Avoid a flaky warning caused by multi-threaded DB access

[Zalathar]

I've noticed that we occasionally see a warning about an uncaught exception thrown on one of the worker threads in test_drawing_from_recursive_strategy_is_thread_safe.

Unfortunately I don't have an example handy, but from memory at least one of the exceptions involved was due to threads not being able to access the database file. Since the database isn't relevant to this test, I've disabled it in the hopes that it will get rid of the warning.

[Zalathar]

Ah, here's a different flaky warning in this test that I remember having seen earlier:

=============================== warnings summary ===============================
hypothesis-python/tests/nocover/test_recursive.py::test_drawing_from_recursive_strategy_is_thread_safe
  /Users/runner/work/hypothesis/hypothesis/hypothesis-python/.tox/py38-full/lib/python3.8/site-packages/_pytest/threadexception.py:75: PytestUnhandledThreadExceptionWarning: Exception in thread Thread-3
  
  Traceback (most recent call last):
    File "/Users/runner/hostedtoolcache/Python/3.8.10/x64/lib/python3.8/threading.py", line 932, in _bootstrap_inner
      self.run()
    File "/Users/runner/hostedtoolcache/Python/3.8.10/x64/lib/python3.8/threading.py", line 870, in run
      self._target(*self._args, **self._kwargs)
    File "/Users/runner/work/hypothesis/hypothesis/hypothesis-python/tests/nocover/test_recursive.py", line 141, in test
      @given(data=st.data())
    File "/Users/runner/work/hypothesis/hypothesis/hypothesis-python/.tox/py38-full/lib/python3.8/site-packages/hypothesis/core.py", line 1174, in wrapped_test
      raise the_error_hypothesis_found
    File "/Users/runner/work/hypothesis/hypothesis/hypothesis-python/.tox/py38-full/lib/python3.8/site-packages/hypothesis/core.py", line 1143, in wrapped_test
      state.run_engine()
    File "/Users/runner/work/hypothesis/hypothesis/hypothesis-python/.tox/py38-full/lib/python3.8/site-packages/hypothesis/core.py", line 806, in run_engine
      self.execute_once(
    File "/Users/runner/work/hypothesis/hypothesis/hypothesis-python/.tox/py38-full/lib/python3.8/site-packages/hypothesis/core.py", line 651, in execute_once
      self.__flaky(
    File "/Users/runner/work/hypothesis/hypothesis/hypothesis-python/.tox/py38-full/lib/python3.8/site-packages/hypothesis/core.py", line 879, in __flaky
      raise Flaky(message)
  hypothesis.errors.Flaky: Hypothesis test(data=data(...)) produces unreliable results: Falsified on the first call but did not on a subsequent one
  
    warnings.warn(pytest.PytestUnhandledThreadExceptionWarning(msg))

[Zac-HD]

Avoiding such a warning is clearly an improvement on the status quo, and we're careful to document that Hypothesis is not threadsafe anyway.

[Zac-HD]

Hey @Zalathar - is this ready to merge? It looks like a marginal improvement on the status quo to me, and of course we can always open another PR with further work 🙂

[Zalathar]

Yeah, feel free to merge this if you want. As you say it's a clear improvement even if it doesn't necessarily fix the original problem, and I don't have any specific plans to keep working on it in the foreseeable future.