syscall: don't cache environment variables on Plan 9

Fixes #8849.

LGTM=bradfitz, aram
R=bradfitz, rsc, aram
CC=golang-codereviews
https://golang.org/cl/158970045

src/syscall/env_plan9.go

@@ -8,28 +8,9 @@ package syscall
 
 import (
 	"errors"
-	"sync"
 )
 
 var (
-	// envOnce guards copyenv, which populates env, envi and envs.
-	envOnce sync.Once
-
-	// envLock guards env, envi and envs.
-	envLock sync.RWMutex
-
-	// env maps from an environment variable to its value.
-	// TODO: remove this? golang.org/issue/8849
-	env = make(map[string]string)
-
-	// envi maps from an environment variable to its index in envs.
-	// TODO: remove this? golang.org/issue/8849
-	envi = make(map[string]int)
-
-	// envs contains elements of env in the form "key=value".
-	// empty strings mean deleted.
-	envs []string
-
 	errZeroLengthKey = errors.New("zero length key")
 	errShortWrite    = errors.New("i/o count too small")
 )
@@ -70,106 +51,58 @@ func writeenv(key, value string) error {
 	return nil
 }
 
-func copyenv() {
-	fd, err := Open("/env", O_RDONLY)
-	if err != nil {
-		return
-	}
-	defer Close(fd)
-	files, err := readdirnames(fd)
-	if err != nil {
-		return
-	}
-	envs = make([]string, len(files))
-	i := 0
-	for _, key := range files {
-		v, err := readenv(key)
-		if err != nil {
-			continue
-		}
-		env[key] = v
-		envs[i] = key + "=" + v
-		envi[key] = i
-		i++
-	}
-}
-
 func Getenv(key string) (value string, found bool) {
 	if len(key) == 0 {
 		return "", false
 	}
-
-	envLock.RLock()
-	defer envLock.RUnlock()
-
-	if v, ok := env[key]; ok {
-		return v, true
-	}
 	v, err := readenv(key)
 	if err != nil {
 		return "", false
 	}
-	env[key] = v
-	envs = append(envs, key+"="+v)
 	return v, true
 }
 
 func Setenv(key, value string) error {
 	if len(key) == 0 {
 		return errZeroLengthKey
 	}
-
-	envLock.Lock()
-	defer envLock.Unlock()
-
 	err := writeenv(key, value)
 	if err != nil {
 		return err
 	}
-	env[key] = value
-	envs = append(envs, key+"="+value)
-	envi[key] = len(envs) - 1
 	return nil
 }
 
 func Clearenv() {
-	envLock.Lock()
-	defer envLock.Unlock()
-
-	env = make(map[string]string)
-	envi = make(map[string]int)
-	envs = []string{}
 	RawSyscall(SYS_RFORK, RFCENVG, 0, 0)
 }
 
 func Unsetenv(key string) error {
 	if len(key) == 0 {
 		return errZeroLengthKey
 	}
-
-	envLock.Lock()
-	defer envLock.Unlock()
-
 	Remove("/env/" + key)
-
-	if i, ok := envi[key]; ok {
-		delete(env, key)
-		delete(envi, key)
-		envs[i] = ""
-	}
 	return nil
 }
 
 func Environ() []string {
-	envLock.RLock()
-	defer envLock.RUnlock()
+	fd, err := Open("/env", O_RDONLY)
+	if err != nil {
+		return nil
+	}
+	defer Close(fd)
+	files, err := readdirnames(fd)
+	if err != nil {
+		return nil
+	}
+	ret := make([]string, 0, len(files))
 
-	envOnce.Do(copyenv)
-	ret := make([]string, 0, len(envs))
-	for _, pair := range envs {
-		if pair != "" {
-			ret = append(ret, pair)
+	for _, key := range files {
+		v, err := readenv(key)
+		if err != nil {
+			continue
 		}
+		ret = append(ret, key+"="+v)
 	}
 	return ret
 }