[gnome-shell/gnome-3-36] shell: Check we are in a systemd environment before starting systemd units

[Florian Müllner <fmuellner src gnome org>]

commit 57e67e0f319965bf5b0ff66575f65ba51e633f12
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.
    
    This changes the return value meaning slightly, TRUE means "service did
    start", FALSE otherwise. The error is only raised if we ought to start,
    but it produced an error somehow.
    
    https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1238
    https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2755

 src/shell-util.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)
---
diff --git a/src/shell-util.c b/src/shell-util.c
index d1a75bc6b4..68af72a76c 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)
@@ -659,7 +660,27 @@ 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 doing '%s' on '%s'", mode, unit);
+      return FALSE;
+    }
+  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);
 
@@ -678,7 +699,9 @@ shell_util_systemd_call (const char  *command,
                           -1, NULL,
                           on_systemd_call_cb,
                           (gpointer) command);
-  return TRUE;
+#endif /* HAVE_SYSTEMD */
+
+  return FALSE;
 }
 
 gboolean