Unsquashing from a merge commit is weird

[cormacrelf]

Say you've got a merge commit.

; jj new knx ttl
; jj log
@    qqyvktys [email protected] 2024-08-26 12:24:32 76ca4eb8
├─╮  (empty) (no description set)
│ ○  ttlomumr [email protected] 2024-08-26 11:29:21 ffbaeec4
│ │  add CONTRIBUTING.md
○ │  knxrzpxl [email protected] 2024-08-26 11:30:01 dfa6e45f
├─╯  add README.md
◆  zzzzzzzz root() 00000000

Then, you make some changes:

; echo 'fn main() {}' > main.rs
; jj log
@    qqyvktys [email protected] 2024-08-26 12:26:21 0f41bee4
├─╮  (no description set)
│ ○  ttlomumr [email protected] 2024-08-26 11:29:21 ffbaeec4
│ │  add CONTRIBUTING.md
○ │  knxrzpxl [email protected] 2024-08-26 11:30:01 dfa6e45f
├─╯  add README.md
◆  zzzzzzzz root() 00000000

Often at this point I realise I didn't want to do the work in the merge commit, since there were no conflicts to resolve or warnings/formatting issues to fix up during the merge, rather this was new work. As a habit it's better to jj new after a merge, but nevertheless.

There is no way to jj split or jj commit with all changes going into the second commit, from the command line. You have to open up a merge tool and manually move changes to the right. That job is basically what unsquash is for -- except this is what happens when you unsquash:

; jj new
; jj unsquash
; jj log
@    womzqlzp [email protected] 2024-08-26 12:32:55 a2592df4
├─╮  (no description set)
│ ○  ttlomumr [email protected] 2024-08-26 11:29:21 ffbaeec4
│ │  add CONTRIBUTING.md
○ │  knxrzpxl [email protected] 2024-08-26 11:30:01 dfa6e45f
├─╯  add README.md
◆  zzzzzzzz root() 00000000

Basically qqy was "empty" after we took all the changes, so it got abandoned, and we end up with a new merge commit that's exactly the same as before. The current behaviour is a bit surprising to me because I don't really think of the parents of a commit as being "changes" that you can move around. jj log shows an empty merge commit as empty, and it will get abandoned when you switch away from an empty @ merge commit (IIRC). So when unsquash says you can take the changes, I don't really expect the parents to be part of that.

Not sure what the right path forward is for this. Maybe merge commits should not be empty for unsquash purposes? Or maybe there should be an unsquash --allow-empty?

Also, it would be nice if jj split could have a flag to invert the default location to the second commit. Don't know if invert is the right name, but:

; jj split --invert         # basically inserts an empty commit before rev, doesn't go into interactive mode
; jj split --invert PATHS   # puts PATHS in the first commit instead

[martinvonz]

I think you can jj new --before qq to insert a new commit before qq. It should become a merge commit because it inherits the parents of qq by default (you could combine it with --after, but that's not what you're looking for in your case).

EDIT: Add --no-edit to just create the commit without editing it afterwards.

[cormacrelf]

Hell yeah, that works. Thanks!