On Mon, Aug 30, 2021 at 09:21:31PM -0400, Derrick Stolee wrote:
> Yes, supporting non-alternates is a historical accident. Supporting
> alternates that are not actually the core object database of a full
> repository is on purpose.
>
> So, hopefully the remaining discussion that I am seeing can be
> solved by a decision such as:
>
> "If we add the restriction that the builtin always runs with a
> repository and --object-dir always points to its objects dir
> or one of its registered alternates, then we have access to a
> local config file to learn how to interpret that object directory."
I left a similar comment in the other part of the thread. :)
> >> I wonder if it is safe to assume that in practice a directory given
> >> to the "--object-dir" option is always the "objects" subdirectory in
> >> a repository, and it is an error if there is no "config" file next
> >> to the directory. Then, we could check ../config relative to the
> >> given directory and error out if they use different hash.
>
> I would say that is not always the case, and we should not error out.
>
> I think taking a look to see if ../config exists to use the data
> might be helpful for some cases, but should not be a blocker for
> completing the requested operation. The config from the non-alternate
> repo should be sufficient for this (somewhat strange) case.
Yes, agreed. We have long supported these kind of "bare" alternates, and
I wouldn't be surprised if they are in wide use (though I do wonder how
folks actually modify them, since most commands that touch objects
really do want to be in a repository).
In other cases where we may benefit from their being a containing repo
(e.g., accessing the ref tips of the alternate), we speculatively look
at ".." and see if there are any refs. See refs_from_alternate_cb()[0].
The natural extension for the hash-format problem would probably be to
call check_repository_format_gently() on the parent directory of the
alternate-objects dir. If it succeeds, then we can pull out the
hash_algo parameter from its repository_format struct. And if not, then
we just assume it matches the main repo.
But I suspect all of this is moot for now, beyond being able to return a
nicer error message. The rest of the code is not at all ready to handle
packs with two different hashes in the same process. And I suspect it
would take a reasonable amount of refactoring to make it so. If somebody
wants to work on that, I won't stop them, but I kind of doubt it is
worth anybody's time.
[0] Looking at refs_from_alternate_cb(), I did wonder if it would work
at all with a reftable alternate, but I suspect it would. I think we
ended up still having a "refs/" directory in that case, so we'd
recognize it as a repo (though really, it ought to be using
is_git_directory() instead of its hacky check). And then we farm out
the actual ref iteration to a separate for-each-ref process, passing
along --git-dir, which will read that alternate repo's config. So it
should Just Work, even with a different ref backend. It's almost
certainly broken if the hash algorithms don't match, though, because
we'd get oddly sized results from for-each-ref's output.
That's all just interesting tangent, though. :)
-Peff
