lock_windows.go

//  Copyright 2019 Google Inc. All Rights Reserved.
//
//  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 main

import (
	"errors"
	"os"
	"syscall"
	"unsafe"

	"golang.org/x/sys/windows"
)

var (
	kernel32         = windows.NewLazySystemDLL("kernel32.dll")
	procLockFileEx   = kernel32.NewProc("LockFileEx")
	procUnlockFileEx = kernel32.NewProc("UnlockFileEx")
)

const (
	// https://docs.microsoft.com/en-us/windows/desktop/api/fileapi/nf-fileapi-lockfileex
	LOCKFILE_EXCLUSIVE_LOCK   = 2
	LOCKFILE_FAIL_IMMEDIATELY = 1
)

func lockFileEx(hFile uintptr, dwFlags, nNumberOfBytesToLockLow, nNumberOfBytesToLockHigh uint32, lpOverlapped *syscall.Overlapped) (err error) {
	ret, _, _ := procLockFileEx.Call(
		hFile,
		uintptr(dwFlags),
		0,
		uintptr(nNumberOfBytesToLockLow),
		uintptr(nNumberOfBytesToLockHigh),
		uintptr(unsafe.Pointer(lpOverlapped)),
	)
	// If the function succeeds, the return value is nonzero.
	if ret == 0 {
		return errors.New("LockFileEx unable to obtain lock")
	}
	return nil
}

func unlockFileEx(hFile uintptr, nNumberOfBytesToLockLow, nNumberOfBytesToLockHigh uint32, lpOverlapped *syscall.Overlapped) (err error) {
	ret, _, _ := procUnlockFileEx.Call(
		hFile,
		0,
		uintptr(nNumberOfBytesToLockLow),
		uintptr(nNumberOfBytesToLockHigh),
		uintptr(unsafe.Pointer(lpOverlapped)),
	)
	// If the function succeeds, the return value is nonzero.
	if ret == 0 {
		return errors.New("UnlockFileEx unable to unlock")
	}
	return nil
}

func lock(f *os.File) error {
	if err := lockFileEx(f.Fd(), LOCKFILE_EXCLUSIVE_LOCK, 1, 0, &syscall.Overlapped{}); err != nil {
		return err
	}

	deferredFuncs = append(deferredFuncs, func() { unlockFileEx(f.Fd(), 1, 0, &syscall.Overlapped{}); f.Close(); os.Remove(lockFile) })
	return nil
}