syscall: handle empty environment variable values properly on windows

Setenv("AN_ENV_VAR", "") deletes AN_ENV_VAR instead of setting it
to "" at this moment. Also Getenv("AN_ENV_VAR") returns "not found",
if AN_ENV_VAR is "". Change it, so they behave like unix.

Fixes #5610

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/10594043

src/pkg/syscall/env_windows.go

@@ -28,20 +28,13 @@ func Getenv(key string) (value string, found bool) {
 			n = 0
 		}
 	}
-	if n == 0 {
-		return "", false
-	}
 	return string(utf16.Decode(b[0:n])), true
 }
 
 func Setenv(key, value string) error {
-	var v *uint16
-	var err error
-	if len(value) > 0 {
-		v, err = UTF16PtrFromString(value)
-		if err != nil {
-			return err
-		}
+	v, err := UTF16PtrFromString(value)
+	if err != nil {
+		return err
 	}
 	keyp, err := UTF16PtrFromString(key)
 	if err != nil {

src/pkg/syscall/syscall_test.go

+// Copyright 2013 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 syscall_test
+
+import (
+	"syscall"
+	"testing"
+)
+
+func testSetGetenv(t *testing.T, key, value string) {
+	err := syscall.Setenv(key, value)
+	if err != nil {
+		t.Fatalf("Setenv failed to set %q: %v", value, err)
+	}
+	newvalue, found := syscall.Getenv(key)
+	if !found {
+		t.Fatalf("Getenv failed to find %v variable (want value %q)", key, value)
+	}
+	if newvalue != value {
+		t.Fatalf("Getenv(%v) = %q; want %q", key, newvalue, value)
+	}
+}
+
+func TestEnv(t *testing.T) {
+	testSetGetenv(t, "TESTENV", "AVALUE")
+	// make sure TESTENV gets set to "", not deleted
+	testSetGetenv(t, "TESTENV", "")
+}