From: Jeff King <peff@peff.net>
Date: Thu, 27 Sep 2012 21:36:02 GMT
Subject: Re: Using bitmaps to accelerate fetch and clone
Message-ID: <20120927213602.GA12512@sigill.intra.peff.net>
In-Reply-To: <7vfw63p2wi.fsf@alter.siamese.dyndns.org>

On Thu, Sep 27, 2012 at 02:33:01PM -0700, Junio C Hamano wrote:

> Jeff King <peff@peff.net> writes:
> 
> > So yeah, we would want to put the pack trailer sha1 into the
> > supplementary index file, and check that it matches when we open it.
> > It's a slight annoyance, but it's O(1).
> 
> Yes.  If I am not mistaken, that is exactly how an .idx file makes
> sure that it describes the matching .pack file (it has packfile
> checksum in its trailer).  Otherwise you can repack the same set of
> objects into a new .pack file and make existing .idx very confused.

Yeah. In theory you wouldn't name the new packfile into place without
also generating a new index for it. But even if you do it right, there's
a race condition, and checking the trailer sha1 at least lets us know
that they don't match (I assume we just reject the index, then; I guess
this can result in an operation failing, but in practice it doesn't
really happen, as we don't bother packing unless there are actually new
objects).

-Peff