time: TestTicker timeouts after CL 232298

[bcmills]

(Issue forked from #42237.)

panic: test timed out after 15m0s

goroutine 219 [running]:
testing.(*M).startAlarm.func1()
	C:/workdir/go/src/testing/testing.go:1680 +0xe6
created by time.goFunc
	C:/workdir/go/src/time/sleep.go:167 +0x4b

goroutine 1 [chan receive, 14 minutes]:
testing.(*T).Run(0xc000058000, 0x4cb903, 0xa, 0x545b78, 0x368a01)
	C:/workdir/go/src/testing/testing.go:1219 +0x2da
testing.runTests.func1(0xc000048f00)
	C:/workdir/go/src/testing/testing.go:1491 +0x7f
testing.tRunner(0xc000048f00, 0xc000133de0)
	C:/workdir/go/src/testing/testing.go:1173 +0xef
testing.runTests(0xc000004558, 0x674680, 0x6f, 0x6f, 0xbfe1343f8f278ecc, 0xd18c5b20e5, 0x678480, 0x4cca88)
	C:/workdir/go/src/testing/testing.go:1489 +0x310
testing.(*M).Run(0xc00001e100, 0x0)
	C:/workdir/go/src/testing/testing.go:1397 +0x1f5
main.main()
	_testmain.go:377 +0x145

goroutine 31 [chan receive, 14 minutes]:
testing.(*T).Parallel(0xc00019d680)
	C:/workdir/go/src/testing/testing.go:1039 +0x119
time_test.TestParseYday(0xc00019d680)
	C:/workdir/go/src/time/format_test.go:762 +0x52
testing.tRunner(0xc00019d680, 0x545ae8)
	C:/workdir/go/src/testing/testing.go:1173 +0xef
created by testing.(*T).Run
	C:/workdir/go/src/testing/testing.go:1218 +0x2b3

goroutine 231 [chan receive, 14 minutes]:
time_test.TestTicker(0xc000058000)
	C:/workdir/go/src/time/tick_test.go:45 +0xc5
testing.tRunner(0xc000058000, 0x545b78)
	C:/workdir/go/src/testing/testing.go:1173 +0xef
created by testing.(*T).Run
	C:/workdir/go/src/testing/testing.go:1218 +0x2b3
FAIL	time	900.029s

2020-11-05T19:22:02-370682a/windows-amd64-longtest
2020-11-05T16:47:45-a19a4dc/windows-amd64-longtest
2020-11-05T15:16:57-34c0969/windows-amd64-longtest
2020-11-02T21:08:14-4fcb506/windows-amd64-longtest
2020-11-02T15:23:43-d5388e2/linux-amd64-longtest
2020-11-02T05:29:59-f2ee58b/windows-amd64-longtest
2020-11-02T03:03:16-0387bed/windows-amd64-longtest
2020-10-30T18:06:13-6d087c8/windows-amd64-longtest
2020-10-30T16:29:11-1af388f/windows-amd64-longtest
2020-10-30T15:25:49-e1faebe/linux-386-longtest
2020-10-29T20:50:02-38d1ec8/windows-amd64-longtest
2020-10-29T19:06:32-5cc43c5/windows-amd64-longtest
2020-10-29T03:52:23-3089ef6/windows-amd64-longtest
2020-10-28T00:48:22-49b017f/windows-amd64-longtest
2020-10-28T00:25:05-40d1ec5/windows-amd64-longtest
2020-10-27T22:57:36-b4b0144/windows-amd64-longtest

CC @prattmic @ChrisHines @aclements @ianlancetaylor

[prattmic]

One of these failures was after http://golang.org/cl/267257, so it seems that's not the fix.

2020-11-06T23:59:04-f7ef5ca/windows-amd64-longtest

[aclements]

@laboger may have an easy reproducer for this on linux/amd64 (and linux/ppc64le) in #42402.

[bcmills]

Sounds like #42402 was fixed by CL 267257, so unfortunately we don't seem to have a lead on a reliable reproducer.

[prattmic]

Still ongoing, as expected:

2020-11-30T03:18:36-e5da18d/windows-amd64-longtest
2020-11-25T04:48:23-750b372/windows-amd64-longtest
2020-11-25T02:18:15-f6dcc97/windows-amd64-longtest
2020-11-24T19:53:18-65dcd15/linux-386-longtest
2020-11-19T04:02:59-5ba1c3f/windows-amd64-longtest
2020-11-17T15:10:45-0968d2d/linux-amd64-longtest
2020-11-14T17:24:37-92c732e/windows-amd64-longtest
2020-11-13T22:01:37-86954d5/linux-amd64-longtest
2020-11-13T15:00:23-35455ff/netbsd-386-9_0
2020-11-12T14:38:38-c167635/windows-amd64-longtest
2020-11-06T23:59:04-f7ef5ca/windows-amd64-longtest

