etcdmain: fix incomplete proxy config file

etcd might generate incomplete proxy config file after a power failure.
It is because we use ioutil.WriteFile. And iotuile.WriteFile does
not call Sync before closing the file.

etcdmain/etcd.go

@@ -40,6 +40,7 @@ import (
 	"github.com/coreos/etcd/etcdserver/etcdserverpb"
 	"github.com/coreos/etcd/pkg/cors"
 	"github.com/coreos/etcd/pkg/fileutil"
+	pkgioutil "github.com/coreos/etcd/pkg/ioutil"
 	"github.com/coreos/etcd/pkg/osutil"
 	runtimeutil "github.com/coreos/etcd/pkg/runtime"
 	"github.com/coreos/etcd/pkg/transport"
@@ -409,7 +410,7 @@ func startProxy(cfg *config) error {
 			return clientURLs
 		}
 
-		err = ioutil.WriteFile(clusterfile+".bak", b, 0600)
+		err = pkgioutil.WriteAndSyncFile(clusterfile+".bak", b, 0600)
 		if err != nil {
 			plog.Warningf("proxy: error on writing urls %s", err)
 			return clientURLs

pkg/ioutil/util.go

@@ -0,0 +1,41 @@
+// Copyright 2015 CoreOS, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ioutil
+
+import (
+	"io"
+	"os"
+)
+
+// WriteAndSyncFile behaviors just like ioutil.WriteFile in standard library
+// but calls Sync before closing the file. WriteAndSyncFile guarantees the data
+// is synced if there is no error returned.
+func WriteAndSyncFile(filename string, data []byte, perm os.FileMode) error {
+	f, err := os.OpenFile(filename, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
+	if err != nil {
+		return err
+	}
+	n, err := f.Write(data)
+	if err == nil && n < len(data) {
+		err = io.ErrShortWrite
+	}
+	if err == nil {
+		err = f.Sync()
+	}
+	if err1 := f.Close(); err == nil {
+		err = err1
+	}
+	return err
+}