Re: [PATCH v3 0/4] cache parent project's gitdir in submodules

From: Emily Shaffer <emilyshaffer@google.com>
To: Derrick Stolee <stolee@gmail.com>
Cc: git@vger.kernel.org, "Albert Cui" <albertcui@google.com>,
	"Phillip Wood" <phillip.wood123@gmail.com>,
	"Johannes Schindelin" <Johannes.Schindelin@gmx.de>,
	"Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
	"Junio C Hamano" <gitster@pobox.com>,
	"Matheus Tavares Bernardino" <matheus.bernardino@usp.br>,
	"Jonathan Nieder" <jrnieder@gmail.com>,
	"Jacob Keller" <jacob.keller@gmail.com>,
	"Atharva Raykar" <raykar.ath@gmail.com>,
	"Jonathan Tan" <jonathantanmy@google.com>
Subject: Re: [PATCH v3 0/4] cache parent project's gitdir in submodules
Date: Wed, 13 Oct 2021 11:51:37 -0700	[thread overview]
Message-ID: <YWcquctPsrRm964k@google.com> (raw)
In-Reply-To: <f5197594-550f-3bd8-2d22-c9e1b3e161fd@gmail.com>

On Thu, Aug 19, 2021 at 09:09:46PM -0400, Derrick Stolee wrote:
> 
> On 8/19/2021 4:09 PM, Emily Shaffer wrote:
> > Since v2, mostly documentation changes and a handful of small nits from
> > Junio and Jonathan Tan. Thanks for the reviews, both.
> 
> Sorry to show up late with many questions (mostly because my understanding
> of submodules is not as strong as yours),

Well, here I am apologizing for showing up even later with a reply ;)

> but I do have some concerns that
> we have not covered all the cases that could lead to the relative path
> needing an update, such as a 'git checkout' across commits in the super-
> project which moves a submodule.
> 
> Leading more to my confusion is that I thought 'git submodule update'
> was the way to move a submodule, but that does not appear to be the case.
> I used 'git mv' to move a submodule and that correctly updated the
> .gitmodules file, but did not run any 'git submodule' subcommands (based
> on GIT_TRACE2_PERF=1).

During a live review a few weeks ago it was pointed out, I forget by
who, that this whole series would make a lot more sense if it was
treated as the path from the submodule's gitdir to the superproject's
gitdir. I think this would also fix your 'git mv' example here, as the
submodule gitdir would not change.

> 
> You mention in an earlier cover letter that this config does not need to
> exist, but it seems to me that if it exists it needs to be correct for us
> to depend on it for future features. I'm not convinced that we can rely
> on it as-written, but you might be able to convince me by pointing out
> why these submodule movements are safe.

Yeah, that is a good point, and I wonder how to be defensive about
this... Maybe it makes sense to BUG() if we don't find the
superproject's gitdir from this config? Maybe it makes sense to warn
(asking users to 'git submodule update') and erase the incorrect config
line?

Both those approaches would require a wrapper around
'git_config_get_string()' to cope with a failure, but it might be worth
it.

I'll try proposing such a wrapper in the reroll, unless I hear back
sooner.

 - Emily