Re: [PATCH] [RFC] setup.c: make bare repo discovery optional

From: Taylor Blau <me@ttaylorr.com>
Date: May 9, 2022, 23:57 UTC
Message-ID: <YnmqgdJUaKCYOT4J@nand.local>
In-Reply-To: <xmqq8rrab9m7.fsf@gitster.g>

On Mon, May 09, 2022 at 03:54:08PM -0700, Junio C Hamano wrote:

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> Taylor Blau <me@ttaylorr.com> writes:
>
> > Thanks for working on this! I'm excited to see some patches here, though
> > I'm not totally convinced of this direction. More below.
> >
> > To summarize, this proposal attempts to work around the problem of
> > embedding bare repositories in non-bare checkouts by providing a way to
> > opt-out of bare repository discovery (which is to only discover things
> > that are listed in the safe.bareRepository configuration).
> >
> > I agree that this would prevent the problem you're trying to solve, but
> > I have significant concerns that this patch is going too far (at the
> > risk of future damage to unrelated workflows) in order to accomplish
> > that goal.
> >
> > My concern is that if we ever flipped the default (i.e. that
> > "safe.bareRepository" might someday be ""), that many legitimate cases
> > of using bare repositories would be broken. I think there are many such
> > legitimate use cases that _do_ rely on discovering bare repositories
> > (i.e., Git invocations that do not have a `--git-dir` in their
> > command-line).
>
> I think 99% of such use is to chdir into the directory with HEAD,
> refs/ and objects/ in it and let git recognise the cwd is a git
> directory.  Am I mistaken, or are there tools that chdir into
> objects/08/ and rely on setup_git_directory_gently_1() to find the
> parent directory of that 'objects' directory to be a git directory?

If you took this change, and then at some point in the future we changed the default value of safe.bareRepository to "", wouldn't that break that 99% of use cases you are talking about?

When I read your "I think 99% of such use is ...", it makes me think that this change won't disrupt bare repo discovery when we only traverse one layer above $CWD. But this change disrupts the case where we don't need to traverse at all to do discovery (i.e., when $CWD is the root of a bare repository).

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> I am wondering if another knob to help that particular use case
> easier may be sufficient.  If you are a forge operator, you'd just
> set a boolean configuration variable to say "it is sufficient to
> chdir into a directory to use it a bare repository without exporting
> the environment variable GIT_DIR=."

Yes, GitHub would almost certainly set safe.bareRepository to "*" regardless of what Git's own default would be.

> It is likely that end-user human users would not want to enable such
> a variable, of course, but I wonder if a simple single knob would be
> sufficient to help other use cases you are worried about?

I'm not sure I agree that end-users wouldn't want to touch this knob. If they have embedded bare repositories that they rely on as test fixtures, for example, wouldn't safe.bareRepository need to be tweaked?

(On a separate but somewhat-related note, I still think that this setting should be read from the repository config, too, i.e., it seems odd that we'd force a user to set safe.bareRepository to some deeply nested repository (in the embedded case) via their global config.)

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> While I wish "extensions and nothing else", i.e. we use "degraded
> access", not "refuse to give access at all", were workable, I am
> pessimistic that it would work well in practice.
>
> Saying "nothing else" is easy, but we do "if X exists, use it" for
> hook, and to implement "nothing else", you'd need to find such a
> code and say "even if X exists, because we are in this strange
> embedded bare thing, ignore this part of the logic" for every X.
> We've been casually saying "potentially risky config" and then
> started mixing "hooks" in the discussion, but who knows what other
> things are used from the repository by third-party tools that we
> need to yet add to the mix?
>
> > I'm still not convinced that just reading repository extensions while
> > ignoring the rest of config and hooks is too confusing, so I'd be more
> > in favor of something like:
>
> I do not think it would be confusing.  I am worried about it being
> error prone.

Yeah, on this and the above quoted hunk, I am fine if our behavior eventually became "call die()" for when we are in an embedded bare repository. But I do think this transition should be gradual, i.e., we should likely emit a warning in those cases that would be broken in the future to say "this will break, run this `git config` invocation if you want it to remain working".

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> >     $ git.compile rev-parse --absolute-git-dir
> >     warning: ignoring repository config and hooks
> >     advice: to permit bare repository discovery (which
> >     advice: will read config and hooks), consider running:
> >     advice:
> >     advice:   $ git config --global --add safe.bareRepository /home/ttaylorr/repo.git
> >     /home/ttaylorr/repo.git
>
> Is the last line meant to be an output from "rev-parse --absolute-git-dir"?
> IOW, the warning says you are ignoring, but we are still recognising
> it as a repository?

In this example, yes. But again, I'm not so deeply attached to the idea that we *have* to run in those cases. So I would equally be OK with the above s/warning/fatal and minus the last line, too (i.e., that we call die(), obviously we'd have to emit the advice before calling die()).

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> By the way, do we need safe.bareRepository?  Shouldn't
> safe.directory cover the same purpose?
>
> If a directory is on the latter, you are saying that (1) the
> directory is OK to use as a repository, and (2) it is so even if the
> directory is owned by somebody else, not you.
>
> Theoretically you can argue that there can be cases where you only
> want (1) and not (2), but as long as you control such a directory
> (like an embedded repository in your project's checkout) yourself,
> you do not have to worry about the "ok even if it is owned by
> somebody else" part.

I'm not sure yet, but will think more about it.

Thanks, Taylor