optionally protect against multiple starts with flock (#1873)

* optionally protect against multiple starts with flock

Fixes #1823

* use traditional .lock extension

* move config key to top level

default.yaml

@@ -746,6 +746,12 @@ debug:
     # set to `null`, "", leave blank, or omit to disable
     # pprof-listener: "localhost:6060"
 
+# lock file preventing multiple instances of Ergo from accidentally being
+# started at once. comment out or set to the empty string ("") to disable.
+# this path is relative to the working directory; you may want to use an
+# absolute path instead:
+lock-file: "ircd.lock"
+
 # datastore configuration
 datastore:
     # path to the datastore

go.mod

@@ -25,6 +25,8 @@ require (
 	gopkg.in/yaml.v2 v2.4.0
 )
 
+require github.com/gofrs/flock v0.8.1
+
 require (
 	github.com/tidwall/btree v0.6.1 // indirect
 	github.com/tidwall/gjson v1.10.2 // indirect

go.sum

@@ -21,6 +21,8 @@ github.com/go-sql-driver/mysql v1.6.0 h1:BCTh4TKNUYmOmMUcQ3IipzF5prigylS7XXjEkfC
 github.com/go-sql-driver/mysql v1.6.0/go.mod h1:DCzpHaOWr8IXmIStZouvnhqoel9Qv2LBy8hT2VhHyBg=
 github.com/go-test/deep v1.0.6 h1:UHSEyLZUwX9Qoi99vVwvewiMC8mM2bf7XEM2nqvzEn8=
 github.com/go-test/deep v1.0.6/go.mod h1:QV8Hv/iy04NyLBxAdO9njL0iVPN1S4d/A3NVv1V36o8=
+github.com/gofrs/flock v0.8.1 h1:+gYjHKf32LDeiEEFhQaotPbLuUXjY5ZqxKgXy7n59aw=
+github.com/gofrs/flock v0.8.1/go.mod h1:F1TvTiK9OcQqauNUHlbJvyl9Qa1QvF/gOUDKA14jxHU=
 github.com/golang-jwt/jwt v3.2.2+incompatible h1:IfV12K8xAKAnZqdXVzCZ+TOjboZ2keLg81eXfW3O+oY=
 github.com/golang-jwt/jwt v3.2.2+incompatible/go.mod h1:8pz2t5EyA70fFQQSrl6XZXzqecmYZeUEB8OUGHkxJ+I=
 github.com/golang/protobuf v1.2.0/go.mod h1:6lQm79b+lXiMfvg/cZm0SGofjICqVBUtrP5yJMmIC1U=

irc/config.go

@@ -616,6 +616,8 @@ type Config struct {
 
 	languageManager *languages.Manager
 
+	LockFile string `yaml:"lock-file"`
+
 	Datastore struct {
 		Path        string
 		AutoUpgrade bool

irc/flock/flock.go

@@ -0,0 +1,24 @@
+//go:build !plan9
+
+package flock
+
+import (
+	"errors"
+
+	"github.com/gofrs/flock"
+)
+
+var (
+	CouldntAcquire = errors.New("Couldn't acquire flock (is another Ergo running?)")
+)
+
+func TryAcquireFlock(path string) (fl Flocker, err error) {
+	f := flock.New(path)
+	success, err := f.TryLock()
+	if err != nil {
+		return nil, err
+	} else if !success {
+		return nil, CouldntAcquire
+	}
+	return f, nil
+}

irc/flock/flock_iface.go

@@ -0,0 +1,14 @@
+package flock
+
+// documentation for github.com/gofrs/flock incorrectly claims that
+// Flock implements sync.Locker; it does not because the Unlock method
+// has a return type (err).
+type Flocker interface {
+	Unlock() error
+}
+
+type noopFlocker struct{}
+
+func (n *noopFlocker) Unlock() error {
+	return nil
+}

irc/flock/flock_plan9.go

@@ -0,0 +1,7 @@
+//go:build plan9
+
+package flock
+
+func TryAcquireFlock(path string) (fl Flocker, err error) {
+	return &noopFlocker{}, nil
+}

irc/server.go

@@ -25,6 +25,7 @@ import (
 	"github.com/ergochat/ergo/irc/caps"
 	"github.com/ergochat/ergo/irc/connection_limits"
 	"github.com/ergochat/ergo/irc/flatip"
+	"github.com/ergochat/ergo/irc/flock"
 	"github.com/ergochat/ergo/irc/history"
 	"github.com/ergochat/ergo/irc/logger"
 	"github.com/ergochat/ergo/irc/modes"
@@ -88,6 +89,7 @@ type Server struct {
 	whoWas            WhoWasList
 	stats             Stats
 	semaphores        ServerSemaphores
+	flock             flock.Flocker
 	defcon            uint32
 }
 
