Review merge train default settings to reduce redundant testing

Note sure if it belongs to https://www.drupal.org/project/infrastructure → as it will affect core as well.

I believe GitLab used to require merge trains to enable merged results pipelines, so they were a requirement at the time. That requirement looks reversed now, so we could disable merge trains without impact, other than for project maintainer’s workflow.

Merge trains enable “merge if tests pass,” so maintainers don’t need to think as much about whether recent upstream commits might have changed the results. It is a nice feature, that I expect many maintainers might not know about, since “merge train” doesn’t clearly communicate what it is.

Combined with testing on git push, it is indeed redundant. Is there a way to skip post-push testing if the commit was a merge that was tested? I suspect there might not be, merging a MR is not too distinguishable from any other push. We can’t skip post-push testing since it is common enough to make commits outside of a MR.

There are some variables defined around merge requests here: https://docs.gitlab.com/ee/ci/variables/predefined_variables.html#predef...

There is no way to skip push testing if if came from a successful MR, as we cannot get that information.

There is the variable CI_MERGE_REQUEST_EVENT_TYPE, which can have the values "detached", "merged_result" or "merge_train". So I am not sure if maybe we can do the condition CI_MERGE_REQUEST_EVENT_TYPE != "merge_train". I'm not sure if this is what we'd want though.

Untested theory:

If we solve #3387117: Enable distributed caching in GitLab Runner → we could investigate setting all the rules for commits to a branch to default to skip and use a launcher job to trigger the full pipeline if a commit key doesn’t exist in the cache.

Will not save 100% of the resources (would have to always launch one checker job for commits to the main branch), and makes the workflow a bit more complex Howber should cut down CI time to a fraction of it current usage for those tested through Merge train.