time: more flake removal in asynctimerchan test

Trying to write a test for the corner cases in the old async timer chan
implementation may have been a mistake, especially since this isn't
going to be the default timer chan implementation anymore.
But let's try one more time to fix the test.

I reproduced the remaining builder failures on my Mac laptop
by overloading the CPU in one window and then running 48 instances
of the flaky test in loops using 'stress' in another window.

It turns out that, contrary to my understanding of async timers
and therefore contrary to what the test expected, it is technically
possible for

	t := time.NewTicker(1)
	t.Reset(1000*time.Hour)
	<-t.C
	<-t.C

to observe two time values on t.C, as opposed to blocking forever.

We always expect the first time value, since the ticker goes off
immediately (after 1ns) and sends that value into the channel buffer.
To get the second value, the ticker has to be in the process of
going off (which it is doing constantly anyway), and the timer
goroutine has to be about to call sendTime and then get rescheduled.
Then t.Reset and the first <-t.C have to happen.
Then the timer goroutine gets rescheduled and can run sendTime's
non-blocking send on t.C, which finds an empty buffer and writes
a value.

This is unlikely, of course, but it definitely happens. This program
always panics in just a second or two on my laptop:

	package main

	import (
		"os"
		"time"
	)

	func main() {
		os.Setenv("GODEBUG", "asynctimerchan=1")
		for  {
			go func() {
				t := time.NewTicker(1)
				t.Reset(1000*time.Hour)
				<-t.C
				select {
				case <-t.C:
					panic("two receives")
				case <-time.After(1*time.Second):
				}
			}()
		}
	}

Because I did not understand this nuance, the test did not expect it.

This CL rewrites the test to expect that possibility. I can no longer
make the test fail under 'stress' on my laptop.

For #66322.

Change-Id: I15c75d2c6f24197c43094da20d6ab55306a0a9f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/585359
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/time/tick_test.go

@@ -361,7 +361,11 @@ func testTimerChan(t *testing.T, tim timer, C <-chan Time, synctimerchan bool) {
 		drainTries = 5
 	)
 
-	drain := func() {
+	// drain1 removes one potential stale time value
+	// from the timer/ticker channel after Reset.
+	// When using Go 1.23 sync timers/tickers, draining is never needed
+	// (that's the whole point of the sync timer/ticker change).
+	drain1 := func() {
 		for range drainTries {
 			select {
 			case <-C:
@@ -371,6 +375,34 @@ func testTimerChan(t *testing.T, tim timer, C <-chan Time, synctimerchan bool) {
 			Sleep(sched)
 		}
 	}
+
+	// drainAsync removes potential stale time values after Stop/Reset.
+	// When using Go 1 async timers, draining one or two values
+	// may be needed after Reset or Stop (see comments in body for details).
+	drainAsync := func() {
+		if synctimerchan {
+			// sync timers must have the right semantics without draining:
+			// there are no stale values.
+			return
+		}
+
+		// async timers can send one stale value (then the timer is disabled).
+		drain1()
+		if isTicker {
+			// async tickers can send two stale values: there may be one
+			// sitting in the channel buffer, and there may also be one
+			// send racing with the Reset/Stop+drain that arrives after
+			// the first drain1 has pulled the value out.
+			// This is rare, but it does happen on overloaded builder machines.
+			// It can also be reproduced on an M3 MacBook Pro using:
+			//
+			//	go test -c strings
+			//	stress ./strings.test &   # chew up CPU
+			//	go test -c -race time
+			//	stress -p 48 ./time.test -test.count=10 -test.run=TestChan/asynctimerchan=1/Ticker
+			drain1()
+		}
+	}
 	noTick := func() {
 		t.Helper()
 		select {
@@ -439,9 +471,7 @@ func testTimerChan(t *testing.T, tim timer, C <-chan Time, synctimerchan bool) {
 		assertTick()
 		Sleep(sched)
 		tim.Reset(10000 * Second)
-		if isTicker {
-			drain()
-		}
+		drainAsync()
 		noTick()
 
 		// Test that len sees an immediate tick arrive
@@ -453,9 +483,7 @@ func testTimerChan(t *testing.T, tim timer, C <-chan Time, synctimerchan bool) {
 		// Test that len sees an immediate tick arrive
 		// for Reset of timer NOT in heap.
 		tim.Stop()
-		if !synctimerchan {
-			drain()
-		}
+		drainAsync()
 		tim.Reset(1)
 		assertLen()
 		assertTick()
@@ -465,9 +493,7 @@ func testTimerChan(t *testing.T, tim timer, C <-chan Time, synctimerchan bool) {
 	// Test that Reset does not lose the tick that should have happened.
 	Sleep(sched)
 	tim.Reset(10000 * Second)
-	if !synctimerchan && isTicker {
-		drain()
-	}
+	drainAsync()
 	noTick()
 
 	notDone := func(done chan bool) {
@@ -494,9 +520,7 @@ func testTimerChan(t *testing.T, tim timer, C <-chan Time, synctimerchan bool) {
 
 	// Reset timer in heap (already reset above, but just in case).
 	tim.Reset(10000 * Second)
-	if !synctimerchan {
-		drain()
-	}
+	drainAsync()
 
 	// Test stop while timer in heap (because goroutine is blocked on <-C).
 	done := make(chan bool)
@@ -526,17 +550,12 @@ func testTimerChan(t *testing.T, tim timer, C <-chan Time, synctimerchan bool) {
 	}
 
 	tim.Stop()
-	if isTicker || !synctimerchan {
-		t.Logf("drain")
-		drain()
-	}
+	drainAsync()
 	noTick()
 
 	// Again using select and with two goroutines waiting.
 	tim.Reset(10000 * Second)
-	if !synctimerchan {
-		drain()
-	}
+	drainAsync()
 	done = make(chan bool, 2)
 	done1 := make(chan bool)
 	done2 := make(chan bool)