nixos/pgbackup: Fix and refactor the postgres backup module

[markuskowa]

Motivation for this change

Fix the postgresqlBackup module. At least in NixOS 18.03 the backup is broken, delivering empty backup files quietly (#41388).

Things done

• Fix PostgressqlBackup delivers empty backup files #41388
• Switch from cron job to systemd
• rotate backup instead of overwriting old one
• Added pgdumpOptions to module

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

---

[Mic92]

@GrahamcOfBorg test postgresql

[GrahamcOfBorg]

Success on aarch64-linux

Attempted: tests.postgresql

No partial log is available.

[GrahamcOfBorg]

Success on x86_64-linux

Attempted: tests.postgresql

No partial log is available.

[Mic92]

Maybe we should rename this option? The type of both is string old values could break silently. Renaming the option would make this more obvious since the format is different.

[Mic92]

startAt could be alternative.

[markuskowa]

Yes, that seems more natural in the new version

[Mic92]

Adding:

(mkRemovedOptionModule [ "services" "postgresqlBackup" "period" ] "A systemd timer is now used instead of cron. The starting time can be configured via services.postgresqlBackup.startAt")

modules/rename.nix would make sense.

[markuskowa]

OK. Do we need to also mention the change in the release notes?

[Mic92]

Probably not, the error message generated from mkRemovedOptionModule should be self-explanatory.