[New-bugs-announce] [issue29094] Regression in zipfile writing in 2.7.13

[Peter Ebden]

New submission from Peter Ebden:

In Python 2.7.13, using zipfile.ZipFile to write into a file with some initial preamble produces a zip file that cannot be read again by some zip implementations.
Our use case is using pex (https://github.com/pantsbuild/pex) which writes a zip that begins with a shebang, and later attempting to manipulate that using Go's standard archive/zip package. In 2.7.12 that works OK, but in 2.7.13 the .pex file is rejected on reading. Linux's command-line unzip tool will read the archive, but issues a warning ("4 extra bytes at beginning or within zipfile") which wasn't present previously. 
zipfile.ZipFile does read the files OK.

I assume this is related to https://bugs.python.org/issue26293 since that's the most obvious zipfile change in 2.7.13. It's pretty easy to reproduce using the example in that issue:

from zipfile import ZipFile
with open('a.zip', 'wb') as base:
    base.write(b'old\n')
    with ZipFile(base, 'a') as myzip:
        myzip.write('eggs.txt')

unzip -t a.zip
Archive:  a.zip
warning [a.zip]:  4 extra bytes at beginning or within zipfile
  (attempting to process anyway)
...

----------
components: Library (Lib)
messages: 284172
nosy: Peter Ebden
priority: normal
severity: normal
status: open
title: Regression in zipfile writing in 2.7.13
versions: Python 2.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue29094>
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