Re: [PATCH 2/5] add object-cache infrastructure

[Jeff King <peff@peff.net>]

On Mon, Jul 11, 2011 at 12:17:54PM -0400, Jeff King wrote:

> +Storage
> +-------
> [...]
> +The disk storage consists of a single file per cache, located in the
> +`$GIT_DIR/cache` directory. Each file contains a list of rows ordered by
> +sha1. Each row contains a binary sha1, followed by the fixed-size data
> +mapped to that sha1.

There's nothing else in the file, not even a header. I should probably
add at least a version byte, in case we ever do want to change what goes
into a cache (in which case we could just blow away what's there and
regenerate).

We could also have a "validity" field that must match for the cache to
be valid. Junio mentioned earlier that you would want to regenerate a
generations cache whenever grafts or replace objects changed. It
wouldn't be hard to do something like:

  validity = sha1(grafts + replace);
  if (validity != cache_validity)
    empty_disk_cache();

And then it would just automatically work, without the user having to
remember to clear the cache.  Calculating the validity would be cheap in
the common case (you have no grafts or replace objects), and not much
more expensive when you do have them (we have to read that data anyway
to respect the replacements).

-Peff