Timer race condition prove.

timer.go

package main

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

var withLock = flag.Bool("lock", false, "use lock on resets")
var withStop = flag.Bool("stop", false, "use stop and channel drain on resets")

func main() {
	flag.Parse()
	fmt.Printf("lock: %v; stop: %v\n", *withLock, *withStop)
	var (
		resetGoroutines sync.WaitGroup
		timers          sync.WaitGroup
		delays          sync.WaitGroup
	)
	var (
		startReset = make(chan struct{})
		stopReset  = make(chan struct{})
	)
	const (
		timersCount          = 4
		resetGoroutinesCount = 32
		resetTo              = time.Millisecond * 600
		resetInterval        = resetTo / 6
	)

	timers.Add(timersCount)
	delays.Add(timersCount)

	for i := 0; i < timersCount; i++ {
		go func(x int) {
			tm := time.AfterFunc(time.Hour, func() {
				fmt.Printf("timer #%d fired\n", x)
				delays.Done()
			})

			timers.Done()

			var mu sync.Mutex

			resetGoroutines.Add(resetGoroutinesCount)

			for i := 0; i < resetGoroutinesCount; i++ {
				go func(i int) {
					defer resetGoroutines.Done()

					<-startReset

					tick := time.NewTicker(resetInterval)
					for {
						select {
						case <-tick.C:
							if *withLock {
								mu.Lock()
							}
							if *withStop {
								if !tm.Stop() {
									select {
									case <-tm.C:
									default:
									}
								}
							}

							tm.Reset(resetTo)

							if *withLock {
								mu.Unlock()
							}
						case <-stopReset:
							return
						}
					}
				}(i)
			}
		}(i)
	}

	timers.Wait()
	fmt.Printf("initialized %d timers\n", timersCount)
	close(startReset)

	fmt.Printf("will sleep for %s x 4 (delay which is set during resets)\n", resetTo)
	time.Sleep(resetTo * 4)
	close(stopReset)
	fmt.Println("done, waiting...")

	resetGoroutines.Wait()
	fmt.Println("reset goroutines are done")
	delays.Wait()
}