ENH: Baseline free threading support

[HaoZeke]

Closes #1105. Closes #1089. Followup.

Draft until:

• Tests pass
• CI is green
• [ ] Audit is complete

[HaoZeke]

It seems like a lot of the flaky tests seem to arbitrarily fail with the ft build.

e.g. https://github.com/HaoZeke/watchdog/actions/runs/15240749477 passes

but https://github.com/HaoZeke/watchdog/actions/runs/15240754178/job/42860520286 does not

[HaoZeke]

CI is green now.

[HaoZeke]

cc @BoboTiG @rgommers

[BoboTiG]

Is it possible to use "py313t" in the current CI file to unify everything?

[HaoZeke]

Is it possible to use "py313t" in the current CI file to unify everything?

Yup, but it might get a it longer / slightly more complex because the coverage removal is only for the FT work. I was thinking it might be cleaner to have them in a separate job (also in case only the FT build is being run, like I often do locally) but I can update this with a single file.

[BoboTiG]

Is it possible to use "py313t" in the current CI file to unify everything?

Yup, but it might get a it longer / slightly more complex because the coverage removal is only for the FT work. I was thinking it might be cleaner to have them in a separate job (also in case only the FT build is being run, like I often do locally) but I can update this with a single file.

Lets keep it as-is: it works fine.

[rgommers]

A comment on the test skips here. pytest-run-parallel auto-detects most thread-unsafe fixtures, including recwarn, but the problem here seems to be that the custom _no_warnings fixture hides it:

watchdog/tests/conftest.py

Lines 35 to 36 in 1068560

	@pytest.fixture(autouse=True)
	def _no_warnings(recwarn):

There is a way to make the code much more concise though, which is to add __thread_safe__ = False as an attribute of _no_warnings (see the docs here).

Also, I'd suggest actually turning that fixture off when testing under free-threading, since it applies to almost all tests (leaving pytest-run-parallel mostly disabled) and it isn't actually necessary to do those "no warnings" checks in all circumstances - running it on the default 3.13 interpreter will already catch anything that's actually interesting. Turning it off then allows actually checking for most thread-safety issues.

So I'd suggest:

1. A cleanup PR adding __thread_safe__ = False to remove all the manual skips
2. As part of the already-planned second PR for free-threading support, use autouse=not sys._is_gil_enabled() for the fixture (with a comment about why).

[HaoZeke]

A comment on the test skips here. pytest-run-parallel auto-detects most thread-unsafe fixtures, including recwarn, but the problem here seems to be that the custom _no_warnings fixture hides it:

watchdog/tests/conftest.py

Lines 35 to 36 in 1068560

	@pytest.fixture(autouse=True)
	def _no_warnings(recwarn):

There is a way to make the code much more concise though, which is to add __thread_safe__ = False as an attribute of _no_warnings (see the docs here).

Also, I'd suggest actually turning that fixture off when testing under free-threading, since it applies to almost all tests (leaving pytest-run-parallel mostly disabled) and it isn't actually necessary to do those "no warnings" checks in all circumstances - running it on the default 3.13 interpreter will already catch anything that's actually interesting. Turning it off then allows actually checking for most thread-safety issues.

So I'd suggest:

1. A cleanup PR adding __thread_safe__ = False to remove all the manual skips
2. As part of the already-planned second PR for free-threading support, use autouse=not sys._is_gil_enabled() for the fixture (with a comment about why).

It seems to be more than _no_warnings, will try to narrow it down.

2 files changed, 5 insertions(+)
pyproject.toml    | 3 +++
tests/conftest.py | 2 ++

modified   pyproject.toml
@@ -38,6 +38,9 @@ disallow_untyped_calls = true
 strict_equality = true
 
 [tool.pytest.ini_options]
+thread_unsafe_fixtures = [
+    '_no_warnings',
+]
 pythonpath = "src"
 addopts = """
     --showlocals
modified   tests/conftest.py
@@ -36,6 +36,8 @@ def _no_thread_leaks():
 def _no_warnings(recwarn):
     """Fail on warning."""
 
