1 files changed, 97 insertions, 0 deletions
diff --git a/src/cmd/go/internal/cache/default.go b/src/cmd/go/internal/cache/default.go
new file mode 100644
index 0000000..0b1c1e0
--- /dev/null
+++ b/src/cmd/go/internal/cache/default.go
@@ -0,0 +1,97 @@
+// 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"
+	"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
+	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" {
+		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.
+		os.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 = cfg.Getenv("GOCACHE")
+		if filepath.IsAbs(defaultDir) || defaultDir == "off" {
+			return
+		}
+		if defaultDir != "" {
+			defaultDir = "off"
+			defaultDirErr = fmt.Errorf("GOCACHE is not an absolute path")
+			return
+		}
+
+		// Compute default location.
+		dir, err := os.UserCacheDir()
+		if err != nil {
+			defaultDir = "off"
+			defaultDirErr = fmt.Errorf("GOCACHE is not defined and %v", err)
+			return
+		}
+		defaultDir = filepath.Join(dir, "go-build")
+	})
+
+	return defaultDir
+}