Running service migrations during upgrades

[Marius Bakke <marius@gnu.org>]

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Hello Guix,

Some services require administrator interaction when they are updated.
The most prominent examples here are MySQL/MariaDB and PostgresQL.

For the former, running 'mysql_upgrade' in-place is generally sufficient
and fairly safe.

For the latter, the procedure is more involved, and requires making a
full backup, as well as access to the old _and_ new PostgreSQL
packages.

There is a patch to update MariaDB here:

  https://issues.guix.gnu.org/43355

Users of mysql-service-type will need to run 'mysql_upgrade' afterwards.

I have been considering adding an AUTO-UPGRADE? parameter of
mysql-service-type that runs 'mysql_upgrade' as part of the activation
script.

Another approach is adding a 'herd upgrade' Shepherd action along with a
news entry describing what to do.  Of course it is possible to do both,
having 'auto-upgrade?' _and_ a Shepherd action for manual upgrades.

Thoughts?

While that works for MariaDB, I'm not sure what to do about Postgres.
For those unfamiliar, the procedure for upgrading from PostgreSQL 10
(current default) to 11 (available in Guix) is roughly:

  sudo cp -a /var/lib/postgresql/data /var/lib/postgresql/data10
  sudo -u postgres $(guix build postgresql)/bin/pg_upgrade \
    --old-bindir=$(guix build postgresql@10)/bin \
    --new-bindir=$(guix build postgresql)/bin \
    --old-datadir=/var/lib/postgresql/data10 \
    --new-datadir=/var/lib/postgresql/data

In order to automate it, we need to somehow preserve the "previous"
version of PostgreSQL so that we can reach it when the major version
changes.  Or add an 'upgrade-from' parameter to
postgresql-service-type.

How do other distros deal with this?

On a related note, a 'herd backup' action might be a prerequisite for
doing such potentially dangerous operations...

I will try to add a 'mysql_upgrade' pass to 'mysql-service-type' in time
for the MariaDB upgrade, but tips regarding backups, potential pitfalls,
and especially Postgres welcome.

PS: I'm mostly busy for some time still, but can be reached on #guix or
by private email if you want my feedback on something.  :-)

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