runtime: do not share underlying envs/argv array

Removes a potential data race between os.Setenv and runtime.GOROOT,
along with a bug where os.Setenv would only sometimes change the
value of runtime.GOROOT.

Change-Id: I7d2a905115c667ea6e73f349f3784a1d3e8f810d
Reviewed-on: https://go-review.googlesource.com/6611Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/env_test.go

+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runtime_test
+
+import (
+	"runtime"
+	"syscall"
+	"testing"
+)
+
+func TestFixedGOROOT(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		t.Skipf("skipping plan9, it is inconsistent by allowing GOROOT to be updated by Setenv")
+	}
+
+	envs := runtime.Envs()
+	oldenvs := append([]string{}, envs...)
+	defer runtime.SetEnvs(oldenvs)
+
+	// attempt to reuse existing envs backing array.
+	want := runtime.GOROOT()
+	runtime.SetEnvs(append(envs[:0], "GOROOT="+want))
+
+	if got := runtime.GOROOT(); got != want {
+		t.Errorf(`initial runtime.GOROOT()=%q, want %q`, got, want)
+	}
+	if err := syscall.Setenv("GOROOT", "/os"); err != nil {
+		t.Fatal(err)
+	}
+	if got := runtime.GOROOT(); got != want {
+		t.Errorf(`after setenv runtime.GOROOT()=%q, want %q`, got, want)
+	}
+	if err := syscall.Unsetenv("GOROOT"); err != nil {
+		t.Fatal(err)
+	}
+	if got := runtime.GOROOT(); got != want {
+		t.Errorf(`after unsetenv runtime.GOROOT()=%q, want %q`, got, want)
+	}
+}

src/runtime/export_test.go

@@ -122,3 +122,6 @@ var Open = open
 var Close = close
 var Read = read
 var Write = write
+
+func Envs() []string     { return envs }
+func SetEnvs(e []string) { envs = e }

src/runtime/runtime.go

@@ -47,7 +47,7 @@ var envs []string
 var argslice []string
 
 //go:linkname syscall_runtime_envs syscall.runtime_envs
-func syscall_runtime_envs() []string { return envs }
+func syscall_runtime_envs() []string { return append([]string{}, envs...) }
 
 //go:linkname os_runtime_args os.runtime_args
-func os_runtime_args() []string { return argslice }
+func os_runtime_args() []string { return append([]string{}, argslice...) }