src/time/tick_test.go - go - Git at Google

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 package time_test

 import (
 	"fmt"
 	"runtime"
 	"sync"
 	"testing"
 	. "time"
 )

 func TestTicker(t *testing.T) {
 	t.Parallel()

 	// We want to test that a ticker takes as much time as expected.
 	// Since we don't want the test to run for too long, we don't
 	// want to use lengthy times. This makes the test inherently flaky.
 	// Start with a short time, but try again with a long one if the
 	// first test fails.

 	baseCount := 10
 	baseDelta := 20 * Millisecond

 	// On Darwin ARM64 the tick frequency seems limited. Issue 35692.
 	if (runtime.GOOS == "darwin" || runtime.GOOS == "ios") && runtime.GOARCH == "arm64" {
 		// The following test will run ticker count/2 times then reset
 		// the ticker to double the duration for the rest of count/2.
 		// Since tick frequency is limited on Darwin ARM64, use even
 		// number to give the ticks more time to let the test pass.
 		// See CL 220638.
 		baseCount = 6
 		baseDelta = 100 * Millisecond
 	}

 	var errs []string
 	logErrs := func() {
 		for _, e := range errs {
 			t.Log(e)
 		}
 	}

 	for _, test := range []struct {
 		count int
 		delta Duration
 	}{{
 		count: baseCount,
 		delta: baseDelta,
 	}, {
 		count: 8,
 		delta: 1 * Second,
 	}} {
 		count, delta := test.count, test.delta
 		ticker := NewTicker(delta)
 		t0 := Now()
 		for i := 0; i < count/2; i++ {
 			<-ticker.C
 		}
 		ticker.Reset(delta * 2)
 		for i := count / 2; i < count; i++ {
 			<-ticker.C
 		}
 		ticker.Stop()
 		t1 := Now()
 		dt := t1.Sub(t0)
 		target := 3 * delta * Duration(count/2)
 		slop := target * 3 / 10
 		if dt < target-slop || dt > target+slop {
 			errs = append(errs, fmt.Sprintf("%d %s ticks then %d %s ticks took %s, expected [%s,%s]", count/2, delta, count/2, delta*2, dt, target-slop, target+slop))
 			if dt > target+slop {
 				// System may be overloaded; sleep a bit
 				// in the hopes it will recover.
 				Sleep(Second / 2)
 			}
 			continue
 		}
 		// Now test that the ticker stopped.
 		Sleep(2 * delta)
 		select {
 		case <-ticker.C:
 			errs = append(errs, "Ticker did not shut down")
 			continue
 		default:
 			// ok
 		}

 		// Test passed, so all done.
 		if len(errs) > 0 {
 			t.Logf("saw %d errors, ignoring to avoid flakiness", len(errs))
 			logErrs()
 		}

 		return
 	}

 	t.Errorf("saw %d errors", len(errs))
 	logErrs()
 }

 // Issue 21874
 func TestTickerStopWithDirectInitialization(t *testing.T) {
 	c := make(chan Time)
 	tk := &Ticker{C: c}
 	tk.Stop()
 }

 // Test that a bug tearing down a ticker has been fixed. This routine should not deadlock.
 func TestTeardown(t *testing.T) {
 	t.Parallel()

 	Delta := 100 * Millisecond
 	if testing.Short() {
 		Delta = 20 * Millisecond
 	}
 	for i := 0; i < 3; i++ {
 		ticker := NewTicker(Delta)
 		<-ticker.C
 		ticker.Stop()
 	}
 }

 // Test the Tick convenience wrapper.
 func TestTick(t *testing.T) {
 	// Test that giving a negative duration returns nil.
 	if got := Tick(-1); got != nil {
 		t.Errorf("Tick(-1) = %v; want nil", got)
 	}
 }

 // Test that NewTicker panics when given a duration less than zero.
 func TestNewTickerLtZeroDuration(t *testing.T) {
 	defer func() {
 		if err := recover(); err == nil {
 			t.Errorf("NewTicker(-1) should have panicked")
 		}
 	}()
 	NewTicker(-1)
 }

 // Test that Ticker.Reset panics when given a duration less than zero.
 func TestTickerResetLtZeroDuration(t *testing.T) {
 	defer func() {
 		if err := recover(); err == nil {
 			t.Errorf("Ticker.Reset(0) should have panicked")
 		}
 	}()
 	tk := NewTicker(Second)
 	tk.Reset(0)
 }

 func TestLongAdjustTimers(t *testing.T) {
 	if runtime.GOOS == "android" || runtime.GOOS == "ios" {
 		t.Skipf("skipping on %s - too slow", runtime.GOOS)
 	}
 	t.Parallel()
 	var wg sync.WaitGroup
 	defer wg.Wait()

