FR: Support --template for jj describe and related commands

[frxstrem]

Is your feature request related to a problem? Please describe.

When I want to update the description of many commits in bulk, for example to add a trailer to each commit, or strip a wip: prefix that I often put in front of my commits while working on them, I need to either call jj describe interatively, or call it once for each commmit in sequence.

Describe the solution you'd like

It would be very nice if jj describe supported --template as an alternative to --message, which could generate the message for each commit based on a template expression.

For example, to add a trailer to all commits, one could then do

jj describe -r 'my_revset()' --template 'description ++ "\n\nMy-trailer: Something Something"'

It would also be nice if this applied to other commands that take --message, such as jj new and jj commit.

Describe alternatives you've considered

As mentioned, I could do the same interactively. If it involves a large number of commits, I would likely need to do some scripting within my editor, though.

Alternatively, I could do this in a three-step process, running several jj commands (possibly scripted):

1. Expand revset to a list of commits
2. For each commit, read the existing description (or other fields that the new description would be based on)
3. For each commit, run jj describe to update the description

[josephlou5]

jj describe is templated, just not with a --template flag; see https://docs.jj-vcs.dev/latest/config/#default-description.

You can still customize this on the command line like so (yes it's kind of verbose):

# `describe` takes positional revsets, btw.
# Using TOML triple quotes to avoid double quote escaping in the template itself...
# Providing configs on the command line is kind of hard :')
$ jj describe 'my_revset()' --config 'templates.draft_commit_description="""self.description().remove_prefix("wip:").trim_start()"""'
$ jj describe 'my_revset()' --config 'templates.draft_commit_description="""self.description() ++ "\n\nMy-trailer: Something Something" """'

If you do this a lot, you can also consider writing a pipeline that does something like this:

#!/usr/bin/env bash

# Use the change ID because rewriting revisions will change descendant commit IDs,
# but `jj describe` should never make change IDs unavailable.
jj log --no-graph -r="${input_revset}" -T 'change_id ++ "\n"' \
  | while read -r change_id; do
      jj log --no-graph -r="${change_id}" -T description \
        | make-your-modifications-here \
        | jj describe --stdin -- "${change_id}"
    done

# Edit: Sorry, just saw that you already identified this as a possibility lol 🙃

Also note https://docs.jj-vcs.dev/latest/config/#commit-trailers for the trailer case, which does some special handling like deduplication:

$ jj describe 'my_revset()' --config 'templates.commit_trailers="My-trailer: Something Something"'

[josephlou5]

Oh I also realize that the --config solution I mentioned above will still try to interactively open the editor, but with the modifications from the template. Adding --config 'ui.editor=["true"]' or JJ_EDITOR=true fixes that I believe.

[josephlou5]

Two other things worth nothing:

jj describe can be given multiple revisions, and all their descriptions will be opened in the same editor. This means you can use your editor to do bulk changes, such as removing wip: at the start of each line. (You might have known this, since your example says jj describe -r 'my_revset()'.)

Adding a --template for jj new doesn't really make sense IMO. I'm not sure about the exact order of operations, but it's possible that the commit is created with the --message set directly, which means the Commit type wouldn't be available in the template. Plus, jj new by default doesn't have a description set in the first place, so manipulating something with a template also doesn't make sense. I have also tested that templates.draft_commit_description does not seem to affect jj new.

[yuja]

Related: #7508 (comment)