FR: Support for trailer separator customization

[dolangish]

Is your feature request related to a problem? Please describe.
In git, I can customize my trailer separator. This is the character that delimits the key from the value. By default it is a colon, but git allows users to customize it in their config.

Describe the solution you'd like
I would like jj to also allow customization of this separator.

Describe alternatives you've considered
To match git's functionality here seems like the most obvious solution, so I haven't considered alternatives.

Additional context
For me, I have a trailer in the format of key-value instead of key:value. I need to extract it for display in my one-line log messages as it's a jira task id.

[ilyagr]

I don't understand the situation well, but I am undecided on whether this is worth doing. In my mind, trailers are mostly used for interacting with the remote, e.g. Co-authored-by tells GiotHub to put more user avatars at a commit. This applies to most examples from this list, at least. Presumably, the remote won't be able to do this if you use a custom character.

Could you elaborate on your use-case with Jira? Does it just demand using - instead of : as a separator? What other situtations are custom separators useful in?

Another useful link I found: https://git-scm.com/docs/git-interpret-trailers#Documentation/git-interpret-trailers.txt-trailerseparators

[jennings]

Some people in Discord were surprised that the commit trailer separator was configurable in Git, including myself.

That seems to me like the kind of thing that is better left unconfigurable for better compatibility. I wonder if it's a good idea to wait for more people to ask for this before making it an official feature? Admittedly, this is a pretty small feature, so it's probably not a terrible maintenance burden.

The person in Discord asked for this because their team had the habit of adding Jira ticket numbers to the end of their commit messages:

feat: Do something cool

A longer explanation of the feature.

CORE-1234

They were using - as the trailer separator so they could get the ticket number. But I think that could also be done in other ways.

[ilyagr]

Would it be possible for them to switch to jira-ticket: 12345, I wonder?

[dolangish]

Trailers are often times automatically added to commits, like a signature. I like to think of them as email headers but for commit messages. Key value pairs that scripts using git can use. In my case, we manually add our Jira task IDs to the end of every commit. Currently I can access this info in JJ by running a blob or regex on the commit body, but it would be cleaner if I could use trailers to do it so it doesn't mistakenly pull in non trailer text. Our trailers use - instead of : as the separator simply because at the time the decision was made we were using RCS and had no idea git even existed much less it's idea of trailers. We have since converted to git and started leveraging trailers.

I can understand not wanting to implement this to avoid feature creep, but keep in mind, this is something git supports, and I think generally JJ should be able to functionally do everything git does, even if JJ may have a different interface for doing it. Otherwise people who use this feature of git, however small, will have reason to not use JJ. And that would be sad.

Switching over to colon is a possibility for us if it's decided that JJ will not implement this enhancement.

[ilyagr]

I think generally JJ should be able to functionally do everything git does, even if JJ may have a different interface for doing it

My sense is that we should seriously consider adding any feature Git has, but not we shouldn't add features that people might regret relying on. We should rely on Git both for positive examples (something works very well) and for negative ones (they implemented something, but it turns out to not be very useful in practice, or people have since thought of something better).

We certainly do want to know if you rely on this Git feature so much that it'd make it difficult to use jj while it doesn't support the feature. Hopefully, we'll figure out something that works well enough for most people and for you.

Our trailers use - instead of : as the separator simply because at the time the decision was made we were using RCS and had no idea git even existed much less it's idea of trailers. We have since converted to git and started leveraging trailers.

Do you ever use the more conventional Git trailers like Co-authored-by, Signed-off-by, etc? How do you deal with the fact that they contain - in the middle of their name?

Perhaps you could have a custom script for dealing with your special trailers (until one day, perhaps, you can migrate)? This would be similar to how people used trailers with jj in the recent past, before there was official trailer support in jj. (TODO: maybe find some links)

In some ways, your example makes me more doubtful that jj should support these kinds of trailers. Your use of them is completely different from the currently established way people use trailers, as I understand it. It's completely understandable for an old code-base; when Git trailers first appeared, it was far less clear how people would be using them, and it's likely they considered something like RCS compatibility when implementing custom trailer separators. But from the perspective of the present day, your use seems custom enough that it might not be out of place for you to write your own script to maintain it instead of relying on the VCS supporting it.

I also don't mean this message to be the final word in this discussion. I'm curious about other ideas, use-cases, etc.

[dolangish]

Real talk, as much as I would like this feature because it keeps me from having to make a company-wide change to how we tag our commits, moving to the : standard is probably the right thing for me to do in this case.

If I were you, I would probably elect to not implement this because you are correct in that it's really just a bandaid for a problem that should be solved in another way.

As an aside, it doesn't look like trailers are available in revsets anyways, only templates, so assuming my company moves over to the colon syntax, probably what I should ask for is the ability to filter revsets by trailer.

[yuja]

My feeling is that CORE-1234 doesn't look like a key-value pair, but is something akin to #1234. Maybe we should extend string substitution/extraction functions in template so the user can easily extract arbitrary tags from commit description?

[ilyagr]

For the part of this issue where you might want to label a commit differently in (say) jj log based on a tag, that seems like a good idea. It also seems spiritually similar to having jj interact with some external system (say CI, review platform) to find something about the PR status/CI status/etc of the commit, as recently discussed on Discord. The main difference is that when the info we need is in the commit description, it's more likely for the lookup to be quick and successful.

The other part of the issue is support for modifying or adding trailers to a commit, which doesn't seem like it could be addressed by the templater (though it can be addressed by combining the existing jj describe with an external script).

[martinvonz]

I have read through this thread but I didn't understand where we would add support for custom separators. The template language already supports writing any character you like. I don't remember where else we have support for trailers, sorry.

[martinvonz]

Ah, after seeing PR #6839, I see that this is about the Commit::trailers() template function, which is responsible for parsing trailers from the commit description. This makes more sense to me now.