test: avoid scheduler races in context waits

[cpunion]

Summary

• check already-canceled contexts synchronously instead of racing Done against a newly created short timer
• give asynchronous AfterFunc delivery a realistic bound and recheck the success channel if the timeout case is selected
• apply the synchronous assertion consistently to cancel and past-deadline cases

This changes tests only; it does not relax or modify context, timer, or scheduler semantics.

Why

The Windows MinGW full-test lane in cpunion#208 reported TestAfterFunc and TestWithTimeoutCause, then passed unchanged on rerun. WithTimeoutCause(parent, -time.Hour, cause) closes Done before returning. In the old assertion, time.After(10ms) was created while entering the select; if the test goroutine was descheduled long enough, both channels were ready and Go could legally choose the timeout case. The 100ms asynchronous callback bound had the same scheduler sensitivity.

The immediate cases now verify the stronger contract directly with a nonblocking receive. The asynchronous helper also checks Done again before reporting a timeout, so a random select choice cannot produce a false failure.

Validation

• go test ./test/std/context -count=100
• go test -race ./test/std/context -count=10
• unchanged failed jobs from cpunion/llgo#208 both passed on rerun: macOS LLGo and Windows MinGW full tests

[fennoai]

Review: avoid scheduler races in context waits

Focused, well-scoped change to test/std/context/context_test.go. It introduces two helpers — requireDone (non-blocking "must already be done") and waitDone (wait up to a timeout, with a recheck) — and replaces the ad-hoc select { case <-ch: ... case <-time.After(...) } patterns.

Assessment: The refactor is correct and improves the tests.

• requireDone is the right (stricter) check at the four synchronous-cancellation sites: an explicit cancel(...) or a past deadline/timeout cancels the context before the constructor/cancel call returns, so a non-blocking select with default is appropriate and would catch a regression where cancellation is not synchronous.
• waitDone correctly uses time.NewTimer + defer timer.Stop() (no leaked timer) and rechecks done after the timer fires to avoid a false timeout when select picks the timer branch even though done became ready.
• Happy-path runtime is unaffected: the larger bounds only apply on the failure path; on success the <-done branch is taken immediately.
• No security or performance concerns.

Two minor, non-blocking notes are inline below.

[github-actions]

