Concurrently running different jobs or concurrent builds of the same job?

As far as I know we already run concurrent builds of different jobs. Eg. on patchset-create mediawiki-core-phpunit-misc runs simultaneously with mediawiki-core-phpunit-api, that is already the case.

Running concurrent builds of the same job (if that is even possible, is it?) will likely get rather tricky.

For one, it will no longer make our gate an effective gate as it will no longer be testing the result as it will be when merged, other conflicting commits could make it into the repository separate from each other, thus breaking the end result with no warning *at all* anywhere to be found (until the next unrelated commit comes in and fails with nobody having a clue).

Secondly, we have various jobs (such as in Parsoid) that have certain hacks in place to run twice in a row to use last-run build artefacts. Not sure how that goes with concurrency.

Except for this last category (Parsoid double-runs) concurrency would be fine as long as the gate pipeline does not use it (for aforementioned reasons). On the check and test pipelines for patchset creations it should be fine.

I have made lot of jobs concurrent in December. The mediawiki-core-phpunit-parsertests had a race condition (bug 58094) which is apparently fixed.

https://gerrit.wikimedia.org/r/#/c/102149/ fixed a race condition with .sqlite files.

Change 102152 had a related patch set uploaded by Hashar:
(WIP) mediawiki-core-phpunit-* made concurrent (WIP)

https://gerrit.wikimedia.org/r/102152

Change 102152 merged by jenkins-bot:
mediawiki-core-phpunit-* made concurrent

https://gerrit.wikimedia.org/r/102152

The Wikibase / Wikidata jobs need to be made concurrent as well.

Wikibase / Wikidata got moved to another CI system.

For mw/core the last non concurrent job was the qunit one which Timo modified last Friday with https://gerrit.wikimedia.org/r/#/c/117603/