_base.py: fix double acquire by TrioExecuto, issue #143

[axiomiety]

Fixes #143

Ran pytest on both master and the branch with the fix - output is the same (there are existing failures):

• on executor-double-acquire-fix

=============================================== short test summary info ================================================
FAILED tests/interop/test_calls.py::TestCalls::test_trio_asyncio_cancel_direct - BaseExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
FAILED tests/test_misc.py::test_keyboard_interrupt_teardown - BaseExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
FAILED tests/test_misc.py::test_cancel_loop[True] - ExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
FAILED tests/test_misc.py::test_run_trio_task_errors - BaseExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
FAILED tests/test_trio_asyncio.py::test_cancel_loop_with_tasks[True-False] - ExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
FAILED tests/test_trio_asyncio.py::test_cancel_loop_with_tasks[True-True] - ExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
=========================== 6 failed, 2504 passed, 62 skipped, 4 xfailed in 71.12s (0:01:11) ===========================

• on master

================================================ short test summary info ================================================
FAILED tests/interop/test_calls.py::TestCalls::test_trio_asyncio_cancel_direct - BaseExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
FAILED tests/test_misc.py::test_keyboard_interrupt_teardown - BaseExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
FAILED tests/test_misc.py::test_cancel_loop[True] - ExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
FAILED tests/test_misc.py::test_run_trio_task_errors - BaseExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
FAILED tests/test_trio_asyncio.py::test_cancel_loop_with_tasks[True-False] - ExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
FAILED tests/test_trio_asyncio.py::test_cancel_loop_with_tasks[True-True] - ExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
=========================== 6 failed, 2504 passed, 62 skipped, 4 xfailed in 71.33s (0:01:11) ============================

[oremanj]

Please add a newsfragment for this change, as well as a test that would have failed before the fix and now doesn't. Once those are set, I should be able to merge this.

[oremanj]

Thanks for the test!

[oremanj]

This will show up in the release notes; please try a little harder to be detailed and clear. One possibility is shown.

Suggested change

	Fix double-acquire by in the executor that could lead to deadlock
	`TrioExecutor` now acquires just one token from its `~trio.CapacityLimiter` for each
	worker thread that it runs, rather than two. The double-acquisition previously could
	cause a deadlock when trying to run a large number of tasks simultaneously using
	:meth:`asyncio.loop.run_in_executor`.