Fix multiprocessing warnings when runnign tests on Python 3.12 #16564
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I saw a bunch of warnings when running tests in parallel using pytest.
When running tests sequentially using `-n0` I didn't see warnings. This
only seems to happen on Linux.
The warnings were like these, which can be fixed by avoiding the use of fork,
and using forkserver instead:
```
mypy/test/teststubgen.py::StubgenPythonSuite::stubgen.test::testNestedClass_inspect
/usr/local/lib/python3.12/multiprocessing/popen_fork.py:66: DeprecationWarning: This process (pid=84587) is multi-threaded, use of fork() may lead to deadlocks in the child.
self.pid = os.fork()
```
Relevant discussion:
https://discuss.python.org/t/concerns-regarding-deprecation-of-fork-with-alive-threads/33555
|
Diff from mypy_primer, showing the effect of this PR on open source code: pyp (https://github.com/hauntsaninja/pyp): typechecking got 6105.34x slower (0.0s -> 11.5s)
(Performance measurements are based on a single noisy sample)
discord.py (https://github.com/Rapptz/discord.py): typechecking got 1.09x faster (190.9s -> 174.9s)
(Performance measurements are based on a single noisy sample)
|
ikonst
approved these changes
Nov 26, 2023
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I saw a bunch of warnings when running tests in parallel using pytest. When running tests sequentially using
-n0I didn't see warnings. This only seems to happen on Linux.The warnings were like these, which can be fixed by avoiding the use of fork, and using forkserver instead:
Relevant discussion:
https://discuss.python.org/t/concerns-regarding-deprecation-of-fork-with-alive-threads/33555