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>

1 files changed, 230 insertions, 24 deletions

diff --git a/src/cmd/go/internal/cfg/cfg.go b/src/cmd/go/internal/cfg/cfg.go
index 35f7f1a173..1060c8f6df 100644
--- a/src/cmd/go/internal/cfg/cfg.go
+++ b/src/cmd/go/internal/cfg/cfg.go
@@ -7,11 +7,15 @@
 package cfg
 
 import (
+	"bytes"
 	"fmt"
 	"go/build"
+	"io/ioutil"
 	"os"
 	"path/filepath"
 	"runtime"
+	"strings"
+	"sync"
 
 	"cmd/internal/objabi"
 )
@@ -46,6 +50,50 @@ var (
 func defaultContext() build.Context {
 	ctxt := build.Default
 	ctxt.JoinPath = filepath.Join // back door to say "do not use go command"
+
+	ctxt.GOROOT = findGOROOT()
+	if runtime.Compiler != "gccgo" {
+		// Note that we must use runtime.GOOS and runtime.GOARCH here,
+		// as the tool directory does not move based on environment
+		// variables. This matches the initialization of ToolDir in
+		// go/build, except for using ctxt.GOROOT rather than
+		// runtime.GOROOT.
+		build.ToolDir = filepath.Join(ctxt.GOROOT, "pkg/tool/"+runtime.GOOS+"_"+runtime.GOARCH)
+	}
+
+	ctxt.GOPATH = envOr("GOPATH", ctxt.GOPATH)
+
+	// Override defaults computed in go/build with defaults
+	// from go environment configuration file, if known.
+	ctxt.GOOS = envOr("GOOS", ctxt.GOOS)
+	ctxt.GOARCH = envOr("GOARCH", ctxt.GOARCH)
+
+	// The go/build rule for whether cgo is enabled is:
+	//	1. If $CGO_ENABLED is set, respect it.
+	//	2. Otherwise, if this is a cross-compile, disable cgo.
+	//	3. Otherwise, use built-in default for GOOS/GOARCH.
+	// Recreate that logic here with the new GOOS/GOARCH setting.
+	if v := Getenv("CGO_ENABLED"); v == "0" || v == "1" {
+		ctxt.CgoEnabled = v[0] == '1'
+	} else if ctxt.GOOS != runtime.GOOS || ctxt.GOARCH != runtime.GOARCH {
+		ctxt.CgoEnabled = false
+	} else {
+		// Use built-in default cgo setting for GOOS/GOARCH.
+		// Note that ctxt.GOOS/GOARCH are derived from the preference list
+		// (1) environment, (2) go/env file, (3) runtime constants,
+		// while go/build.Default.GOOS/GOARCH are derived from the preference list
+		// (1) environment, (2) runtime constants.
+		// We know ctxt.GOOS/GOARCH == runtime.GOOS/GOARCH;
+		// no matter how that happened, go/build.Default will make the
+		// same decision (either the environment variables are set explicitly
+		// to match the runtime constants, or else they are unset, in which
+		// case go/build falls back to the runtime constants), so
+		// go/build.Default.GOOS/GOARCH == runtime.GOOS/GOARCH.
+		// So ctxt.CgoEnabled (== go/build.Default.CgoEnabled) is correct
+		// as is and can be left unmodified.
+		// Nothing to do here.
+	}
+
 	return ctxt
 }
 
@@ -70,9 +118,10 @@ var CmdEnv []EnvVar
 
 // Global build parameters (used during package load)
 var (
-	Goarch    = BuildContext.GOARCH
-	Goos      = BuildContext.GOOS
-	ExeSuffix string
+	Goarch = BuildContext.GOARCH
+	Goos   = BuildContext.GOOS
+
+	ExeSuffix = exeSuffix()
 
 	// ModulesEnabled specifies whether the go command is running
 	// in module-aware mode (as opposed to GOPATH mode).
@@ -85,40 +134,195 @@ var (
 	GoModInGOPATH string
 )
 