@@ -585,6 +587,19 @@ func (server *Server) applyConfig(config *Config) (err error) {
 
 	server.logger.Info("server", "Using config file", server.configFilename)
 
+	if initial {
+		if config.LockFile != "" {
+			server.flock, err = flock.TryAcquireFlock(config.LockFile)
+			if err != nil {
+				return fmt.Errorf("failed to acquire flock on %s: %w",
+					config.LockFile, err)
+			}
+		}
+		// the lock is never released until quit; we need to save a pointer
+		// to the (*flock.Flock) object so it doesn't get GC'ed, which would
+		// close the file and surrender the lock
+	}
+
 	// first, reload config sections for functionality implemented in subpackages:
 	wasLoggingRawIO := !initial && server.logger.IsLoggingRawIO()
 	err = server.logger.ApplyConfig(config.Logging)

traditional.yaml

@@ -718,6 +718,12 @@ debug:
     # set to `null`, "", leave blank, or omit to disable
     # pprof-listener: "localhost:6060"
 
+# lock file preventing multiple instances of Ergo from accidentally being
+# started at once. comment out or set to the empty string ("") to disable.
+# this path is relative to the working directory; you may want to use an
+# absolute path instead:
+lock-file: "ircd.lock"
+
 # datastore configuration
 datastore:
     # path to the datastore

vendor/github.com/gofrs/flock/.gitignore

@@ -0,0 +1,24 @@
+# Compiled Object files, Static and Dynamic libs (Shared Objects)
+*.o
+*.a
+*.so
+
+# Folders
+_obj
+_test
+
+# Architecture specific extensions/prefixes
+*.[568vq]
+[568vq].out
+
+*.cgo1.go
+*.cgo2.c
+_cgo_defun.c
+_cgo_gotypes.go
+_cgo_export.*
+
+_testmain.go
+
+*.exe
+*.test
+*.prof

vendor/github.com/gofrs/flock/.travis.yml

@@ -0,0 +1,10 @@
+language: go
+go:
+  - 1.14.x
+  - 1.15.x
+script: go test -v -check.vv -race ./...
+sudo: false
+notifications:
+  email:
+    on_success: never
+    on_failure: always

vendor/github.com/gofrs/flock/LICENSE

@@ -0,0 +1,27 @@
+Copyright (c) 2015-2020, Tim Heckman
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+* Redistributions of source code must retain the above copyright notice, this
+  list of conditions and the following disclaimer.
+
+* Redistributions in binary form must reproduce the above copyright notice,
+  this list of conditions and the following disclaimer in the documentation
+  and/or other materials provided with the distribution.
+
+* Neither the name of gofrs nor the names of its contributors may be used
+  to endorse or promote products derived from this software without
+  specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

vendor/github.com/gofrs/flock/README.md

@@ -0,0 +1,41 @@
+# flock
+[![TravisCI Build Status](https://img.shields.io/travis/gofrs/flock/master.svg?style=flat)](https://travis-ci.org/gofrs/flock)
+[![GoDoc](https://img.shields.io/badge/godoc-flock-blue.svg?style=flat)](https://godoc.org/github.com/gofrs/flock)
+[![License](https://img.shields.io/badge/license-BSD_3--Clause-brightgreen.svg?style=flat)](https://github.com/gofrs/flock/blob/master/LICENSE)
+[![Go Report Card](https://goreportcard.com/badge/github.com/gofrs/flock)](https://goreportcard.com/report/github.com/gofrs/flock)
+
+`flock` implements a thread-safe sync.Locker interface for file locking. It also
+includes a non-blocking TryLock() function to allow locking without blocking execution.
+
+## License
+`flock` is released under the BSD 3-Clause License. See the `LICENSE` file for more details.
+
+## Go Compatibility
+This package makes use of the `context` package that was introduced in Go 1.7. As such, this
+package has an implicit dependency on Go 1.7+.
+
+## Installation
+```
+go get -u github.com/gofrs/flock
+```
+
+## Usage
+```Go
+import "github.com/gofrs/flock"
+
+fileLock := flock.New("/var/lock/go-lock.lock")
+
+locked, err := fileLock.TryLock()
+
+if err != nil {
+	// handle locking error
+}
+
+if locked {
+	// do work
+	fileLock.Unlock()
+}
+```
+
+For more detailed usage information take a look at the package API docs on
+[GoDoc](https://godoc.org/github.com/gofrs/flock).

