testing: parallel fuzzing hangs with -parallel=1

go version devel go1.18-7f2314530e Thu Dec 16 00:34:10 2021 +0000 darwin/arm64

The following test hangs when run with go test -parallel=1:

func FuzzParallel(f *testing.F) {
	f.Add(42)

	f.Fuzz(func(t *testing.T, i int) {
		t.Parallel()
	})
}

It works with -parallel=2 and other values. I think it should work in any case.

Small self-contained example is there: https://github.com/AlekSi/go-bug/tree/master/50217

Bisected down to b2b6be7

cc @golang/fuzzing

@katiehockman

This seems like one of the things that should just be disabled, both within the f.Fuzz function, and disallowing the flag, since I'm not really sure what it would do. @katiehockman what do you think?

FWIW, in terms of what it could do, the design doc includes:

// Run the fuzz test
f.Fuzz(func(t *testing.T, a string, num *big.Int) {
	t.Parallel() // seed corpus tests can run in parallel

Certainly seems reasonable to disallow it though, especially for a first version.

Running seed corpus entries in parallel actually works (as far as I can tell) and is very useful for me. If there is not enough time to fix it for -parallel=1 edge case, maybe just explicitly fail in that case instead of hanging or removing the whole feature.

t.Parallel() in a fuzz target should be a no-op during fuzzing, but is similar to a t.Parallel() inside a t.Run() in that it runs the seed corpus in parallel when -fuzz isn't set.
This is just a bug I think. If we can't fix it, then just removing that functionality for 1.18 also seems fine. Will take a look.

Change https://golang.org/cl/374054 mentions this issue: testing: fix deadlock with t.Parallel in testing seed corpus

testing: fix deadlock with t.Parallel in testing seed corpus

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testing: parallel fuzzing hangs with -parallel=1 #50217

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testing: parallel fuzzing hangs with -parallel=1 #50217

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AlekSi commented on Dec 16, 2021

AlekSi commented on Dec 16, 2021

cherrymui commented on Dec 16, 2021

rolandshoemaker commented on Dec 16, 2021

thepudds commented on Dec 17, 2021

AlekSi commented on Dec 17, 2021

katiehockman commented on Dec 20, 2021

gopherbot commented on Dec 22, 2021

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