cmd/go: add env -w and env -u to set and unset default env vars

Setting environment variables for go command configuration
is too difficult and system-specific. This CL adds go env -w,
to change the default settings more easily, in a portable way.
It also adds go env -u, to unset those changes.

See https://golang.org/design/30411-env for details.

Fixes #30411.

Change-Id: I36e83f55b666459f8f7f482432a4a6ee015da71d
Reviewed-on: https://go-review.googlesource.com/c/go/+/171137
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

Author: rsc

src/cmd/go/alldocs.go

@@ -6,8 +6,6 @@ package main_test
 
 import (
 	"bytes"
-	"cmd/go/internal/cache"
-	"cmd/internal/sys"
 	"context"
 	"debug/elf"
 	"debug/macho"
@@ -28,6 +26,10 @@ import (
 	"strings"
 	"testing"
 	"time"
+
+	"cmd/go/internal/cache"
+	"cmd/go/internal/cfg"
+	"cmd/internal/sys"
 )
 
 var (
@@ -119,6 +121,8 @@ var testCtx = context.Background()
 // The TestMain function creates a go command for testing purposes and
 // deletes it after the tests have been run.
 func TestMain(m *testing.M) {
+	// $GO_GCFLAGS a compiler debug flag known to cmd/dist, make.bash, etc.
+	// It is not a standard go command flag; use os.Getenv, not cfg.Getenv.
 	if os.Getenv("GO_GCFLAGS") != "" {
 		fmt.Fprintf(os.Stderr, "testing: warning: no tests to run\n") // magic string for cmd/go
 		fmt.Printf("cmd/go test is not compatible with $GO_GCFLAGS being set\n")
@@ -256,6 +260,7 @@ func TestMain(m *testing.M) {
 		}
 	}
 	// Don't let these environment variables confuse the test.
+	os.Setenv("GOENV", "off")
 	os.Unsetenv("GOBIN")
 	os.Unsetenv("GOPATH")
 	os.Unsetenv("GIT_ALLOW_PROTOCOL")
@@ -264,7 +269,7 @@ func TestMain(m *testing.M) {
 	// Setting HOME to a non-existent directory will break
 	// those systems. Disable ccache and use real compiler. Issue 17668.
 	os.Setenv("CCACHE_DISABLE", "1")
-	if os.Getenv("GOCACHE") == "" {
+	if cfg.Getenv("GOCACHE") == "" {
 		os.Setenv("GOCACHE", testGOCACHE) // because $HOME is gone
 	}
 
@@ -5258,7 +5263,7 @@ func TestCacheVet(t *testing.T) {
 	if strings.Contains(os.Getenv("GODEBUG"), "gocacheverify") {
 		t.Skip("GODEBUG gocacheverify")
 	}
-	if os.Getenv("GOCACHE") == "off" {
+	if cfg.Getenv("GOCACHE") == "off" {
 		tooSlow(t)
 		tg.makeTempdir()
 		tg.setenv("GOCACHE", tg.path("cache"))

src/cmd/go/go_test.go

@@ -7,7 +7,6 @@ package base
 import (
 	"flag"
 	"fmt"
-	"os"
 	"runtime"
 	"strings"
 
@@ -62,7 +61,7 @@ func InitGOFLAGS() {
 	// (Both will show the GOFLAGS setting if let succeed.)
 	hideErrors := cfg.CmdName == "env" || cfg.CmdName == "bug"
 
-	goflags = strings.Fields(os.Getenv("GOFLAGS"))
+	goflags = strings.Fields(cfg.Getenv("GOFLAGS"))
 	if goflags == nil {
 		goflags = []string{} // avoid work on later InitGOFLAGS call
 	}

src/cmd/go/internal/base/goflags.go

@@ -12,6 +12,7 @@ import (
 	"sync"
 
 	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
 )
 
 // Default returns the default cache to use, or nil if no cache should be used.
@@ -73,7 +74,7 @@ func DefaultDir() string {
 	// otherwise distinguish between an explicit "off" and a UserCacheDir error.
 
 	defaultDirOnce.Do(func() {
-		defaultDir = os.Getenv("GOCACHE")
+		defaultDir = cfg.Getenv("GOCACHE")
 		if filepath.IsAbs(defaultDir) || defaultDir == "off" {
 			return
 		}