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: default.go
    path of file: ./repos/under-the-hood/gosrc/cmd/go/internal/cache
    
    the code of the file until where you have to start completion: // Copyright 2017 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package cache

import (
	"fmt"
	"io/ioutil"
	"os"
	"path/filepath"
	"sync"

	"cmd/go/internal/base"
	"cmd/go/internal/cfg"
)

// Default returns the default cache to use, or nil if no cache should be used.
func Default() *Cache {
	defaultOnce.Do(initDefaultCache)
	return defaultCache
}

var (
	defaultOnce  sync.Once
	
    
func initDefaultCache() {
	dir := DefaultDir()
	if dir == "off" {
		if defaultDirErr != nil {
			base.Fatalf("build cache is required, but could not be located: %v", defaultDirErr)
		}
		base.Fatalf("build cache is disabled by GOCACHE=off, but required as of Go 1.12")
	}
	if err := os.MkdirAll(dir, 0777); err != nil {
		base.Fatalf("failed to initialize build cache at %s: %s\n", dir, err)
	}
	if _, err := os.Stat(filepath.Join(dir, "README")); err != nil {
		// Best effort.
		ioutil.WriteFile(filepath.Join(dir, "README"), []byte(cacheREADME), 0666)
	}

	c, err := Open(dir)
	if err != nil {
		base.Fatalf("failed to initialize build cache at %s: %s\n", dir, err)
	}
	defaultCache = c
}