[RFC/PATCH 0/8] Add configuration options for split-index

Goal ~~~~

We want to make it possible to use the split-index feature automatically by just setting a new "core.splitIndex" configuration variable to true.

This can be valuable as split-index can help significantly speed up `git rebase` especially along with the current work to libify `git apply`.

Design ~~~~~~

The design is similar as the previous work that introduced "core.untrackedCache".

The new "core.splitIndex" configuration option can be either true, false or undefined which is the default.

When it is true, the split index is created, if it does not already exists, when the index is read. When it is false, the split index is removed if it exists, when the index is read. Otherwise it is left as is.

Highlevel view of the patches in the series ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    - Patches 1/8 to 4/8 introduce the functions that are reading the
      configuration variable and tweaking the split index depending on
      its value.

    - Patches 5/8 and 6/8 add some documentation for the new feature.

    - Patch 7/8 adds a few simple tests.

    - Patch 8/8 cleans up the old "sharedindex.XXXX" previously
      created by split-index.

Future work ~~~~~~~~~~~

One thing that is probably missing is a mechanism to avoid having too many changes accumulating in the (split) index while in split index mode. The git-update-index documentation says:

	If split-index mode is already enabled and `--split-index` is
	given again, all changes in $GIT_DIR/index are pushed back to
	the shared index file.

but it is probably better to not expect the user to think about it and to have a mechanism that pushes back all changes to the shared index file automatically when some threshold is reached. The threshold could be for example when $GIT_DIR/index size is larger than 25% of the shared index size. Opinions, test results or test ideas are welcome on this.

Links ~~~~~

The last iteration of the git apply libification patch series is there:

http://thread.gmane.org/gmane.comp.version-control.git/298344/

This patch series is also available here:

https://github.com/chriscool/git/commits/config-split-index

Christian Couder (8):
  config: add git_config_get_split_index()
  split-index: add {add,remove}_split_index() functions
  read-cache: add and then use tweak_split_index()
  update-index: warn in case of split-index incoherency
  Documentation/config: add information for core.splitIndex
  Documentation/git-update-index: talk about core.splitIndex config var
  t1700: add tests for core.splitIndex
  read-cache: unlink old sharedindex files

 Documentation/config.txt           |  4 ++++
 Documentation/git-update-index.txt |  6 ++++++
 builtin/update-index.c             | 20 ++++++++---------
 cache.h                            |  1 +
 config.c                           | 10 +++++++++
 read-cache.c                       | 44 +++++++++++++++++++++++++++++++++++++-
 split-index.c                      | 18 ++++++++++++++++
 split-index.h                      |  2 ++
 t/t1700-split-index.sh             | 44 ++++++++++++++++++++++++++++++++++++++
 9 files changed, 138 insertions(+), 11 deletions(-)