New CI does not rebase roottest branch on top of roottest master

[pcanal]

Check duplicate issues.

• Checked for duplicates

Description

The new CI does not rebase roottest branch on top of roottest master. However it does rebase the code branch on top of the roottest master. In practice this means that if the latest development in the ROOT code requires a change in roottest (and it has already been merged into the main branch) then the PR will spuriously fail.

For example in https://github.com/root-project/root/actions/runs/6751316012/job/18355186874?pr=13900

We have:

  git fetch origin refs/pull/13900/head:master-13574
  git checkout master-13574
  git rebase master
  
  From https://github.com/root-project/root

but we have as part of the cmake invocation

  -- Found roottest: /github/home/ROOT-CI/src/roottest
  -- Updating roottest from 'https://github.com/pcanal/roottest' (master-13574)
  From https://github.com/pcanal/roottest
   * [new branch]        master-13574 -> master-13574
  Switched to branch 'master-13574'

A possible solution is to treat the roottest repository the same way as the ROOT repository and check it out and rebase outside of cmake.

Reproducer

See description

ROOT version

does not matter.

Installation method

New CI

Operating system

any

Additional context

No response

[pcanal]

I'll take a look. What is the best way to test new code in build_root.py?

[Axel-Naumann]

Create a dev branch in your fork of root.git for your modifications in build_root.py. Create a branch with the name name in your fork of roottest.git but from an old commit that cannot work with your root.git branch. If things work, the rebasing of your roottest branch in build_root.py should make the CI pass nonetheless.

[ferdymercury]

I guess this issue can be closed due to the merge prepared at #18338 by @dpiparo ?

[pcanal]

Yes, this can be closed when #18338 is merged.

[dpiparo]

Most likely, one will have to directly push the changes to the main branch directly immediately after branching v6-36-00-patches. We verified that rebasing and merging of such a big set of changes causes problems.
All that to say that @ferdymercury and @pcanal are absolutely right. The merge will be a nice simplification.