build: add elogind support

Checking the uid of the user seat also works if elogind is used. Add
this as an option to the build and make it mutually exclusive with
enabling systemd.

dbus/dbus-userdb-util.c

@@ -42,6 +42,10 @@
 #include <systemd/sd-login.h>
 #endif
 
+#ifdef HAVE_ELOGIND
+#include <elogind/sd-login.h>
+#endif
+
 /**
  * @addtogroup DBusInternalsUtils
  * @{
@@ -67,7 +71,7 @@ dbus_bool_t
 _dbus_is_console_user (dbus_uid_t uid,
 		       DBusError *error)
 {
-#ifdef HAVE_SYSTEMD
+#if defined(HAVE_SYSTEMD) || defined(HAVE_ELOGIND)
   /* check if we have logind */
   if (access ("/run/systemd/seats/", F_OK) >= 0)
     {

dbus/meson.build

@@ -24,6 +24,7 @@ dbus_dependencies = [
     adt_libs,
     network_libs,
     systemd,
+    elogind,
     valgrind.partial_dependency(compile_args: true),
 ]
 

meson.build

@@ -535,6 +535,17 @@ else
 endif
 data_config.set('SYSTEMCTL', systemctl)
 
+elogind_opt = get_option('elogind')
+if use_systemd
+    if elogind_opt.enabled()
+        error('-Dsystemd and -Delogind are mutually exclusive')
+    endif
+    elogind = dependency('', required: false)
+else
+    elogind = dependency('libelogind', version: '>=209', required: elogind_opt)
+endif
+use_elogind = elogind.found()
+config.set('HAVE_ELOGIND', use_elogind)
 
 
 use_traditional_activation = message_bus and get_option('traditional_activation')
@@ -1378,6 +1389,7 @@ summary_dict += {
     'Building inotify support': use_inotify,
     'Building kqueue support':  use_kqueue,
     'Building systemd support': use_systemd,
+    'Building elogind support': use_elogind,
     'Traditional activation':   use_traditional_activation,
     'Building X11 code':        config.get('DBUS_BUILD_X11'),
     'Building Doxygen docs':    doxygen.found(),

meson_options.txt

@@ -95,6 +95,13 @@ option(
   description: 'Enable tests that require insecure extra code in the library and binaries'
 )
 
+option(
+  'elogind',
+  type: 'feature',
+  value: 'disabled',
+  description: 'Use elogind for detecting user sessions'
+)
+
 option(
   'epoll',
   type: 'feature',