[ERROR] Can't open and lock privilege tables: Got error 9 from storage engine

Nice error. Googling for it doesn’t reveal much on how to fix it. The good news is that I only use MySQL for Phabricator and PostreSQL for everything else. The bad news is that my Phabricator instance is no longer working. The worse news is that I get the same error when trying to use backups, so there must be something else in play here.

Ideas on how to resolve this are welcome!