vendor/github.com/gofrs/flock/appveyor.yml

@@ -0,0 +1,25 @@
+version: '{build}'
+
+build: false
+deploy: false
+
+clone_folder: 'c:\gopath\src\github.com\gofrs\flock'
+
+environment:
+  GOPATH: 'c:\gopath'
+  GOVERSION: '1.15'
+
+init:
+  - git config --global core.autocrlf input
+
+install:
+  - rmdir c:\go /s /q
+  - appveyor DownloadFile https://storage.googleapis.com/golang/go%GOVERSION%.windows-amd64.msi
+  - msiexec /i go%GOVERSION%.windows-amd64.msi /q
+  - set Path=c:\go\bin;c:\gopath\bin;%Path%
+  - go version
+  - go env
+
+test_script:
+  - go get -t ./...
+  - go test -race -v ./...

vendor/github.com/gofrs/flock/flock.go

@@ -0,0 +1,144 @@
+// Copyright 2015 Tim Heckman. All rights reserved.
+// Use of this source code is governed by the BSD 3-Clause
+// license that can be found in the LICENSE file.
+
+// Package flock implements a thread-safe interface for file locking.
+// It also includes a non-blocking TryLock() function to allow locking
+// without blocking execution.
+//
+// Package flock is released under the BSD 3-Clause License. See the LICENSE file
+// for more details.
+//
+// While using this library, remember that the locking behaviors are not
+// guaranteed to be the same on each platform. For example, some UNIX-like
+// operating systems will transparently convert a shared lock to an exclusive
+// lock. If you Unlock() the flock from a location where you believe that you
+// have the shared lock, you may accidentally drop the exclusive lock.
+package flock
+
+import (
+	"context"
+	"os"
+	"runtime"
+	"sync"
+	"time"
+)
+
+// Flock is the struct type to handle file locking. All fields are unexported,
+// with access to some of the fields provided by getter methods (Path() and Locked()).
+type Flock struct {
+	path string
+	m    sync.RWMutex
+	fh   *os.File
+	l    bool
+	r    bool
+}
+
+// New returns a new instance of *Flock. The only parameter
+// it takes is the path to the desired lockfile.
+func New(path string) *Flock {
+	return &Flock{path: path}
+}
+
+// NewFlock returns a new instance of *Flock. The only parameter
+// it takes is the path to the desired lockfile.
+//
+// Deprecated: Use New instead.
+func NewFlock(path string) *Flock {
+	return New(path)
+}
+
+// Close is equivalent to calling Unlock.
+//
+// This will release the lock and close the underlying file descriptor.
+// It will not remove the file from disk, that's up to your application.
+func (f *Flock) Close() error {
+	return f.Unlock()
+}
+
+// Path returns the path as provided in NewFlock().
+func (f *Flock) Path() string {
+	return f.path
+}
+
+// Locked returns the lock state (locked: true, unlocked: false).
+//
+// Warning: by the time you use the returned value, the state may have changed.
+func (f *Flock) Locked() bool {
+	f.m.RLock()
+	defer f.m.RUnlock()
+	return f.l
+}
+
+// RLocked returns the read lock state (locked: true, unlocked: false).
+//
+// Warning: by the time you use the returned value, the state may have changed.
+func (f *Flock) RLocked() bool {
+	f.m.RLock()
+	defer f.m.RUnlock()
+	return f.r
+}
+
+func (f *Flock) String() string {
+	return f.path
+}
+
+// TryLockContext repeatedly tries to take an exclusive lock until one of the
+// conditions is met: TryLock succeeds, TryLock fails with error, or Context
+// Done channel is closed.
+func (f *Flock) TryLockContext(ctx context.Context, retryDelay time.Duration) (bool, error) {
+	return tryCtx(ctx, f.TryLock, retryDelay)
+}
+
+// TryRLockContext repeatedly tries to take a shared lock until one of the
+// conditions is met: TryRLock succeeds, TryRLock fails with error, or Context
+// Done channel is closed.
+func (f *Flock) TryRLockContext(ctx context.Context, retryDelay time.Duration) (bool, error) {
+	return tryCtx(ctx, f.TryRLock, retryDelay)
+}
+
+func tryCtx(ctx context.Context, fn func() (bool, error), retryDelay time.Duration) (bool, error) {
+	if ctx.Err() != nil {
+		return false, ctx.Err()
+	}
+	for {
+		if ok, err := fn(); ok || err != nil {
+			return ok, err
+		}
+		select {
+		case <-ctx.Done():
+			return false, ctx.Err()
+		case <-time.After(retryDelay):
+			// try again
+		}
+	}
+}
+
+func (f *Flock) setFh() error {
+	// open a new os.File instance
+	// create it if it doesn't exist, and open the file read-only.
+	flags := os.O_CREATE
+	if runtime.GOOS == "aix" {
+		// AIX cannot preform write-lock (ie exclusive) on a
+		// read-only file.
+		flags |= os.O_RDWR
+	} else {
+		flags |= os.O_RDONLY
+	}
+	fh, err := os.OpenFile(f.path, flags, os.FileMode(0600))
+	if err != nil {
+		return err
+	}
+
+	// set the filehandle on the struct
+	f.fh = fh
+	return nil
+}
+
+// ensure the file handle is closed if no lock is held
+func (f *Flock) ensureFhState() {
+	if !f.l && !f.r && f.fh != nil {
+		f.fh.Close()
+		f.fh = nil
+	}
+}

