core/dbus: "RequestName" of NetworkManager D-Bus API later to fix race

NetworkManager.service is "Type=dbus". Systemd takes that as indication
for declaring the service as started when the D-Bus name is acquired.

Currently, we acquire the name very early. The benefit is, that the
service appears to start very fast. However, most the D-Bus API is not
yet populated or ready to use. So if you order your service
`After=NetworkManager.service`, then there is a race that NetworkManager
might not yet be fully usable.

Another benefit was that requesting a D-Bus name is atomic. That means,
we could take that to ensure only one NetworkManager daemon was running.
If we noticed that NetworkManager is already running, we would quit
without doing anything. In practice, systemd already ensures that the
daemon is not running in parallel. This was still useful for catching
misuse when testing manually. This is now no longer done. We will notice
a concurrent NetworkManager only very late, at which point we might have
already broken things (e.g. rewrite wrong state files).

Fix the race with `After=` by acquiring the name much later.

Note that NetworkManager is pretty slow during initialization. This
easily adds several hundreds of milliseconds to the startup.

src/core/main.c

@@ -508,6 +508,9 @@ main(int argc, char *argv[])
     nm_log_dbg(LOGD_CORE, "setting up local loopback");
     nm_platform_link_change_flags(NM_PLATFORM_GET, 1, IFF_UP, TRUE);
 
+    if (!nm_dbus_manager_request_name_sync(nm_dbus_manager_get()))
+        goto done;
+
     success = TRUE;
 
     if (configure_and_quit == FALSE) {

src/core/nm-dbus-manager.c

@@ -1421,6 +1421,11 @@ nm_dbus_manager_request_name_sync(NMDBusManager *self)
 
     priv = NM_DBUS_MANAGER_GET_PRIVATE(self);
 
+    if (priv->objmgr_registration_id == 0) {
+        /* Do nothing. We're presumably in the configure-and-quit mode. */
+        return TRUE;
+    }
+
     g_return_val_if_fail(G_IS_DBUS_CONNECTION(priv->main_dbus_connection), FALSE);
 
     ret = g_dbus_connection_call_sync(
@@ -1499,9 +1504,6 @@ nm_dbus_manager_setup(NMDBusManager *self)
 
     _LOGD("D-Bus connection created and ObjectManager object registered");
 
-    if (!nm_dbus_manager_request_name_sync(self))
-        return FALSE;
-
     return TRUE;
 }