nixos/postgresql: add option for domain sockets

[shivak]

Adds a socketDir option corresponding to PostgreSQL's unix_socket_directories setting, and uses it in the setup scripts run post-start.

Previously, in these scripts, psql tried to connect to the PostgreSQL via its TCP port. In a typical pg_hba.conf, all TCP connections, even those of localhost, are password protected. Since these scripts run unattended with no passwords provided, they actually worked by falling back to the domain socket. But when a non-standard socket directory is used, it must be explicitly passed to psql. Thus, these scripts were previously broken for those using non-standard socket directories.

PostgreSQL actually supports multiple socket directories, as well as @-prefixed paths for sockets in the "abstract namespace". However, supporting these would upset the types and serialization within the module.

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.05 Release Notes (or backporting 22.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[shivak]

Bump: this module is still broken when custom socket directories are desired.

This becomes a problem when running two PostgreSQL instances on the same host. In NixOS, this is accomplished by running a container with its own PostgreSQL service enabled. In order to make this instance externally accessible, one bind-mounts a host directory, which must then be configured as the socket directory.

[wolfgangwalther]

I think this change makes sense. Could you rebase?