lock_linux.go

// Copyright 2016 The etcd Authors
//
// 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 linux

package fileutil

import (
	"io"
	"os"
	"syscall"
)

// This used to call syscall.Flock() but that call fails with EBADF on NFS.
// An alternative is lockf() which works on NFS but that call lets a process lock
// the same file twice. Instead, use Linux's non-standard open file descriptor
// locks which will block if the process already holds the file lock.
//
// constants from /usr/include/bits/fcntl-linux.h
const (
	F_OFD_GETLK  = 37
	F_OFD_SETLK  = 37
	F_OFD_SETLKW = 38
)

var (
	wrlck = syscall.Flock_t{
		Type:   syscall.F_WRLCK,
		Whence: int16(io.SeekStart),
		Start:  0,
		Len:    0,
	}

	linuxTryLockFile = flockTryLockFile
	linuxLockFile    = flockLockFile
)

func init() {
	// use open file descriptor locks if the system supports it
	getlk := syscall.Flock_t{Type: syscall.F_RDLCK}
	if err := syscall.FcntlFlock(0, F_OFD_GETLK, &getlk); err == nil {
		linuxTryLockFile = ofdTryLockFile
		linuxLockFile = ofdLockFile
	}
}

func TryLockFile(path string, flag int, perm os.FileMode) (*LockedFile, error) {
	return linuxTryLockFile(path, flag, perm)
}

func ofdTryLockFile(path string, flag int, perm os.FileMode) (*LockedFile, error) {
	f, err := os.OpenFile(path, flag, perm)
	if err != nil {
		return nil, err
	}

	flock := wrlck
	if err = syscall.FcntlFlock(f.Fd(), F_OFD_SETLK, &flock); err != nil {
		f.Close()
		if err == syscall.EWOULDBLOCK {
			err = ErrLocked
		}
		return nil, err
	}
	return &LockedFile{f}, nil
}

func LockFile(path string, flag int, perm os.FileMode) (*LockedFile, error) {
	return linuxLockFile(path, flag, perm)
}

func ofdLockFile(path string, flag int, perm os.FileMode) (*LockedFile, error) {
	f, err := os.OpenFile(path, flag, perm)
	if err != nil {
		return nil, err
	}

	flock := wrlck
	err = syscall.FcntlFlock(f.Fd(), F_OFD_SETLKW, &flock)

	if err != nil {
		f.Close()
		return nil, err
	}
	return &LockedFile{f}, err
}