context: misuse of `sync.Cond` in `ExampleAfterFunc_cond`

[bcmills]

#!watchflakes
post <- `^goroutine \d+ \[sync\.Cond\.Wait, .*\]:` && `^context_test\.ExampleAfterFunc_cond`

(Pulled out from #62165 (comment).)

https://build.golang.org/log/6040fd74316c6fe15adcca97b7b68a471db61575:

goroutine 1 [sync.Cond.Wait, 14 minutes]:
runtime.gopark(0x40002800e0?, 0x40000021a0?, 0x88?, 0x59?, 0xc731c?)
	/home/gopher/build/go/src/runtime/proc.go:398 +0xc8 fp=0x4000235940 sp=0x4000235920 pc=0x4a178
runtime.goparkunlock(...)
	/home/gopher/build/go/src/runtime/proc.go:404
sync.runtime_notifyListWait(0x40000ce010, 0x0)
	/home/gopher/build/go/src/runtime/sema.go:527 +0x150 fp=0x4000235990 sp=0x4000235940 pc=0x78580
sync.(*Cond).Wait(0x40000ce000)
	/home/gopher/build/go/src/sync/cond.go:70 +0xcc fp=0x40002359d0 sp=0x4000235990 pc=0x8093c
context_test.ExampleAfterFunc_cond.func1({0x1d3bf8, 0x40000b00e0}, 0x40000ce000)
	/home/gopher/build/go/src/context/example_test.go:131 +0xa8 fp=0x4000235a30 sp=0x40002359d0 pc=0x1469d8
context_test.ExampleAfterFunc_cond()
	/home/gopher/build/go/src/context/example_test.go:142 +0x120 fp=0x4000235ab0 sp=0x4000235a30 pc=0x13f190
testing.runExample({{0x19d3c8, 0x15}, 0x1a78e0, {0x19f322, 0x1a}, 0x0})
	/home/gopher/build/go/src/testing/run_example.go:63 +0x244 fp=0x4000235bb0 sp=0x4000235ab0 pc=0xd58f4
testing.runExamples(0x1d4c78?, {0x2ca040, 0x7, 0x25?})
	/home/gopher/build/go/src/testing/example.go:40 +0x104 fp=0x4000235c50 sp=0x4000235bb0 pc=0xd1ad4
testing.(*M).Run(0x400011a280)
	/home/gopher/build/go/src/testing/testing.go:1927 +0x5e4 fp=0x4000235ea0 sp=0x4000235c50 pc=0xdb404
main.main()
	_testmain.go:149 +0x1a8 fp=0x4000235f30 sp=0x4000235ea0 pc=0x147888
runtime.main()
	/home/gopher/build/go/src/runtime/proc.go:267 +0x2b8 fp=0x4000235fd0 sp=0x4000235f30 pc=0x49d18
runtime.goexit()
	/home/gopher/build/go/src/runtime/asm_arm64.s:1197 +0x4 fp=0x4000235fd0 sp=0x4000235fd0 pc=0x7c4c4

This test / example was added in https://go.dev/cl/482695 (attn @dneil @Sajmani) for #57928; it's new as of Go 1.21.

Given that this example involves a sync.Cond and a context.WithTimeout, I'm about 98% confident that the bug is in the example somewhere. 😅

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
post <- `^goroutine \d+ \[sync\.Cond\.Wait, .*\]:` && `^context_test\.ExampleAfterFunc_cond`

2023-08-07 22:48 openbsd-arm64-jsing go@b9153f6e context (log)

panic: test timed out after 15m0s

goroutine 304 [running]:
panic({0x161860?, 0x40001c4010?})
	/home/gopher/build/go/src/runtime/panic.go:621 +0x140 fp=0x4000047f00 sp=0x4000047e50 pc=0x46800
testing.(*M).startAlarm.func1()
	/home/gopher/build/go/src/testing/testing.go:2259 +0x30c fp=0x4000047fd0 sp=0x4000047f00 pc=0xdd86c
runtime.goexit()
	/home/gopher/build/go/src/runtime/asm_arm64.s:1197 +0x4 fp=0x4000047fd0 sp=0x4000047fd0 pc=0x7c4c4
created by time.goFunc
	/home/gopher/build/go/src/time/sleep.go:177 +0x38

— watchflakes

[bcmills]

@gopherbot, please backport to Go 1.21. This example in the context documentation for an API added in Go 1.21 demonstrates incorrect use of a sync.Cond.

[gopherbot]

Backport issue(s) opened: #62189 (for 1.21).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.

[gopherbot]

Change https://go.dev/cl/521596 mentions this issue: context: fix synchronization in ExampleAfterFunc_cond

[bcmills]

I will send a fix. Further evidence that sync.Cond should be avoided.