extensions-tool: Split out get_extension_property() helper

For the prefs command, we first fetch the extension info to check
whether the extension exists and actually has preferences. This
pattern can be useful for other commands and properties, so split
out a generic helper function.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/2391

subprojects/extensions-tool/src/main.c

@@ -124,6 +124,41 @@ get_shell_settings (void)
   return g_settings_new_full (schema, NULL, NULL);
 }
 
+GVariant *
+get_extension_property (GDBusProxy *proxy,
+                        const char *uuid,
+                        const char *property)
+{
+  g_autoptr (GVariant) response = NULL;
+  g_autoptr (GVariant) asv = NULL;
+  g_autoptr (GVariantDict) info = NULL;
+  g_autoptr (GError) error = NULL;
+
+  response = g_dbus_proxy_call_sync (proxy,
+                                     "GetExtensionInfo",
+                                     g_variant_new ("(s)", uuid),
+                                     0,
+                                     -1,
+                                     NULL,
+                                     &error);
+  if (response == NULL)
+    {
+      g_printerr (_("Failed to connect to GNOME Shell"));
+      return NULL;
+    }
+
+  asv = g_variant_get_child_value (response, 0);
+  info = g_variant_dict_new (asv);
+
+  if (!g_variant_dict_contains (info, "uuid"))
+    {
+      g_printerr (_("Extension “%s” doesn't exist\n"), uuid);
+      return NULL;
+    }
+
+  return g_variant_dict_lookup_value (info, property, NULL);
+}
+
 gboolean
 settings_list_add (GSettings  *settings,
                    const char *key,