You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: fs_site.go
    path of file: ./repos/arvados/sdk/go/arvados
    
    the code of the file until where you have to start completion: // Copyright (C) The Arvados Authors. All rights reserved.
//
// SPDX-License-Identifier: Apache-2.0

package arvados

import (
	"net/http"
	"os"
	"strings"
	"sync"
	"time"
)

type CustomFileSystem interface {
	FileSystem
	MountByID(mount string)
	MountProject(mount, uuid string)
	MountUsers(mount string)
	ForwardSlashNameSubstitution(string)
}

type customFileSystem struct {
	fileSystem
	root *vdirnode
	thr  *throttle

	staleThreshold time.Time
	staleLock      s
    
func (fs *customFileSystem) newCollectionOrProjectHardlink(parent inode, id string) (inode, error) {
	if strings.Contains(id, "-4zz18-") || pdhRegexp.MatchString(id) {
		node, err := fs.collectionSingleton(id)
		if os.IsNotExist(err) {
			return nil, nil
		} else if err != nil {
			return nil, err
		}
		return &hardlink{inode: node, parent: parent, name: id}, nil
	} else if strings.Contains(id, "-j7d0g-") || strings.Contains(id, "-tpzed-") {
		fs.byIDLock.Lock()
		node := fs.byID[id]
		fs.byIDLock.Unlock()
		if node == nil {
			// Look up the project synchronously before
			// calling projectSingleton (otherwise we
			// wouldn't detect a nonexistent project until
			// it's too late to return ErrNotExist).
			proj, err := fs.getProject(id)
			if os.IsNotExist(err) {
				return nil, nil
			} else if err != nil {
				return nil, err
			}
			node = fs.projectSingleton(id, proj)
		}
		return &hardlink{inode: node, parent: parent, name: id}, nil
	} else {
		return nil, nil
	}
}