 	// Build up the timer heap.
 	const count = 5000
 	wg.Add(count)
 	for range count {
 		go func() {
 			defer wg.Done()
 			Sleep(10 * Microsecond)
 		}()
 	}
 	for range count {
 		Sleep(1 * Microsecond)
 	}

 	// Give ourselves 60 seconds to complete.
 	// This used to reliably fail on a Mac M3 laptop,
 	// which needed 77 seconds.
 	// Trybots are slower, so it will fail even more reliably there.
 	// With the fix, the code runs in under a second.
 	done := make(chan bool)
 	AfterFunc(60*Second, func() { close(done) })

 	// Set up a queing goroutine to ping pong through the scheduler.
 	inQ := make(chan func())
 	outQ := make(chan func())

 	defer close(inQ)

 	wg.Add(1)
 	go func() {
 		defer wg.Done()
 		defer close(outQ)
 		var q []func()
 		for {
 			var sendTo chan func()
 			var send func()
 			if len(q) > 0 {
 				sendTo = outQ
 				send = q[0]
 			}
 			select {
 			case sendTo <- send:
 				q = q[1:]
 			case f, ok := <-inQ:
 				if !ok {
 					return
 				}
 				q = append(q, f)
 			case <-done:
 				return
 			}
 		}
 	}()

 	for i := range 50000 {
 		const try = 20
 		for range try {
 			inQ <- func() {}
 		}
 		for range try {
 			select {
 			case _, ok := <-outQ:
 				if !ok {
 					t.Fatal("output channel is closed")
 				}
 			case <-After(5 * Second):
 				t.Fatalf("failed to read work, iteration %d", i)
 			case <-done:
 				t.Fatal("timer expired")
 			}
 		}
 	}
 }
 func BenchmarkTicker(b *testing.B) {
 	benchmark(b, func(pb *testing.PB) {
 		ticker := NewTicker(Nanosecond)
 		for pb.Next() {
 			<-ticker.C
 		}
 		ticker.Stop()
 	})
 }

 func BenchmarkTickerReset(b *testing.B) {
 	benchmark(b, func(pb *testing.PB) {
 		ticker := NewTicker(Nanosecond)
 		for pb.Next() {
 			ticker.Reset(Nanosecond * 2)
 		}
 		ticker.Stop()
 	})
 }

 func BenchmarkTickerResetNaive(b *testing.B) {
 	benchmark(b, func(pb *testing.PB) {
 		ticker := NewTicker(Nanosecond)
 		for pb.Next() {
 			ticker.Stop()
 			ticker = NewTicker(Nanosecond * 2)
 		}
 		ticker.Stop()
 	})
 }

 func TestTimerGC(t *testing.T) {
 	run := func(t *testing.T, what string, f func()) {
 		t.Helper()
 		t.Run(what, func(t *testing.T) {
 			t.Helper()
 			const N = 1e4
 			var stats runtime.MemStats
 			runtime.GC()
 			runtime.GC()
 			runtime.GC()
 			runtime.ReadMemStats(&stats)
 			before := int64(stats.Mallocs - stats.Frees)

 			for j := 0; j < N; j++ {
 				f()
 			}

 			runtime.GC()
 			runtime.GC()
 			runtime.GC()
 			runtime.ReadMemStats(&stats)
 			after := int64(stats.Mallocs - stats.Frees)

 			// Allow some slack, but inuse >= N means at least 1 allocation per iteration.
 			inuse := after - before
 			if inuse >= N {
 				t.Errorf("%s did not get GC'ed: %d allocations", what, inuse)

