[Distutils] PEP 426: proposed metadata caching convention

[Nick Coghlan]

(This probably belongs in a successor to PEP 376, but I'll leave it
under the PEP 426 umbrella for now)

One of the points raised regarding PEP 426's integrated metadata
format is the potential for runtime issues with pkg_resources as it
reads and processes the metadata during startup, particularly if it
needs to process any environment markers. While I acknowledge the
suggestions I have received that we should really be moving away from
the current filesystem based distributed installation information to a
real database that properly handle import hooks, I'm looking for
something simpler that will make it easier for setuptools and
distribute to consume the new metadata format (and thus hopefully make
them more amenable to generating it as well)

Assuming we add an Entry-Points field as I have proposed in another
message, I'd like to propose that installers generate three additional
cache files as part of the installation process:

    <dist-info-dir>/__cache__/version.txt
    <dist-info-dir>/__cache__/requires-dist.txt
    <dist-info-dir>/__cache__/entry-points.txt

version.txt would just be the version of the installed distribution
(no need to parse the main metadata file just to read the version
field)

requires-dist.txt would be similar to the pkg_resources requires.txt
format, but use PEP 426 version specifiers. It would:
- only contain runtime requirements where the environment markers
match the current system
- be split into sections based on the "extras" definition needed to
get the environment marker to pass

entry-points.txt would be the same format as the pkg_resources entry_points.txt

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia