Re: [PATCH v6 0/5] teach submodules to know they're submodules

From: Junio C Hamano <gitster@pobox.com>
To: Emily Shaffer <emilyshaffer@google.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>,
	"Matheus Tavares Bernardino" <matheus.bernardino@usp.br>,
	"Jonathan Nieder" <jrnieder@gmail.com>,
	"Jacob Keller" <jacob.keller@gmail.com>,
	"Atharva Raykar" <raykar.ath@gmail.com>,
	"Derrick Stolee" <stolee@gmail.com>,
	"Jonathan Tan" <jonathantanmy@google.com>
Subject: Re: [PATCH v6 0/5] teach submodules to know they're submodules
Date: Thu, 03 Feb 2022 14:39:17 -0800	[thread overview]
Message-ID: <xmqqczk3r2wa.fsf@gitster.g> (raw)
In-Reply-To: <20220203215914.683922-1-emilyshaffer@google.com> (Emily Shaffer's message of "Thu, 3 Feb 2022 13:59:10 -0800")

Emily Shaffer <emilyshaffer@google.com> writes:

> A couple things. Firstly, a semantics change *back* to the semantics of
> v3 - we map from gitdir to gitdir, *not* from common dir to common dir,
> so that theoretically a submodule with multiple worktrees in multiple
> superproject worktrees will be able to figure out which worktree of the
> superproject it's in. (Realistically, that's not really possible right
> now, but I'd like to change that soon.)

Sounds sensible.

> Secondly, a rewording of comments and commit messages to indicate that
> this isn't a cache of some expensive operation, but rather intended to
> be the source of truth for all submodules.

I'd expect that there is a way (e.g. "git fsck") that helps the
users notice when the actual filesystem layout contradicts with what
the gitdir-to-gitdir link says, and repair the repositories when
they go out of sync if possible.

It would be similar to "git worktree", where a link between the
".git" file that records "gitdir:" in a secondary worktree and the
repository's $GIT_DIR/worktrees/*/gitdir, and the "repair" command
can be used to bring them back in sync after moving the real
repository without telling the secondary worktree about the move.

> I did discuss Ævar's idea of relying on in-process filesystem digging to
> find the superproject's gitdir with the rest of the Google team, but in
> the end decided that there are some worries about filesystem digging in
> this way (namely, some ugly interactions with network drives that are
> actually already an issue for Googler Linux machines). Plus, the allure
> of being able to definitively know that we're a submodule is pretty
> strong.

The other side of the coin is that, even when a configuration
variable says that you are a submodule of the superproject at
location X, if such a submodule gets moved out of the superproject
(perhaps because the end-user wanted to concentrate on that
submodule project alone as an independent project) and the
superproject that used to be at location X got archived away,
trusting and relying on what the configuration variable says would
not help us access the now-gone superproject.  And that would not
change no matter how strongly we declare that it is the source of
truth.

Unless we have a very good way to detect inconsistency and stop
spreading the damage (e.g. the setting thought our superproject sits
at directory X, but that location is now occupied by a different
repository that is not related), I am still skeptical about the
"setting is the sole truth" design.