cmd/go: don't clobber `go env GOGCCFLAGS`

When CC is set in the environment, the mkEnv function sets its version
of CC to the first word $CC and sets GOGCCFLAGS to the remainder. That
worked since Go 1 but was broken accidentally by
https://golang.org/cl/6409, which changed the code such that `go env`
calls mkEnv twice. The second call to mkEnv would clobber GOGCCFLAGS
based on the value of CC set by the first call. Go back to the old
handling by only calling mkEnv once.

Fixes #15457.

Change-Id: I000a1ebcc48684667e48f2b9b24605867b9e06cd
Reviewed-on: https://go-review.googlesource.com/33293Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/bug.go

@@ -39,7 +39,7 @@ func runBug(cmd *Command, args []string) {
 	fmt.Fprint(&buf, "#### System details\n\n")
 	fmt.Fprintln(&buf, "```")
 	fmt.Fprintf(&buf, "go version %s %s/%s\n", runtime.Version(), runtime.GOOS, runtime.GOARCH)
-	env := mkEnv()
+	env := newEnv
 	env = append(env, extraEnvVars()...)
 	for _, e := range env {
 		fmt.Fprintf(&buf, "%s=\"%s\"\n", e.name, e.value)

src/cmd/go/env.go

@@ -104,7 +104,7 @@ func extraEnvVars() []envVar {
 }
 
 func runEnv(cmd *Command, args []string) {
-	env := mkEnv()
+	env := newEnv
 	env = append(env, extraEnvVars()...)
 	if len(args) > 0 {
 		for _, name := range args {

src/cmd/go/go_test.go

@@ -3586,6 +3586,12 @@ func TestGoEnv(t *testing.T) {
 	tg.setenv("CGO_CFLAGS", "-foobar")
 	tg.run("env", "CGO_CFLAGS")
 	tg.grepStdout("^-foobar$", "CGO_CFLAGS not honored")
+
+	tg.setenv("CC", "gcc -fmust -fgo -ffaster")
+	tg.run("env", "CC")
+	tg.grepStdout("gcc", "CC not found")
+	tg.run("env", "GOGCCFLAGS")
+	tg.grepStdout("-ffaster", "CC arguments not found")
 }
 
 const (

src/cmd/go/main.go

@@ -115,6 +115,7 @@ func setExitStatus(n int) {
 }
 
 var origEnv []string
+var newEnv []envVar
 
 func main() {
 	_ = go11tag
@@ -164,7 +165,8 @@ func main() {
 	// but in practice there might be skew
 	// This makes sure we all agree.
 	origEnv = os.Environ()
-	for _, env := range mkEnv() {
+	newEnv = mkEnv()
+	for _, env := range newEnv {
 		if os.Getenv(env.name) != env.value {
 			os.Setenv(env.name, env.value)
 		}