LLGo baseline benchmarks

cd564c6d258e | workflow run | long-term charts

Program measurements

Platform	Workload	File size	vs base	Text size	vs base	Build	vs base	Run	vs base
Linux	cprintf	19496 B	0 B / +0.0%	387 B	0 B / +0.0%	357.014 ms	+4.735 ms / +1.3% (worse)	1.338 ms	-33.78 us / -2.5% (better)
Linux	cprintf-lto	19328 B	0 B / +0.0%	368 B	0 B / +0.0%	369.691 ms	+18.55 ms / +5.3% (worse)	1.404 ms	-257.9 us / -15.5% (better)
Linux	fmtprintf	1607888 B	-8 B / -0.0004975% (better)	490042 B	0 B / +0.0%	2.435 s	+4.893 ms / +0.2% (worse)	3.449 ms	-100.9 us / -2.8% (better)
Linux	fmtprintf-lto	1485616 B	0 B / +0.0%	450888 B	0 B / +0.0%	8.232 s	+187.8 ms / +2.3% (worse)	3.402 ms	+48.95 us / +1.5% (worse)
Linux	println	62344 B	0 B / +0.0%	15286 B	0 B / +0.0%	338.747 ms	-3.426 ms / -1.0% (better)	1.717 ms	+7.881 us / +0.5% (worse)
Linux	println-lto	54160 B	0 B / +0.0%	12914 B	0 B / +0.0%	532.185 ms	+12.37 ms / +2.4% (worse)	1.743 ms	+19.52 us / +1.1% (worse)
macOS	cprintf	84480 B	0 B / +0.0%	16733 B	0 B / +0.0%	489.452 ms	-8.041 ms / -1.6% (better)	3.488 ms	0 ns / +0.0%
macOS	cprintf-lto	100704 B	0 B / +0.0%	16713 B	0 B / +0.0%	718.485 ms	+160.5 ms / +28.8% (worse)	3.790 ms	-4.662 ms / -55.2% (better)
macOS	fmtprintf	1470304 B	0 B / +0.0%	867088 B	0 B / +0.0%	3.206 s	+274.5 ms / +9.4% (worse)	7.059 ms	-1.214 ms / -14.7% (better)
macOS	fmtprintf-lto	1175536 B	0 B / +0.0%	862804 B	0 B / +0.0%	8.651 s	-1.16 s / -11.8% (better)	8.553 ms	+1.436 ms / +20.2% (worse)
macOS	println	114784 B	0 B / +0.0%	35181 B	0 B / +0.0%	578.547 ms	-276.9 ms / -32.4% (better)	4.788 ms	-1.498 ms / -23.8% (better)
macOS	println-lto	118656 B	0 B / +0.0%	32833 B	0 B / +0.0%	757.851 ms	-282.4 ms / -27.2% (better)	4.494 ms	-969.3 us / -17.7% (better)
Windows MinGW	cprintf	20480 B	0 B / +0.0%	4662 B	0 B / +0.0%	890.689 ms	+92.65 ms / +11.6% (worse)	3.918 ms	+591.8 us / +17.8% (worse)
Windows MinGW	cprintf-lto	18432 B	0 B / +0.0%	4582 B	0 B / +0.0%	908.830 ms	+81.43 ms / +9.8% (worse)	3.860 ms	+411.6 us / +11.9% (worse)
Windows MinGW	fmtprintf	1936384 B	0 B / +0.0%	597398 B	0 B / +0.0%	3.683 s	+251.1 ms / +7.3% (worse)	9.119 ms	+1.066 ms / +13.2% (worse)
Windows MinGW	fmtprintf-lto	1990656 B	0 B / +0.0%	590870 B	0 B / +0.0%	9.940 s	+660.6 ms / +7.1% (worse)	8.069 ms	+393.8 us / +5.1% (worse)
Windows MinGW	println	74240 B	0 B / +0.0%	25062 B	0 B / +0.0%	852.072 ms	-399.4 us / -0.04685% (better)	6.876 ms	+370.6 us / +5.7% (worse)
Windows MinGW	println-lto	67584 B	0 B / +0.0%	21734 B	0 B / +0.0%	1.084 s	+81.85 ms / +8.2% (worse)	7.011 ms	+211 us / +3.1% (worse)
Windows MSVC	cprintf	12288 B	0 B / +0.0%	4438 B	0 B / +0.0%	560.287 ms	+33.57 ms / +6.4% (worse)	3.448 ms	+29.1 us / +0.9% (worse)
Windows MSVC	cprintf-lto	11776 B	0 B / +0.0%	4278 B	0 B / +0.0%	575.293 ms	+17.93 ms / +3.2% (worse)	3.383 ms	-517.2 us / -13.3% (better)
Windows MSVC	fmtprintf	1475584 B	0 B / +0.0%	596982 B	0 B / +0.0%	2.738 s	+71.74 ms / +2.7% (worse)	7.529 ms	-760.4 us / -9.2% (better)
Windows MSVC	fmtprintf-lto	1522688 B	0 B / +0.0%	597782 B	0 B / +0.0%	7.550 s	+118.1 ms / +1.6% (worse)	7.871 ms	+409.7 us / +5.5% (worse)
Windows MSVC	println	47104 B	0 B / +0.0%	25062 B	0 B / +0.0%	533.161 ms	-113.6 ms / -17.6% (better)	6.005 ms	-1.545 ms / -20.5% (better)
Windows MSVC	println-lto	44032 B	0 B / +0.0%	22102 B	0 B / +0.0%	742.255 ms	-13.11 ms / -1.7% (better)	5.988 ms	+417.2 us / +7.5% (worse)

