Re: [PATCH 2/3] t1405: mark test that checks existence as REFFILES - Han-Wen Nienhuys

From: Han-Wen Nienhuys <hanwen@google.com>
To: Junio C Hamano <gitster@pobox.com>
Cc: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
	"Taylor Blau" <me@ttaylorr.com>,
	"Han-Wen Nienhuys via GitGitGadget" <gitgitgadget@gmail.com>,
	git@vger.kernel.org, "Han-Wen Nienhuys" <hanwenn@gmail.com>
Subject: Re: [PATCH 2/3] t1405: mark test that checks existence as REFFILES
Date: Thu, 3 Feb 2022 17:02:47 +0100	[thread overview]
Message-ID: <CAFQ2z_NSCvRbj1bxirxhqSWD+LadzCa8VNOsxGCmFCNT3GUU0g@mail.gmail.com> (raw)
In-Reply-To: <xmqqsft2b5jl.fsf@gitster.g>

On Tue, Feb 1, 2022 at 11:12 PM Junio C Hamano <gitster@pobox.com> wrote:
>
> Ævar Arnfjörð Bjarmason <avarab@gmail.com> writes:
>
> > We could surely add magic record types, but how would such a dance be
> > performed while keeping compatibility with existing JGit clients?
>
> Yes.  It is exactly the point of the question I asked.  If it is
> simple and easy to add such a new type that is ignored/skipped by
> existing clients, then we can go that route.  If it is simple and
> easy to add a new bit per ref that existing clients would not barf,
> we can use that as an alternative implementation strategy.

I'm not sure that there are any JGit clients: I committed reftable
support at the end of 2019. Before that time, we were running it
internally at Google, but only ref storage, and without the posix
part. Reflogs were never stored in refable, and I actually found a
couple of bugs in Shawn's Java code.

Gerrit has increasingly started using Git as a database, and the
packed/loose system is just not a very good database, so that
motivates the work reftable in general. But the folks who run Gerrit
on a POSIX filesystem want to be sure that isn't a fringe feature, so
they only want to start using it once Git itself supports it. So there
is a chicken & egg problem.

It's sad that we have to introduce an existence bit to make things
work, but overall it is probably easier for me to do than trying to
make sense of sequencer.c and how it uses refs/stash@{0}.

Technically, the only obstacle I see is that we'd need to treat an
existence entry especially for the purpose of compaction/gc: we can
discard older entries, but we shouldn't discard the existence bit, no
matter how old it is.

-- 
Han-Wen Nienhuys - Google Munich
I work 80%. Don't expect answers from me on Fridays.
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