From https://lists.wikimedia.org/pipermail/wikitech-l/2015-May/081644.html
> We're planning on deploying a global user merge tool to Wikimedia sites shortly. As the name suggests, it merges multiple users into one.
>
> This means that if your extension is storing user ids or user names, it will need to listen to one of the UserMerge hooks (UserMergeAccountFields, MergeAccountFromTo, UserMergeAccountDeleteTables, or DeleteAccount) to make sure it isn't referring to non-existent users. Reedy & I previously did an audit last November of all deployed extensions, however new ones have been deployed since then. Please check your extension(s) and if they need updating, file bugs that block T49918 and T69758.
This should be documented:
- Mention in [[ https://www.mediawiki.org/wiki/Writing_an_extension_for_deployment | Writing an extension for deployment ]] or [[ https://www.mediawiki.org/wiki/Review_queue#Checklist | Review queue checklist ]]
- Document this on other mw.org pages about extension development... //TBD!// Create a new manual page about usernames and IDs? Mention on Database design pages? Where else?
- Identify good extensions to link to as source code examples from the documentation.
Is it a general guideline for any extension storing user IDs and usernames, or does it only apply to wikis with a global rename extension, or only to extensions on Wikimedia wikis?