go.sys/unix: implement the environment functions by wrapping syscall

The environment is global state that is owned by the standard syscall package.
With this change, go test passes on darwin in the unix directory.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/129900043

unix/env_unix.go

@@ -8,112 +8,20 @@
 
 package unix
 
-import "sync"
-
-var (
-	// envOnce guards initialization by copyenv, which populates env.
-	envOnce sync.Once
-
-	// envLock guards env and envs.
-	envLock sync.RWMutex
-
-	// env maps from an environment variable to its first occurrence in envs.
-	env map[string]int
-
-	// envs is provided by the runtime. elements are expected to be
-	// of the form "key=value".
-	envs []string
-)
-
-// setenv_c is provided by the runtime, but is a no-op if cgo isn't
-// loaded.
-func setenv_c(k, v string)
-
-func copyenv() {
-	env = make(map[string]int)
-	for i, s := range envs {
-		for j := 0; j < len(s); j++ {
-			if s[j] == '=' {
-				key := s[:j]
-				if _, ok := env[key]; !ok {
-					env[key] = i
-				}
-				break
-			}
-		}
-	}
-}
+import "syscall"
 
 func Getenv(key string) (value string, found bool) {
-	envOnce.Do(copyenv)
-	if len(key) == 0 {
-		return "", false
-	}
-
-	envLock.RLock()
-	defer envLock.RUnlock()
-
-	i, ok := env[key]
-	if !ok {
-		return "", false
-	}
-	s := envs[i]
-	for i := 0; i < len(s); i++ {
-		if s[i] == '=' {
-			return s[i+1:], true
-		}
-	}
-	return "", false
+	return syscall.Getenv(key)
 }
 
 func Setenv(key, value string) error {
-	envOnce.Do(copyenv)
-	if len(key) == 0 {
-		return EINVAL
-	}
-	for i := 0; i < len(key); i++ {
-		if key[i] == '=' || key[i] == 0 {
-			return EINVAL
-		}
-	}
-	for i := 0; i < len(value); i++ {
-		if value[i] == 0 {
-			return EINVAL
-		}
-	}
-
-	envLock.Lock()
-	defer envLock.Unlock()
-
-	i, ok := env[key]
-	kv := key + "=" + value
-	if ok {
-		envs[i] = kv
-	} else {
-		i = len(envs)
-		envs = append(envs, kv)
-	}
-	env[key] = i
-	setenv_c(key, value)
-	return nil
+	return syscall.Setenv(key, value)
 }
 
 func Clearenv() {
-	envOnce.Do(copyenv) // prevent copyenv in Getenv/Setenv
-
-	envLock.Lock()
-	defer envLock.Unlock()
-
-	env = make(map[string]int)
-	envs = []string{}
-	// TODO(bradfitz): pass through to C
+	syscall.Clearenv()
 }
 
 func Environ() []string {
-	envOnce.Do(copyenv)
-	envLock.RLock()
-	defer envLock.RUnlock()
-	a := make([]string, len(envs))
-	copy(a, envs)
-	return a
+	return syscall.Environ()
 }