testing: parallel fuzzing hangs with -parallel=1

[AlekSi]

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

[AlekSi]

@gopherbot fuzz

[AlekSi]

Bisected down to b2b6be7

[cherrymui]

cc @golang/fuzzing

[rolandshoemaker]

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?

[thepudds]

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.

[AlekSi]

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.

[katiehockman]

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.

[gopherbot]

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