vendor/github.com/gofrs/flock/flock_aix.go

@@ -0,0 +1,281 @@
+// Copyright 2019 Tim Heckman. All rights reserved. Use of this source code is
+// governed by the BSD 3-Clause license that can be found in the LICENSE file.
+
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This code implements the filelock API using POSIX 'fcntl' locks, which attach
+// to an (inode, process) pair rather than a file descriptor. To avoid unlocking
+// files prematurely when the same file is opened through different descriptors,
+// we allow only one read-lock at a time.
+//
+// This code is adapted from the Go package:
+// cmd/go/internal/lockedfile/internal/filelock
+
+//+build aix
+
+package flock
+
+import (
+	"errors"
+	"io"
+	"os"
+	"sync"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+type lockType int16
+
+const (
+	readLock  lockType = unix.F_RDLCK
+	writeLock lockType = unix.F_WRLCK
+)
+
+type cmdType int
+
+const (
+	tryLock  cmdType = unix.F_SETLK
+	waitLock cmdType = unix.F_SETLKW
+)
+
+type inode = uint64
+
+type inodeLock struct {
+	owner *Flock
+	queue []<-chan *Flock
+}
+
+var (
+	mu     sync.Mutex
+	inodes = map[*Flock]inode{}
+	locks  = map[inode]inodeLock{}
+)
+
+// Lock is a blocking call to try and take an exclusive file lock. It will wait
+// until it is able to obtain the exclusive file lock. It's recommended that
+// TryLock() be used over this function. This function may block the ability to
+// query the current Locked() or RLocked() status due to a RW-mutex lock.
+//
+// If we are already exclusive-locked, this function short-circuits and returns
+// immediately assuming it can take the mutex lock.
+//
+// If the *Flock has a shared lock (RLock), this may transparently replace the
+// shared lock with an exclusive lock on some UNIX-like operating systems. Be
+// careful when using exclusive locks in conjunction with shared locks
+// (RLock()), because calling Unlock() may accidentally release the exclusive
+// lock that was once a shared lock.
+func (f *Flock) Lock() error {
+	return f.lock(&f.l, writeLock)
+}
+
+// RLock is a blocking call to try and take a shared file lock. It will wait
+// until it is able to obtain the shared file lock. It's recommended that
+// TryRLock() be used over this function. This function may block the ability to
+// query the current Locked() or RLocked() status due to a RW-mutex lock.
+//
+// If we are already shared-locked, this function short-circuits and returns
+// immediately assuming it can take the mutex lock.
+func (f *Flock) RLock() error {
+	return f.lock(&f.r, readLock)
+}
+
+func (f *Flock) lock(locked *bool, flag lockType) error {
+	f.m.Lock()
+	defer f.m.Unlock()
+
+	if *locked {
+		return nil
+	}
+
+	if f.fh == nil {
+		if err := f.setFh(); err != nil {
+			return err
+		}
+		defer f.ensureFhState()
+	}
+
+	if _, err := f.doLock(waitLock, flag, true); err != nil {
+		return err
+	}
+
+	*locked = true
+	return nil
+}
+
+func (f *Flock) doLock(cmd cmdType, lt lockType, blocking bool) (bool, error) {
+	// POSIX locks apply per inode and process, and the lock for an inode is
+	// released when *any* descriptor for that inode is closed. So we need to
+	// synchronize access to each inode internally, and must serialize lock and
+	// unlock calls that refer to the same inode through different descriptors.
+	fi, err := f.fh.Stat()
+	if err != nil {
+		return false, err
+	}
+	ino := inode(fi.Sys().(*syscall.Stat_t).Ino)
+
+	mu.Lock()
+	if i, dup := inodes[f]; dup && i != ino {
+		mu.Unlock()
+		return false, &os.PathError{
+			Path: f.Path(),
+			Err:  errors.New("inode for file changed since last Lock or RLock"),
+		}
