[Python-Dev] Tests failing on Windows with TESTFN

[Chris Jerdonek]

On Fri, Jul 27, 2018 at 6:41 AM, Giampaolo Rodola' <g.rodola at gmail.com> wrote:
>
> Being TESTFN a global name it appears not suited for parallel testing.

It was designed for parallel testing though:

# Disambiguate TESTFN for parallel testing, while letting it remain a valid
# module name.
TESTFN = "{}_{}_tmp".format(TESTFN, os.getpid())

https://github.com/python/cpython/blob/aee632dfbb0abbc0d2bcc988c43a736afd568c55/Lib/test/support/__init__.py#L807-L809

> Windows makes this more noticeable than POSIX as it's more strict, but
> accessing the same file from 2 unit tests is technically a problem
> regardless of the platform. I don't think that means we should ditch
> TESTFN in favor of tempfile.mktemp() though. Instead the file cleanup
> functions (support.unlink() and support.rmtree()) may be more clever
> and (important) they should always be used in setUp / tearDown. For
> instance, the traceback you pasted refers to a test class which
> doesn't do this.

The "test_file" test method referenced in the traceback calls
os.remove(TESTFN) in finally blocks preceding its calls to
open(TESTFN, "wb"), and inspecting the method shows that it must have
been able to open TESTFN earlier in the method (the same test method
uses TESTFN multiple times):

https://github.com/python/cpython/blob/aee632dfbb0abbc0d2bcc988c43a736afd568c55/Lib/test/test_urllib2.py#L811-L830

So I think one should investigate what can be causing the error / how
it can be happening.

TESTFN uses the pid of the process, so it doesn't seem like another
test case could be interfering and opening the same TESTFN while the
"test_file" test method is running. On Stack Overflow, there are some
comments suggesting that in some cases os.remove() doesn't always
complete right away (e.g. because of anti-malware software briefly
holding a second reference).

--Chris


>
> In psutil I've had occasional Windows failures like this for years
> then I got tired of it and came up with this:
> https://github.com/giampaolo/psutil/blob/1b09b5fff78f705dfb42458726ff9789c26f6f21/psutil/tests/__init__.py#L686
> ...which basically aggressively retries os.unlink or shutil.rmtree for
> 1 sec in case of (any) error, and I haven't had this problem since
> then.
>
> I suppose test.support's unlink() and rmtree() can do something
> similar, maybe just by using a better exception handling, and all unit
> tests should use them in setUp / tearDown. I think this will diminish
> the occasional failures on Windows, although not completely.
>
> --
> Giampaolo - http://grodola.blogspot.com
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