don't run autologin
    display on tty1"). So simply replace all usages of login_vt with
    GDM_INITIAL_VT to solve the above problem.
    
    This is a simpler version of the patch by Iain Lane <iainl gnome org>,
    taking into account that we can make the assumption about the login_vt.
    
    Closes #515

 daemon/gdm-session-worker.c | 43 ++++++++-----------------------------------
 1 file changed, 8 insertions(+), 35 deletions(-)
---
diff --git a/daemon/gdm-session-worker.c b/daemon/gdm-session-worker.c
index 0e854158..4f9083c3 100644
--- a/daemon/gdm-session-worker.c
+++ b/daemon/gdm-session-worker.c
@@ -146,7 +146,6 @@ struct GdmSessionWorkerPrivate
         char            **extensions;
 
         int               cred_flags;
-        int               login_vt;
         int               session_vt;
         int               session_tty_fd;
 
@@ -1052,13 +1051,15 @@ gdm_session_worker_uninitialize_pam (GdmSessionWorker *worker,
 
         gdm_session_worker_stop_auditor (worker);
 
+#ifdef ENABLE_USER_DISPLAY_SERVER
         if (g_strcmp0 (worker->priv->display_seat_id, "seat0") == 0) {
-                if (worker->priv->login_vt != worker->priv->session_vt) {
-                        jump_to_vt (worker, worker->priv->login_vt);
+                /* Switch to the login VT if we are not the login screen. */
+                if (worker->priv->session_vt != GDM_INITIAL_VT) {
+                        jump_to_vt (worker, GDM_INITIAL_VT);
                 }
         }
+#endif
 
-        worker->priv->login_vt = 0;
         worker->priv->session_vt = 0;
 
         g_debug ("GdmSessionWorker: state NONE");
@@ -1107,32 +1108,6 @@ _get_xauth_for_pam (const char *x11_authority_file)
 }
 #endif
 
-static gboolean
-ensure_login_vt (GdmSessionWorker *worker)
-{
-        int fd;
-        struct vt_stat vt_state = { 0 };
-        gboolean got_login_vt = FALSE;
-
-        fd = open ("/dev/tty0", O_RDWR | O_NOCTTY);
-
-        if (fd < 0) {
-                g_debug ("GdmSessionWorker: couldn't open VT master: %m");
-                return FALSE;
-        }
-
-        if (ioctl (fd, VT_GETSTATE, &vt_state) < 0) {
-                g_debug ("GdmSessionWorker: couldn't get current VT: %m");
-                goto out;
-        }
-
-        worker->priv->login_vt = vt_state.v_active;
-        got_login_vt = TRUE;
-out:
-        close (fd);
-        return got_login_vt;
-}
-
 static gboolean
 gdm_session_worker_initialize_pam (GdmSessionWorker   *worker,
                                    const char         *service,
@@ -1227,12 +1202,10 @@ gdm_session_worker_initialize_pam (GdmSessionWorker   *worker,
         g_debug ("GdmSessionWorker: state SETUP_COMPLETE");
         gdm_session_worker_set_state (worker, GDM_SESSION_WORKER_STATE_SETUP_COMPLETE);
 
-        /* Temporarily set PAM_TTY with the currently active VT (login screen) 
+        /* Temporarily set PAM_TTY with the login VT,
            PAM_TTY will be reset with the users VT right before the user session is opened */
-        if (ensure_login_vt (worker)) {
-                g_snprintf (tty_string, 256, "/dev/tty%d", worker->priv->login_vt);
-                pam_set_item (worker->priv->pam_handle, PAM_TTY, tty_string);
-        }
+        g_snprintf (tty_string, 256, "/dev/tty%d", GDM_INITIAL_VT);
+        pam_set_item (worker->priv->pam_handle, PAM_TTY, tty_string);
         if (!display_is_local)
                 worker->priv->password_is_required = TRUE;