There are also a few TestTeardown timeouts, which I suspect are related:

2020-11-28T03:50:50-358d354/windows-amd64-longtest
2020-11-02T05:29:59-f2ee58b/windows-amd64-longtest
2020-10-30T16:29:11-1af388f/windows-amd64-longtest
2020-10-29T19:06:32-5cc43c5/windows-amd64-longtest

[prattmic]

The root cause is a timer in TestCheckRuntimeTimerOverflow, where we explictly set t.when = 0: https://cs.opensource.google/go/go/+/master:src/time/internal_test.go;l=67;bpv=0;bpt=1.

This timer is on top of the heap, and in turn, pp.timer0When = 0, and then checkTimers will decide there are no timers at all, thus missing any timers further down the heap.

I'm still looking into when we broke this, because as I recall, we've treated timer0When == 0 as "no timers" for a while.

As an aside, @mknyszek notes that we may want timer0When's sentinel value to be maxWhen to make this more obvious (regardless if it fixes the issue directly).

---

[5.158222300 P 2] M 4 modtimer: modify t 0xc0000661e0 when 9223372036854775807 nextwhen 0 P 2 
        0xec3288 [time.resetTimer+0x68 C:/workdir/go/src/runtime/time.go:611]
        0xec3244 [time.resetTimer+0x24 C:/workdir/go/src/runtime/time.go:224]
        0xef58c4 [time.CheckRuntimeTimerOverflow.func1+0x64 C:/workdir/go/src/time/internal_test.go:67]
        0xef56bb [time.CheckRuntimeTimerOverflow+0xfb C:/workdir/go/src/time/internal_test.go:78]
        0xfe5cee [time_test.TestOverflowRuntimeTimer+0x4e C:/workdir/go/src/time/sleep_test.go:482]
        0xf4de2e [testing.tRunner+0xee C:/workdir/go/src/testing/testing.go:1195]
        0xec6100 [runtime.goexit+0x0 C:/workdir/go/src/runtime/asm_amd64.s:1367]
...
[5.158222300 P 2] M 4 cleantimers: modified t 0xc0000661e0 when 9223372036854775807 nextwhen 0
...
[5.158222300 P 2] P 2 dodeltimer0: t 0xc0000661e0
...
[5.158222300 P 2] P 2 doaddtimer: t 0xc0000661e0 when 0 now 7187394008100 until -7187394008100 len(pp.timers) 0 
... (when = 0 at top of heap)
[5.158222300 P 2] P 2 doaddtimer: t 0xc000066378 when 7187414008100 now 7187394008100 until 20000000 len(pp.timers) 1 
        0xeacabc [runtime.addtimer+0xbc C:/workdir/go/src/runtime/time.go:260]
        0xec3191 [time.startTimer+0x31 C:/workdir/go/src/runtime/time.go:207]
        0xeeaa64 [time.NewTicker+0xe4 C:/workdir/go/src/time/tick.go:39]
        0xfe6c89 [time_test.TestTicker+0x349 C:/workdir/go/src/time/tick_test.go:42]
        0xf4de2e [testing.tRunner+0xee C:/workdir/go/src/testing/testing.go:1195]
        0xec6100 [runtime.goexit+0x0 C:/workdir/go/src/runtime/asm_amd64.s:1367]
[5.158222300 P 2] P 2 doaddtimer done: t 0xc000066378 len(pp.timers) 2 timer0When 0 numTimers 2
... (added new timer, timer0When still 0)
[5.158222300 P 2] M 4 checkTimers: P 2
[5.158222300 P 2] M 4 checkTimers: no timers: timer0When 0 timerModifiedEarliest 0
... (no timers!)

[prattmic]

Bisect shows that this was broken by golang.org/cl/258303 (cc @ianlancetaylor). If I understand correctly, previously this happened:

1. TestCheckRuntimeTimerOverflow timer sets nextwhen = 0 in modtimer, and increments adjustTimers.
2. checkTimers sees adjustTimers != 0, adjusts timer, and then immediately runs the when = 0 timer, removing it from the heap.

Now we've only updated timerModifiedEarliest. Since timer0When and timerModifiedEarliest are 0, checkTimers will never run the when = 0 timer, causing the stuckness.

Now, how to fix? The simple fix is to change https://cs.opensource.google/go/go/+/master:src/time/internal_test.go;l=67;bpv=1;bpt=0 to set when to 1 instead of 0. The overflow test still does it's job, the only question is could a timer ever legitimately get when = 0? I don't think so, but I may be missing something.

[gopherbot]

Change https://golang.org/cl/274632 mentions this issue: time, runtime: don't set timer when = 0