+    __thread_safe__ = False
+
     yield
 
     warnings = []

Was tested on CI (and locally). However this doesn't seem prevent failures.

Failure log

tests/test_delayed_queue.py::test_delayed_get PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_delayed_queue.py::test_nondelayed_get PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_echo.py::test_format_arg_value[value0-x=(1, 2, 3)] PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_emitter.py::test_create FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_emitter.py::test_closed FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_emitter.py::test_create_wrong_encoding FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_emitter.py::test_delete FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_emitter.py::test_modify FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_emitter.py::test_chmod PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_emitter.py::test_move FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_emitter.py::test_case_change FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_emitter.py::test_move_to FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_emitter.py::test_move_to_full FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_emitter.py::test_move_from FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_emitter.py::test_move_from_full FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_emitter.py::test_separate_consecutive_moves FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_emitter.py::test_delete_self PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_emitter.py::test_fast_subdirectory_creation_deletion FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_emitter.py::test_passing_unicode_should_give_unicode PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_emitter.py::test_passing_bytes_should_give_bytes PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_emitter.py::test_recursive_on FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_emitter.py::test_recursive_off FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_emitter.py::test_renaming_top_level_directory FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_emitter.py::test_move_nested_subdirectories FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_emitter.py::test_move_nested_subdirectories_on_windows SKIPPED
tests/test_emitter.py::test_file_lifecyle FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_events.py::test_file_deleted_event PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_events.py::test_file_delete_event_is_directory PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_events.py::test_file_modified_event PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_events.py::test_file_modified_event_is_directory PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_events.py::test_file_created_event PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_events.py::test_file_moved_event PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_events.py::test_file_closed_event PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_events.py::test_file_closed_no_write_event PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_events.py::test_file_opened_event PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_events.py::test_dir_deleted_event PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_events.py::test_dir_modified_event PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_events.py::test_dir_created_event PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_events.py::test_file_system_event_handler_dispatch PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_events.py::test_event_comparison PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_inotify_c.py::test_late_double_deletion FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_inotify_c.py::test_raise_error[28-inotify watch limit reached] PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_inotify_c.py::test_raise_error[24-inotify instance limit reached] PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_inotify_c.py::test_raise_error[2-No such file or directory] PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_inotify_c.py::test_raise_error[-1-error] PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_inotify_c.py::test_non_ascii_path FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_inotify_c.py::test_watch_file PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_inotify_c.py::test_event_equality PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_inotify_c.py::test_select_fd FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_inotify_watch_group.py::test_move_from FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_inotify_watch_group.py::test_move_to FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_inotify_watch_group.py::test_move_internal FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_inotify_watch_group.py::test_move_internal_symlink_followed FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_inotify_watch_group.py::test_move_internal_batch FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_inotify_watch_group.py::test_delete_watched_directory PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_inotify_watch_group.py::test_delete_watched_directory_symlink_followed FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_inotify_watch_group.py::test_delete_watched_directory_symlink_followed_recursive FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_inotify_watch_group.py::test_unmount_watched_directory_filesystem SKIPPED
tests/test_inotify_watch_group.py::test_watch_groups_are_independent PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_isolated.py::test_observer_stops_in_eventlet SKIPPED (eventlet not
installed)
tests/test_isolated.py::test_eventlet_skip_repeat_queue SKIPPED (eventlet not
installed)
