[New-bugs-announce] [issue47082] No protection: `import numpy` in two different threads can lead to race-condition

[Jens Henrik Goebbert]

New submission from Jens Henrik Goebbert <j.goebbert at fz-juelich.de>:

While using [Xpra](https://github.com/Xpra-org/xpra) we came across a bug which might be a Python or a NumPy issue.
Perhaps some of you can help us understanding some internals.

Calling `import numpy` at the same time in two different threads of the Python program can lead to a race-condition.
This happens for example with Xpra when loading the encoder nvjpeg:
```
2022-03-20 12:54:59,298  cannot load enc_nvjpeg (nvjpeg encoder)
Traceback (most recent call last):
  File "<pythondir>/lib/python3.9/site-packages/xpra/codecs/loader.py", line 52, in codec_import_check
    ic =  __import__(class_module, {}, {}, classnames)
  File "xpra/codecs/nvjpeg/encoder.pyx", line 8, in init xpra.codecs.nvjpeg.encoder
  File "<pythondir>/lib/python3.9/site-packages/numpy/__init__.py", line 150, in <module>
    from . import core
  File "<pythondir>/lib/python3.9/site-packages/numpy/core/__init__.py", line 51, in <module>
    del os.environ[envkey]
  File "<pythondir>/lib/python3.9/os.py", line 695, in __delitem__
    raise KeyError(key) from None
KeyError: 'OPENBLAS_MAIN_FREE'
```

Here the environment variable OPENBLAS_MAIN_FREE is set in the `numpy` code:
https://github.com/numpy/numpy/blob/maintenance/1.21.x/numpy/core/__init__.py#L18
and short after that it is deleted
https://github.com/numpy/numpy/blob/maintenance/1.21.x/numpy/core/__init__.py#L51
But this deletion fails ... perhaps because the initialization runs at the same time in two threads :thinking:

Shouldn't Python protect us by design?

@seberg comments [HERE](https://github.com/numpy/numpy/issues/21223#issuecomment-1074008386):
```
So, my current hypothesis (and I have briefly checked the Python code) is that Python does not do manual locking. But it effectively locks due to this going into C and thus holding the GIL. But somewhere during the import of NumPy, NumPy probably releases the GIL briefly and that could allow the next thread to go into the import machinery.
[..]
NumPy may be doing some worse than typical stuff here, but right now it seems to me that Python should be protecting us.
```

Can anyone comment on this?

----------
components: Interpreter Core
messages: 415687
nosy: jhgoebbert
priority: normal
severity: normal
status: open
title: No protection: `import numpy` in two different threads can lead to race-condition
type: behavior
versions: Python 3.11

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue47082>
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