#0 sync.Once

go version 1.18.9

// Once is an object that will perform exactly one action.
//
// A Once must not be copied after first use. 因为使用之后内部状态已经改变，但是就算copy使用也不会导致panic.
type Once struct {
	// done indicates whether the action has been performed.
	// It is first in the struct because it is used in the hot path.
	// The hot path is inlined at every call site.
	// Placing done first allows more compact instructions on some architectures (amd64/386),
	// and fewer instructions (to calculate offset) on other architectures.
	done uint32
	m    Mutex
}

// Do calls the function f if and only if Do is being called for the
// first time for this instance of Once. In other words, given
// 	var once Once
// if once.Do(f) is called multiple times, only the first call will invoke f,
// even if f has a different value in each invocation. A new instance of
// Once is required for each function to execute.
//
// Do is intended for initialization that must be run exactly once. Since f
// is niladic, it may be necessary to use a function literal to capture the
// arguments to a function to be invoked by Do:
// 	config.once.Do(func() { config.init(filename) })
//
// Because no call to Do returns until the one call to f returns, if f causes
// Do to be called, it will deadlock.
//
// If f panics, Do considers it to have returned; future calls of Do return
// without calling f.
//
func (o *Once) Do(f func()) {
	// Note: Here is an incorrect implementation of Do:
	//
	//	if atomic.CompareAndSwapUint32(&o.done, 0, 1) {
	//		f()
	//	}
	//
	// Do guarantees that when it returns, f has finished. [这是重点，如果同时执行多次Do，
	// 只有其中一个会获取锁并执行f，等f返回之后再标记done，之后不再执行f。这就是doSlow的由来。]
	// This implementation would not implement that guarantee:
	// given two simultaneous calls, the winner of the cas would
	// call f, and the second would return immediately, without
	// waiting for the first's call to f to complete.
	// This is why the slow path falls back to a mutex, and why
	// the atomic.StoreUint32 must be delayed until after f returns.

	if atomic.LoadUint32(&o.done) == 0 {
		// Outlined slow-path to allow inlining of the fast-path.
		o.doSlow(f)
	}
}

func (o *Once) doSlow(f func()) {
	o.m.Lock()
	defer o.m.Unlock()
	if o.done == 0 {
		defer atomic.StoreUint32(&o.done, 1)
		f()
	}
}

eg:

package main

import (
	"fmt"
	"sync"
)

func main() {
	var a sync.Once
	a.Do(func() {
		fmt.Println(1)
	})

	// 如果有个智能的IDE，就会出现:
	// Variable declaration copies a lock value to 'b': type 'sync.Once' contains 'sync.Mutex' which is 'sync.Locker' 
	b := a

	b.Do(func() {
		fmt.Println(2)
	})
}
程序输出: 1