nixos/nix-daemon: Ensure continued availability of daemon socket

[roberth]

Motivation for this change

This fixes the error messages

error: cannot connect to socket at '/nix/var/nix/daemon-socket/socket': Connection refused

and

error: opening a connection to remote store 'daemon' previously failed

in nix programs that are active during a NixOS switch.

As nix-daemon.service does not make use of ExecStop, we prefer to keep the socket up and available. This is important for machines that run Nix-based services, such as automated build, test, and deploy services, that expect the daemon socket to be available at all times.

See committed inline comment for further explanation.

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/)
• 22.05 Release Notes (or backporting 21.11 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.

cc @tomberek @dasJ

[roberth]

@ofborg test switchTest installer.simple

[arianvp]

Hmm I was under the assumption that the .socket unit would keep requests being buffered as the .service restarts. Are we accidentally restarting the .socket ? I know we had some bugs about that in the activation logic but I recall those were squashed recently.

[dasJ]

@arianvp No, .socket triggers the .service unit and buffers until the .service unit is activated. We are not accidentially restarting sockets because that got reverted: https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/system/activation/switch-to-configuration.pl#L313

[dasJ]

Looks good. I'll merge this in a few days when nobody has any strong point why I shouldn't

[dasJ]

Can you confirm that the issue is also resolved by #161838?
Both PRs solve the same issue in different ways and are not conflicting with each other.
#161838 is preferrable IMO because it fixes the root cause of the problem for all services instead of just for nix-daemon.service

[github-actions]

Successfully created backport PR #163323 for release-21.11.