os, syscall: add Unsetenv

Also address a TODO, making Clearenv pass through to cgo.

Based largely on Minux's earlier https://codereview.appspot.com/82040044

Fixes Issue 6423

LGTM=iant, alex.brainman, r, rsc
R=rsc, iant, r, alex.brainman
CC=golang-codereviews
https://codereview.appspot.com/148370043

2 files changed, 31 insertions, 0 deletions

diff --git a/src/os/env.go b/src/os/env.go
index db7fc72b8..d0494a476 100644
--- a/src/os/env.go
+++ b/src/os/env.go
@@ -91,6 +91,11 @@ func Setenv(key, value string) error {
 	return nil
 }
 
+// Unsetenv unsets a single environment variable.
+func Unsetenv(key string) error {
+	return syscall.Unsetenv(key)
+}
+
 // Clearenv deletes all environment variables.
 func Clearenv() {
 	syscall.Clearenv()
diff --git a/src/os/env_test.go b/src/os/env_test.go
index 991fa4d05..e61806751 100644
--- a/src/os/env_test.go
+++ b/src/os/env_test.go
@@ -7,6 +7,7 @@ package os_test
 import (
 	. "os"
 	"reflect"
+	"strings"
 	"testing"
 )
 
@@ -68,3 +69,28 @@ func TestConsistentEnviron(t *testing.T) {
 		}
 	}
 }
+
+func TestUnsetenv(t *testing.T) {
+	const testKey = "GO_TEST_UNSETENV"
+	set := func() bool {
+		prefix := testKey + "="
+		for _, key := range Environ() {
+			if strings.HasPrefix(key, prefix) {
+				return true
+			}
+		}
+		return false
+	}
+	if err := Setenv(testKey, "1"); err != nil {
+		t.Fatalf("Setenv: %v", err)
+	}
+	if !set() {
+		t.Error("Setenv didn't set TestUnsetenv")
+	}
+	if err := Unsetenv(testKey); err != nil {
+		t.Fatalf("Unsetenv: %v", err)
+	}
+	if set() {
+		t.Fatal("Unsetenv didn't clear TestUnsetenv")
+	}
+}