+	}
+
+	inodes[f] = ino
+
+	var wait chan *Flock
+	l := locks[ino]
+	if l.owner == f {
+		// This file already owns the lock, but the call may change its lock type.
+	} else if l.owner == nil {
+		// No owner: it's ours now.
+		l.owner = f
+	} else if !blocking {
+		// Already owned: cannot take the lock.
+		mu.Unlock()
+		return false, nil
+	} else {
+		// Already owned: add a channel to wait on.
+		wait = make(chan *Flock)
+		l.queue = append(l.queue, wait)
+	}
+	locks[ino] = l
+	mu.Unlock()
+
+	if wait != nil {
+		wait <- f
+	}
+
+	err = setlkw(f.fh.Fd(), cmd, lt)
+
+	if err != nil {
+		f.doUnlock()
+		if cmd == tryLock && err == unix.EACCES {
+			return false, nil
+		}
+		return false, err
+	}
+
+	return true, nil
+}
+
+func (f *Flock) Unlock() error {
+	f.m.Lock()
+	defer f.m.Unlock()
+
+	// if we aren't locked or if the lockfile instance is nil
+	// just return a nil error because we are unlocked
+	if (!f.l && !f.r) || f.fh == nil {
+		return nil
+	}
+
+	if err := f.doUnlock(); err != nil {
+		return err
+	}
+
+	f.fh.Close()
+
+	f.l = false
+	f.r = false
+	f.fh = nil
+
+	return nil
+}
+
+func (f *Flock) doUnlock() (err error) {
+	var owner *Flock
+	mu.Lock()
+	ino, ok := inodes[f]
+	if ok {
+		owner = locks[ino].owner
+	}
+	mu.Unlock()
+
+	if owner == f {
+		err = setlkw(f.fh.Fd(), waitLock, unix.F_UNLCK)
+	}
+
+	mu.Lock()
+	l := locks[ino]
+	if len(l.queue) == 0 {
+		// No waiters: remove the map entry.
+		delete(locks, ino)
+	} else {
+		// The first waiter is sending us their file now.
+		// Receive it and update the queue.
+		l.owner = <-l.queue[0]
+		l.queue = l.queue[1:]
+		locks[ino] = l
+	}
+	delete(inodes, f)
+	mu.Unlock()
+
+	return err
+}
+
+// TryLock is the preferred function for taking an exclusive file lock. This
+// function takes an RW-mutex lock before it tries to lock the file, so there is
+// the possibility that this function may block for a short time if another
+// goroutine is trying to take any action.
+//
+// The actual file lock is non-blocking. If we are unable to get the exclusive
+// file lock, the function will return false instead of waiting for the lock. If
+// we get the lock, we also set the *Flock instance as being exclusive-locked.
+func (f *Flock) TryLock() (bool, error) {
+	return f.try(&f.l, writeLock)
+}
+
+// TryRLock is the preferred function for taking a shared file lock. This
+// function takes an RW-mutex lock before it tries to lock the file, so there is
+// the possibility that this function may block for a short time if another
+// goroutine is trying to take any action.
+//
+// The actual file lock is non-blocking. If we are unable to get the shared file
+// lock, the function will return false instead of waiting for the lock. If we
+// get the lock, we also set the *Flock instance as being share-locked.
+func (f *Flock) TryRLock() (bool, error) {
+	return f.try(&f.r, readLock)
+}
+
+func (f *Flock) try(locked *bool, flag lockType) (bool, error) {
+	f.m.Lock()
+	defer f.m.Unlock()
+
+	if *locked {
+		return true, nil
+	}
+
+	if f.fh == nil {
+		if err := f.setFh(); err != nil {
+			return false, err
+		}
+		defer f.ensureFhState()
+	}
+
+	haslock, err := f.doLock(tryLock, flag, false)
+	if err != nil {
+		return false, err
+	}
+
+	*locked = haslock
+	return haslock, nil
+}
+
+// setlkw calls FcntlFlock with cmd for the entire file indicated by fd.
+func setlkw(fd uintptr, cmd cmdType, lt lockType) error {
+	for {
+		err := unix.FcntlFlock(fd, int(cmd), &unix.Flock_t{
+			Type:   int16(lt),
+			Whence: io.SeekStart,
+			Start:  0,
+			Len:    0, // All bytes.
+		})
+		if err != unix.EINTR {
+			return err
+		}
+	}
+}

