From: Jeff King <peff@peff.net>
Date: Mon, 24 Aug 2026 04:55:29 GMT
Subject: Re: [PATCH 2/2] packfile: recover when a multi-pack-index names a removed pack
Message-ID: <20260824045529.GB142844@coredump.intra.peff.net>
In-Reply-To: <5792c08f4ee0f9627ab1432d91299fe676e0a2f5.1787092446.git.gitgitgadget@gmail.com>

On Tue, Aug 18, 2026 at 10:34:06PM +0000, Elijah Newren via GitGitGadget wrote:

> Teach find_pack_entry() to recover.  After the normal multi-pack-index
> lookup and the regular pack fallback both miss, check whether the object
> is nonetheless present in a covered multi-pack-index (bsearch_midx()).
> If it is, its recorded owner must have become unavailable, so scan that
> index's packs directly for a surviving copy.  The bsearch gate keeps
> genuine misses (i.e. objects absent from the index) on the fast path, and
> because the recovery lives in find_pack_entry() itself it also fixes the
> OBJECT_INFO_QUICK callers that never reprepare.

You don't even have to pay the bsearch() again. We'd already have looked
in the midx earlier in the function. We just need to distinguish three
cases:

  1. it was not in the midx (or there is no midx)

  2. it was in the midx but we could not load it (pack invalid, or
     object in the bad_objects list)

  3. it was in the midx and is available

In fill_midx_entry() we return a boolean that lumps cases 1+2 together,
versus case 3. It could return a tri-state that would let us distinguish
all three. And then your fallback would kick in only for case 2 (case 3
already returned with success, and case 1 means the midx does not even
mention the object).

This is all assuming the fallback is worth pursuing. I'm still puzzled
why this specific case would matter when we have the same (already
solved) problem of reading a regular .idx whose .pack has gone away.

-Peff