[Bug 1490071] Re: MariaDB writes to /var/lib/mysql after crossgrade, preventing users from reverting to MySQL without manual intervention

[Otto Kekäläinen]

"I expect a user to be able to switch from MySQL to MariaDB and back
again."

It works for MySQL 5.5 and MariaDB 5.5, the latter promising to be
binary compatible wiith 5.5 The Debian packaging flag system in MySQL
packages is compatible with this. The preinstall flag check will allow
installation to proceed when switching back and forth between 5.5
variants.

A downgrade from MySQL 5.7 to 5.6 or MariaDB 10.0 to MySQL 5.6 is not
guarenteed to work due to possible changes in database format. A user
needs to downgrade manually via exportin and improting database dumps.

The bug is about the user experience: how users are notified what
happens and how dpkg is aborted into a broken state.

..and the bug/feature really is in MySQL preinstall script
(http://anonscm.debian.org/cgit/pkg-mysql/mysql-5.6.git/tree/debian
/mysql-server-5.6.preinst#n67 is the exact line), so stop putting
MariaDB in the bug title or assigning this to MariaDB package.

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Title:
  MariaDB writes to /var/lib/mysql after crossgrade, preventing users
  from reverting to MySQL without manual intervention

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