fileutil: Sync on HFS/OSX needs to be handled differently.

A call file.Sync on OSX doesn't guarantee actual persistence on
physical drive media as the data can be cached in physical drive's
buffers. Hence calls to file.Sync need to be replaced with
fcntl(F_FULLFSYNC).

etcdctl/ctlv3/command/snapshot_command.go

@@ -28,6 +28,7 @@ import (
 	"github.com/coreos/etcd/etcdserver"
 	"github.com/coreos/etcd/etcdserver/etcdserverpb"
 	"github.com/coreos/etcd/etcdserver/membership"
+	"github.com/coreos/etcd/pkg/fileutil"
 	"github.com/coreos/etcd/pkg/types"
 	"github.com/coreos/etcd/raft"
 	"github.com/coreos/etcd/raft/raftpb"
@@ -121,7 +122,7 @@ func snapshotSaveCommandFunc(cmd *cobra.Command, args []string) {
 		ExitWithError(ExitInterrupted, rerr)
 	}
 
-	f.Sync()
+	fileutil.Fsync(f)
 
 	if rerr := os.Rename(partpath, path); rerr != nil {
 		exiterr := fmt.Errorf("could not rename %s to %s (%v)", partpath, path, rerr)

pkg/fileutil/sync.go

@@ -12,15 +12,18 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// +build !linux
+// +build !linux,!darwin
 
 package fileutil
 
 import "os"
 
-// Fdatasync is similar to fsync(), but does not flush modified metadata
+// Fsync is a wrapper around file.Sync(). Special handling is needed on darwin platform.
-// unless that metadata is needed in order to allow a subsequent data retrieval
+func Fsync(f *os.File) error {
-// to be correctly handled.
+	return f.Sync()
+}
+
+// Fdatasync is a wrapper around file.Sync(). Special handling is needed on linux platform.
 func Fdatasync(f *os.File) error {
 	return f.Sync()
 }

pkg/fileutil/sync_darwin.go

@@ -0,0 +1,40 @@
+// Copyright 2016 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.
+
+// +build darwin
+
+package fileutil
+
+import (
+	"os"
+	"syscall"
+)
+
+// Fsync on HFS/OSX flushes the data on to the physical drive but the drive
+// may not write it to the persistent media for quite sometime and it may be
+// written in out-of-order sequence. Using F_FULLFSYNC ensures that the
+// physical drive's buffer will also get flushed to the media.
+func Fsync(f *os.File) error {
+	_, _, errno := syscall.Syscall(syscall.SYS_FCNTL, f.Fd(), uintptr(syscall.F_FULLFSYNC), uintptr(0))
+	if errno == 0 {
+		return nil
+	}
+	return errno
+}
+
+// Fdatasync on darwin platform invokes fcntl(F_FULLFSYNC) for actual persistence
+// on physical drive media.
+func Fdatasync(f *os.File) error {
+	return Fsync(f)
+}

pkg/fileutil/sync_linux.go

@@ -21,6 +21,11 @@ import (
 	"syscall"
 )
 
+// Fsync is a wrapper around file.Sync(). Special handling is needed on darwin platform.
+func Fsync(f *os.File) error {
+	return f.Sync()
+}
+
 // Fdatasync is similar to fsync(), but does not flush modified metadata
 // unless that metadata is needed in order to allow a subsequent data retrieval
 // to be correctly handled.

pkg/ioutil/util.go

@@ -17,6 +17,8 @@ package ioutil
 import (
 	"io"
 	"os"
+
+	"github.com/coreos/etcd/pkg/fileutil"
 )
 
 // WriteAndSyncFile behaves just like ioutil.WriteFile in the standard library,
@@ -32,7 +34,7 @@ func WriteAndSyncFile(filename string, data []byte, perm os.FileMode) error {
 		err = io.ErrShortWrite
 	}
 	if err == nil {
-		err = f.Sync()
+		err = fileutil.Fsync(f)
 	}
 	if err1 := f.Close(); err == nil {
 		err = err1

snap/db.go

@@ -34,7 +34,7 @@ func (s *Snapshotter) SaveDBFrom(r io.Reader, id uint64) error {
 	var n int64
 	n, err = io.Copy(f, r)
 	if err == nil {
-		err = f.Sync()
+		err = fileutil.Fsync(f)
 	}
 	f.Close()
 	if err != nil {

wal/repair.go

@@ -78,7 +78,7 @@ func Repair(dirpath string) bool {
 				plog.Errorf("could not repair %v, failed to truncate file", f.Name())
 				return false
 			}
-			if err = f.Sync(); err != nil {
+			if err = fileutil.Fsync(f); err != nil {
 				plog.Errorf("could not repair %v, failed to sync file", f.Name())
 				return false
 			}