diff --git a/util/flock.go b/util/flock.go new file mode 100644 index 00000000..af3da58c --- /dev/null +++ b/util/flock.go @@ -0,0 +1,75 @@ +// Mgmt +// Copyright (C) 2013-2024+ James Shubin and the project contributors +// Written by James Shubin and the project contributors +// +// This program is free software: you can redistribute it and/or modify +// it under the terms of the GNU General Public License as published by +// the Free Software Foundation, either version 3 of the License, or +// (at your option) any later version. +// +// 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 . +// +// Additional permission under GNU GPL version 3 section 7 +// +// If you modify this program, or any covered work, by linking or combining it +// with embedded mcl code and modules (and that the embedded mcl code and +// modules which link with this program, contain a copy of their source code in +// the authoritative form) containing parts covered by the terms of any other +// license, the licensors of this program grant you additional permission to +// convey the resulting work. Furthermore, the licensors of this program grant +// the original author, James Shubin, additional permission to update this +// additional permission if he deems it necessary to achieve the goals of this +// additional permission. + +package util + +import ( + "os" + "syscall" +) + +// TODO: Consider replacing this with: github.com/gofrs/flock if it's better. + +// NewFlock creates a new flock. Details available in the struct documentation. +func NewFlock(path string) *Flock { + return &Flock{ + Path: path, + } +} + +// Flock is a structure for building a lock file. This can be used to prevent a +// binary from running more than one copy at a time on a single machine. +type Flock struct { + Path string + + file *os.File +} + +// TryLock attempts to take the lock. If it errors, it means we couldn't build a +// lock today. If it returns (nil, nil) it means someone is already locked. If +// it returns a non-nil function, then you are now locked. Call that function to +// unlock. Note that it can error if it fails to unlock things. +func (obj *Flock) TryLock() (func() error, error) { + var err error + obj.file, err = os.Create(obj.Path) + if err != nil { + return nil, err // actual error, can't continue + } + + // Exclusive lock in non-blocking mode, we add a shared lock. + if err := syscall.Flock(int(obj.file.Fd()), syscall.LOCK_EX|syscall.LOCK_NB); err != nil { + return nil, nil // can't lock + } + + // We're locked! + return func() error { + defer obj.file.Close() + return syscall.Flock(int(obj.file.Fd()), syscall.LOCK_UN) + }, nil +} diff --git a/util/flock_test.go b/util/flock_test.go new file mode 100644 index 00000000..5f707955 --- /dev/null +++ b/util/flock_test.go @@ -0,0 +1,65 @@ +// Mgmt +// Copyright (C) 2013-2024+ James Shubin and the project contributors +// Written by James Shubin and the project contributors +// +// This program is free software: you can redistribute it and/or modify +// it under the terms of the GNU General Public License as published by +// the Free Software Foundation, either version 3 of the License, or +// (at your option) any later version. +// +// 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 . +// +// Additional permission under GNU GPL version 3 section 7 +// +// If you modify this program, or any covered work, by linking or combining it +// with embedded mcl code and modules (and that the embedded mcl code and +// modules which link with this program, contain a copy of their source code in +// the authoritative form) containing parts covered by the terms of any other +// license, the licensors of this program grant you additional permission to +// convey the resulting work. Furthermore, the licensors of this program grant +// the original author, James Shubin, additional permission to update this +// additional permission if he deems it necessary to achieve the goals of this +// additional permission. + +//go:build !root + +package util + +import ( + "fmt" + "os" + "time" +) + +func ExampleFlock() { + lockFile := "/tmp/flock.lock" + fmt.Printf("hello\n") + defer fmt.Printf("goodbye\n") + defer os.Remove(lockFile) + + m := NewFlock(lockFile) + unlock, err := m.TryLock() + if err != nil { + fmt.Printf("err: %+v\n", err) + return + } + if unlock == nil { + fmt.Printf("already locked\n") + return + } + fmt.Printf("locked...\n") + time.Sleep(1 * time.Second) + unlock() + fmt.Printf("unlocked...\n") + + // Output: hello + // locked... + // unlocked... + // goodbye +}