1// Copyright 2009 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5package sync
6
7import (
8	"sync/atomic"
9)
10
11// Once is an object that will perform exactly one action.
12//
13// A Once must not be copied after first use.
14type Once struct {
15	// done indicates whether the action has been performed.
16	// It is first in the struct because it is used in the hot path.
17	// The hot path is inlined at every call site.
18	// Placing done first allows more compact instructions on some architectures (amd64/386),
19	// and fewer instructions (to calculate offset) on other architectures.
20	done uint32
21	m    Mutex
22}
23
24// Do calls the function f if and only if Do is being called for the
25// first time for this instance of Once. In other words, given
26// 	var once Once
27// if once.Do(f) is called multiple times, only the first call will invoke f,
28// even if f has a different value in each invocation. A new instance of
29// Once is required for each function to execute.
30//
31// Do is intended for initialization that must be run exactly once. Since f
32// is niladic, it may be necessary to use a function literal to capture the
33// arguments to a function to be invoked by Do:
34// 	config.once.Do(func() { config.init(filename) })
35//
36// Because no call to Do returns until the one call to f returns, if f causes
37// Do to be called, it will deadlock.
38//
39// If f panics, Do considers it to have returned; future calls of Do return
40// without calling f.
41//
42func (o *Once) Do(f func()) {
43	// Note: Here is an incorrect implementation of Do:
44	//
45	//	if atomic.CompareAndSwapUint32(&o.done, 0, 1) {
46	//		f()
47	//	}
48	//
49	// Do guarantees that when it returns, f has finished.
50	// This implementation would not implement that guarantee:
51	// given two simultaneous calls, the winner of the cas would
52	// call f, and the second would return immediately, without
53	// waiting for the first's call to f to complete.
54	// This is why the slow path falls back to a mutex, and why
55	// the atomic.StoreUint32 must be delayed until after f returns.
56
57	if atomic.LoadUint32(&o.done) == 0 {
58		// Outlined slow-path to allow inlining of the fast-path.
59		o.doSlow(f)
60	}
61}
62
63func (o *Once) doSlow(f func()) {
64	o.m.Lock()
65	defer o.m.Unlock()
66	if o.done == 0 {
67		defer atomic.StoreUint32(&o.done, 1)
68		f()
69	}
70}
71