This is rework of part of https://github.com/NixOS/nixpkgs/pull/46670.
My usecase was to be able to inspect PG datadir as wheel user.
PG11 now allows starting server with 0750 mask for data dir.
`groupAccess = true` now does this automatically. The only thing you have to do
is to set group ownership.
For PG10 and below, I've described a hack how this can be done. Before this PR
hack was impossible. The hack isn't ideal, because there is short
period of time when dir mode is 0700, so I didn't want to make it official.
Test/example is present too.
* postgresql: reorganize package and it's extensions
Extracts some useful parts of https://github.com/NixOS/nixpkgs/pull/38698,
in particular, it's vision that postgresql plugins should be namespaced.
For large setups it is useful to list all databases explicit
(for example if temporary databases are also present) and store them in extra
files.
For smaller setups it is more convenient to just backup all databases at once,
because it is easy to forget to update configuration when adding/renaming
databases. pg_dumpall also has the advantage that it backups users/passwords.
As a result the module becomes easier to use because it is sufficient
in the default case to just set one option (services.postgresqlBackup.enable).
1. Needs to call makeTest or else nothing happens when you run
`nix-build nixos/tests/postgresql.nix`.
2. Tests run as root, so there needs to be a corresponding user in
PostgreSQL.
