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// 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 (
// Default returns the default cache to use, or nil if no cache should be used.
func Default() *Cache {
return defaultCache
var (
defaultOnce sync.Once
defaultCache *Cache
// cacheREADME is a message stored in a README in the cache directory.
// Because the cache lives outside the normal Go trees, we leave the
// README as a courtesy to explain where it came from.
const cacheREADME = `This directory holds cached build artifacts from the Go build system.
Run "go clean -cache" if the directory is getting too large.
See to learn more about Go.
// initDefaultCache does the work of finding the default cache
// the first time Default is called.
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
var (
defaultDirOnce sync.Once
defaultDir string
defaultDirErr error
// DefaultDir returns the effective GOCACHE setting.
// It returns "off" if the cache is disabled.
func DefaultDir() string {
// Save the result of the first call to DefaultDir for later use in
// initDefaultCache. cmd/go/main.go explicitly sets GOCACHE so that
// subprocesses will inherit it, but that means initDefaultCache can't
// otherwise distinguish between an explicit "off" and a UserCacheDir error.
defaultDirOnce.Do(func() {
defaultDir = os.Getenv("GOCACHE")
if filepath.IsAbs(defaultDir) || defaultDir == "off" {
if defaultDir != "" {
defaultDir = "off"
defaultDirErr = fmt.Errorf("GOCACHE is not an absolute path")
// Compute default location.
dir, err := os.UserCacheDir()
if err != nil {
defaultDir = "off"
defaultDirErr = fmt.Errorf("GOCACHE is not defined and %v", err)
defaultDir = filepath.Join(dir, "go-build")
return defaultDir