Core language and compiler benchmarks

Platform	Benchmark	ns/op	vs base
Linux	BenchmarkLookupPCRandom	12.580 ns/op	-0.02 ns/op / -0.2% (better)
Linux	BenchmarkMergeCompilerFlags	135.700 ns/op	+1.1 ns/op / +0.8% (worse)
Linux	BenchmarkMergeLinkerFlags	89.770 ns/op	+0.41 ns/op / +0.5% (worse)
Linux	BenchmarkChannelBuffered	37.320 ns/op	-0.3 ns/op / -0.8% (better)
Linux	BenchmarkChannelHandoff	24230 ns/op	-594 ns/op / -2.4% (better)
Linux	BenchmarkDefer	48.760 ns/op	+1.11 ns/op / +2.3% (worse)
Linux	BenchmarkDirectCall	1.759 ns/op	+0.001 ns/op / +0.1% (worse)
Linux	BenchmarkGlobalRead	1.758 ns/op	-0.003 ns/op / -0.2% (better)
Linux	BenchmarkGlobalWrite	2.807 ns/op	+0.002 ns/op / +0.1% (worse)
Linux	BenchmarkGoroutine	36337 ns/op	+4790 ns/op / +15.2% (worse)
Linux	BenchmarkInterfaceCall	8.801 ns/op	+0.03 ns/op / +0.3% (worse)
Linux	BenchmarkRuntimeGetG	2.111 ns/op	-0.02 ns/op / -0.9% (better)
macOS	BenchmarkLookupPCRandom	18.130 ns/op	+1.99 ns/op / +12.3% (worse)
macOS	BenchmarkMergeCompilerFlags	140.600 ns/op	-8.4 ns/op / -5.6% (better)
macOS	BenchmarkMergeLinkerFlags	100.200 ns/op	+13.08 ns/op / +15.0% (worse)
macOS	BenchmarkChannelBuffered	33.610 ns/op	+1.59 ns/op / +5.0% (worse)
macOS	BenchmarkChannelHandoff	7807 ns/op	-3568 ns/op / -31.4% (better)
macOS	BenchmarkDefer	41.420 ns/op	-2.43 ns/op / -5.5% (better)
macOS	BenchmarkDirectCall	1.280 ns/op	-0.001 ns/op / -0.1% (better)
macOS	BenchmarkGlobalRead	1.270 ns/op	+0.01 ns/op / +0.8% (worse)
macOS	BenchmarkGlobalWrite	1.371 ns/op	-0.087 ns/op / -6.0% (better)
macOS	BenchmarkGoroutine	37885 ns/op	+1833 ns/op / +5.1% (worse)
macOS	BenchmarkInterfaceCall	5.846 ns/op	-0.282 ns/op / -4.6% (better)
macOS	BenchmarkRuntimeGetG	2.687 ns/op	+0.049 ns/op / +1.9% (worse)
Windows MinGW	BenchmarkLookupPCRandom	12.380 ns/op	+0.01 ns/op / +0.1% (worse)
Windows MinGW	BenchmarkMergeCompilerFlags	534 ns/op	-1.8 ns/op / -0.3% (better)
Windows MinGW	BenchmarkMergeLinkerFlags	467.200 ns/op	+3.9 ns/op / +0.8% (worse)
Windows MinGW	BenchmarkChannelBuffered	39.330 ns/op	-0.14 ns/op / -0.4% (better)
Windows MinGW	BenchmarkChannelHandoff	1444 ns/op	-67 ns/op / -4.4% (better)
Windows MinGW	BenchmarkDefer	60.530 ns/op	+0.13 ns/op / +0.2% (worse)
Windows MinGW	BenchmarkDirectCall	1.745 ns/op	-0.001 ns/op / -0.1% (better)
Windows MinGW	BenchmarkGlobalRead	1.746 ns/op	-0.002 ns/op / -0.1% (better)
Windows MinGW	BenchmarkGlobalWrite	2.786 ns/op	0 ns/op / +0.0%
Windows MinGW	BenchmarkGoroutine	74129 ns/op	-3562 ns/op / -4.6% (better)
Windows MinGW	BenchmarkInterfaceCall	10.310 ns/op	+0.05 ns/op / +0.5% (worse)
Windows MinGW	BenchmarkRuntimeGetG	2.448 ns/op	0 ns/op / +0.0%
Windows MSVC	BenchmarkLookupPCRandom	8.107 ns/op	+0.031 ns/op / +0.4% (worse)
Windows MSVC	BenchmarkMergeCompilerFlags	386.700 ns/op	+3.6 ns/op / +0.9% (worse)
Windows MSVC	BenchmarkMergeLinkerFlags	347.100 ns/op	+11 ns/op / +3.3% (worse)
Windows MSVC	BenchmarkChannelBuffered	38.810 ns/op	-0.03 ns/op / -0.1% (better)
Windows MSVC	BenchmarkChannelHandoff	3825 ns/op	+756 ns/op / +24.6% (worse)
Windows MSVC	BenchmarkDefer	44.260 ns/op	+5.96 ns/op / +15.6% (worse)
Windows MSVC	BenchmarkDirectCall	0.248 ns/op	+0.0021 ns/op / +0.9% (worse)
Windows MSVC	BenchmarkGlobalRead	0.361 ns/op	-0.0147 ns/op / -3.9% (better)
Windows MSVC	BenchmarkGlobalWrite	6.928 ns/op	-0.033 ns/op / -0.5% (better)
Windows MSVC	BenchmarkGoroutine	63334 ns/op	+5861 ns/op / +10.2% (worse)
Windows MSVC	BenchmarkInterfaceCall	4.400 ns/op	-0.047 ns/op / -1.1% (better)
Windows MSVC	BenchmarkRuntimeGetG	0.894 ns/op	+0.0021 ns/op / +0.2% (worse)

Compared with e637682c8772 measured in the same runner job.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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