GNU bug report logs - #22533
Non-determinism in python-3 ".pyc" bytecode

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From: Ricardo Wurmus <rekado <at> elephly.net>
To: Marius Bakke <mbakke <at> fastmail.com>
Cc: 22533 <at> debbugs.gnu.org
Subject: bug#22533: Python bytecode reproducibility
Date: Sat, 03 Mar 2018 23:37:29 +0100

Hi Guix,

Marius Bakke <mbakke <at> fastmail.com> writes:

> It would be great to revive this longstanding bug!

Indeed.

Here’s another attempt.  As far as I understand, the timestamp in the
pyc files only affects the header.

Up until Python 3.6 (incl) the header looks like this:

  magic | timestamp | size

Since Python 3.7 the header may either contain a timestamp or a hash:

  magic | 00000000000000000000000000000000 | timestamp | size
  magic | 00000000000000000000000000000001 | hash      | size

This means we likely won’t have this problem any more with Python 3.7.
For Python 3.6 I guess we could add a final build phase that overwrites
the timestamp in the *binary*.  This needs to happen before any of the
compiled files are wrapped up in a wheel.

Should we just wait for Python 3.7 which is expected to be released in
June 2018?  We’d still have to deal with this problem in Python 2,
though.

Is it a bad idea to override the timestamps in the generated binaries?
I think that we could avoid the recency check then, which was an
obstacle to resetting the timestamps of the source files.

--
Ricardo

GPG: BCA6 89B6 3655 3801 C3C6  2150 197A 5888 235F ACAC
https://elephly.net

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