一个Golang并发同步问题

main.go

package main

import (
	"fmt"
	"math/rand"
	"sync"
	"time"
)

func a(i int, cs chan<- int) {
	time.Sleep(time.Duration(rand.Intn(1e3)) * time.Millisecond)
	fmt.Printf("performing a(%+v)...\n", i)
	cs <- i
}

func b(i int) {
	time.Sleep(time.Duration(rand.Intn(1e3)) * time.Millisecond)
	fmt.Printf("performing        b(%+v)...\n", i)
}

func c(i int) {
	time.Sleep(time.Duration(rand.Intn(1e3)) * time.Millisecond)
	fmt.Printf("performing c(%+v)..........\n", i)
}

func d(i int) {
	fmt.Printf("performing               d(%+v)\n", i)
}

func main() {
	list := []int{4, 1, 5, 2, 6, 3, 7}
	cs := make(chan int, len(list))
	var final sync.WaitGroup
	final.Add(len(list))
	for _, i := range list {
		var wg sync.WaitGroup
		wg.Add(2)
		go a(i, cs)
		go func(i int, wg *sync.WaitGroup) {
			defer wg.Done()
			b(i)
		}(<-cs, &wg)
		go func(i int, wg *sync.WaitGroup) {
			defer wg.Done()
			c(i)
		}(i, &wg)
		go func(i int, wg *sync.WaitGroup) {
			defer final.Done()
			wg.Wait()
			d(i)
		}(i, &wg)
	}
	final.Wait()
	fmt.Println("all done.")
}