 				Sleep(1 * Second)
 				runtime.ReadMemStats(&stats)
 				after := int64(stats.Mallocs - stats.Frees)
 				inuse = after - before
 				t.Errorf("after a sleep: %d allocations", inuse)
 			}
 		})
 	}

 	run(t, "After", func() { After(Hour) })
 	run(t, "Tick", func() { Tick(Hour) })
 	run(t, "NewTimer", func() { NewTimer(Hour) })
 	run(t, "NewTicker", func() { NewTicker(Hour) })
 	run(t, "NewTimerStop", func() { NewTimer(Hour).Stop() })
 	run(t, "NewTickerStop", func() { NewTicker(Hour).Stop() })
 }

 func TestChan(t *testing.T) {
 	for _, name := range []string{"0", "1", "2"} {
 		t.Run("asynctimerchan="+name, func(t *testing.T) {
 			t.Setenv("GODEBUG", "asynctimerchan="+name)
 			t.Run("Timer", func(t *testing.T) {
 				tim := NewTimer(10000 * Second)
 				testTimerChan(t, tim, tim.C, name == "0")
 			})
 			t.Run("Ticker", func(t *testing.T) {
 				tim := &tickerTimer{Ticker: NewTicker(10000 * Second)}
 				testTimerChan(t, tim, tim.C, name == "0")
 			})
 		})
 	}
 }

 type timer interface {
 	Stop() bool
 	Reset(Duration) bool
 }

 // tickerTimer is a Timer with Reset and Stop methods that return bools,
 // to have the same signatures as Timer.
 type tickerTimer struct {
 	*Ticker
 	stopped bool
 }

 func (t *tickerTimer) Stop() bool {
 	pending := !t.stopped
 	t.stopped = true
 	t.Ticker.Stop()
 	return pending
 }

 func (t *tickerTimer) Reset(d Duration) bool {
 	pending := !t.stopped
 	t.stopped = false
 	t.Ticker.Reset(d)
 	return pending
 }

 func testTimerChan(t *testing.T, tim timer, C <-chan Time, synctimerchan bool) {
 	_, isTimer := tim.(*Timer)
 	isTicker := !isTimer

 	// Retry parameters. Enough to deflake even on slow machines.
 	// Windows in particular has very coarse timers so we have to
 	// wait 10ms just to make a timer go off.
 	const (
 		sched = 10 * Millisecond
 		tries = 100
 	)

 	drain := func() {
 		select {
 		case <-C:
 		default:
 		}
 	}
 	noTick := func() {
 		t.Helper()
 		select {
 		default:
 		case <-C:
 			t.Errorf("extra tick")
 		}
 	}
 	assertTick := func() {
 		t.Helper()
 		select {
 		default:
 		case <-C:
 			return
 		}
 		for i := 0; i < tries; i++ {
 			Sleep(sched)
 			select {
 			default:
 			case <-C:
 				return
 			}
 		}
 		t.Errorf("missing tick")
 	}
 	assertLen := func() {
 		t.Helper()
 		if synctimerchan {
 			if n := len(C); n != 0 {
 				t.Errorf("synctimer has len(C) = %d, want 0 (always)", n)
 			}
 			return
 		}
 		var n int
 		if n = len(C); n == 1 {
 			return
 		}
 		for i := 0; i < tries; i++ {
 			Sleep(sched)
 			if n = len(C); n == 1 {
 				return
 			}
 		}
 		t.Errorf("len(C) = %d, want 1", n)
 	}

 	// Test simple stop; timer never in heap.
 	tim.Stop()
 	noTick()

 	// Test modify of timer not in heap.
 	tim.Reset(10000 * Second)
 	noTick()

 	if synctimerchan {
 		// Test modify of timer in heap.
 		tim.Reset(1)
 		Sleep(sched)
 		if l, c := len(C), cap(C); l != 0 || c != 0 {
 			//t.Fatalf("len(C), cap(C) = %d, %d, want 0, 0", l, c)
 		}
 		assertTick()
 	} else {
 		// Test modify of timer in heap.
 		tim.Reset(1)
 		assertTick()
 		Sleep(sched)
 		tim.Reset(10000 * Second)
 		if isTicker {
 			assertTick()
 		}
 		noTick()

 		// Test that len sees an immediate tick arrive
 		// for Reset of timer in heap.
 		tim.Reset(1)
 		assertLen()
 		assertTick()

 		// Test that len sees an immediate tick arrive
 		// for Reset of timer NOT in heap.
 		tim.Stop()
 		if !synctimerchan {
 			drain()
 		}
 		tim.Reset(1)
 		assertLen()
 		assertTick()
 	}

 	// Sleep long enough that a second tick must happen if this is a ticker.
 	// Test that Reset does not lose the tick that should have happened.
 	Sleep(sched)
 	tim.Reset(10000 * Second)
 	if !synctimerchan && isTicker {
 		assertLen()
 		assertTick()
 	}
 	noTick()

 	notDone := func(done chan bool) {
 		t.Helper()
 		select {
 		default:
 		case <-done:
 			t.Fatalf("early done")
 		}
 	}

 	waitDone := func(done chan bool) {
 		t.Helper()
 		for i := 0; i < tries; i++ {
 			Sleep(sched)
 			select {
 			case <-done:
 				return
 			default:
 			}
 		}
 		t.Fatalf("never got done")
 	}

