[issue22107] tempfile module misinterprets access denied error on Windows

[And Clover]

And Clover <and at doxdesk.com> added the comment:

Attempting to answer the question "did this open call fail because the path was a directory" by implication from "do we think we ought to be able to write a file to this directory" is IMO doomed. There's no reliable way to determine whether one should be able to write to a location, short of trying to write to it. There isn't in general and there especially isn't on Windows, for the reasons discussed by Eryk and more.

I believe Billy's patch is an improvement over what we have, in as much as it's specifically checking for the condition we care about to work around a shortcoming of Windows error reporting. I would further remove the use of os.access, which does nothing useful (it reflects the read-only file attribute, which no-one uses, and which doesn't exist for directories anyway).

Yes, there is a race condition if the directory goes away between use and check, but it seems vanishingly unlikely this could happen by accident, and it could only happen on purpose if an attacker can guess the random filenames in which case they already have worse attacks than just making mkstemp fail. In general failure is a much better outcome than hanging for hours on end.

We may be overthinking this. Maybe it is OK to treat all permission errors as maybe-file-exists errors, like issue 18849 originally did, and just retry without trying to pick apart the entrails.

...just not quite as many as 2147483647 times. Given how unlikely an accidental filename clash is in the first place, I'm thinking a more realistic number of retries might be something like, 2.

`os.TMP_MAX` probably isn't a good choice in any case, as it indicates the number of names C's tmpnam() can come up with, which (a) is unrelated to the number of names _RandomNameSequence can come up with and (b) we have no particular reason to try to completely exhaust anyway.

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nosy: +aclover

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue22107>
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