The index (Re: Please default to 'commit -a' when no changes were added)

[Jonathan Nieder <jrnieder@gmail.com>]

Goswin von Brederlow wrote:

> Other RCS use an index of files they track. Git uses an index of patch
> chunks to commit. Same name, totaly different concept.

What you say is correct in terms of how some people use the tools.
But underneath, the index is a cache that tracks the content of all
files.  To make the index not match the work tree is to deliberately
let it go stale (or to cheat and poison it, or whatever).

I am making an assumption about other version control systems and it
is probably wrong for some.  Here is the logic: suppose I

 1. Make some changes to files.
 2. Invoke “vcs commit”
 3. Pull out the power plug.

What happens?  If the version control system is sane, then either the
entire commit takes place or nothing visible happens; because
otherwise, the result is that I screwed everyone over.  The easiest
way to implement this is to make “vcs commit” two steps:

 1. Prepare the proposed changes in a staging area.
 2. Atomically commit them.

Traditionally, “atomically” means “with a lock, on the remote server
which has a steady power supply”.

In particular, the index I am talking about tends to be on the _remote_
machine.  Making it local leads to a lot of improvements.

Early in the design of git’s user interface, it took some time to
figure out how visible to make the index [1].  Personally I am happier
with the modern approach of letting people dirty the index if they
want to, but if you believe that is wrong, maybe you would like to
look at the Cogito scripts for inspiration [2].  There are still many
lessons to learn from them, I suspect.  More importantly, it might be
fun or interesting.

Hopefully that is a little clearer.
Jonathan

[1] http://thread.gmane.org/gmane.comp.version-control.git/780/focus=918
As you can see, cogito, the most widely used front-end in early
history, did hide the index from the user.
[2] http://git.or.cz/cogito/
(warning: they have not been maintained for a while)