vendor/github.com/gofrs/flock/flock_unix.go

@@ -0,0 +1,197 @@
+// Copyright 2015 Tim Heckman. All rights reserved.
+// Use of this source code is governed by the BSD 3-Clause
+// license that can be found in the LICENSE file.
+
+// +build !aix,!windows
+
+package flock
+
+import (
+	"os"
+	"syscall"
+)
+
+// Lock is a blocking call to try and take an exclusive file lock. It will wait
+// until it is able to obtain the exclusive file lock. It's recommended that
+// TryLock() be used over this function. This function may block the ability to
+// query the current Locked() or RLocked() status due to a RW-mutex lock.
+//
+// If we are already exclusive-locked, this function short-circuits and returns
+// immediately assuming it can take the mutex lock.
+//
+// If the *Flock has a shared lock (RLock), this may transparently replace the
+// shared lock with an exclusive lock on some UNIX-like operating systems. Be
+// careful when using exclusive locks in conjunction with shared locks
+// (RLock()), because calling Unlock() may accidentally release the exclusive
+// lock that was once a shared lock.
+func (f *Flock) Lock() error {
+	return f.lock(&f.l, syscall.LOCK_EX)
+}
+
+// RLock is a blocking call to try and take a shared file lock. It will wait
+// until it is able to obtain the shared file lock. It's recommended that
+// TryRLock() be used over this function. This function may block the ability to
+// query the current Locked() or RLocked() status due to a RW-mutex lock.
+//
+// If we are already shared-locked, this function short-circuits and returns
+// immediately assuming it can take the mutex lock.
+func (f *Flock) RLock() error {
+	return f.lock(&f.r, syscall.LOCK_SH)
+}
+
+func (f *Flock) lock(locked *bool, flag int) error {
+	f.m.Lock()
+	defer f.m.Unlock()
+
+	if *locked {
+		return nil
+	}
+
+	if f.fh == nil {
+		if err := f.setFh(); err != nil {
+			return err
+		}
+		defer f.ensureFhState()
+	}
+
+	if err := syscall.Flock(int(f.fh.Fd()), flag); err != nil {
+		shouldRetry, reopenErr := f.reopenFDOnError(err)
+		if reopenErr != nil {
+			return reopenErr
+		}
+
+		if !shouldRetry {
+			return err
+		}
+
+		if err = syscall.Flock(int(f.fh.Fd()), flag); err != nil {
+			return err
+		}
+	}
+
+	*locked = true
+	return nil
+}
+
+// Unlock is a function to unlock the file. This file takes a RW-mutex lock, so
+// while it is running the Locked() and RLocked() functions will be blocked.
+//
+// This function short-circuits if we are unlocked already. If not, it calls
+// syscall.LOCK_UN on the file and closes the file descriptor. It does not
+// remove the file from disk. It's up to your application to do.
+//
+// Please note, if your shared lock became an exclusive lock this may
+// unintentionally drop the exclusive lock if called by the consumer that
+// believes they have a shared lock. Please see Lock() for more details.
+func (f *Flock) Unlock() error {
+	f.m.Lock()
+	defer f.m.Unlock()
+
+	// if we aren't locked or if the lockfile instance is nil
+	// just return a nil error because we are unlocked
+	if (!f.l && !f.r) || f.fh == nil {
+		return nil
+	}
+
+	// mark the file as unlocked
+	if err := syscall.Flock(int(f.fh.Fd()), syscall.LOCK_UN); err != nil {
+		return err
+	}
+
+	f.fh.Close()
+
+	f.l = false
+	f.r = false
+	f.fh = nil
+
+	return nil
+}
+
+// TryLock is the preferred function for taking an exclusive file lock. This
+// function takes an RW-mutex lock before it tries to lock the file, so there is
+// the possibility that this function may block for a short time if another
+// goroutine is trying to take any action.
+//
+// The actual file lock is non-blocking. If we are unable to get the exclusive
+// file lock, the function will return false instead of waiting for the lock. If
+// we get the lock, we also set the *Flock instance as being exclusive-locked.
+func (f *Flock) TryLock() (bool, error) {
+	return f.try(&f.l, syscall.LOCK_EX)
+}
+
+// TryRLock is the preferred function for taking a shared file lock. This
+// function takes an RW-mutex lock before it tries to lock the file, so there is
+// the possibility that this function may block for a short time if another
+// goroutine is trying to take any action.
+//
+// The actual file lock is non-blocking. If we are unable to get the shared file
+// lock, the function will return false instead of waiting for the lock. If we
+// get the lock, we also set the *Flock instance as being share-locked.
+func (f *Flock) TryRLock() (bool, error) {
+	return f.try(&f.r, syscall.LOCK_SH)
+}
+
+func (f *Flock) try(locked *bool, flag int) (bool, error) {
+	f.m.Lock()
+	defer f.m.Unlock()
+
+	if *locked {
+		return true, nil
+	}
+
+	if f.fh == nil {
+		if err := f.setFh(); err != nil {
+			return false, err
+		}
+		defer f.ensureFhState()
+	}
+
+	var retried bool
+retry:
+	err := syscall.Flock(int(f.fh.Fd()), flag|syscall.LOCK_NB)
+
+	switch err {
+	case syscall.EWOULDBLOCK:
+		return false, nil
+	case nil:
+		*locked = true
+		return true, nil
+	}
+	if !retried {
+		if shouldRetry, reopenErr := f.reopenFDOnError(err); reopenErr != nil {
+			return false, reopenErr
+		} else if shouldRetry {
+			retried = true
+			goto retry
+		}
+	}
+
+	return false, err
+}
+
+// reopenFDOnError determines whether we should reopen the file handle
+// in readwrite mode and try again. This comes from util-linux/sys-utils/flock.c:
+//  Since Linux 3.4 (commit 55725513)
+//  Probably NFSv4 where flock() is emulated by fcntl().
+func (f *Flock) reopenFDOnError(err error) (bool, error) {
+	if err != syscall.EIO && err != syscall.EBADF {
+		return false, nil
+	}
+	if st, err := f.fh.Stat(); err == nil {
+		// if the file is able to be read and written
+		if st.Mode()&0600 == 0600 {
+			f.fh.Close()
+			f.fh = nil
+
+			// reopen in read-write mode and set the filehandle
+			fh, err := os.OpenFile(f.path, os.O_CREATE|os.O_RDWR, os.FileMode(0600))
+			if err != nil {
+				return false, err
+			}
+			f.fh = fh
+			return true, nil
+		}
+	}
+
+	return false, nil
+}

