GitLab: rebaseWhen=auto should not update the branch when merge trains are enabled

[cjpearson]

When merge trains are enabled on GitLab, the branch does not need to be up to date in order to merge. However, renovate will still rebase old branches when rebaseWhen=auto. This can cause MRs which are on the merge train to be removed.

There was a previous discussion on the topic here, but was closed when the author found a workaround.
#29842

The workaround is simple (set rebaseWhen=conflicted) but it's a bit inconsistent with the docs which state that rebasing will only occur if necessary due to a conflict or up-to-date requirement.

My assumption is that the logic here could be updated to something like const forceRebase = config?.mergeMethod !== 'merge' && !config.mergeTrainsEnabled. (mergeTrainsEnabled is not on the config, but it is returned from the API) Does this make sense?

[rarkins]

rebaseWhen=auto converts to rebaseWhen=behind-base-branch whenever automerge=true. This is an opinionated decision in Renovate to only automerge branches which are up-to-date. There are some other reasons not related to automerge too, such as branch protection policies.

I support this idea but it might require a slight change in the calls from Renovate's worker layer to platform layer to indicate not to rebase

[cjpearson]

Thanks for the feedback. I've looked a bit more into this. With the proposed change nothing should change with automerge. These are the results with a dry run on a repo with automerge and non-automerge dependencies.

Before:

• automerge dep: Converting rebaseWhen=auto to rebaseWhen=behind-base-branch because automerge=true
• non-automerge dep: Converting rebaseWhen=auto to rebaseWhen=behind-base-branch because platform is configured to require up-to-date branches

After:

• automerge dep: Converting rebaseWhen=auto to rebaseWhen=behind-base-branch because automerge=true
• non-automerge dep: Converting rebaseWhen=auto to rebaseWhen=conflicted because no rule for behind-base-branch applies

I've added a PR here: #36960