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ed75533cb1
* optionally protect against multiple starts with flock Fixes #1823 * use traditional .lock extension * move config key to top level
282 lines
6.9 KiB
Go
282 lines
6.9 KiB
Go
// Copyright 2019 Tim Heckman. All rights reserved. Use of this source code is
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// governed by the BSD 3-Clause license that can be found in the LICENSE file.
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// Copyright 2018 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// This code implements the filelock API using POSIX 'fcntl' locks, which attach
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// to an (inode, process) pair rather than a file descriptor. To avoid unlocking
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// files prematurely when the same file is opened through different descriptors,
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// we allow only one read-lock at a time.
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//
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// This code is adapted from the Go package:
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// cmd/go/internal/lockedfile/internal/filelock
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//+build aix
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package flock
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import (
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"errors"
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"io"
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"os"
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"sync"
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"syscall"
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"golang.org/x/sys/unix"
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)
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type lockType int16
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const (
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readLock lockType = unix.F_RDLCK
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writeLock lockType = unix.F_WRLCK
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)
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type cmdType int
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const (
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tryLock cmdType = unix.F_SETLK
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waitLock cmdType = unix.F_SETLKW
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)
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type inode = uint64
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type inodeLock struct {
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owner *Flock
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queue []<-chan *Flock
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}
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var (
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mu sync.Mutex
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inodes = map[*Flock]inode{}
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locks = map[inode]inodeLock{}
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)
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// Lock is a blocking call to try and take an exclusive file lock. It will wait
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// until it is able to obtain the exclusive file lock. It's recommended that
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// TryLock() be used over this function. This function may block the ability to
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// query the current Locked() or RLocked() status due to a RW-mutex lock.
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//
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// If we are already exclusive-locked, this function short-circuits and returns
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// immediately assuming it can take the mutex lock.
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//
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// If the *Flock has a shared lock (RLock), this may transparently replace the
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// shared lock with an exclusive lock on some UNIX-like operating systems. Be
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// careful when using exclusive locks in conjunction with shared locks
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// (RLock()), because calling Unlock() may accidentally release the exclusive
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// lock that was once a shared lock.
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func (f *Flock) Lock() error {
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return f.lock(&f.l, writeLock)
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}
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// RLock is a blocking call to try and take a shared file lock. It will wait
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// until it is able to obtain the shared file lock. It's recommended that
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// TryRLock() be used over this function. This function may block the ability to
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// query the current Locked() or RLocked() status due to a RW-mutex lock.
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//
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// If we are already shared-locked, this function short-circuits and returns
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// immediately assuming it can take the mutex lock.
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func (f *Flock) RLock() error {
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return f.lock(&f.r, readLock)
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}
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func (f *Flock) lock(locked *bool, flag lockType) error {
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f.m.Lock()
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defer f.m.Unlock()
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if *locked {
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return nil
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}
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if f.fh == nil {
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if err := f.setFh(); err != nil {
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return err
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}
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defer f.ensureFhState()
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}
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if _, err := f.doLock(waitLock, flag, true); err != nil {
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return err
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}
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*locked = true
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return nil
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}
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func (f *Flock) doLock(cmd cmdType, lt lockType, blocking bool) (bool, error) {
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// POSIX locks apply per inode and process, and the lock for an inode is
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// released when *any* descriptor for that inode is closed. So we need to
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// synchronize access to each inode internally, and must serialize lock and
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// unlock calls that refer to the same inode through different descriptors.
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fi, err := f.fh.Stat()
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if err != nil {
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return false, err
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}
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ino := inode(fi.Sys().(*syscall.Stat_t).Ino)
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mu.Lock()
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if i, dup := inodes[f]; dup && i != ino {
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mu.Unlock()
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return false, &os.PathError{
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Path: f.Path(),
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Err: errors.New("inode for file changed since last Lock or RLock"),
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}
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}
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inodes[f] = ino
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var wait chan *Flock
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l := locks[ino]
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if l.owner == f {
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// This file already owns the lock, but the call may change its lock type.
