RFC GSoC idea: git configuration caching (needs co-mentor!)

[Michael Haggerty <mhagger@alum.mit.edu>]

I just wrote up the idea that fell out of the discussion [1] about the
other configuration features that I proposed.  As far as I am concerned,
it can be merged as soon as somebody volunteers as a co-mentor.  The
idea is embodied in a pull request against the git.github.io repository
[2]; the text is also appended below for your convenience.

Michael

[1] http://article.gmane.org/gmane.comp.version-control.git/242952
[2] https://github.com/git/git.github.io/pull/7

### git configuration API improvements

There are many places in Git that need to read a configuration value.
Currently, each such site calls `git_config()`, which reads and parses
the configuration files every time that it is called.  This is
wasteful, because it results in the configuration files being
processed multiple times during a single `git` invocation.  It also
prevents the implementation of potential new features, like adding
syntax to allow a configuration file to unset a previously-set value.

This goal of this project is to make configuration work as follows:

* Read the configuration from files once and cache the results in an
  appropriate data structure in memory.

* Change `git_config()` to iterate through the pre-read values in
  memory rather than re-reading the configuration files.

* Add new API calls that allow the cache to be inquired easily and
  efficiently.  Rewrite other functions like `git_config_int()` to be
  cache-aware.

* Rewrite callers to use the new API wherever possible.

You will need to consider how to handle other config API entry points
like `git_config_early()` and `git_config_from_file()`, as well as how
to invalidate the cache correctly in the case that the configuration
is changed while `git` is executing.

See
[this mailing list
thread](http://article.gmane.org/gmane.comp.version-control.git/242952)
for some discussion about this and related ideas.

 - Language: C
 - Difficulty: medium
 - Possible mentors: Michael Haggerty

-- 
Michael Haggerty
mhagger@alum.mit.edu
http://softwareswirl.blogspot.com/