version parser refuses to recognize foolscap's "in between releases" versions

[zooko]

If you install foolscap from its source, it will have a version number like 0.6.1+. If you then run Tahoe-LAFS, it will exit at startup with:

Traceback (most recent call last):
  File "/Users/zooko/playground/tahoe-lafs/pristine/support/bin/tahoe", line 9, in <module>
    load_entry_point('allmydata-tahoe==1.8.2-r5006', 'console_scripts', 'tahoe')()
  File "/Users/zooko/playground/tahoe-lafs/pristine/support/lib/python2.6/site-packages/setuptools-0.6c16dev3.egg/pkg_resources.py", line 318, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/Users/zooko/playground/tahoe-lafs/pristine/support/lib/python2.6/site-packages/setuptools-0.6c16dev3.egg/pkg_resources.py", line 2245, in load_entry_point
    return ep.load()
  File "/Users/zooko/playground/tahoe-lafs/pristine/support/lib/python2.6/site-packages/setuptools-0.6c16dev3.egg/pkg_resources.py", line 1978, in load
    entry = __import__(self.module_name, globals(),globals(), ['__name__'])
  File "/Users/zooko/playground/tahoe-lafs/pristine/src/allmydata/__init__.py", line 375, in <module>
    check_all_requirements()
  File "/Users/zooko/playground/tahoe-lafs/pristine/src/allmydata/__init__.py", line 373, in check_all_requirements
    raise PackagingError(get_error_string(errors, debug=True))
allmydata.PackagingError: 
TypeError: expected string or buffer

For debugging purposes, the PYTHONPATH was
  '/Users/zooko/playground/tahoe-lafs/pristine/support/lib/python2.6/site-packages:/usr/local/lib/python2.6/site-packages:/Users/zooko/lib/python2.6/site-packages'
install_requires was
  ['setuptools >= 0.6c6', 'zfec >= 1.1.0', 'simplejson >= 1.4', 'zope.interface', 'Twisted >= 2.4.0', 'foolscap[secure_connections] >= 0.6.1', 'Nevow >= 0.6.0', 'pycrypto == 2.0.1, == 2.1.0, >= 2.3', 'pyasn1 >= 0.0.8a', 'mock', 'pycryptopp >= 0.5.20']
sys.path after importing pkg_resources was
  /Users/zooko/playground/tahoe-lafs/pristine/support/bin:
  /Users/zooko/playground/tahoe-lafs/pristine/support/lib/python2.6/site-packages/setuptools_darcs-1.2.12.egg:
  /Users/zooko/playground/tahoe-lafs/pristine/src:
  /Users/zooko/playground/tahoe-lafs/pristine/support/lib/python2.6/site-packages/mock-0.6.0-py2.6.egg:
  /Users/zooko/playground/tahoe-lafs/pristine/support/lib/python2.6/site-packages/pyasn1-0.0.13b-py2.6.egg:
  /Users/zooko/playground/tahoe-lafs/pristine/support/lib/python2.6/site-packages/pycrypto-2.3-py2.6-macosx-10.6-universal.egg:
  /Users/zooko/playground/tahoe-lafs/pristine/support/lib/python2.6/site-packages/Nevow-0.10.0-py2.6.egg:
  /Users/zooko/playground/tahoe-lafs/pristine/support/lib/python2.6/site-packages/simplejson-2.1.3-py2.6-macosx-10.6-universal.egg:
  /Users/zooko/playground/tahoe-lafs/pristine/support/lib/python2.6/site-packages/zfec-1.4.10-py2.6-macosx-10.6-universal.egg:
  /Library/Python/2.6/site-packages/foolscap-0.6.1_-py2.6.egg:
  /Users/zooko/playground/tahoe-lafs/pristine/support/lib/python2.6/site-packages/setuptools-0.6c16dev3.egg:
  /Library/Python/2.6/site-packages/argparse-1.1-py2.6.egg:
  /Library/Python/2.6/site-packages/pexpect-2.4-py2.6.egg:
  /Library/Python/2.6/site-packages/zbase32-1.1.3-py2.6.egg:
  /Library/Python/2.6/site-packages/docutils-0.7-py2.6.egg:
  /Library/Python/2.6/site-packages/setuptools_darcs-1.2.11-py2.6.egg:
  /Library/Python/2.6/site-packages/pycryptopp-0.5.29-py2.6-macosx-10.6-universal.egg:
  /Library/Python/2.6/site-packages/Twisted-10.2.0-py2.6-macosx-10.6-universal.egg:
  /Library/Python/2.6/site-packages/pyutil-1.8.4-py2.6.egg:
  /Library/Python/2.6/site-packages/coverage-3.4-py2.6-macosx-10.6-universal.egg:
  /Library/Python/2.6/site-packages/setuptools-0.6c11-py2.6.egg:
  /Library/Python/2.6/site-packages/txAWS-0.0.1-py2.6.egg:
  /Library/Python/2.6/site-packages/Epsilon-0.6.0-py2.6.egg:
  /Users/zooko/playground/tahoe-lafs/pristine/support/lib/python2.6/site-packages:
  /usr/local/lib/python2.6/site-packages:
  /Users/zooko/lib/python2.6/site-packages:
  /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python26.zip:
  /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6:
  /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/plat-darwin:
  /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/plat-mac:
  /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/plat-mac/lib-scriptpackages:
  /System/Library/Frameworks/Python.framework/Versions/2.6/Extras/lib/python:
  /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/lib-tk:
  /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/lib-old:
  /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/lib-dynload:
  /Library/Python/2.6/site-packages:
  /System/Library/Frameworks/Python.framework/Versions/2.6/Extras/lib/python/PyObjC:
  /System/Library/Frameworks/Python.framework/Versions/2.6/Extras/lib/python/wx-2.8-mac-unicode:
  /Users/zooko/lib/python2.6/site-packages