vendor/github.com/gofrs/flock/flock_winapi.go

@@ -0,0 +1,76 @@
+// Copyright 2015 Tim Heckman. All rights reserved.
+// Use of this source code is governed by the BSD 3-Clause
+// license that can be found in the LICENSE file.
+
+// +build windows
+
+package flock
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+var (
+	kernel32, _         = syscall.LoadLibrary("kernel32.dll")
+	procLockFileEx, _   = syscall.GetProcAddress(kernel32, "LockFileEx")
+	procUnlockFileEx, _ = syscall.GetProcAddress(kernel32, "UnlockFileEx")
+)
+
+const (
+	winLockfileFailImmediately = 0x00000001
+	winLockfileExclusiveLock   = 0x00000002
+	winLockfileSharedLock      = 0x00000000
+)
+
+// Use of 0x00000000 for the shared lock is a guess based on some the MS Windows
+// `LockFileEX` docs, which document the `LOCKFILE_EXCLUSIVE_LOCK` flag as:
+//
+// > The function requests an exclusive lock. Otherwise, it requests a shared
+// > lock.
+//
+// https://msdn.microsoft.com/en-us/library/windows/desktop/aa365203(v=vs.85).aspx
+
+func lockFileEx(handle syscall.Handle, flags uint32, reserved uint32, numberOfBytesToLockLow uint32, numberOfBytesToLockHigh uint32, offset *syscall.Overlapped) (bool, syscall.Errno) {
+	r1, _, errNo := syscall.Syscall6(
+		uintptr(procLockFileEx),
+		6,
+		uintptr(handle),
+		uintptr(flags),
+		uintptr(reserved),
+		uintptr(numberOfBytesToLockLow),
+		uintptr(numberOfBytesToLockHigh),
+		uintptr(unsafe.Pointer(offset)))
+
+	if r1 != 1 {
+		if errNo == 0 {
+			return false, syscall.EINVAL
+		}
+
+		return false, errNo
+	}
+
+	return true, 0
+}
+
+func unlockFileEx(handle syscall.Handle, reserved uint32, numberOfBytesToLockLow uint32, numberOfBytesToLockHigh uint32, offset *syscall.Overlapped) (bool, syscall.Errno) {
+	r1, _, errNo := syscall.Syscall6(
+		uintptr(procUnlockFileEx),
+		5,
+		uintptr(handle),
+		uintptr(reserved),
+		uintptr(numberOfBytesToLockLow),
+		uintptr(numberOfBytesToLockHigh),
+		uintptr(unsafe.Pointer(offset)),
+		0)
+
+	if r1 != 1 {
+		if errNo == 0 {
+			return false, syscall.EINVAL
+		}
+
+		return false, errNo
+	}
+
+	return true, 0
+}