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} else if l.owner == nil {
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// No owner: it's ours now.
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l.owner = f
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} else if !blocking {
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// Already owned: cannot take the lock.
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mu.Unlock()
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return false, nil
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} else {
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// Already owned: add a channel to wait on.
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wait = make(chan *Flock)
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l.queue = append(l.queue, wait)
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}
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locks[ino] = l
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mu.Unlock()
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if wait != nil {
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wait <- f
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}
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err = setlkw(f.fh.Fd(), cmd, lt)
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if err != nil {
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f.doUnlock()
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if cmd == tryLock && err == unix.EACCES {
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return false, nil
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}
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return false, err
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}
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return true, nil
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}
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func (f *Flock) Unlock() error {
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f.m.Lock()
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defer f.m.Unlock()
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// if we aren't locked or if the lockfile instance is nil
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// just return a nil error because we are unlocked
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if (!f.l && !f.r) || f.fh == nil {
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return nil
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}
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if err := f.doUnlock(); err != nil {
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return err
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}
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f.fh.Close()
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f.l = false
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f.r = false
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f.fh = nil
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return nil
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}
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func (f *Flock) doUnlock() (err error) {
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var owner *Flock
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mu.Lock()
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ino, ok := inodes[f]
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if ok {
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owner = locks[ino].owner
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}
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mu.Unlock()
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if owner == f {
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err = setlkw(f.fh.Fd(), waitLock, unix.F_UNLCK)
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}
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mu.Lock()
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l := locks[ino]
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if len(l.queue) == 0 {
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// No waiters: remove the map entry.
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delete(locks, ino)
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} else {
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// The first waiter is sending us their file now.
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// Receive it and update the queue.
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l.owner = <-l.queue[0]
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l.queue = l.queue[1:]
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locks[ino] = l
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}
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delete(inodes, f)
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mu.Unlock()
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return err
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}
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// TryLock is the preferred function for taking an exclusive file lock. This
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// function takes an RW-mutex lock before it tries to lock the file, so there is
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// the possibility that this function may block for a short time if another
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// goroutine is trying to take any action.
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//
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// The actual file lock is non-blocking. If we are unable to get the exclusive
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// file lock, the function will return false instead of waiting for the lock. If
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// we get the lock, we also set the *Flock instance as being exclusive-locked.
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func (f *Flock) TryLock() (bool, error) {
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return f.try(&f.l, writeLock)
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}
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// TryRLock is the preferred function for taking a shared file lock. This
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// function takes an RW-mutex lock before it tries to lock the file, so there is
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// the possibility that this function may block for a short time if another
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// goroutine is trying to take any action.
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//
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// The actual file lock is non-blocking. If we are unable to get the shared file
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// lock, the function will return false instead of waiting for the lock. If we
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// get the lock, we also set the *Flock instance as being share-locked.
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func (f *Flock) TryRLock() (bool, error) {
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return f.try(&f.r, readLock)
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}
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func (f *Flock) try(locked *bool, flag lockType) (bool, error) {
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f.m.Lock()
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defer f.m.Unlock()
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if *locked {
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return true, nil
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}
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if f.fh == nil {
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if err := f.setFh(); err != nil {
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return false, err
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}
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defer f.ensureFhState()
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}
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haslock, err := f.doLock(tryLock, flag, false)
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if err != nil {
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return false, err
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}
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*locked = haslock
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return haslock, nil
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}
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// setlkw calls FcntlFlock with cmd for the entire file indicated by fd.
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func setlkw(fd uintptr, cmd cmdType, lt lockType) error {
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for {
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err := unix.FcntlFlock(fd, int(cmd), &unix.Flock_t{
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Type: int16(lt),
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Whence: io.SeekStart,
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Start: 0,
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Len: 0, // All bytes.
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})
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if err != unix.EINTR {
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return err
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}
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}
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}
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