Does jj have git hook support?

[zygoloid]

Does jj have support for either git hooks or a similar mechanism?

https://github.com/martinvonz/jj/blob/main/docs/git-compatibility.md doesn't seem to mention this one way or the other, so it'd be nice if it were discussed there even if the answer is no for now.

[martinvonz]

There's no support for git hooks yet. Good point about updating the docs. I'll send a PR for that.

Because the workflow in jj is quite different, some of the hooks don't seem to make much sense. For example, because we implicitly create commits from the working copy and/or without touching the working copy (e.g. when you change the description of an ancestor commit), Git's pre-commit hook would usually not make sense. I think even Git itself is considering skipping the pre-commit hook when rebasing, once they start doing it without touching the working copy. The post-commit doesn't make much sense either, since it also relies on HEAD being for the just-created commit.

We could probably call the prepare-commit-msg hook. However, if the hook tries to inspect the working copy or Git's HEAD, it will typically not do what you want. Also, I just experimented a bit with it and the third "SHA1" argument it gets doesn't seem to actually be a SHA1 (it's been empty or the string "HEAD" in my experimentation). So maybe it only makes sense for jj to call it when the working copy is colocated with Git (i.e. created by jj init --git-repo=.) and we're snapshotting the working copy. That may be true for most hooks.

Which hook(s) do you miss having support for?

FWIW, I've planned to eventually add native (i.e. not Git-specific) hooks. There would be at least a transaction-level hook, which would let you inspect and approve the changes in a transaction (there's typically one transaction per command, plus maybe one more for snapshotting the working copy).

[chandlerc]

For myself at least, I'm not so much looking for support of all the hooks, but more the hooks that make sense in the new workflow. Ideally, supporting the same hook interface (so existing hooks can be used), provided they fit into the meaningful slot in the workflow like you describe for prepare-commit-msg. But no idea how to bridge the API gap if it exists.

For me, the hooks I care most about are things like https://pre-commit.com/ -- while it is called "pre-commit", it isn't actually specific to the "pre-commit" hook in Git. Personally, I most often use it as a pre-push hook.

So selfishly, just having the pre-push hook from pre-commit.com would be pretty helpful for me... but others probably have some other more interesting needs here.

[martinvonz]

I filed #405 for integrating with pre-commit.com in some way. Thanks for pointing me to that.

[ucirello]

Echoing the support to add support to some of the git-hooks. The ones I use that I believe fit well with jj's workflow:

• commit-msg: that allows me to ensure that the message I wrote fits a certain standard.
• pre-push: which I believe should play well with jj git push - unless I am missing something, the push operation between git and jj git push use the same underlying constructs? In my naïveté, they should play well together.

[PhilipMetzger]

Hello thanks for your interest in hooks. See this comment for an analysis of the feasibility of hooks #3577 (comment) in jj. I'm informing you because I'm largely responsible for it (#405, #1869).

[ucirello]

@PhilipMetzger -- thank you for the pointers.