waiter.go

package main

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

// Waiter allows multiple clients to wait until a certain sequence number has been reached
type Waiter struct {
	cond *sync.Cond
	seq  int64
}

// NewWaiter creates a new Waiter instance
func NewWaiter() *Waiter {
	var m sync.Mutex
	return &Waiter{
		cond: sync.NewCond(&m),
	}
}

// Seq returns the current sequence number
func (w *Waiter) Seq() int64 {
	w.cond.L.Lock()
	defer w.cond.L.Unlock()
	return w.seq
}

// Wait until the current sequence number is greater than or
// equal to seq
func (w *Waiter) Wait(seq int64) {
	w.cond.L.Lock()
	for w.seq < seq {
		w.cond.Wait()
	}
	w.cond.L.Unlock()
}

// Update the current sequence number
// This method panics if seq is less than the previous one
func (w *Waiter) Update(seq int64) {
	w.cond.L.Lock()
	if seq < w.seq {
		panic("sequence numbers cannot decrease")
	}
	w.seq = seq
	w.cond.Broadcast()
	w.cond.L.Unlock()
}