py3: race condition with .pth files #26160
Description
I am seeing this in the occasional log file, here fpylll-0.4.1dev.log:
Error processing line 1 of /Users/jpalmier/Desktop/Sage/sage_builds/PYTHON3/sage-8.4.beta2/local/lib/python3.6/site-packages/matplotlib-2.2.2-py3.6-nspkg.pth:
Traceback (most recent call last):
File "/Users/jpalmier/Desktop/Sage/sage_builds/PYTHON3/sage-8.4.beta2/local/lib/python3.6/site.py", line 168, in addpackage
exec(line)
File "<string>", line 1, in <module>
File "<frozen importlib._bootstrap>", line 568, in module_from_spec
AttributeError: 'NoneType' object has no attribute 'loader'
Remainder of file ignored
The installation reports success, despite this warning.
I'm guessing that the matplotlib installation is writing the .pth file while the fpylll installation is trying to read it. I've only seen this with a Python 3 build, so it may be Python 3 specific.
CC: @fchapoton @jdemeyer @embray
Component: build
Reviewer: Matthias Koeppe
Issue created by migration from https://trac.sagemath.org/ticket/26160