I just had the same problem on a fresh debian 9 installation. The upgrade from 3.1.1.4 to 3.1.2 ended in:
ERROR: Upgrade failed: DB: Specified key was too long; max key length is 767 bytes
I’m running:
~ ᐅ php --version
PHP 7.0.33-0+deb9u3 (cli) (built: Mar 8 2019 10:01:24) ( NTS )
~ ᐅ mariadb --version
mariadb Ver 15.1 Distrib 10.1.37-MariaDB, for debian-linux-gnu (x86_64) using readline 5.2
Those versions are the current versions provided by apt.
@asmecher: Do you know what version will be sufficient?
It think it would be nice to update the system requirements on the downloads page as long this is an issue happening with the default versions provided by apt.
I finally was able to successfully upgrade, avoiding the error. The solution for me was to use real mySQL. So I removed mariaDB, installed mySQL and then the update worked.