I don't really understand why some of code made by the WMF is not hosted on its git platform, actually Gerrit. So I hope that all reasons used for the exceptions (for example notebooks previews) will be solved for official Wikimedia Gitlab creation.
May it be possible to define a rule saying that all developments made during WMF employees worktime be made on our Gitlab instance exclusively? Of course excepting pull requests for improvements on external repositories hosted somewhere else.
When we have migrated from Subversion to git, we have selected Gerrit as the code review system. As part of the project we also had the repositories mirrored to GitHub https://phabricator.wikimedia.org/T37429.
Why? Well I am not quite sure, but most likely to open the possibility to submit a pull request via GitHub: https://phabricator.wikimedia.org/T37497 . At the time (2012), some wanted additional tooling to make it very easy to contribute. I would argue the complexity of the tooling and the reviewing workflow itself are more to blame as a barrier of entry rather than the tooling itself, but that is really just my point of view.
Before that subversion migration, we already had repositories on Github mostly for mobile applications:
And after the migration to git/Gerrit, we still had repositories created on Github instead of Gerrit. For example: Limn, a data visualization framework https://github.com/wikimedia/limn . Groups got created, people added to them and eventually more repositories have been created.
In short we do not have a policy to enforce Gerrit has the canonical code hosting place. Although anything touching MediaWiki on production is definitely on Gerrit (we do not deploy from GitHub hosted repositories), anything else is a gray area at the discretion of the team, and sometime due to technical limitations such as testing the IOS based applications.
The point you have raised to have a rule to exclusively host on Gitlab is covered on the consultation page:
So essentially the same situation: still mirroring and GitHub is not explicitly forbidden. Then given Gitlab and Github have essentially the same workflow, one can imagine that repositories might want to migrate from GitHub to Gitlab unless they rely on tooling which is only available at GitHub (such as issue tracker, see https://github.com/issues?q=is%3Aissue+org%3Awikimedia for currently opened issues on the Github organization).
