[Distutils] Better version pinning in buildout (buildout-versions)

[Jim Fulton]

Lots of people agree that buildout-versions us useful
and the author has volunteered to contribute it to core buildout.

Cool!

Of course, as a part of buildout, rather than an extension, it can be
streamlined a bit. Also, based on experience, I think we can simplify
versions management a bit further.

When I implemented the versions mechanism in buildout, I decided to
build on the configuration model.  This has worked pretty well,
especially for complex use cases like layering version specifications
(e.g. building on "known good sets"), but this approach has added a
little bit of drudgery.  Because I didn't want to stomp on existing
versions sections, I made the name of the section containing versions
require configuration, which means you end up with::

  versions = versions

  [versions]
  ...

I also think it's a little awkward to have buildout update files that
are also edited by users.

Based on this, I propose that buildout-versions get incorporated into
buildout in the following way:

1. New buildout option named ``versions-file`` which takes the name of
   a file. to contain version information.  It is not a configuration
   file.  It is a file consisting of comments (#...)  and version
   specifications::

     # whatever
     foo = 1.3

   If it doesn't exist, it wil be created.  (I'm not sure it's a good
   idea to create the file implicitly though.) Any version constraints
   found in the file are added to the buildout version constraints.
   Version constraints found in the versions-file override version
   constraints obtained via a versions option, if any.

2. New buildout option: ``update-versions-file``. If this is true,
   then any picked/unpinned versions are appended to the versions file
   and reported in the output. There's a command-line option, ``-V``
   to set this to true for a run.  It's an error to use this if
   ``versions-file`` isn't set.

3. The ``allow-picked-versions`` option gets a new allowed value of
   ``warn``. if there are unpicked versions and this option is set to
   ``warn``, then picked/unpinned versions are reported.  Also, if
   ``allow-picked-versions`` is true, there will be no error if
   ``update-versions-file`` is true.

4. New buildout option: ``python-version`` that restricts the Python
   version, with the same semantics as buildout-version provides now.

Thoughts? Did I miss anything?

Jim

--
Jim Fulton
http://www.linkedin.com/in/jimfulton
Jerky is better than bacon! http://zo.pe/Kqm