Any extension that is going to be deployed to SUL wikis on the Wikimedia cluster needs to support UserMerge (user_id, user_name) and Renameuser (user_name).

Since Renameuser is shipped in the tarball it would be a good idea for extensions in general to support it, but it's not *required* since it's not a core feature (there's a bug somewhere to merge it into core).

Started a small section at https://www.mediawiki.org/w/index.php?title=Writing_an_extension_for_deployment&type=revision&diff=1651094&oldid=1403328

Some guidance on which hook to use would be nice; different hooks which seemingly perform the exact same thing create developer angst.
I'm guessing the fields/tables hooks should be preferred as they stay inside the transaction?

UserMergeAccountFields is preferred, as it'll automatically take care of transactions and batching. But in some cases you have to use MergeAccountFromTo (ex: FlaggedRevs) if merging requires some logic besides just updating the id.

For some tables, we use UPDATE IGNORE in the UserMergeAccountFields hook, and then delete any duplicates using the UserMergeAccountDeleteTables hook. and DeleteAccount is the equivalent of MergeAccountFromTo but for deletion.

For some extensions like Translate's sandbox feature, they don't listen to the merge hook and just act on the account deletion since we decided that retaining the data wasn't useful.