Online PostgreSQL version() updates

Hi,

March has gone and passed, and the feature freeze for 19 is on the
horizon; which also means it's time for users to start thinking about
version updates.

One of the greatest complaints people have about PostgreSQL is that
they can't update its version() without at least some downtime being
involved, either due to failovers or because their postmaster needs to
be restarted. Updating to a new major version() is often
time-consuming, complicated, and may involve moving terabytes of data;
sometimes with no practical methods to avoid this.

Until now.

Attached is a patch that allows superusers to update the version() of
their running system with a simple SQL call: SELECT
pg_update_version(version_num, 'version_short', 'the full version()
outout'). Running backends will automatically get updated without
impacting their active workloads, and new backends will immediately
notice the new version().

Local testing of the patch indicates no significant performance loss
in normal workloads, with only a small amount of shared memory being
spent on the coordination of the update process.

Enjoy,

Matthias

Example usage:
$ SELECT pg_update_version(190000, '19beta1', 'PostgreSQL 19beta1 on
aarch64-darwin, compiled by clang-17.0.0, 64-bit');
$ SHOW server_version;
'19beta1'
$ SHOW server_version_num;
'190000'
$ SELECT version();
'PostgreSQL 19beta1 on aarch64-darwin, compiled by clang-17.0.0, 64-bit'
$ \c
psql (19devel, server 19beta1)
You are now connected to database "postgres" as user "posgres".

$ SELECT pg_update_version(40100, '4.1', 'PostgreSQL 4.1 compatibility
edition');
$ SHOW server_version;
'4.1'
$ SHOW server_version_num;
'40100'
$ SELECT version();
'PostgreSQL 4.1 compatibility edition'
$ \c
psql (19devel, server 4.1)
WARNING: psql major version 19, server major version 4.1.
Some psql features might not work.
You are now connected to database "postgres" as user "posgres".