presto-main/src/main/java/io/prestosql/server/PluginManager.java/PluginManager/loadPlugins
class PluginManager:    public void loadPlugins()
            throws Exception
    {
        if (!pluginsLoading.compareAndSet(false, true)) {
            return;
        }

        for (File file : listFiles(installedPluginsDir)) {
            if (file.isDirectory()) {
                loadPlugin(file.getAbsolutePath());
            }
        }

        for (String plugin : plugins) {
            loadPlugin(plugin);
        }

        metadataManager.verifyComparableOrderableContract();

        pluginsLoaded.set(true);
    }