Another example of sync.cond.

limitgroup.go

package xsync

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

type LimitGroup struct {
	wg   sync.WaitGroup
	mu   *sync.Mutex
	c    *sync.Cond
	l, n int
}

func NewLimitGroup(n int) *LimitGroup {
	mu := new(sync.Mutex)
	return &LimitGroup{
		mu: mu,
		c:  sync.NewCond(mu),
		l:  n,
		n:  n,
	}
}

func (lg *LimitGroup) Add(delta int) {
	lg.mu.Lock()
	defer lg.mu.Unlock()
	if delta > lg.l {
		panic(`LimitGroup: delta must not exceed limit`)
	}
	for lg.n < 1 {
		lg.c.Wait()
	}
	lg.n -= delta
	lg.wg.Add(delta)
}

func (lg *LimitGroup) Done() {
	lg.mu.Lock()
	defer lg.mu.Unlock()
	lg.n++
	lg.c.Signal()
	lg.wg.Done()
}

func (lg *LimitGroup) Wait() { lg.wg.Wait() }