Create xdg-app-info file in user runtime dir with effective state

This shows the name of the app, the runtime in use and the permissions
that the app has. This is useful for a client to know if e.g. it needs
to use a portal of some sort.

app/xdg-app-builtins-run.c

@@ -176,6 +176,33 @@ xdg_app_builtin_run (int argc, char **argv, GCancellable *cancellable, GError **
 
   xdg_app_context_merge (app_context, arg_context);
 
+    {
+      g_autofree char *tmp_path = NULL;
+      g_autofree char *path = NULL;
+      int fd;
+
+      fd = g_file_open_tmp ("xdg-app-context-XXXXXX", &tmp_path, NULL);
+      if (fd >= 0)
+        {
+          g_autoptr(GKeyFile) keyfile = NULL;
+
+          close (fd);
+
+          keyfile = g_key_file_new ();
+
+          g_key_file_set_string (keyfile, "Application", "name", app);
+          g_key_file_set_string (keyfile, "Application", "runtime", runtime_ref);
+
+          xdg_app_context_save_metadata (app_context, keyfile);
+
+          if (!g_key_file_save_to_file (keyfile, tmp_path, error))
+            return FALSE;
+
+          g_ptr_array_add (argv_array, g_strdup ("-M"));
+          g_ptr_array_add (argv_array, g_strdup_printf ("/run/user/%d/xdg-app-info=%s", getuid(), tmp_path));
+        }
+    }
+
   if (!xdg_app_run_add_extension_args (argv_array, runtime_metakey, runtime_ref, cancellable, error))
     return FALSE;