Part of the problem is that verlib (the nascent Python standard for version numbers) doesn't like 0.6.1+:

python -i verlib.py
>>> NormalizedVersion('0.6.1+')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/zooko/playground/tahoe-lafs/fileutil/tp/tarek-distutilsversion-22a8cdc67ac8/verlib.py", line 86, in __init__
    self._parse(s, error_on_huge_major_num)
  File "/Users/zooko/playground/tahoe-lafs/fileutil/tp/tarek-distutilsversion-22a8cdc67ac8/verlib.py", line 97, in _parse
    raise IrrationalVersionError(s)
__main__.IrrationalVersionError: 0.6.1+

Ways to improve this:

1. We could extend the copy of verlib that we bundle in Tahoe-LAFS to accept 0.6.1+ and let it sort higher than 0.6.1 and lower than 0.6.2. How should 0.6.1+ compare to 0.6.1.1?
2. Brian (the maintainer of foolscap) could change it to use the Python-nascent-standard notation. I think the closest analogue to 0.6.1+ would be 0.6.1.post0:

   >>> NormalizedVersion('0.6.1.post0')
   NormalizedVersion('0.6.1.post0.f')
   

Then we could just not worry about the fact that foolscap 0.6.1+ is incompatible with Tahoe-LAFS, and instead quickly make foolscap trunk describe itself as 0.6.1.post0. (I'll open a separate ticket saying that Tahoe-LAFS should more nicely deal with unparseable version numbers among its dependencies.)

3. We could switch back from using verlib as version number parser to using pkg_resources, which handles foolscap's version numbers in a way that has always worked for Tahoe-LAFS's purposes. I don't know if doing that would run afoul of #1258 or if we could separate the version-number-parsing from the sys.path-manipulation.

[zooko]

I opened #1388 (nicer reporting of unparseable version numbers in deps).

[zooko]

Foolscap no longer uses irrational version numbers -- ​http://foolscap.lothar.com/trac/ticket/175 -- so we don't need to deal with it.