cmd/go: add "go env GOCACHE"

This lets users see the effective GOCACHE setting.

Change-Id: I0b6dd2945d54611be89ed68fe2fd99110b9a25f6
Reviewed-on: https://go-review.googlesource.com/75293Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/dist/deps.go

@@ -138,6 +138,7 @@ var builddeps = map[string][]string{
 
 	"cmd/go/internal/envcmd": {
 		"cmd/go/internal/base",  // cmd/go/internal/envcmd
+		"cmd/go/internal/cache", // cmd/go/internal/envcmd
 		"cmd/go/internal/cfg",   // cmd/go/internal/envcmd
 		"cmd/go/internal/load",  // cmd/go/internal/envcmd
 		"cmd/go/internal/work",  // cmd/go/internal/envcmd

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

@@ -26,11 +26,28 @@ var (
 // initDefaultCache does the work of finding the default cache
 // the first time Default is called.
 func initDefaultCache() {
-	dir := os.Getenv("GOCACHE")
+	dir := DefaultDir()
 	if dir == "off" {
 		return
 	}
-	if dir == "" {
+	if err := os.MkdirAll(dir, 0777); err != nil {
+		base.Fatalf("initializing cache in $GOCACHE: %s", err)
+	}
+	c, err := Open(dir)
+	if err != nil {
+		base.Fatalf("initializing cache in $GOCACHE: %s", 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.
 	// TODO(rsc): This code belongs somewhere else,
 	// like maybe ioutil.CacheDir or os.CacheDir.
@@ -41,15 +58,17 @@ func initDefaultCache() {
 	case "darwin":
 		dir = os.Getenv("HOME")
 		if dir == "" {
-				return
+			return "off"
 		}
 		dir += "/Library/Caches"
 
 	case "plan9":
 		dir = os.Getenv("home")
 		if dir == "" {
-				return
+			return "off"
 		}
+		// Plan 9 has no established per-user cache directory,
+		// but $home/lib/xyz is the usual equivalent of $HOME/.xyz on Unix.
 		dir += "/lib/cache"
 
 	default: // Unix
@@ -58,20 +77,10 @@ func initDefaultCache() {
 		if dir == "" {
 			dir = os.Getenv("HOME")
 			if dir == "" {
-					return
+				return "off"
 			}
 			dir += "/.cache"
 		}
 	}
-		dir = filepath.Join(dir, "go-build")
-		if err := os.MkdirAll(dir, 0777); err != nil {
-			return
-		}
-	}
-
-	c, err := Open(dir)
-	if err != nil {
-		base.Fatalf("initializing cache in $GOCACHE: %s", err)
-	}
-	defaultCache = c
+	return filepath.Join(dir, "go-build")
 }

src/cmd/go/internal/envcmd/env.go

@@ -13,6 +13,7 @@ import (
 	"strings"
 
 	"cmd/go/internal/base"
+	"cmd/go/internal/cache"
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/load"
 	"cmd/go/internal/work"
@@ -55,6 +56,7 @@ func MkEnv() []cfg.EnvVar {
 		{Name: "GORACE", Value: os.Getenv("GORACE")},
 		{Name: "GOROOT", Value: cfg.GOROOT},
 		{Name: "GOTOOLDIR", Value: base.ToolDir},
+		{Name: "GOCACHE", Value: cache.DefaultDir()},
 
 		// disable escape codes in clang errors
 		{Name: "TERM", Value: "dumb"},