Adding a Postgres plugin restarts Postgres

[jamessewell]

Issue description

Using the new Postgresql design when extensions are added to Postgresql using extraPlugins a new postgresql-and-plugins derivation is created.

While this solves a lot of other issues, one it creates is that adding extensions causes the Postgresql service to be restarted (which isn't strictly needed).

Steps to reproduce

Start a postgresql service, add

extraPlugins = [ pg_hll ] 

to the config and switch to it - a new postgresql-and-plugins derivation will be created and activated, which will cause a service stop / start.

Technical details

• system: "x86_64-linux"
• host os: Linux 4.19.37, NixOS, 19.09pre179307.bc94dcf5002 (Loris)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.2.2
• channels(jirotech): ""
• channels(root): "nixos-19.09pre179307.bc94dcf5002"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos

[jbboehr]

You can probably disable this behavior for your own use case by setting systemd.services.postgresql.restartIfChanged = false. I believe this will apply to any change that would normally restart the service, however.

Example current usage:

nixpkgs/nixos/modules/services/security/vault.nix

Line 133 in 3464b50

restartIfChanged = false; # do not restart on "nixos-rebuild switch". It would seal the storage and disrupt the clients.

[danbst]

@jamessewell unfortunately, this would be left as by-design for now.

There are a few problems, some PG related, some Nix related.

• not every plugin can be added without restart (shared_preload_libraries). We want to distinguish "requires restart" event during normal activation, but NixOS doesn't handle this very good now. For example, I don't want PG restarts, but if I have to, I want to be notified. NixOS doesn't notify about this currently
• the design of Nix and NixOS is to be purely functional/atomic, but reloadable PG can only be implemented with some kind of state. Currently, the only place in NixOS which manages state correctly, is environment.etc, or /etc/static directory. So the implementation should put plugins there and then wrap PG with NIX_PGLIBDIR=/etc/static/pg-plugins/.... :/

So stick to restarts for now. I've created meta-issue #65296 to track this