From: Junio C Hamano <gitster@pobox.com>
Date: Wed, 03 Feb 2021 19:42:48 GMT
Subject: Re: [PATCH 19/25] pickaxe -G: set -U0 for diff generation
Message-ID: <xmqqy2g5aqfb.fsf@gitster.c.googlers.com>
In-Reply-To: <874kitdy6r.fsf@evledraar.gmail.com>

Ævar Arnfjörð Bjarmason <avarab@gmail.com> writes:

> I since discovered Junio's f01cae918f (diff: teach --stat/--numstat to
> honor -U$num, 2011-09-22) (as an aside we have no test for that
> behavior).
>
> I haven't looked carefully, but I don't think we'll have the same issue
> here, as pickaxe currently doesn't care about whether something is on
> the + or - line, when briefly looking at the diffstat edge cases it
> seems that's what differs based on -U<n> for the diffstat.

With -U0 or different <n> in general, the matching between preimage
and postimage may become different, and both -U3 (usual) and -U0 may
express the same change "correctly" from the point of view of a
program like "git apply", but humans would see them as different
patches, and "diffstat" that counts number of +/- would give
different results.  The patch IDs may also be different.  The old
commit was to pessimize the logic (because we do not need context
just to count +/- lines for the purpose of diffstat) to match human
expectations.  They expect "'diffstat' must be counting 'diff -p'
output" and we were counting "diff -p -U0" instead, resulting in
different numbers.

With internally using -U0, the updated "pickaxe -G" is likely to get
the same complaints: "'pickaxe -G<token>' found this commit, but in
the 'git show' output, the token does not seem to be affected".

You'd respond to "try 'git show -U0' and now you'd see the <token>",
but again that is probably breaking human expectations.