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

[Derrick Stolee <stolee@gmail.com>]

On 10/13/2021 2:51 PM, Emily Shaffer wrote:
> On Thu, Aug 19, 2021 at 09:09:46PM -0400, Derrick Stolee wrote:
>> 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.

I think that's a great way to deliver similar functionality without
the issues I was thinking about.

>> 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?

I think we can complain with something like a die() if the config points
to data that doesn't make sense (not a .git directory), but a BUG() is
too heavy-handed because it can just be a user modifying their config
file incorrectly.

I'm happy to shut down a process because a user messed with config
incorrectly. Since your proposed change allows operations like 'git mv'
to move submodules without needing to change this config, I'm much
happier with the design. It becomes impossible for the config to get
out of sync unless the user does something outside of a Git command.

Thanks,
-Stolee