time: racy Timer access should either work or throw, not panic

[bcmills]

2020-02-21T20:22:19-13d73b2/linux-arm

panic: runtime error: racy use of timers
	panic: err must be non-nil

goroutine 3372 [running]:
net/http.(*http2pipe).closeWithError(0xd6f774, 0xd6f7a8, 0x0, 0x0, 0x0)
	/workdir/go/src/net/http/h2_bundle.go:3558 +0x210
net/http.(*http2pipe).CloseWithError(...)
	/workdir/go/src/net/http/h2_bundle.go:3545
net/http.(*http2clientConnReadLoop).cleanup(0xc43fd4)
	/workdir/go/src/net/http/h2_bundle.go:8228 +0x21c
panic(0x467b60, 0x580c18)
	/workdir/go/src/runtime/panic.go:967 +0x118
time.stopTimer(0x10c9e84, 0x10a9ca8)
	/workdir/go/src/runtime/time.go:224 +0x1c
time.(*Timer).Stop(...)
	/workdir/go/src/time/sleep.go:78
net/http.http2bodyWriterState.on100(0xd6f760, 0x10c9e80, 0x10a9c80, 0xefc780, 0x10c9e40, 0x3b9aca00, 0x0)
	/workdir/go/src/net/http/h2_bundle.go:9057 +0x38
net/http.(*http2clientConnReadLoop).handleResponse(0xc43fd4, 0xd6f760, 0xefc800, 0xd6f760, 0x58a650, 0x2cdc78)
	/workdir/go/src/net/http/h2_bundle.go:8433 +0x2fc
net/http.(*http2clientConnReadLoop).processHeaders(0xc43fd4, 0xefc800, 0x0, 0x0)
	/workdir/go/src/net/http/h2_bundle.go:8355 +0x14c
net/http.(*http2clientConnReadLoop).run(0xc43fd4, 0x0, 0x0)
	/workdir/go/src/net/http/h2_bundle.go:8277 +0x648
net/http.(*http2ClientConn).readLoop(0xdb2000)
	/workdir/go/src/net/http/h2_bundle.go:8174 +0x60
created by net/http.(*http2Transport).newClientConn
	/workdir/go/src/net/http/h2_bundle.go:7174 +0x538
FAIL	net/http	6.612s

CC @ianlancetaylor @bradfitz @tombergan

[bcmills]

Could this be related to #37196? (I'm not sure.)

[rsc]

The panic inside time.Timer.Stop should really be runtime.throw. Code does not expect panic from Stop. If there's really a racy use of timers and timers are corrupt, stop executing.

[rsc]

Also, time.stopTimer clearly called deltimer, and deltimer called badTimer, which panicked. Do we not have a stack frame for deltimer showing what line it is at? Do we not have a stack frame for badTimer? Why not?

[bcmills]

That may be #26206..?

[rsc]

This program crashes very quickly:

package main

import "time"

func main() {
	for i := 0; i < 1000; i++ {
		t := time.AfterFunc(1*time.Second, func() {})
		go t.Stop()
		go t.Reset(1 * time.Second)
		go t.Stop()
		go t.Reset(1 * time.Second)
	}
	time.Sleep(10 * time.Second)
}

I don't believe this program should panic. The claim in the runtime code is that the runtime timer data structures may be corrupted, but if so then there is not enough locking protecting against bad user call sequences. But the bad user call sequences should find a way to complete (serialize them somehow), no?

[rsc]

Seems like a possible Go 1.14 release blocker.

[rsc]

/cc @aclements

[dmitshur]

This program crashes very quickly

I can reproduce this with go1.14rc1, go1.14beta1, but also with go1.13.7 on darwin/amd64:

$ go run .
panic: runtime error: racy use of timers

goroutine 195 [running]:
time.stopTimer(0xc00005af08, 0x1)
	/usr/local/go/src/runtime/time.go:224 +0x2b
time.(*Timer).Stop(0xc00005af00, 0x3b9aca00)
	/usr/local/go/src/time/sleep.go:78 +0x36
created by main.main
	/var/folders/zb/5p8cwfhj29gf_m8vdy8ylmlr00jwcj/T/tmp.5VGZN9cY/main.go:10 +0xb5
exit status 2

$ go1.14beta1 run .
panic: runtime error: racy use of timers

goroutine 177 [running]:
time.stopTimer(0xc000060c88, 0x1)
	/Users/dmitshur/sdk/go1.14beta1/src/runtime/time.go:219 +0x2b
time.(*Timer).Stop(0xc000060c80, 0x3b9aca00)
	/Users/dmitshur/sdk/go1.14beta1/src/time/sleep.go:78 +0x36
created by main.main
	/var/folders/zb/5p8cwfhj29gf_m8vdy8ylmlr00jwcj/T/tmp.5VGZN9cY/main.go:10 +0xb5
panic: runtime error: racy use of timers

goroutine 198 [running]:
time.resetTimer(0xc000060e18, 0xb9845fda967c)
	/Users/dmitshur/sdk/go1.14beta1/src/runtime/time.go:228 +0x35
time.(*Timer).Reset(0xc000060e10, 0x3b9aca00, 0x0)
	/Users/dmitshur/sdk/go1.14beta1/src/time/sleep.go:126 +0x7d
created by main.main
	/var/folders/zb/5p8cwfhj29gf_m8vdy8ylmlr00jwcj/T/tmp.5VGZN9cY/main.go:11 +0xe0
exit status 2

$ go1.13.7 run .   
panic: runtime error: racy use of timers

goroutine 355 [running]:
time.startTimer(0xc000087548)
	/Users/dmitshur/sdk/go1.13.7/src/runtime/time.go:114 +0x2b
time.(*Timer).Reset(0xc000087540, 0x3b9aca00, 0x0)
	/Users/dmitshur/sdk/go1.13.7/src/time/sleep.go:126 +0x81
created by main.main
	/var/folders/zb/5p8cwfhj29gf_m8vdy8ylmlr00jwcj/T/tmp.5VGZN9cY/main.go:9 +0x93
exit status 2