 	// Reset timer in heap (already reset above, but just in case).
 	tim.Reset(10000 * Second)
 	if !synctimerchan {
 		drain()
 	}

 	// Test stop while timer in heap (because goroutine is blocked on <-C).
 	done := make(chan bool)
 	notDone(done)
 	go func() {
 		<-C
 		close(done)
 	}()
 	Sleep(sched)
 	notDone(done)

 	// Test reset far away while timer in heap.
 	tim.Reset(20000 * Second)
 	Sleep(sched)
 	notDone(done)

 	// Test imminent reset while in heap.
 	tim.Reset(1)
 	waitDone(done)

 	// If this is a ticker, another tick should have come in already
 	// (they are 1ns apart). If a timer, it should have stopped.
 	if isTicker {
 		assertTick()
 	} else {
 		noTick()
 	}

 	tim.Stop()
 	if isTicker || !synctimerchan {
 		t.Logf("drain")
 		drain()
 	}
 	noTick()

 	// Again using select and with two goroutines waiting.
 	tim.Reset(10000 * Second)
 	if !synctimerchan {
 		drain()
 	}
 	done = make(chan bool, 2)
 	done1 := make(chan bool)
 	done2 := make(chan bool)
 	stop := make(chan bool)
 	go func() {
 		select {
 		case <-C:
 			done <- true
 		case <-stop:
 		}
 		close(done1)
 	}()
 	go func() {
 		select {
 		case <-C:
 			done <- true
 		case <-stop:
 		}
 		close(done2)
 	}()
 	Sleep(sched)
 	notDone(done)
 	tim.Reset(sched / 2)
 	Sleep(sched)
 	waitDone(done)
 	tim.Stop()
 	close(stop)
 	waitDone(done1)
 	waitDone(done2)
 	if isTicker {
 		// extra send might have sent done again
 		// (handled by buffering done above).
 		select {
 		default:
 		case <-done:
 		}
 		// extra send after that might have filled C.
 		select {
 		default:
 		case <-C:
 		}
 	}
 	notDone(done)

 	// Test enqueueTimerChan when timer is stopped.
 	stop = make(chan bool)
 	done = make(chan bool, 2)
 	for i := 0; i < 2; i++ {
 		go func() {
 			select {
 			case <-C:
 				panic("unexpected data")
 			case <-stop:
 			}
 			done <- true
 		}()
 	}
 	Sleep(sched)
 	close(stop)
 	waitDone(done)
 	waitDone(done)

 	// Test that Stop and Reset block old values from being received.
 	// (Proposal go.dev/issue/37196.)
 	if synctimerchan {
 		tim.Reset(1)
 		Sleep(10 * Millisecond)
 		if pending := tim.Stop(); pending != true {
 			t.Errorf("tim.Stop() = %v, want true", pending)
 		}
 		noTick()

 		tim.Reset(Hour)
 		noTick()
 		if pending := tim.Reset(1); pending != true {
 			t.Errorf("tim.Stop() = %v, want true", pending)
 		}
 		assertTick()
 		Sleep(10 * Millisecond)
 		if isTicker {
 			assertTick()
 			Sleep(10 * Millisecond)
 		} else {
 			noTick()
 		}
 		if pending, want := tim.Reset(Hour), isTicker; pending != want {
 			t.Errorf("tim.Stop() = %v, want %v", pending, want)
 		}
 		noTick()
 	}
 }

 func TestManualTicker(t *testing.T) {
 	// Code should not do this, but some old code dating to Go 1.9 does.
 	// Make sure this doesn't crash.
 	// See go.dev/issue/21874.
 	c := make(chan Time)
 	tick := &Ticker{C: c}
 	tick.Stop()
 }

 func TestAfterTimes(t *testing.T) {
 	t.Parallel()
 	// Using After(10ms) but waiting for 500ms to read the channel
 	// should produce a time from start+10ms, not start+500ms.
 	// Make sure it does.
 	// To avoid flakes due to very long scheduling delays,
 	// require 10 failures in a row before deciding something is wrong.
 	for i := 0; i < 10; i++ {
 		start := Now()
 		c := After(10 * Millisecond)
 		Sleep(500 * Millisecond)
 		dt := (<-c).Sub(start)
 		if dt < 400*Millisecond {
 			return
 		}
 		t.Logf("After(10ms) time is +%v, want <400ms", dt)
 	}
 	t.Errorf("not working")
 }