vendor/github.com/gofrs/flock/flock_windows.go

@@ -0,0 +1,142 @@
+// Copyright 2015 Tim Heckman. All rights reserved.
+// Use of this source code is governed by the BSD 3-Clause
+// license that can be found in the LICENSE file.
+
+package flock
+
+import (
+	"syscall"
+)
+
+// ErrorLockViolation is the error code returned from the Windows syscall when a
+// lock would block and you ask to fail immediately.
+const ErrorLockViolation syscall.Errno = 0x21 // 33
+
+// Lock is a blocking call to try and take an exclusive file lock. It will wait
+// until it is able to obtain the exclusive file lock. It's recommended that
+// TryLock() be used over this function. This function may block the ability to
+// query the current Locked() or RLocked() status due to a RW-mutex lock.
+//
+// If we are already locked, this function short-circuits and returns
+// immediately assuming it can take the mutex lock.
+func (f *Flock) Lock() error {
+	return f.lock(&f.l, winLockfileExclusiveLock)
+}
+
+// RLock is a blocking call to try and take a shared file lock. It will wait
+// until it is able to obtain the shared file lock. It's recommended that
+// TryRLock() be used over this function. This function may block the ability to
+// query the current Locked() or RLocked() status due to a RW-mutex lock.
+//
+// If we are already locked, this function short-circuits and returns
+// immediately assuming it can take the mutex lock.
+func (f *Flock) RLock() error {
+	return f.lock(&f.r, winLockfileSharedLock)
+}
+
+func (f *Flock) lock(locked *bool, flag uint32) error {
+	f.m.Lock()
+	defer f.m.Unlock()
+
+	if *locked {
+		return nil
+	}
+
+	if f.fh == nil {
+		if err := f.setFh(); err != nil {
+			return err
+		}
+		defer f.ensureFhState()
+	}
+
+	if _, errNo := lockFileEx(syscall.Handle(f.fh.Fd()), flag, 0, 1, 0, &syscall.Overlapped{}); errNo > 0 {
+		return errNo
+	}
+
+	*locked = true
+	return nil
+}
+
+// Unlock is a function to unlock the file. This file takes a RW-mutex lock, so
+// while it is running the Locked() and RLocked() functions will be blocked.
+//
+// This function short-circuits if we are unlocked already. If not, it calls
+// UnlockFileEx() on the file and closes the file descriptor. It does not remove
+// the file from disk. It's up to your application to do.
+func (f *Flock) Unlock() error {
+	f.m.Lock()
+	defer f.m.Unlock()
+
+	// if we aren't locked or if the lockfile instance is nil
+	// just return a nil error because we are unlocked
+	if (!f.l && !f.r) || f.fh == nil {
+		return nil
+	}
+
+	// mark the file as unlocked
+	if _, errNo := unlockFileEx(syscall.Handle(f.fh.Fd()), 0, 1, 0, &syscall.Overlapped{}); errNo > 0 {
+		return errNo
+	}
+
+	f.fh.Close()
+
+	f.l = false
+	f.r = false
+	f.fh = nil
+
+	return nil
+}
+
+// TryLock is the preferred function for taking an exclusive file lock. This
+// function does take a RW-mutex lock before it tries to lock the file, so there
+// is the possibility that this function may block for a short time if another
+// goroutine is trying to take any action.
+//
+// The actual file lock is non-blocking. If we are unable to get the exclusive
+// file lock, the function will return false instead of waiting for the lock. If
+// we get the lock, we also set the *Flock instance as being exclusive-locked.
+func (f *Flock) TryLock() (bool, error) {
+	return f.try(&f.l, winLockfileExclusiveLock)
+}
+
+// TryRLock is the preferred function for taking a shared file lock. This
+// function does take a RW-mutex lock before it tries to lock the file, so there
+// is the possibility that this function may block for a short time if another
+// goroutine is trying to take any action.
+//
+// The actual file lock is non-blocking. If we are unable to get the shared file
+// lock, the function will return false instead of waiting for the lock. If we
+// get the lock, we also set the *Flock instance as being shared-locked.
+func (f *Flock) TryRLock() (bool, error) {
+	return f.try(&f.r, winLockfileSharedLock)
+}
+
+func (f *Flock) try(locked *bool, flag uint32) (bool, error) {
+	f.m.Lock()
+	defer f.m.Unlock()
+
+	if *locked {
+		return true, nil
+	}
+
+	if f.fh == nil {
+		if err := f.setFh(); err != nil {
+			return false, err
+		}
+		defer f.ensureFhState()
+	}
+
+	_, errNo := lockFileEx(syscall.Handle(f.fh.Fd()), flag|winLockfileFailImmediately, 0, 1, 0, &syscall.Overlapped{})
+
+	if errNo > 0 {
+		if errNo == ErrorLockViolation || errNo == syscall.ERROR_IO_PENDING {
+			return false, nil
+		}
+
+		return false, errNo
+	}
+
+	*locked = true
+
+	return true, nil
+}

vendor/modules.txt

@@ -27,6 +27,9 @@ github.com/ergochat/irc-go/ircutils
 github.com/go-sql-driver/mysql
 # github.com/go-test/deep v1.0.6
 ## explicit; go 1.13
+# github.com/gofrs/flock v0.8.1
+## explicit
+github.com/gofrs/flock
 # github.com/golang-jwt/jwt v3.2.2+incompatible
 ## explicit
 github.com/golang-jwt/jwt