[at-spi2-core/mcatanzaro#25] Don't use dbus-broker if not running under systemd

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 260a4414ac26cc5e91dc56b6a10b5dda3dae22cd
Author: Michael Catanzaro <mcatanzaro gnome org>
Date:   Thu Sep 24 15:06:44 2020 -0500

    Don't use dbus-broker if not running under systemd
    
    Since gdm@febeb9a9, gdm no longer runs a systemd user session, because
    gdm supports multiseat but systemd only allows one graphical session per
    user. Since gdm currently runs as the gdm user, that means we cannot use
    systemd there. Benjamin Berg says we could fix that by changing gdm to
    use temporary users for each seat, but that would be a lot of work.
    
    Meanwhile, dbus-broker relies on systemd to autostart D-Bus services. So
    if we are not running a systemd user session, nothing gets autostarted
    in response to D-Bus calls. That means orca never gets any response to
    its method calls to org.a11y.atspi.Registry, and we wind up with no
    accessibility on the gnome-shell login screen.
    
    Fix this by implementing Benjamin's suggested check to see if we are
    running under systemd before using dbus-broker. So now we will use
    dbus-daemon on the login screen, but we will still use dbus-broker for
    the user session (except in distros that still prefer dbus-daemon...
    which is actually the default configuration). libsystemd is added as a
    build dependency whenever built with dbus-broker support, which should
    be uncontroversial because it won't work without systemd.
    
    I expect dbus-daemon is going to live alongside dbus-broker for a long
    time, because it seems very hard for us to migrate fully.
    
    Big thanks to Benjamin Berg for discovering the problem and suggesting
    this solution.
    
    Fixes #25

 bus/at-spi-bus-launcher.c | 18 ++++++++++++++++++
 bus/meson.build           | 11 ++++++++++-
 2 files changed, 28 insertions(+), 1 deletion(-)
---
diff --git a/bus/at-spi-bus-launcher.c b/bus/at-spi-bus-launcher.c
index 362fd05f..d7c66900 100644
--- a/bus/at-spi-bus-launcher.c
+++ b/bus/at-spi-bus-launcher.c
@@ -39,6 +39,9 @@
 #include <X11/Xlib.h>
 #include <X11/Xatom.h>
 #endif
+#ifdef DBUS_BROKER
+#include <systemd/sd-login.h>
+#endif
 
 typedef enum {
   A11Y_BUS_STATE_IDLE = 0,
@@ -392,11 +395,26 @@ static gboolean
 ensure_a11y_bus_broker (A11yBusLauncher *app, char *config_path)
 {
   char *argv[] = { DBUS_BROKER, config_path, "--scope", "user", NULL };
+  char *unit;
   struct sockaddr_un addr = { .sun_family = AF_UNIX };
   socklen_t addr_len = sizeof(addr);
   GPid pid;
   GError *error = NULL;
 
+  /* This detects whether we are running under systemd. We only try to
+   * use dbus-broker if we are running under systemd because D-Bus
+   * service activation won't work otherwise.
+   */
+  if (sd_pid_get_user_unit (getpid (), &unit) >= 0)
+    {
+      free (unit);
+    }
+  else
+    {
+      app->state = A11Y_BUS_STATE_ERROR;
+      return FALSE;
+    }
+
   if ((app->listenfd = socket (PF_UNIX, SOCK_STREAM | SOCK_NONBLOCK, 0)) < 0)
     g_error ("Failed to create listening socket: %s", strerror (errno));
 
diff --git a/bus/meson.build b/bus/meson.build
index 0fff5a89..f6c32c99 100644
--- a/bus/meson.build
+++ b/bus/meson.build
@@ -48,13 +48,16 @@ else
   endif
 endif
 
+needs_systemd = false
 if get_option('dbus_broker') != 'default'
   launcher_args += '-DDBUS_BROKER="@0@"'.format(get_option('dbus_broker'))
+  needs_systemd = true
 else
   dbus_broker = find_program('dbus-broker-launch',
                              required: false)
   if dbus_broker.found()
     launcher_args += '-DDBUS_BROKER="@0@"'.format(dbus_broker.path())
+    needs_systemd = true
   endif
 endif
 
@@ -62,9 +65,15 @@ if get_option('default_bus') == 'dbus-broker'
   launcher_args += '-DWANT_DBUS_BROKER'
 endif
 
+if needs_systemd
+  systemd_dep = dependency('libsystemd')
+else
+  systemd_dep = dependency('', required: false)
+endif
+
 executable('at-spi-bus-launcher', 'at-spi-bus-launcher.c',
            include_directories: [ root_inc, include_directories('.') ],
-           dependencies: [ gio_dep, x11_deps ],
+           dependencies: [ gio_dep, systemd_dep, x11_deps ],
            c_args: launcher_args,
            install: true,
            install_dir: atspi_libexecdir)