Add a lock to deterministic_PRNG

[tybug]

Part of #4451.

To reproduce: pytest -k test_zero_contained_examples --parallel-threads 2.

Pushing this lock down to after the hypothesis.core._hypothesis_global_random is None check would in theory avoid the described issue, and reduce overhead to once per session instead of once per test. But pushing it down could create two Random instances, one of which gets overwritten. This makes me nervous since it's possible for a thread to use the overwritten random before it gets overwritten, which may break determinism assumptions.

[Zac-HD]

• I'd like a test for this
• Solution seems fine to me as an immediate fix, but I wonder whether we should make the _hypothesis_global_random into a threadlocal - that seems like it might better express the intention that this is "engine-global", to coin a term.

[tybug]

but I wonder whether we should make the _hypothesis_global_random into a threadlocal

nice, I like this a lot!

I don't have a great test for this other than the generic "run hypothesis concurrently in lots of threads", which I have written locally, but won't be able to PR until hypothesis is fully thread safe

[Zac-HD]

I don't have a great test for this other than the generic "run hypothesis concurrently in lots of threads", which I have written locally, but won't be able to PR until hypothesis is fully thread safe

So long as you're tracking tests to eventually land in our CI, I'm pretty relaxed if those don't pass yet - since you're working on the contract and I'm confident that they will in fact land eventually! If it makes sense we could land some xfail tests as we go, tagged so that we un-xfail them all before declaring thread-safety?

[tybug]

If it makes sense we could land some xfail tests as we go, tagged so that we un-xfail them all before declaring thread-safety?

I'll do this for anything more targeted, but for generic thread safety tests I'm going to iterate on them a bunch so I'll keep those for the end

[Zac-HD]

ooh, the thread-local works really nicely here 👍