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.
14//
15// In the terminology of [the Go memory model],
16// the return from f “synchronizes before”
17// the return from any call of once.Do(f).
18//
19// [the Go memory model]: https://go.dev/ref/mem
20type Once struct {
21	// done indicates whether the action has been performed.
22	// It is first in the struct because it is used in the hot path.
23	// The hot path is inlined at every call site.
24	// Placing done first allows more compact instructions on some architectures (amd64/386),
25	// and fewer instructions (to calculate offset) on other architectures.
26	done atomic.Uint32
27	m    Mutex
28}
29
30// Do calls the function f if and only if Do is being called for the
31// first time for this instance of [Once]. In other words, given
32//
33//	var once Once
34//
35// if once.Do(f) is called multiple times, only the first call will invoke f,
36// even if f has a different value in each invocation. A new instance of
37// Once is required for each function to execute.
38//
39// Do is intended for initialization that must be run exactly once. Since f
40// is niladic, it may be necessary to use a function literal to capture the
41// arguments to a function to be invoked by Do:
42//
43//	config.once.Do(func() { config.init(filename) })
44//
45// Because no call to Do returns until the one call to f returns, if f causes
46// Do to be called, it will deadlock.
47//
48// If f panics, Do considers it to have returned; future calls of Do return
49// without calling f.
50func (o *Once) Do(f func()) {
51	// Note: Here is an incorrect implementation of Do:
52	//
53	//	if o.done.CompareAndSwap(0, 1) {
54	//		f()
55	//	}
56	//
57	// Do guarantees that when it returns, f has finished.
58	// This implementation would not implement that guarantee:
59	// given two simultaneous calls, the winner of the cas would
60	// call f, and the second would return immediately, without
61	// waiting for the first's call to f to complete.
62	// This is why the slow path falls back to a mutex, and why
63	// the o.done.Store must be delayed until after f returns.
64
65	if o.done.Load() == 0 {
66		// Outlined slow-path to allow inlining of the fast-path.
67		o.doSlow(f)
68	}
69}
70
71func (o *Once) doSlow(f func()) {
72	o.m.Lock()
73	defer o.m.Unlock()
74	if o.done.Load() == 0 {
75		defer o.done.Store(1)
76		f()
77	}
78}
79