[Distutils] bin/buildout hoses itself when the buildout directory has a long path

[PJ Eby]

AFAICT, this problem was only reported once, here:
https://mail.zope.org/pipermail/zope-dev/2012-February/044108.html but
nobody identified the cause or steps to reproduce.

What I've figured out is this: if your buildout's absolute path is
sufficiently long (I believe it's just if it's longer than your system
site-packages' absolute path), then running bin/buildout (vs. the system
buildout) will decide NOT to include eggs/zc.buildout in the buildout_paths
list in parts/buildout/site.py.  The result of this is that the zc.buildout
egg gets left out of pkg_resources.working_set, which means that when
buildout tries to find itself in the working set, it fails, resulting in
the following error:

$ bin/buildout
Upgraded:
  zc.buildout version 1.5.2;
restarting.
Traceback (most recent call last):
  File "bin/buildout", line 17, in <module>
    import zc.buildout.buildout
  File
"/usr/lib/python2.6/site-packages/zc.buildout-1.5.2-py2.6.egg/zc/buildout/buildout.py",
line 39, in <module>
    import zc.buildout.download
  File
"/usr/lib/python2.6/site-packages/zc.buildout-1.5.2-py2.6.egg/zc/buildout/download.py",
line 20, in <module>
    from zc.buildout.easy_install import realpath
  File
"/usr/lib/python2.6/site-packages/zc.buildout-1.5.2-py2.6.egg/zc/buildout/easy_install.py",
line 81, in <module>
    pkg_resources.Requirement.parse('zc.buildout')).location
AttributeError: 'NoneType' object has no attribute 'location'


Notice that the buildout is using the site-packages zc.buildout egg,
instead of the one in the local eggs directory.

Steps to reproduce:

$ cd really/really/long/directory/name/with/lots/of/stuff/emptydir
$ buildout init
$ bin/buildout

Workaround:  use "include-site-packages = false" in the [buildout] section.

This strongly suggests to me that some part of the site-packages isolation
mechanism is broken, at least for zc.buildout itself.  It is successfully
allowing zc.buildout to be imported from site-packages, but not letting
setuptools see that it is present in site-packages.  If it does this for
other packages that refer to themselves in the way zc.buildout.easy_install
does, they will similarly break.  (Likewise anything expecting to find
entry points, etc.)

On a somewhat unrelated note, I am as of yesterday a new zc.buildout
user...  and I already have a buildout that compiles Erlang, Spidermonkey,
CouchDB, and RabbitMQ, and sets them all up with a Celery daemon under
Supervisord, all in about 63 lines of .ini file.  To say that I am
impressed with buildout is an extreme understatement.  ;-)
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