 func TestTickTimes(t *testing.T) {
 	t.Parallel()
 	// See comment in TestAfterTimes
 	for i := 0; i < 10; i++ {
 		start := Now()
 		c := Tick(10 * Millisecond)
 		Sleep(500 * Millisecond)
 		dt := (<-c).Sub(start)
 		if dt < 400*Millisecond {
 			return
 		}
 		t.Logf("Tick(10ms) time is +%v, want <400ms", dt)
 	}
 	t.Errorf("not working")
 }
	// Copyright 2009 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	package time_test

	import (
	"fmt"
	"runtime"
	"sync"
	"testing"
	. "time"
	)

	func TestTicker(t *testing.T) {
	t.Parallel()

	// We want to test that a ticker takes as much time as expected.
	// Since we don't want the test to run for too long, we don't
	// want to use lengthy times. This makes the test inherently flaky.
	// Start with a short time, but try again with a long one if the
	// first test fails.

	baseCount := 10
	baseDelta := 20 * Millisecond

	// On Darwin ARM64 the tick frequency seems limited. Issue 35692.
	if (runtime.GOOS == "darwin" \|\| runtime.GOOS == "ios") && runtime.GOARCH == "arm64" {
	// The following test will run ticker count/2 times then reset
	// the ticker to double the duration for the rest of count/2.
	// Since tick frequency is limited on Darwin ARM64, use even
	// number to give the ticks more time to let the test pass.
	// See CL 220638.
	baseCount = 6
	baseDelta = 100 * Millisecond
	}

	var errs []string
	logErrs := func() {
	for _, e := range errs {
	t.Log(e)
	}
	}

	for _, test := range []struct {
	count int
	delta Duration
	}{{
	count: baseCount,
	delta: baseDelta,
	}, {
	count: 8,
	delta: 1 * Second,
	}} {
	count, delta := test.count, test.delta
	ticker := NewTicker(delta)
	t0 := Now()
	for i := 0; i < count/2; i++ {
	<-ticker.C
	}
	ticker.Reset(delta * 2)
	for i := count / 2; i < count; i++ {
	<-ticker.C
	}
	ticker.Stop()
	t1 := Now()
	dt := t1.Sub(t0)
	target := 3 * delta * Duration(count/2)
	slop := target * 3 / 10
	if dt < target-slop \|\| dt > target+slop {
	errs = append(errs, fmt.Sprintf("%d %s ticks then %d %s ticks took %s, expected [%s,%s]", count/2, delta, count/2, delta*2, dt, target-slop, target+slop))
	if dt > target+slop {
	// System may be overloaded; sleep a bit
	// in the hopes it will recover.
	Sleep(Second / 2)
	}
	continue
	}
	// Now test that the ticker stopped.
	Sleep(2 * delta)
	select {
	case <-ticker.C:
	errs = append(errs, "Ticker did not shut down")
	continue
	default:
	// ok
	}

	// Test passed, so all done.
	if len(errs) > 0 {
	t.Logf("saw %d errors, ignoring to avoid flakiness", len(errs))
	logErrs()
	}

	return
	}

	t.Errorf("saw %d errors", len(errs))
	logErrs()
	}

	// Issue 21874
	func TestTickerStopWithDirectInitialization(t *testing.T) {
	c := make(chan Time)
	tk := &Ticker{C: c}
	tk.Stop()
	}

	// Test that a bug tearing down a ticker has been fixed. This routine should not deadlock.
	func TestTeardown(t *testing.T) {
	t.Parallel()

	Delta := 100 * Millisecond
	if testing.Short() {
	Delta = 20 * Millisecond
	}
	for i := 0; i < 3; i++ {
	ticker := NewTicker(Delta)
	<-ticker.C
	ticker.Stop()
	}
	}

	// Test the Tick convenience wrapper.
	func TestTick(t *testing.T) {
	// Test that giving a negative duration returns nil.
	if got := Tick(-1); got != nil {
	t.Errorf("Tick(-1) = %v; want nil", got)
	}
	}

	// Test that NewTicker panics when given a duration less than zero.
	func TestNewTickerLtZeroDuration(t *testing.T) {
	defer func() {
	if err := recover(); err == nil {
	t.Errorf("NewTicker(-1) should have panicked")
	}
	}()
	NewTicker(-1)
	}

