Interest in integrating hg-git into Mercurial

Gregory Szorc gregory.szorc at gmail.com
Thu Aug 1 22:30:32 EDT 2019


On Thu, Aug 1, 2019 at 10:38 AM Augie Fackler <raf at durin42.com> wrote:

>
>
> > On Aug 1, 2019, at 13:01, Gregory Szorc <gregory.szorc at gmail.com> wrote:
> >
> > Is there any interest in integrating hg-git (or hg-git functionality)
> into the Mercurial distribution as an officially supported extension?
> >
> > Given the popularity of Git and the difficulty of installing
> semi-complex Python software like hg-git, I was thinking it would be
> beneficial to end-users for Mercurial to support interacting with Git
> repositories out-of-the-box with as little set-up pain as possible. hg-git
> feels like the path of least resistance towards attaining that goal. (I
> would eventually like to see support for Mercurial opening Git repositories
> natively. While I think this is technically viable, it is probably a year
> or two away, as it needs significant work to shore up Mercurial's storage
> interfaces and internal code contracts to support such a significant
> invariant as interfacing with Git repositories.)
> >
> > Vendoring hg-git would like also entail vendoring its dependencies:
> urllib3, certifi, and dulwich. Vendoring urllib3 and certifi is probably
> not the worst thing in the world, as it would give us an excuse to refactor
> Mercurial's HTTP internals to move off the ugly hacks we employ to use
> Python's standard library.
> >
> > I'm not promising I will follow through and do this work. At this time
> I'm mostly interested in taking a quick pulse to see if there is any
> interest in doing it. If there is general support, I may follow through :)
>
> I'm semi-enthusiastic, but I'd rather we took a storage-abstraction
> approach than the hg-git "convert the repo" approach. With our newfound
> storage abstractions, I think it's reasonable to index a git repository and
> present its data in an hg-friendly format. A few places in hg would likely
> need patching to handle more than two parents in a merge, but other than
> that I think things basically work.
>

I would prefer we interface with Git repositories using the storage
abstraction and not have to maintain a shadow Mercurial repository as well.
And I do think that is technically viable. But I think there is a long,
long tail of issues that will prevent that from working as robustly as we
all want it to - and as many end-users will want it to. The handling of
octopus merges alone will be a giant PITA because there is code all over
Mercurial that assumes exactly 2 parents. Plus, the storage abstraction
work stopped short of the changelog and locks/transactions, which I think
will be the hardest parts to abstract. I think there's a lot of work there
and we shouldn't block Git repo integration on solving that general problem.

If we were to move forward with integrating hg-git, I would do so by making
the hg<->git repo coupling stronger. For example, I would install hooks
into the Git repository that prevented mutations unless Mercurial were
driving them (e.g. by looking for the presence of an environment variable).
And I would define repo requirements that denoted special behavior in the
presence of an associated Git repository. This would be designed such that
in a future world one could run `hg debugupgraderepo` and replace the
Mercurial full conversion repo with storage abstractions that allow us to
write directly into the Git repo.


>
> Note that I'm also willing to try and push this forward, and have some
> experimental hacks lying around that should make a proof of concept fairly
> easy. The reaction at the last sprint took away some of my enthusiasm, but
> if people are receptive to the idea I'll carve out some time in August...
>
> (I think your "a year or two" estimate is pessimistic on this front: I
> think it could be done very quickly by indexing the git repo at load time,
> and the indexing can be made reasonably quick, especially given that I had
> this _working_ in the 3.x era, albeit slowly.)
>
> > _______________________________________________
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>
>
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