rate_limiter.go

// Limits concurrent tasks to a limit of n. If limit is 
// reached, return error instead of running task
type RateLimiter struct {
  MaxConcurrent int
  // Add whatever you need
  taskChan chan struct{}
}

func (l *RateLimiter) Limit(task func()) error {
  select {
  case l.taskChan <- struct{}{}:
    go func() {
      task()
      <-l.taskChan
    }()
    return nil
  default:
      return errors.New("Rate limit met")
  }
}

func NewRateLimiter(limit int) *RateLimiter {
  return &RateLimiter{
    MaxConcurrent: limit,
    taskChan: make(chan struct{}, limit),
  }
}