	// Test that Ticker.Reset panics when given a duration less than zero.
	func TestTickerResetLtZeroDuration(t *testing.T) {
	defer func() {
	if err := recover(); err == nil {
	t.Errorf("Ticker.Reset(0) should have panicked")
	}
	}()
	tk := NewTicker(Second)
	tk.Reset(0)
	}

	func TestLongAdjustTimers(t *testing.T) {
	if runtime.GOOS == "android" \|\| runtime.GOOS == "ios" {
	t.Skipf("skipping on %s - too slow", runtime.GOOS)
	}
	t.Parallel()
	var wg sync.WaitGroup
	defer wg.Wait()

	// Build up the timer heap.
	const count = 5000
	wg.Add(count)
	for range count {
	go func() {
	defer wg.Done()
	Sleep(10 * Microsecond)
	}()
	}
	for range count {
	Sleep(1 * Microsecond)
	}

	// Give ourselves 60 seconds to complete.
	// This used to reliably fail on a Mac M3 laptop,
	// which needed 77 seconds.
	// Trybots are slower, so it will fail even more reliably there.
	// With the fix, the code runs in under a second.
	done := make(chan bool)
	AfterFunc(60*Second, func() { close(done) })

	// Set up a queing goroutine to ping pong through the scheduler.
	inQ := make(chan func())
	outQ := make(chan func())

	defer close(inQ)

	wg.Add(1)
	go func() {
	defer wg.Done()
	defer close(outQ)
	var q []func()
	for {
	var sendTo chan func()
	var send func()
	if len(q) > 0 {
	sendTo = outQ
	send = q[0]
	}
	select {
	case sendTo <- send:
	q = q[1:]
	case f, ok := <-inQ:
	if !ok {
	return
	}
	q = append(q, f)
	case <-done:
	return
	}
	}
	}()

	for i := range 50000 {
	const try = 20
	for range try {
	inQ <- func() {}
	}
	for range try {
	select {
	case _, ok := <-outQ:
	if !ok {
	t.Fatal("output channel is closed")
	}
	case <-After(5 * Second):
	t.Fatalf("failed to read work, iteration %d", i)
	case <-done:
	t.Fatal("timer expired")
	}
	}
	}
	}
	func BenchmarkTicker(b *testing.B) {
	benchmark(b, func(pb *testing.PB) {
	ticker := NewTicker(Nanosecond)
	for pb.Next() {
	<-ticker.C
	}
	ticker.Stop()
	})
	}

	func BenchmarkTickerReset(b *testing.B) {
	benchmark(b, func(pb *testing.PB) {
	ticker := NewTicker(Nanosecond)
	for pb.Next() {
	ticker.Reset(Nanosecond * 2)
	}
	ticker.Stop()
	})
	}

	func BenchmarkTickerResetNaive(b *testing.B) {
	benchmark(b, func(pb *testing.PB) {
	ticker := NewTicker(Nanosecond)
	for pb.Next() {
	ticker.Stop()
	ticker = NewTicker(Nanosecond * 2)
	}
	ticker.Stop()
	})
	}

	func TestTimerGC(t *testing.T) {
	run := func(t *testing.T, what string, f func()) {
	t.Helper()
	t.Run(what, func(t *testing.T) {
	t.Helper()
	const N = 1e4
	var stats runtime.MemStats
	runtime.GC()
	runtime.GC()
	runtime.GC()
	runtime.ReadMemStats(&stats)
	before := int64(stats.Mallocs - stats.Frees)

	for j := 0; j < N; j++ {
	f()
	}

	runtime.GC()
	runtime.GC()
	runtime.GC()
	runtime.ReadMemStats(&stats)
	after := int64(stats.Mallocs - stats.Frees)

	// Allow some slack, but inuse >= N means at least 1 allocation per iteration.
	inuse := after - before
	if inuse >= N {
	t.Errorf("%s did not get GC'ed: %d allocations", what, inuse)

