• Add() 和 Wait() 最好不要放在不同的 goroutine, 避免并发执行.
• 同一个 Once 实例调用多次 Do 方法, 只有一个函数会被调用, 但是其他的函数会阻塞, 等待 Unlock.

上代码, 自己品:

package main

import (
	"fmt"
	"sync"
)

func main() {
	var wg sync.WaitGroup
	wg.Add(2)
	go func() {
		defer wg.Done()
		fmt.Println(1)
	}()
	go func() {
		defer wg.Done()
		fmt.Println(2)
	}()
	wg.Wait()
	fmt.Println("Done")
}

package main

import (
	"fmt"
	"sync"
	"time"
)

func main() {
	var wg sync.WaitGroup
	wg.Add(1)
	go func() {
		//defer wg.Done()
		fmt.Println(1)
		wg.Done()
		wg.Add(1)
		time.Sleep(time.Second * 1)
		wg.Done()
	}()
	wg.Wait() // panic: sync: WaitGroup is reused before previous Wait has returned
	fmt.Println("Done")
}

package main

import (
	"fmt"
	"sync"
)

func main() {
	var once sync.Once
	once.Do(
		func() {
			fmt.Println(1)
		},
	)
	once.Do(
		func() {
			fmt.Println(2)
		},
	)
	fmt.Println("Done")
}

/*
1
Done
*/

package main

import (
	"fmt"
	"sync"
)

func main() {
	var once sync.Once

	go func() {
		once.Do(
			func() {
				fmt.Println(1)
			},
		)
	}()

	go func() {
		once.Do(
			func() {
				fmt.Println(2)
			},
		)
	}()

	fmt.Println("Done")
}

/*
Done
*/

package main

import (
	"fmt"
	"sync"
	"time"
)

func main() {
	var once sync.Once

	go func() {
		// 会阻塞
		once.Do(
			func() {
				fmt.Println(2)
			},
		)
		fmt.Println(2, "done")
	}()
	once.Do(
		func() {
			time.Sleep(time.Second * 3)
			fmt.Println(1)
		},
	)
	fmt.Println("Done")

	time.Sleep(time.Second * 5)
}

/*
1
Done
2 done
*/