Update default datadir for debian to be /var/lib/mariadb

[lvoytek]

Match the existing datadir setting for Debian and Ubuntu, which was changed from /var/lib/mysql to /var/lib/mariadb in 1:11.8.6-5.

Existing Debian patch: https://salsa.debian.org/mariadb-team/mariadb-server/-/blob/debian/latest/debian/patches/set-default-datadir-to-var-lib-mariadb.patch

[CLAassistant]

All committers have signed the CLA.

[gemini-code-assist]

Code Review

This pull request updates the default data directory for Debian-based installations in cmake/install_layout.cmake by changing INSTALL_MYSQLDATADIR_DEB from /var/lib/mysql to /var/lib/mariadb. No review comments were provided for this change, and I have no feedback to provide.

[vuvova]

Questions:

• this is the main branch, so 13.0. A bit too late for Ubuntu?
• what about upgrades 12.3→13.0 ? May be not a problem for Ubuntu, but the change affects our (vendor) packages too.
• How does Ubuntu build MariaDB packages? You likely don't use our autobake.sh script, do you? In that case
  • we can set INSTALL_MYSQLDATADIR=/var/lib/mysql from our autobake.sh and keep the old path in vendor builds and the new path in Ubuntu builds
  • or Ubuntu can set INSTALL_MYSQLDATADIR=/var/lib/mariadb and get a new path without affecting vendor builds. I think the first approach might be easier.
  • or we can change all, Ubuntu and vendor, builds, and then pre/post-inst scriptlets need to handle the upgrade and move the data to the new location

@ottok, what do you think?

[grooverdan]

Thanks @lvoytek

Questions:

* this is the `main` branch, so 13.0. A bit too late for Ubuntu?

Ubuntu 26.04 already carrying the patch for 11.8_+
bugs.launchpad.net/ubuntu/+source/mariadb/+bug/2148632

* what about upgrades 12.3→13.0 ? May be not a problem for Ubuntu, but the change affects our (vendor) packages too.

I opened https://jira.mariadb.org/browse/MDEV-39461

But yes, we'll need the scripts too.

* How does Ubuntu build MariaDB packages? You likely don't use our `autobake.sh` script, do you? In that case

Like Debian - straight from debian/rules

  * we can set `INSTALL_MYSQLDATADIR=/var/lib/mysql` from our `autobake.sh` and keep the old path in vendor builds and the new path in Ubuntu builds
  * or Ubuntu can set `INSTALL_MYSQLDATADIR=/var/lib/mariadb` and get a new path without affecting vendor builds. I think the first approach might be easier.
  * or we can change all, Ubuntu and vendor, builds, and then pre/post-inst scriptlets need to handle the upgrade and move the data to the new location

I do like the last plan.

Question is, do we stop on Deb changes or include RPM too?

@ottok, what do you think?

Its already in Debian sid.

https://salsa.debian.org/mariadb-team/mariadb-server/-/merge_requests/155

I was hoping to see more of the script changes from that here.

Having a modified subset in 11.8 to handle Ubuntu upgrade to our vendor ones would be good too to solve MDEV-39461. Probably like a earlier version in salsa merge request 155 that dropped the config file in the post install without changing the default.