	Sleep(1 * Second)
	runtime.ReadMemStats(&stats)
	after := int64(stats.Mallocs - stats.Frees)
	inuse = after - before
	t.Errorf("after a sleep: %d allocations", inuse)
	}
	})
	}

	run(t, "After", func() { After(Hour) })
	run(t, "Tick", func() { Tick(Hour) })
	run(t, "NewTimer", func() { NewTimer(Hour) })
	run(t, "NewTicker", func() { NewTicker(Hour) })
	run(t, "NewTimerStop", func() { NewTimer(Hour).Stop() })
	run(t, "NewTickerStop", func() { NewTicker(Hour).Stop() })
	}

	func TestChan(t *testing.T) {
	for _, name := range []string{"0", "1", "2"} {
	t.Run("asynctimerchan="+name, func(t *testing.T) {
	t.Setenv("GODEBUG", "asynctimerchan="+name)
	t.Run("Timer", func(t *testing.T) {
	tim := NewTimer(10000 * Second)
	testTimerChan(t, tim, tim.C, name == "0")
	})
	t.Run("Ticker", func(t *testing.T) {
	tim := &tickerTimer{Ticker: NewTicker(10000 * Second)}
	testTimerChan(t, tim, tim.C, name == "0")
	})
	})
	}
	}

	type timer interface {
	Stop() bool
	Reset(Duration) bool
	}

	// tickerTimer is a Timer with Reset and Stop methods that return bools,
	// to have the same signatures as Timer.
	type tickerTimer struct {
	*Ticker
	stopped bool
	}

	func (t *tickerTimer) Stop() bool {
	pending := !t.stopped
	t.stopped = true
	t.Ticker.Stop()
	return pending
	}

	func (t *tickerTimer) Reset(d Duration) bool {
	pending := !t.stopped
	t.stopped = false
	t.Ticker.Reset(d)
	return pending
	}

	func testTimerChan(t *testing.T, tim timer, C <-chan Time, synctimerchan bool) {
	_, isTimer := tim.(*Timer)
	isTicker := !isTimer

	// Retry parameters. Enough to deflake even on slow machines.
	// Windows in particular has very coarse timers so we have to
	// wait 10ms just to make a timer go off.
	const (
	sched = 10 * Millisecond
	tries = 100
	)

	drain := func() {
	select {
	case <-C:
	default:
	}
	}
	noTick := func() {
	t.Helper()
	select {
	default:
	case <-C:
	t.Errorf("extra tick")
	}
	}
	assertTick := func() {
	t.Helper()
	select {
	default:
	case <-C:
	return
	}
	for i := 0; i < tries; i++ {
	Sleep(sched)
	select {
	default:
	case <-C:
	return
	}
	}
	t.Errorf("missing tick")
	}
	assertLen := func() {
	t.Helper()
	if synctimerchan {
	if n := len(C); n != 0 {
	t.Errorf("synctimer has len(C) = %d, want 0 (always)", n)
	}
	return
	}
	var n int
	if n = len(C); n == 1 {
	return
	}
	for i := 0; i < tries; i++ {
	Sleep(sched)
	if n = len(C); n == 1 {
	return
	}
	}
	t.Errorf("len(C) = %d, want 1", n)
	}

	// Test simple stop; timer never in heap.
	tim.Stop()
	noTick()

	// Test modify of timer not in heap.
	tim.Reset(10000 * Second)
	noTick()

	if synctimerchan {
	// Test modify of timer in heap.
	tim.Reset(1)
	Sleep(sched)
	if l, c := len(C), cap(C); l != 0 \|\| c != 0 {
	//t.Fatalf("len(C), cap(C) = %d, %d, want 0, 0", l, c)
	}
	assertTick()
	} else {
	// Test modify of timer in heap.
	tim.Reset(1)
	assertTick()
	Sleep(sched)
	tim.Reset(10000 * Second)
	if isTicker {
	assertTick()
	}
	noTick()

	// Test that len sees an immediate tick arrive
	// for Reset of timer in heap.
	tim.Reset(1)
	assertLen()
	assertTick()

	// Test that len sees an immediate tick arrive
	// for Reset of timer NOT in heap.
	tim.Stop()
	if !synctimerchan {
	drain()
	}
	tim.Reset(1)
	assertLen()
	assertTick()
	}

	// Sleep long enough that a second tick must happen if this is a ticker.
	// Test that Reset does not lose the tick that should have happened.
	Sleep(sched)
	tim.Reset(10000 * Second)
	if !synctimerchan && isTicker {
	assertLen()
	assertTick()
	}
	noTick()

	notDone := func(done chan bool) {
	t.Helper()
	select {
	default:
	case <-done:
	t.Fatalf("early done")
	}
	}

	waitDone := func(done chan bool) {
	t.Helper()
	for i := 0; i < tries; i++ {
	Sleep(sched)
	select {
	case <-done:
	return
	default:
	}
	}
	t.Fatalf("never got done")
	}

