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// -*- Mode: Go; indent-tabs-mode: t -*-
/*
* Copyright (C) 2017 Canonical Ltd
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 3 as
* published by the Free Software Foundation.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
*/
package osutil
import (
"errors"
"os"
"syscall"
)
// FileLock describes a file system lock
type FileLock struct {
file *os.File
}
var ErrAlreadyLocked = errors.New("cannot acquire lock, already locked")
// OpenExistingLockForReading opens an existing lock file given by "path".
// The lock is opened in read-only mode.
func OpenExistingLockForReading(path string) (*FileLock, error) {
flag := syscall.O_RDONLY | syscall.O_NOFOLLOW | syscall.O_CLOEXEC
file, err := os.OpenFile(path, flag, 0)
if err != nil {
return nil, err
}
l := &FileLock{file: file}
return l, nil
}
// NewFileLockWithMode creates and opens the lock file given by "path" with the given mode.
func NewFileLockWithMode(path string, mode os.FileMode) (*FileLock, error) {
flag := syscall.O_RDWR | syscall.O_CREAT | syscall.O_NOFOLLOW | syscall.O_CLOEXEC
file, err := os.OpenFile(path, flag, mode)
if err != nil {
return nil, err
}
l := &FileLock{file: file}
return l, nil
}
// NewFileLockWithFile wraps an open file with a file lock which can be used for
// locking.
func NewFileLockWithFile(f *os.File) *FileLock {
return &FileLock{file: f}
}
// NewFileLock creates and opens the lock file given by "path" with mode 0600.
func NewFileLock(path string) (*FileLock, error) {
return NewFileLockWithMode(path, 0600)
}
// Path returns the path of the lock file.
func (l *FileLock) Path() string {
return l.file.Name()
}
// File returns the underlying file.
func (l *FileLock) File() *os.File {
return l.file
}
// Close closes the lock, unlocking it automatically if needed.
func (l *FileLock) Close() error {
return l.file.Close()
}
// Lock acquires an exclusive lock and blocks until the lock is free.
//
// Only one process can acquire an exclusive lock at a given time, preventing
// shared or exclusive locks from being acquired.
func (l *FileLock) Lock() error {
return syscall.Flock(int(l.file.Fd()), syscall.LOCK_EX)
}
// Lock acquires an shared lock and blocks until the lock is free.
//
// Multiple processes can acquire a shared lock at the same time, unless an
// exclusive lock is held.
func (l *FileLock) ReadLock() error {
return syscall.Flock(int(l.file.Fd()), syscall.LOCK_SH)
}
// TryLock acquires an exclusive lock and errors if the lock cannot be acquired.
func (l *FileLock) TryLock() error {
err := syscall.Flock(int(l.file.Fd()), syscall.LOCK_EX|syscall.LOCK_NB)
if err == syscall.EWOULDBLOCK {
return ErrAlreadyLocked
}
return err
}
// Unlock releases an acquired lock.
func (l *FileLock) Unlock() error {
return syscall.Flock(int(l.file.Fd()), syscall.LOCK_UN)
}
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