non-deterministic deadlock in test_subprocess::test_warn_on_failed_cancel_terminate

See https://github.com/python-trio/trio/runs/756580934?check_suite_focus=true. I was able to retrieve the test name thanks to the traceback. The test timed out on an acquire() call.

2020-06-10T05:22:22.9465010Z Timeout (0:01:00)!
2020-06-10T05:22:22.9465567Z Thread 0x00007feaed267700 (most recent call first):
2020-06-10T05:22:22.9466805Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/jedi/inference/compiled/subprocess/__init__.py", line 37 in _enqueue_output
2020-06-10T05:22:22.9467371Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/threading.py", line 864 in run
2020-06-10T05:22:22.9467782Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/threading.py", line 916 in _bootstrap_inner
2020-06-10T05:22:22.9468195Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/threading.py", line 884 in _bootstrap
2020-06-10T05:22:22.9468437Z 
2020-06-10T05:22:22.9468695Z Thread 0x00007feaef1be700 (most recent call first):
2020-06-10T05:22:22.9469487Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/trio/_core/tests/test_thread_cache.py", line 102 in acquire
2020-06-10T05:22:22.9470319Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/trio/_core/_thread_cache.py", line 62 in _work
2020-06-10T05:22:22.9470793Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/threading.py", line 864 in run
2020-06-10T05:22:22.9471195Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/threading.py", line 916 in _bootstrap_inner
2020-06-10T05:22:22.9471607Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/threading.py", line 884 in _bootstrap
2020-06-10T05:22:22.9471852Z 
2020-06-10T05:22:22.9472088Z Thread 0x00007feb08802680 (most recent call first):
2020-06-10T05:22:22.9472826Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/trio/_core/_io_epoll.py", line 223 in get_events
2020-06-10T05:22:22.9473561Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/trio/_core/_run.py", line 1890 in run
2020-06-10T05:22:22.9474782Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/trio/testing/_trio_test.py", line 27 in wrapper
2020-06-10T05:22:22.9475534Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/_pytest/python.py", line 182 in pytest_pyfunc_call
2020-06-10T05:22:22.9476332Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/pluggy/callers.py", line 187 in _multicall
2020-06-10T05:22:22.9477195Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/pluggy/manager.py", line 87 in <lambda>
2020-06-10T05:22:22.9478034Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/pluggy/manager.py", line 93 in _hookexec
2020-06-10T05:22:22.9478759Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/pluggy/hooks.py", line 286 in __call__
2020-06-10T05:22:22.9479528Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/_pytest/python.py", line 1477 in runtest
2020-06-10T05:22:22.9480285Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/_pytest/runner.py", line 135 in pytest_runtest_call
2020-06-10T05:22:22.9481053Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/pluggy/callers.py", line 187 in _multicall
2020-06-10T05:22:22.9481766Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/pluggy/manager.py", line 87 in <lambda>
2020-06-10T05:22:22.9482536Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/pluggy/manager.py", line 93 in _hookexec
2020-06-10T05:22:22.9483258Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/pluggy/hooks.py", line 286 in __call__
2020-06-10T05:22:22.9484024Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/_pytest/runner.py", line 217 in <lambda>
2020-06-10T05:22:22.9484897Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/_pytest/runner.py", line 244 in from_call
2020-06-10T05:22:22.9485766Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/_pytest/runner.py", line 217 in call_runtest_hook
2020-06-10T05:22:22.9486511Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/_pytest/runner.py", line 186 in call_and_report
2020-06-10T05:22:22.9487294Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/_pytest/runner.py", line 100 in runtestprotocol
2020-06-10T05:22:22.9488044Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/_pytest/runner.py", line 85 in pytest_runtest_protocol
2020-06-10T05:22:22.9488841Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/pluggy/callers.py", line 187 in _multicall
2020-06-10T05:22:22.9489558Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/pluggy/manager.py", line 87 in <lambda>
2020-06-10T05:22:22.9490321Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/pluggy/manager.py", line 93 in _hookexec
2020-06-10T05:22:22.9491048Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/pluggy/hooks.py", line 286 in __call__
2020-06-10T05:22:22.9491830Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/_pytest/main.py", line 272 in pytest_runtestloop
2020-06-10T05:22:22.9492548Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/pluggy/callers.py", line 187 in _multicall
2020-06-10T05:22:22.9493315Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/pluggy/manager.py", line 87 in <lambda>
2020-06-10T05:22:22.9494033Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/pluggy/manager.py", line 93 in _hookexec
2020-06-10T05:22:22.9494796Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/pluggy/hooks.py", line 286 in __call__
2020-06-10T05:22:22.9495505Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/_pytest/main.py", line 247 in _main
2020-06-10T05:22:22.9496435Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/_pytest/main.py", line 191 in wrap_session
2020-06-10T05:22:22.9497225Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/_pytest/main.py", line 240 in pytest_cmdline_main
2020-06-10T05:22:22.9497954Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/pluggy/callers.py", line 187 in _multicall
2020-06-10T05:22:22.9498777Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/pluggy/manager.py", line 87 in <lambda>
2020-06-10T05:22:22.9499607Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/pluggy/manager.py", line 93 in _hookexec
2020-06-10T05:22:22.9500722Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/pluggy/hooks.py", line 286 in __call__
2020-06-10T05:22:22.9501514Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/_pytest/config/__init__.py", line 125 in main
2020-06-10T05:22:22.9501986Z   File "/opt/hostedtoolcache/Python/3.6.10/x64/bin/pytest", line 8 in <module>
2020-06-10T05:30:20.1060105Z ##[error]The operation was canceled.

The report points to test_subprocess::test_warn_on_failed_cancel_terminate and shows waits in trio/_core/tests/test_thread_cache.py:102 and trio/_core/_thread_cache.py:62. Start by running that test and tracing the acquire() timeout alongside the linked CI log. Done means the test no longer deadlocks or times out.