How can I step a rebase forward in time?

[xobs]

I'm unclear of the terminology here, so my question may be a little bit ill-formed.

I would like to be able to rebase a branch on top of the thing it was originally branched from by repeatedly moving it forward one step at a time.

For example:

 C
 |  Y
 B  |
 |  Z
 | /
 A

Ordinarily, a rebase of Z onto C will apply the changes from Z to the state of C, which may or may not work. There may be multiple changes that occurred in B that make it not cleanly merge, and there may be additional changes in C that further complicate things.

I'd like to have a way to rebase the entire tree first onto B, and if that applies cleanly reapply it onto C.

I find this approach has a number of uses:

• There are some forks to gcc and llvm to add additional compilers, and I'd like to update them to the latest version. By doing this incremental rebase, I can catch where the API changes, allowing for smaller updates and breaking the process up.
• I have patches against some projects (e.g. Rust) that are unstable, and I'd like to be able to modernize these patches by catching the change as it happens, which at least gives me some visibility into how other parts of the project were updated to match the new API

My current approach is to perform nightly builds against the projects in question, but if I forget something then I have to manually step things forward in time.

Is there any way to get jj to incrementally rebase a branch on every commit along the way to get to the tip, and pause if the rebase fails?

[joyously]

Would jj run or jj fix help with that?

[xobs]

What would that look like? I'm not familiar with those commands.

I'm now thinking I might need to wrap it in a shell script to iteratively ratchet the rebase forwards.

[bnjmnt4n]

This can probably be done using a shell script.

Given the BASE and the BRANCH containing changes, here’s some pseudo code:

1. For each COMMIT_ID in between the fork point and the new BASE location (jj log -r "fork_point(BASE | BRANCH)..BASE" -T 'commit_id ++ "\n"' --no-graph --reversed):
   1. Rebase the commits from the branch onto the “incremental” commit: jj rebase -r "BASE..BRANCH" -o "$COMMIT_ID"
   2. Check if the rebase introduced any conflicts, by storing jj log -r "BASE..BRANCH & conflicts()" --count as a COUNT variable:
      1. If the $COUNT is 0, continue the loop.
      2. If the $COUNT is greater than 0, the rebase introduced new conflicts.
         1. Run jj undo to go back to the last non-conflicted state.
         2. Exit the loop.

[xobs]

Thank you for the help! I didn't know about some of those commands, and I think this will do the trick.