-func init() {
+func exeSuffix() string {
 	if Goos == "windows" {
-		ExeSuffix = ".exe"
+		return ".exe"
+	}
+	return ""
+}
+
+var envCache struct {
+	once sync.Once
+	m    map[string]string
+}
+
+// EnvFile returns the name of the Go environment configuration file.
+func EnvFile() (string, error) {
+	if file := os.Getenv("GOENV"); file != "" {
+		if file == "off" {
+			return "", fmt.Errorf("GOENV=off")
+		}
+		return file, nil
+	}
+	dir, err := os.UserConfigDir()
+	if err != nil {
+		return "", err
+	}
+	if dir == "" {
+		return "", fmt.Errorf("missing user-config dir")
+	}
+	return filepath.Join(dir, "go/env"), nil
+}
+
+func initEnvCache() {
+	envCache.m = make(map[string]string)
+	file, _ := EnvFile()
+	if file == "" {
+		return
+	}
+	data, err := ioutil.ReadFile(file)
+	if err != nil {
+		return
+	}
+
+	for len(data) > 0 {
+		// Get next line.
+		line := data
+		i := bytes.IndexByte(data, '\n')
+		if i >= 0 {
+			line, data = line[:i], data[i+1:]
+		} else {
+			data = nil
+		}
+
+		i = bytes.IndexByte(line, '=')
+		if i < 0 || line[0] < 'A' || 'Z' < line[0] {
+			// Line is missing = (or empty) or a comment or not a valid env name. Ignore.
+			// (This should not happen, since the file should be maintained almost
+			// exclusively by "go env -w", but better to silently ignore than to make
+			// the go command unusable just because somehow the env file has
+			// gotten corrupted.)
+			continue
+		}
+		key, val := line[:i], line[i+1:]
+		envCache.m[string(key)] = string(val)
+	}
+}
+
+// Getenv gets the value for the configuration key.
+// It consults the operating system environment
+// and then the go/env file.
+// If Getenv is called for a key that cannot be set
+// in the go/env file (for example GODEBUG), it panics.
+// This ensures that CanGetenv is accurate, so that
+// 'go env -w' stays in sync with what Getenv can retrieve.
+func Getenv(key string) string {
+	if !CanGetenv(key) {
+		switch key {
+		case "CGO_TEST_ALLOW", "CGO_TEST_DISALLOW", "CGO_test_ALLOW", "CGO_test_DISALLOW":
+			// used by internal/work/security_test.go; allow
+		default:
+			panic("internal error: invalid Getenv " + key)
+		}
+	}
+	val := os.Getenv(key)
+	if val != "" {
+		return val
 	}
+	envCache.once.Do(initEnvCache)
+	return envCache.m[key]
 }
 
+// CanGetenv reports whether key is a valid go/env configuration key.
+func CanGetenv(key string) bool {
+	return strings.Contains(knownEnv, "\t"+key+"\n")
+}
+
+var knownEnv = `
+	AR
+	CC
+	CGO_CFLAGS
+	CGO_CFLAGS_ALLOW
+	CGO_CFLAGS_DISALLOW
+	CGO_CPPFLAGS
+	CGO_CPPFLAGS_ALLOW
+	CGO_CPPFLAGS_DISALLOW
+	CGO_CXXFLAGS
+	CGO_CXXFLAGS_ALLOW
+	CGO_CXXFLAGS_DISALLOW
+	CGO_ENABLED
+	CGO_FFLAGS
+	CGO_FFLAGS_ALLOW
+	CGO_FFLAGS_DISALLOW
+	CGO_LDFLAGS
+	CGO_LDFLAGS_ALLOW
+	CGO_LDFLAGS_DISALLOW
+	CXX
+	FC
+	GCCGO
+	GO111MODULE
+	GO386
+	GOARCH
+	GOARM
+	GOBIN
+	GOCACHE
+	GOENV
+	GOEXE
+	GOFLAGS
+	GOGCCFLAGS
+	GOHOSTARCH
+	GOHOSTOS
+	GOMIPS
+	GOMIPS64
+	GONOVERIFY
+	GOOS
+	GOPATH
+	GOPPC64
+	GOPROXY
+	GOROOT
+	GOTMPDIR
+	GOTOOLDIR
+	GOWASM
+	GO_EXTLINK_ENABLED
+	PKG_CONFIG
+`
+
 var (
-	GOROOT       = findGOROOT()
-	GOBIN        = os.Getenv("GOBIN")
+	GOROOT       = BuildContext.GOROOT
+	GOBIN        = Getenv("GOBIN")
 	GOROOTbin    = filepath.Join(GOROOT, "bin")
 	GOROOTpkg    = filepath.Join(GOROOT, "pkg")
 	GOROOTsrc    = filepath.Join(GOROOT, "src")
 	GOROOT_FINAL = findGOROOT_FINAL()
 
