cmd/go: add "go env GOCACHE"

This lets users see the effective GOCACHE setting.

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

src/cmd/dist/deps.go

@@ -137,15 +137,16 @@ var builddeps = map[string][]string{
 	},
 
 	"cmd/go/internal/envcmd": {
-		"cmd/go/internal/base", // 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
-		"encoding/json",        // cmd/go/internal/envcmd
-		"fmt",                  // cmd/go/internal/envcmd
-		"os",                   // cmd/go/internal/envcmd
-		"runtime",              // cmd/go/internal/envcmd
-		"strings",              // 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
+		"encoding/json",         // cmd/go/internal/envcmd
+		"fmt",                   // cmd/go/internal/envcmd
+		"os",                    // cmd/go/internal/envcmd
+		"runtime",               // cmd/go/internal/envcmd
+		"strings",               // cmd/go/internal/envcmd
 	},
 
 	"cmd/go/internal/fix": {

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

@@ -26,52 +26,61 @@ 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 == "" {
-		// Compute default location.
-		// TODO(rsc): This code belongs somewhere else,
-		// like maybe ioutil.CacheDir or os.CacheDir.
-		switch runtime.GOOS {
-		case "windows":
-			dir = os.Getenv("LocalAppData")
-
-		case "darwin":
-			dir = os.Getenv("HOME")
-			if dir == "" {
-				return
-			}
-			dir += "/Library/Caches"
-
-		case "plan9":
-			dir = os.Getenv("home")
-			if dir == "" {
-				return
-			}
-			dir += "/lib/cache"
-
-		default: // Unix
-			// https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
-			dir = os.Getenv("XDG_CACHE_HOME")
-			if dir == "" {
-				dir = os.Getenv("HOME")
-				if dir == "" {
-					return
-				}
-				dir += "/.cache"
-			}
-		}
-		dir = filepath.Join(dir, "go-build")
-		if err := os.MkdirAll(dir, 0777); err != nil {
-			return
-		}
+	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.
+	switch runtime.GOOS {
+	case "windows":
+		dir = os.Getenv("LocalAppData")
+
+	case "darwin":
+		dir = os.Getenv("HOME")
+		if dir == "" {
+			return "off"
+		}
+		dir += "/Library/Caches"
+
+	case "plan9":
+		dir = os.Getenv("home")
+		if dir == "" {
+			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
+		// https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
+		dir = os.Getenv("XDG_CACHE_HOME")
+		if dir == "" {
+			dir = os.Getenv("HOME")
+			if dir == "" {
+				return "off"
+			}
+			dir += "/.cache"
+		}
+	}
+	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"},