Don't have root bypass the daemon by default

[thufschmitt]

Is your feature request related to a problem? Please describe.

When the store option is set to auto (the default), Nix will guess whether to use the daemon or the local store.
However, this detection logic will use the local store if Nix has access to it, even if there's a daemon socket available. This reduces the isolation that the clent-daemon logic provides. Some examples from a quick issue search:

• nix lock can modify lock files in the nix store and corrupt it #10139
• Running Nix as root ignores nix-damon's TMPDIR #7154
• nix-shell as root cd's to / #6083
• /nix/store is writable in a nix-shell started by root #4337

Describe the solution you'd like

Have the logic be: “If there's a socket I can connect to, use the daemon that listens there, otherwise try to directly access the store”.

Describe alternatives you've considered

• Keep the statu-quo
• Have the multi-user installer configure nix.conf to set store = daemon (and pass --store local to the daemon invocation). That would reduce the breaking change, at the cost of more complexity on the installations in the long run.

Additional context

• IIRC @edolstra mentioned somewhere (couldn't find the source again), that directly connecting to the store as root was an optimisation for some systems (like hydra) where the overhead of the daemon was noticeable. I think that can be avoided by just configuring these systems to use forcibly use the local store

Priorities

Add 👍 to issues you find important.

[thufschmitt]

Extra issue due to that: #10158

[roberth]

Issues like #10158 will still happen in a use case like nixos-enter.
I don't think we can get rid of the non-daemon use case completely, and by switching to daemon by default, we risk letting it bitrot.
The status quo could be considered anti-fragile.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2024-03-11-nix-team-meeting-132/42960/1

[roberth]

Bypassing the daemon is also useful when trying out a build feature, such as described here NixOS/nixpkgs#318013 (comment).