os: Rename error to fit IsExist

Fixes #3828.

R=golang-dev, iant, rsc
CC=golang-dev
https://golang.org/cl/6420056

src/pkg/os/error_windows.go

@@ -10,6 +10,9 @@ func isExist(err error) bool {
 	if pe, ok := err.(*PathError); ok {
 		err = pe.Err
 	}
+	if pe, ok := err.(*LinkError); ok {
+		err = pe.Err
+	}
 	return err == syscall.ERROR_ALREADY_EXISTS ||
 		err == syscall.ERROR_FILE_EXISTS || err == ErrExist
 }

src/pkg/os/error_windows_test.go

+// Copyright 2012 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 os_test
+
+import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestErrIsExistAfterRename(t *testing.T) {
+	dir, err := ioutil.TempDir("", "go-build")
+	if err != nil {
+		t.Fatalf("Create temp directory: %v", err)
+	}
+	defer os.RemoveAll(dir)
+
+	src := filepath.Join(dir, "src")
+	dest := filepath.Join(dir, "dest")
+
+	f, err := os.Create(src)
+	if err != nil {
+		t.Fatalf("Create file %v: %v", src, err)
+	}
+	f.Close()
+	err = os.Rename(src, dest)
+	if err != nil {
+		t.Fatalf("Rename %v to %v: %v", src, dest, err)
+	}
+
+	f, err = os.Create(src)
+	if err != nil {
+		t.Fatalf("Create file %v: %v", src, err)
+	}
+	f.Close()
+	err = os.Rename(src, dest)
+	if err == nil {
+		t.Fatal("Rename should have failed")
+	}
+	if s := checkErrorPredicate("os.IsExist", os.IsExist, err); s != "" {
+		t.Fatal(s)
+		return
+	}
+}