spuriouswakeup.go

package main

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

func main() {
	const num = 100000
	var wg sync.WaitGroup
	wg.Add(num * 2)
	var duration time.Duration
	sem := make(chan struct{}, 5)
	for i := 0; i < num; i++ {
		sem <- struct{}{}
		ctx, cancel := context.WithCancel(context.Background())
		go func() {
			// Sleep a bit before canceling the context. The goal is to race
			// the cancellation against the "connection refused" packet from
			// the remote host, so adjust the constant below based on ping
			// time.
			time.Sleep(time.Duration(rand.Int63n(int64(5 * time.Millisecond))))
			cancel()
			wg.Done()
		}()
		go func(i int) {
			start := time.Now()
			// Attempt to connect to a port where nothing is listening. This
			// should be a responsive host to ensure that it fails with
			// ECONNREFUSED instead of a timeout. It must not be localhost,
			// because we need connection attempts to not be instantaneous.
			_, err := (&net.Dialer{}).DialContext(ctx, "tcp4", "192.168.1.1:1")
			duration += time.Since(start)
			if err == nil {
				panic(fmt.Sprintf("Dial to unbound port succeeded on attempt %d", i))
			}
			wg.Done()
			<-sem
		}(i)
	}
	wg.Wait()
	fmt.Printf("avg duration: %s\n", duration/num)
}