[gnome-shell/wip/carlosg/check-systemd] shell: Check we are in a systemd environment before starting systemd units

[Carlos Garnacho <carlosg src gnome org>]

commit fca4c9f601acdf8bfc53a9f99bf50188f539e93a
Author: Carlos Garnacho <carlosg gnome org>
Date:   Wed May 6 10:57:54 2020 +0200

    shell: Check we are in a systemd environment before starting systemd units
    
    Avoid starting/stopping the gsd-xsettings systemd unit if the shell itself
    was not started via systemd. In the lack of a user session manager, we
    have no means to neatly start/stop services, so should rely on the good
    ol' gnome-session to do this for us.
    
    Fixes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2755

 src/shell-util.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)
---
diff --git a/src/shell-util.c b/src/shell-util.c
index bb2329536b..4690dbd5cd 100644
--- a/src/shell-util.c
+++ b/src/shell-util.c
@@ -31,6 +31,7 @@
 
 #ifdef HAVE_SYSTEMD
 #include <systemd/sd-daemon.h>
+#include <systemd/sd-login.h>
 #else
 /* So we don't need to add ifdef's everywhere */
 #define sd_notify(u, m)            do {} while (0)
@@ -673,7 +674,27 @@ shell_util_systemd_call (const char  *command,
                          const char  *mode,
                          GError     **error)
 {
+#ifdef HAVE_SYSTEMD
   g_autoptr (GDBusConnection) connection = NULL;
+  g_autofree char *self_unit = NULL;
+  int res;
+
+  res = sd_pid_get_user_unit (getpid (), &self_unit);
+
+  if (res == -ENODATA)
+    {
+      g_debug ("Not systemd-managed, not starting '%s'", unit);
+      return TRUE;
+    }
+  else if (res < 0)
+    {
+      g_set_error (error,
+                   G_IO_ERROR,
+                   g_io_error_from_errno (-res),
+                   "Error trying to start systemd unit '%s': %s",
+                   unit, g_strerror (-res));
+      return FALSE;
+    }
 
   connection = g_bus_get_sync (G_BUS_TYPE_SESSION, NULL, error);
 
@@ -692,6 +713,8 @@ shell_util_systemd_call (const char  *command,
                           -1, NULL,
                           on_systemd_call_cb,
                           (gpointer) command);
+#endif /* HAVE_SYSTEMD */
+
   return TRUE;
 }