bpo-32713: Fix tarfile.itn for large/negative float values

[shin-]

This PR changes the behavior of tarfile.itn to cast the input argument n to an integer for bitwise operations.

https://bugs.python.org/issue32713

[the-knights-who-say-ni]

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[serhiy-storchaka]

Wouldn't be better to convert n to int at the begin of the function?

[shin-]

My concern in doing so was that the loss of precision might lead to unwanted results if it is done before the comparisons (e.g. -0.1 becomes 0). That said, it may not matter in any practical case.

[serhiy-storchaka]

This is a good example why this should be done before the comparisons. With this patch itn() produces incorrect results for values between -1 and 0.

>>> tarfile.itn(-0.1)
bytearray(b'\xff\x00\x00\x00\x00\x00\x00\x00')
>>> tarfile.nti(tarfile.itn(-0.1))
-72057594037927936

Please add this case to tests.

[serhiy-storchaka]

This is a good example why this should be done before the comparisons. With this patch itn() produces incorrect results for values between -1 and 0.

>>> tarfile.itn(-0.1)
bytearray(b'\xff\x00\x00\x00\x00\x00\x00\x00')
>>> tarfile.nti(tarfile.itn(-0.1))
-72057594037927936

Please add this case to tests.

[bedevere-bot]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[shin-]

Thank you for the review!

I have made the requested changes; please review again

[bedevere-bot]

Thanks for making the requested changes!

@serhiy-storchaka: please review the changes made to this pull request.

[serhiy-storchaka]

Please add a period at the end of the sentence and "Patch by yourname."

[shin-]

done 👍

[miss-islington]

Thanks @shin- for the PR, and @serhiy-storchaka for merging it 🌮🎉.. I'm working now to backport this PR to: 2.7, 3.6, 3.7.
🐍🍒⛏🤖

[bedevere-bot]

GH-5917 is a backport of this pull request to the 3.7 branch.

[miss-islington]

Sorry, @shin- and @serhiy-storchaka, I could not cleanly backport this to 2.7 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker 72d9b2be36f091793ae7ffc5ad751f040c6e6ad3 2.7

[bedevere-bot]

GH-5918 is a backport of this pull request to the 3.6 branch.