tests/test_logging_event_handler.py::test_logging_event_handler_dispatch PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_observer.py::test_schedule_should_start_emitter_if_running FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_observer.py::test_schedule_should_not_start_emitter_if_not_running PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_observer.py::test_start_should_start_emitter FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_observer.py::test_stop_should_stop_emitter FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_observer.py::test_unschedule_self FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_observer.py::test_schedule_after_unschedule_all FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_observer.py::test_2_observers_on_the_same_path PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_observer.py::test_start_failure_should_not_prevent_further_try FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_observer.py::test_schedule_failure_should_not_prevent_future_schedules FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_observers_api.py::test_observer_constructor PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_observers_api.py::test_observer__eq__ PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_observers_api.py::test_observer__ne__ PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_observers_api.py::test_observer__repr__ PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_observers_api.py::test_event_emitter PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_observers_api.py::test_event_dispatcher PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_observers_api.py::test_observer_basic PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_observers_polling.py::test___init__ FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_observers_polling.py::test_delete_watched_dir FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_pattern_matching_event_handler.py::test_dispatch PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_pattern_matching_event_handler.py::test_handler PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_pattern_matching_event_handler.py::test_ignore_directories PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_pattern_matching_event_handler.py::test_ignore_patterns PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_pattern_matching_event_handler.py::test_patterns PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_patterns.py::test_match_path[/users/gorakhargosh/foobar.py-included_patterns0-excluded_patterns0-True-True] PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_patterns.py::test_match_path[/users/gorakhargosh/foobar.py-included_patterns1-excluded_patterns1-True-False] PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_patterns.py::test_match_path[/users/gorakhargosh/-included_patterns2-excluded_patterns2-False-False] PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_patterns.py::test_match_path[/users/gorakhargosh/foobar.py-included_patterns3-excluded_patterns3-False-ValueError] PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_patterns.py::test_filter_paths[None-None-True-None] PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_patterns.py::test_filter_paths[None-None-False-None] PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_patterns.py::test_filter_paths[included_patterns2-excluded_patterns2-True-expected2] PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_patterns.py::test_match_any_paths[None-None-True-True] PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_patterns.py::test_match_any_paths[None-None-False-True] PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_patterns.py::test_match_any_paths[included_patterns2-excluded_patterns2-True-True] PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_patterns.py::test_match_any_paths[included_patterns3-None-False-False] PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_patterns.py::test_match_any_paths[included_patterns4-None-True-False] PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_regex_matching_event_handler.py::test_dispatch PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_regex_matching_event_handler.py::test_handler PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_regex_matching_event_handler.py::test_ignore_directories PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_regex_matching_event_handler.py::test_ignore_regexes PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_regex_matching_event_handler.py::test_regexes PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_regex_matching_event_handler.py::test_str_regexes PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_regex_matching_event_handler.py::test_logging_event_handler_dispatch PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_skip_repeats_queue.py::test_basic_queue PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_skip_repeats_queue.py::test_allow_nonconsecutive PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_skip_repeats_queue.py::test_put_with_watchdog_events PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_skip_repeats_queue.py::test_prevent_consecutive PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_skip_repeats_queue.py::test_consecutives_allowed_across_empties PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_snapshot_diff.py::test_pickle FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_snapshot_diff.py::test_move_to FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_snapshot_diff.py::test_move_to_with_context_manager FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_snapshot_diff.py::test_move_from FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_snapshot_diff.py::test_move_internal FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_snapshot_diff.py::test_move_replace FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_snapshot_diff.py::test_dir_modify_on_create FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_snapshot_diff.py::test_dir_modify_on_move FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_snapshot_diff.py::test_detect_modify_for_moved_files PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_snapshot_diff.py::test_replace_dir_with_file FAILED ([thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'})
tests/test_snapshot_diff.py::test_permission_error PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_snapshot_diff.py::test_ignore_device PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}
tests/test_snapshot_diff.py::test_empty_snapshot PASSED [thread-unsafe]: uses thread-unsafe fixture(s): {'_no_warnings', 'recwarn'}

Reduces the failures from 54 (the number of added skips) to 45.

[rgommers]

It would be good to consider what should actually be thread-safe and used from multiple threads - everything in watchdog, or some (possibly small) subset?. I haven't looked very closely, just started from the observation that the skips are a lot of boilerplate and it's not useful to add pytest-run-parallel and then skip every single test.