Re: [PATCH 1/4] odb: store ODB source in `struct odb_transaction`

[Justin Tobler <jltobler@gmail.com>]

On 26/01/29 11:25AM, Junio C Hamano wrote:
> Patrick Steinhardt <ps@pks.im> writes:
> > This makes me wonder whether we should first refactor the `tmp_objdir`
> > subsystem to receive a source instead of a repository as input.
> > Otherwise we "pretend" that the transaction is on the source level, but
> > we ultimately still end up creating the temporary directory in the
> > repository's object directory unconditionally.
> >
> > It wouldn't really change anything right now as we only ever write
> > objects via the primary object source anyway, so the end result would be
> > the same. But it just feels like a good first step to me to fix this
> > conceptual inconsistency, and it shouldn't be too involved either as
> > `tmp_objdir_create()` only has three callsites.
> 
> I agree with your "not really change anything right now" comment,
> but a new odb source that will be invented in the future may not
> even be file based, and a generic-sounding tmp_objdir_create() that
> creates a temporary directory on the filesystem may not even be an
> appropriate abstraction.

Yup, the tmp-objdir system is really only useful in the context of file
based ODB sources. I figure future ODB sources will have to have a
different mechanism to temporarly store objects.

Interestingly, it looks like there are only three users of odb-tmpdir:
remerge-diffs, git-recieve-pack, and ODB transactions. All of these
use-cases seems like a reasonble fit to create an ODB transaction
instead of managing the tmpdir directly. In the case of remerge-diffs
the transaction would need to always be aborted. If this is done, then a
tmpdir could become an internal detail of the ODB transaction for the
files backend.

> If we have two or more odb sources both are filesystem based, on the
> other hand, I do not think it is particulary bad if these two odb
> sources belonging to the same repository took a temporary directory
> out of that repository.  As long as one temporary object directory
> taken by one odb source is not used to commit the transaction into
> the other odb source, it would be fine, no?

I believe that we only support having a single tmpdir per Git process
via `the_tmp_objdir` global anyway. So maybe on second thought
configuring per source doesn't make much sense especially if only the
"files" ODB source would make use of it. If we do go down the route of
merging the tmpdir with the "files" ODB transaction implementation, we
could probably just pass the concrete "files" ODB source to it instead.

-Justin