Jason R. Coombs <[email protected]> added the comment:
My bad. I probably could have been more proactive about providing a reproducer.
The problem, as described above (msg335220) and in the associated cmdix ticket,
is that invocation of `platform.(anything)` causes shelling out to execute
"uname", so it's not possible to implement uname on Python unless one can
guarantee that `platform.(anything)` is not invoked prior to the Python uname
implementation executing.
Here's a Dockerfile replicating the issue:
```
FROM ubuntu:focal
# Install Python
RUN apt update
RUN apt install -y python3
RUN ln -s /usr/bin/python3 /usr/bin/python
# Simulate something on the system that invokes platform early.
RUN printf 'print(__import__("platform").system())' > sitecustomize.py
ENV PYTHONPATH=/
# Create a stubbed 'uname' command build in Python
ENV PATH=/:$PATH
RUN printf '#!/usr/bin/env python\nprint("getting ready to patch platform",
flush=True)' > uname
RUN chmod u+x uname
CMD uname
```
As you can see, this reproducer creates a _very_ simple 'uname' implementation.
All it does is print that it's about to patch the platform module (because
maybe that will make things work). Unfortunately, that behavior is never
reached because before that code has a chance to run, `sitecustomize` is
imported and calls `platform.system()`, which invokes `platform.uname()` which
attempts to resolve the processor, which attempts to invoke `uname -p` (even on
Windows), which invokes the stubbed uname command, and infinite recursion
begins.
The `sitecustomize` might seem a little contrived, except that a very similar
behavior occurs in a very typical environment:
- pip, when installing a package for editing, invokes setuptools to `develop`
the package.
- setuptools, when installing a package for developing, creates command-line
entry points using a routine that imports `pkg_resources` to ensure that the
relevant packages are present before invoking the command's functionality.
- pkg_resources imports packaging to evaluate markers.
- packaging uses `platform.system()` and other behaviors to evaluate the
markers.
So ultimately, the same behavior is triggered before the user's code is ever
executed.
But more importantly, why should "uname -p" be invoked in a subprocess on
Windows to get "platform.system()"?
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<https://bugs.python.org/issue35967>
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