[Python-Dev] Tests failing on Windows with TESTFN

[Tim Golden]

On 25/07/2018 16:07, Tim Golden wrote:
> One problem is that certain tests use support.TESTFN (a local directory 
> constructed from the pid) for output files etc. However this can cause 
> issues on Windows when recreating the folders / files for multiple 
> tests, especially when running in parallel.
> 
> Here's an example on my laptop deliberately running 3 tests with -j0 
> which I know will generate an error about one time in three:
> 
> C:\work-in-progress\cpython>python -mtest -j0 test_urllib2 test_bz2 
> test_importlib
> 

[... snip ...]

> [...] I wanted to ask first
> whether there was any objection to my converting tests to using tempfile 
> functions which should avoid the problem?

Thanks to those who responded here and/or on the two bpo issues I've 
already raised:

https://bugs.python.org/issue34239 -- test_bz2
https://bugs.python.org/issue34240 -- test_mmap

Two key questions have been posed in different ways:

1) Why are these errors occurring? ie are we dodging a root cause issue

2) Isn't this what test.support.unlink is there to solve?

For (1) I'm putting together a test run using code which I originally 
wrote for https://bugs.python.org/issue7443 to force the issues out into 
the open.

For (2), yes: test.support.unlink is supposed to solve that. But it's 
either not doing enough retries etc. or it's missing a trick.

To be clear: my motivation here isn't some housekeeping or modernisation 
exercise. I want to get a clear test run on a fresh build on Windows. At 
present I never get that. I'd be interested to hear from other Windows 
devs whether they have the same experience?

TJG