[BUG] Background thread can break test teardown

[mawbid]

In one of my test suites using pytest-recording, I sometimes get one of two exceptions in vcr.patch.ConnectionRemover.__exit__():

>       for pool, connections in self._connection_pool_to_connections.items():
                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E       RuntimeError: dictionary changed size during iteration

or

>           for connection in connections:
                              ^^^^^^^^^^^
E           RuntimeError: Set changed size during iteration

More complete pytest output for one of them is included below.

This code clearly works for a lot of pytest-recording users, so there's got to be something special about the tests or the code under test in my case. I think this is what it is:

Some of the code exercised by the tests pushes work items onto a queue. A background thread picks them up and processes them, and that involves making HTTP requests.

If I call .join() on the queue right after pushing the work item, the problem goes away. I imagine that without the join, some connection handling done in the background thread can end up running concurrently with the ConnectionRemover.__exit__() call, causing the error.

Versions:

vcrpy                        7.0.0
pytest                       8.4.2
pytest-recording             0.13.4
pytest-xdist                 3.8.0
python                       3.13.7

Redacted pytest output:

================================================================ ERRORS =================================================================
___________________________________________ ERROR at teardown of test_XXX ___________________________________________
[gw3] darwin -- Python 3.13.7 /XXX/.venv/bin/python

request = <SubRequest 'vcr' for <Function XXX>>, vcr_markers = [Mark(name='vcr', args=(), kwargs={})]
vcr_cassette_dir = '/XXX/XXX'
record_mode = 'new_episodes', disable_recording = False, pytestconfig = <_pytest.config.Config object at 0x10579d6a0>

    @pytest.fixture(autouse=True)  # type: ignore
    def vcr(
        request: SubRequest,
        vcr_markers: List[Mark],
        vcr_cassette_dir: str,
        record_mode: str,
        disable_recording: bool,
        pytestconfig: Config,
    ) -> Iterator[Optional["Cassette"]]:
        """Install a cassette if a test is marked with `pytest.mark.vcr`."""
        if disable_recording:
            yield None
        elif vcr_markers:
            from ._vcr import use_cassette

            config = request.getfixturevalue("vcr_config")
            default_cassette = request.getfixturevalue("default_cassette_name")
>           with use_cassette(
                default_cassette,
                vcr_cassette_dir,
                record_mode,
                vcr_markers,
                config,
                pytestconfig,
            ) as cassette:

.venv/lib/python3.13/site-packages/pytest_recording/plugin.py:158:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
.venv/lib/python3.13/site-packages/vcr/cassette.py:98: in __exit__
    next(self.__finish, None)
.venv/lib/python3.13/site-packages/vcr/cassette.py:57: in _patch_generator
    with contextlib.ExitStack() as exit_stack:
         ^^^^^^^^^^^^^^^^^^^^^^
../../.local/share/uv/python/cpython-3.13.7-macos-aarch64-none/lib/python3.13/contextlib.py:619: in __exit__
    raise exc
../../.local/share/uv/python/cpython-3.13.7-macos-aarch64-none/lib/python3.13/contextlib.py:604: in __exit__
    if cb(*exc_details):
       ^^^^^^^^^^^^^^^^
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <vcr.patch.ConnectionRemover object at 0x11019d130>, args = (None, None, None)
pool = <urllib3.connectionpool.HTTPConnectionPool object at 0x110414c80>
connections = {<vcr.patch.VCRRequestsHTTPConnection/XXX/XXX.yaml object at 0x10fb1e350>, <vcr.patch.VCRRequestsHTTPConnection/XXX/XXX.yaml object at 0x10fa902f0>}
readd_connections = [None, None, None, None, None, None, ...]

    def __exit__(self, *args):
>       for pool, connections in self._connection_pool_to_connections.items():
                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E       RuntimeError: dictionary changed size during iteration

.venv/lib/python3.13/site-packages/vcr/patch.py:380: RuntimeError
--------------------------------------------------------- Captured stderr call ----------------------------------------------------------
XXX
----------------------------------------------------------- Captured log call -----------------------------------------------------------
XXX
======================================================== short test summary info ========================================================
ERROR XXX - RuntimeError: dictionary changed size during iteration
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! stopping after 1 failures !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! xdist.dsession.Interrupted: stopping after 1 failures !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
========================================== 187 passed, 2 skipped, 1 xfailed, 1 error in 11.92s ==========================================

(This pytest run was with pytest -n5 using pytest-xdist. I later discovered that concurrent test execution is not required to trigger the exception.)