Re: [PATCH 5/5] load_ref_decorations(): avoid parsing non-tag objects

From: Jeff King <peff@peff.net>
To: Taylor Blau <me@ttaylorr.com>
Cc: git@vger.kernel.org
Subject: Re: [PATCH 5/5] load_ref_decorations(): avoid parsing non-tag objects
Date: Wed, 23 Jun 2021 17:51:59 -0400	[thread overview]
Message-ID: <YNOs/w5JwfBDLbUD@coredump.intra.peff.net> (raw)
In-Reply-To: <YNKgkGkPiMgNubNE@nand.local>

On Tue, Jun 22, 2021 at 10:46:40PM -0400, Taylor Blau wrote:

> On Tue, Jun 22, 2021 at 12:08:40PM -0400, Jeff King wrote:
> > If the packed-refs file ever learns to store all of the peeled layers,
> > then we could switch to it. Or even if it stored a flag to indicate the
> > peel was not multi-layer (because most of them aren't), then we could
> > use it most of the time and fall back to a manual peel for the rare
> > cases.
> 
> Yeah, I would be in favor of either of these. Of the two, the latter
> seems like the simplest thing, but the former provides you all of the
> information you could hope for.
> 
> I suppose that if you are already changing the format of packed-refs,
> then we might as well do the thing which provides the most information
> and allows us to optimize *all* cases, not just the vast majority of
> them.

One reason not to include all of them is that the list can be
arbitrarily long, and regular readers of packed-refs (who may not even
care about peeling at all) have to skip past it. That matters a little
less these since we binary-search it (but you still might be iterating
over the ref).

So I think either way it is a tradeoff, and you are making assumptions
about which cases are less likely.

If I were to work on this (and I don't have any immediate plans to do
so), I'd probably do whichever is easiest to implement, and to maintain
backwards-compatibility. And I suspect that is the "flag" approach, but
a lot would depend on the details of our parser and what it permits.

-Peff