 	// Used in envcmd.MkEnv and build ID computations.
-	GOARM    = fmt.Sprint(objabi.GOARM)
-	GO386    = objabi.GO386
-	GOMIPS   = objabi.GOMIPS
-	GOMIPS64 = objabi.GOMIPS64
-	GOPPC64  = fmt.Sprintf("%s%d", "power", objabi.GOPPC64)
-	GOWASM   = objabi.GOWASM
+	GOARM    = envOr("GOARM", fmt.Sprint(objabi.GOARM))
+	GO386    = envOr("GO386", objabi.GO386)
+	GOMIPS   = envOr("GOMIPS", objabi.GOMIPS)
+	GOMIPS64 = envOr("GOMIPS64", objabi.GOMIPS64)
+	GOPPC64  = envOr("GOPPC64", fmt.Sprintf("%s%d", "power", objabi.GOPPC64))
+	GOWASM   = envOr("GOWASM", fmt.Sprint(objabi.GOWASM))
 )
 
-// Update build context to use our computed GOROOT.
-func init() {
-	BuildContext.GOROOT = GOROOT
-	if runtime.Compiler != "gccgo" {
-		// Note that we must use runtime.GOOS and runtime.GOARCH here,
-		// as the tool directory does not move based on environment
-		// variables. This matches the initialization of ToolDir in
-		// go/build, except for using GOROOT rather than
-		// runtime.GOROOT.
-		build.ToolDir = filepath.Join(GOROOT, "pkg/tool/"+runtime.GOOS+"_"+runtime.GOARCH)
+// GetArchEnv returns the name and setting of the
+// GOARCH-specific architecture environment variable.
+// If the current architecture has no GOARCH-specific variable,
+// GetArchEnv returns empty key and value.
+func GetArchEnv() (key, val string) {
+	switch Goarch {
+	case "arm":
+		return "GOARM", GOARM
+	case "386":
+		return "GO386", GO386
+	case "mips", "mipsle":
+		return "GOMIPS", GOMIPS
+	case "mips64", "mips64le":
+		return "GOMIPS64", GOMIPS64
+	case "ppc64", "ppc64le":
+		return "GOPPC64", GOPPC64
+	case "wasm":
+		return "GOWASM", GOWASM
+	}
+	return "", ""
+}
+
+// envOr returns Getenv(key) if set, or else def.
+func envOr(key, def string) string {
+	val := Getenv(key)
+	if val == "" {
+		val = def
 	}
+	return val
 }
 
 // There is a copy of findGOROOT, isSameDir, and isGOROOT in
@@ -132,7 +336,7 @@ func init() {
 //
 // There is a copy of this code in x/tools/cmd/godoc/goroot.go.
 func findGOROOT() string {
-	if env := os.Getenv("GOROOT"); env != "" {
+	if env := Getenv("GOROOT"); env != "" {
 		return filepath.Clean(env)
 	}
 	def := filepath.Clean(runtime.GOROOT())
@@ -168,6 +372,8 @@ func findGOROOT() string {
 }
 
 func findGOROOT_FINAL() string {
+	// $GOROOT_FINAL is only for use during make.bash
+	// so it is not settable using go/env, so we use os.Getenv here.
 	def := GOROOT
 	if env := os.Getenv("GOROOT_FINAL"); env != "" {
 		def = filepath.Clean(env)