src/cmd/go/internal/cache/default.go

// 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"
)

// 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
	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 golang.org 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" {
		die()
	}
	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
}

// DefaultDir returns the effective GOCACHE setting.
// It returns "off" if the cache is disabled.
func DefaultDir() string {
	dir := os.Getenv("GOCACHE")
	if dir != "" {
		return dir
	}

	// Compute default location.
	dir, err := os.UserCacheDir()
	if err != nil {
		return "off"
	}
	return filepath.Join(dir, "go-build")
}

// die calls base.Fatalf with a message explaining why DefaultDir was "off".
func die() {
	if os.Getenv("GOCACHE") == "off" {
		base.Fatalf("build cache is disabled by GOCACHE=off, but required as of Go 1.12")
	}
	if _, err := os.UserCacheDir(); err != nil {
		base.Fatalf("build cache is required, but could not be located: %v", err)
	}
	panic(fmt.Sprintf("cache.die called unexpectedly with cache.DefaultDir() = %s", DefaultDir()))
}