syscall: use sync.OnceFunc for copyenv

Change-Id: I64f658c1962878685ba7736f19d58e10fbdcb94a
Reviewed-on: https://go-review.googlesource.com/c/go/+/651835
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

Author: tklauser

src/syscall/env_unix.go

@@ -14,9 +14,6 @@ import (
 )
 
 var (
-	// envOnce guards initialization by copyenv, which populates env.
-	envOnce sync.Once
-
 	// envLock guards env and envs.
 	envLock sync.RWMutex
 
@@ -31,7 +28,7 @@ var (
 
 func runtime_envs() []string // in package runtime
 
-func copyenv() {
+var copyenv = sync.OnceFunc(func() {
 	env = make(map[string]int)
 	for i, s := range envs {
 		for j := 0; j < len(s); j++ {
@@ -50,10 +47,10 @@ func copyenv() {
 			}
 		}
 	}
-}
+})
 
 func Unsetenv(key string) error {
-	envOnce.Do(copyenv)
+	copyenv()
 
 	envLock.Lock()
 	defer envLock.Unlock()
@@ -67,7 +64,7 @@ func Unsetenv(key string) error {
 }
 
 func Getenv(key string) (value string, found bool) {
-	envOnce.Do(copyenv)
+	copyenv()
 	if len(key) == 0 {
 		return "", false
 	}
@@ -89,7 +86,7 @@ func Getenv(key string) (value string, found bool) {
 }
 
 func Setenv(key, value string) error {
-	envOnce.Do(copyenv)
+	copyenv()
 	if len(key) == 0 {
 		return EINVAL
 	}
@@ -124,7 +121,7 @@ func Setenv(key, value string) error {
 }
 
 func Clearenv() {
-	envOnce.Do(copyenv)
+	copyenv()
 
 	envLock.Lock()
 	defer envLock.Unlock()
@@ -137,7 +134,7 @@ func Clearenv() {
 }
 
 func Environ() []string {
-	envOnce.Do(copyenv)
+	copyenv()
 	envLock.RLock()
 	defer envLock.RUnlock()
 	a := make([]string, 0, len(envs))