Barrier for concurrency workers to do tasks synchronously.

barrier.go

package main

import "sync"

type barrier struct {
	wc int
	c  chan int
	wg sync.WaitGroup
	sync.Mutex
}

func (br *barrier) fence(w int) {
	id := 0
	br.wg.Wait()
	br.Lock()
	br.wc++
	id = br.wc
	if id == 1 {
		br.c = make(chan int)
	}
	if id == w {
		br.wg.Add(1)
		close(br.c)
	}
	br.Unlock()
	<-br.c
	if id == w {
		br.wc = 0
		br.wg.Done()
	}
}

Inspired by this article to create barriers for concurrency workers.

The barrier can block workers until all of them ran into br.fence(w).
br.fence can be used multi-times in one worker loop to synchronous several steps, like this:

func worker(c *counter, w int, br *barrier, wg *sync.WaitGroup) {
	for i := 0; i < 3; i++ {
		// wait for all workers to finish previous loop
		br.fence(w)
		c.Incr()
		// wait for other workers to do bootstrap
		br.fence(w)
		time.Sleep(time.Duration(rand.Intn(100)) * time.Millisecond)
		fmt.Println(c.Get())
	}
	wg.Done()
}