src/pkg/once/once.go

// Copyright 2009 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 package provides a single function, Do, to run a function
// exactly once, usually used as part of initialization.
package once

import "sync"

type job struct {
	done       bool
	sync.Mutex // should probably be sync.Notification or some such
}

var jobs = make(map[func()]*job)
var joblock sync.Mutex

// Do is the the only exported piece of the package.
// For one-time initialization that is not done during init,
// wrap the initialization in a niladic function f() and call
//	Do(f)
// If multiple processes call Do(f) simultaneously
// with the same f argument, only one will call f, and the
// others will block until f finishes running.
//
// Since a func() expression typically evaluates to a differerent
// function value each time it is evaluated, it is incorrect to
// pass such values to Do.  For example,
//	func f(x int) {
//		Do(func() { fmt.Println(x) })
//	}
// behaves the same as
//	func f(x int) {
//		fmt.Println(x)
//	}
// because the func() expression in the first creates a new
// func each time f runs, and each of those funcs is run once.
func Do(f func()) {
	joblock.Lock()
	j, present := jobs[f]
	if !present {
		// run it
		j = new(job)
		j.Lock()
		jobs[f] = j
		joblock.Unlock()
		f()
		j.done = true
		j.Unlock()
	} else {
		// wait for it
		joblock.Unlock()
		if j.done != true {
			j.Lock()
			j.Unlock()
		}
	}
}