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)