Using tuned_gzip.GzipFile.readline corrupts data on Python 2.7

[Martin Packman]

On Python 2.7 bt.test_multiparent.TestMultiVersionedFile.test_save_load fails in a mysterious manner.

<http://babune.ladeuil.net:24842/job/selftest-maverick/37/testReport/junit/bzrlib.tests.test_multiparent/TestMultiVersionedFile/test_save_load/>

Traceback (most recent call last):
  File "/home/babune/lib/python/testtools/runtest.py", line 144, in _run_user
    return fn(*args)
  File "/home/babune/lib/python/testtools/testcase.py", line 465, in _run_test_method
    testMethod()
  File "/home/babune/babune/slaves/maverick64.local/workspace/selftest-maverick/bzrlib/tests/test_multiparent.py", line 263, in test_save_load
    self.assertEqual('a\nb\nc\nd', ''.join(newvf.get_line_list(['a'])[0]))
  File "/home/babune/babune/slaves/maverick64.local/workspace/selftest-maverick/bzrlib/multiparent.py", line 512, in get_line_list
    return [self.cache_version(v) for v in version_ids]
  File "/home/babune/babune/slaves/maverick64.local/workspace/selftest-maverick/bzrlib/multiparent.py", line 519, in cache_version
    diff = self.get_diff(version_id)
  File "/home/babune/babune/slaves/maverick64.local/workspace/selftest-maverick/bzrlib/multiparent.py", line 567, in get_diff
    return MultiParent.from_patch(zip_file.read())
  File "/home/babune/babune/slaves/maverick64.local/workspace/selftest-maverick/bzrlib/multiparent.py", line 192, in from_patch
    return cls._from_patch(StringIO(text))
  File "/home/babune/babune/slaves/maverick64.local/workspace/selftest-maverick/bzrlib/multiparent.py", line 211, in _from_patch
    hunks[-1].lines[-1] += '\n'
IndexError: list index out of range

The root cause here is scary, and caused by the partial replacement of gzip internals in tuned_gzip clashing with later upstream changes, similar to bug 614476. Can cause all sorts of mayhem:

Python 2.7.0+ (trunk, Oct 1 2010, 18:14:22) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from bzrlib.tuned_gzip import bytes_to_gzip, GzipFile
>>> from StringIO import StringIO
>>> GzipFile(fileobj=StringIO(bytes_to_gzip("a\nb\n"))).readlines()
['a\n', 'b\n']
>>> g = GzipFile(fileobj=StringIO(bytes_to_gzip("a\nb\n")))
>>> g.readline()
'a\n'
>>> g.read()
'\na\n'

The revision that broke us is <http://svn.python.org/view?view=rev&revision=77288> which is attempting to improve perf as well <http://bugs.python.org/issue7471>. It introduces a new 'extrastart' member and changes the semantics of the internal buffering.