	// Reset timer in heap (already reset above, but just in case).
	tim.Reset(10000 * Second)
	if !synctimerchan {
	drain()
	}

	// Test stop while timer in heap (because goroutine is blocked on <-C).
	done := make(chan bool)
	notDone(done)
	go func() {
	<-C
	close(done)
	}()
	Sleep(sched)
	notDone(done)

	// Test reset far away while timer in heap.
	tim.Reset(20000 * Second)
	Sleep(sched)
	notDone(done)

	// Test imminent reset while in heap.
	tim.Reset(1)
	waitDone(done)

	// If this is a ticker, another tick should have come in already
	// (they are 1ns apart). If a timer, it should have stopped.
	if isTicker {
	assertTick()
	} else {
	noTick()
	}

	tim.Stop()
	if isTicker \|\| !synctimerchan {
	t.Logf("drain")
	drain()
	}
	noTick()

	// Again using select and with two goroutines waiting.
	tim.Reset(10000 * Second)
	if !synctimerchan {
	drain()
	}
	done = make(chan bool, 2)
	done1 := make(chan bool)
	done2 := make(chan bool)
	stop := make(chan bool)
	go func() {
	select {
	case <-C:
	done <- true
	case <-stop:
	}
	close(done1)
	}()
	go func() {
	select {
	case <-C:
	done <- true
	case <-stop:
	}
	close(done2)
	}()
	Sleep(sched)
	notDone(done)
	tim.Reset(sched / 2)
	Sleep(sched)
	waitDone(done)
	tim.Stop()
	close(stop)
	waitDone(done1)
	waitDone(done2)
	if isTicker {
	// extra send might have sent done again
	// (handled by buffering done above).
	select {
	default:
	case <-done:
	}
	// extra send after that might have filled C.
	select {
	default:
	case <-C:
	}
	}
	notDone(done)

	// Test enqueueTimerChan when timer is stopped.
	stop = make(chan bool)
	done = make(chan bool, 2)
	for i := 0; i < 2; i++ {
	go func() {
	select {
	case <-C:
	panic("unexpected data")
	case <-stop:
	}
	done <- true
	}()
	}
	Sleep(sched)
	close(stop)
	waitDone(done)
	waitDone(done)

	// Test that Stop and Reset block old values from being received.
	// (Proposal go.dev/issue/37196.)
	if synctimerchan {
	tim.Reset(1)
	Sleep(10 * Millisecond)
	if pending := tim.Stop(); pending != true {
	t.Errorf("tim.Stop() = %v, want true", pending)
	}
	noTick()

	tim.Reset(Hour)
	noTick()
	if pending := tim.Reset(1); pending != true {
	t.Errorf("tim.Stop() = %v, want true", pending)
	}
	assertTick()
	Sleep(10 * Millisecond)
	if isTicker {
	assertTick()
	Sleep(10 * Millisecond)
	} else {
	noTick()
	}
	if pending, want := tim.Reset(Hour), isTicker; pending != want {
	t.Errorf("tim.Stop() = %v, want %v", pending, want)
	}
	noTick()
	}
	}

	func TestManualTicker(t *testing.T) {
	// Code should not do this, but some old code dating to Go 1.9 does.
	// Make sure this doesn't crash.
	// See go.dev/issue/21874.
	c := make(chan Time)
	tick := &Ticker{C: c}
	tick.Stop()
	}

	func TestAfterTimes(t *testing.T) {
	t.Parallel()
	// Using After(10ms) but waiting for 500ms to read the channel
	// should produce a time from start+10ms, not start+500ms.
	// Make sure it does.
	// To avoid flakes due to very long scheduling delays,
	// require 10 failures in a row before deciding something is wrong.
	for i := 0; i < 10; i++ {
	start := Now()
	c := After(10 * Millisecond)
	Sleep(500 * Millisecond)
	dt := (<-c).Sub(start)
	if dt < 400*Millisecond {
	return
	}
	t.Logf("After(10ms) time is +%v, want <400ms", dt)
	}
	t.Errorf("not working")
	}

	func TestTickTimes(t *testing.T) {
	t.Parallel()
	// See comment in TestAfterTimes
	for i := 0; i < 10; i++ {
	start := Now()
	c := Tick(10 * Millisecond)
	Sleep(500 * Millisecond)
	dt := (<-c).Sub(start)
	if dt < 400*Millisecond {
	return
	}
	t.Logf("Tick(10ms) time is +%v, want <400ms", dt)
	}
	t.Errorf("not working")
	}