From: Jeff King <peff@peff.net>
Date: Mon, 24 Aug 2026 07:23:22 GMT
Subject: Re: [PATCH 2/2] packfile: recover when a multi-pack-index names a removed pack
Message-ID: <20260824072322.GA155433@coredump.intra.peff.net>
In-Reply-To: <20260824070601.GC149254@coredump.intra.peff.net>

On Mon, Aug 24, 2026 at 03:06:01AM -0400, Jeff King wrote:

> On Mon, Aug 24, 2026 at 02:55:39AM -0400, Jeff King wrote:
> 
> > Right. It would be OK to skip Elijah's fallback workaround when
> > SECOND_READ is not set; the QUICK callers are prepared to accept the
> > false negative. But since it is cheap-ish to do the fallback check, it
> > is perhaps OK to just do it on the first pass?
> > 
> > I wonder how true that is. Imagine you had a midx covering a million
> > packs, and you notice an object is missing, but you're in QUICK mode. Do
> > you really want to individually check each of those million pack idx
> > files (that were otherwise not even opened or mmap'd because they're
> > covered by the midx!).
> > 
> > I think it's mostly academic. You'd have to do the million-pack search
> > if we are not in QUICK mode. And the point of QUICK mode is mostly
> > avoiding tons of fruitless searches for objects we don't actually have.
> > The bsearch() conditional means that we _know_ this is a racy negative
> > and not just some object we never even had. So it would trigger
> > generally only when the search is useful.
> 
> Actually, thinking on this more: we _don't_ usually scan the million
> packs for an object we actually have. If the object is available in a
> new pack, the SECOND_READ scan should find that pack and put it at the
> front of the packfile list (because they sort by reverse mtime), and
> we'd find the object immediately, without having to open the new packs.

Er, this final sentence should be "without having to open the (million)
old packs".

-Peff