nixos/resolvconf: Enable by default

Fixes #353225

The value of `networking.resolvconf.enable` was previously
predicated on the existence `environment.etc."resolv.conf"`.
This could lead to infinite recursion when an attribute defined
within the resolvconf module was also required to define
`environment.etc` (see example below).

Instead, set the default to an explicit true and set the value to
false where required in-tree. This turns out to only be two places:
systemd-resolved and tetrd.

Example infrec (from building .#nixosTests.containers-hosts):

```
       … while evaluating definitions from `/nix/store/5djjm1dipmdh82issrmpfjdplp7hk0pp-source/nixos/modules/services/logging/logrotate.nix':

       … while evaluating the option `containers.netmask.services.logrotate.enable':

       … while evaluating the option `containers.netmask.users.groups':

       … while evaluating definitions from `/nix/store/5djjm1dipmdh82issrmpfjdplp7hk0pp-source/nixos/modules/config/resolvconf.nix':

       … while evaluating the option `containers.netmask.networking.resolvconf.enable':

       … while evaluating the option `containers.netmask.environment.etc':

       … while evaluating definitions from `/nix/store/5djjm1dipmdh82issrmpfjdplp7hk0pp-source/nixos/modules/services/logging/logrotate.nix':

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: infinite recursion encountered
       at /nix/store/5djjm1dipmdh82issrmpfjdplp7hk0pp-source/lib/modules.nix:1512:13:
         1511|     _type = "if";
         1512|     inherit condition content;
             |             ^
         1513|   };
```

nixos/doc/manual/release-notes/rl-2605.section.md

@@ -356,6 +356,8 @@ See <https://github.com/NixOS/nixpkgs/issues/481673>.
 
 - `systemd.network.*` has been updated to support all configuration options from upstream `networkd` version 259.
 
+- `networking.resolvconf.enable` now defaults to `true` unconditionally instead of `!(config.environment.etc ? "resolv.conf")`.If you set `environment.etc."resolv.conf"` yourself, then you should also set `networking.resolvconf.enable = false`.
+
 - `services.openssh` now supports generating host SSH keys by setting `services.openssh.generateHostKeys = true` while leaving `services.openssh.enable` disabled.  This is particularly useful for systems that have no need of an SSH daemon but want SSH host keys for other purposes such as using agenix or sops-nix.
 
 - `services.openssh.enableRecommendedAlgorithms` has been added to allow users to opt out of NixOS's curated set of recommended algorithms. This set to true by default, and thus is not a breaking change. Users may want to set this to false if they prefer upstream's default algorithms. See <https://github.com/NixOS/nixpkgs/pull/471330>.

nixos/modules/config/resolvconf.nix

@@ -69,8 +69,7 @@ in
 
       enable = lib.mkOption {
         type = lib.types.bool;
-        default = !(config.environment.etc ? "resolv.conf");
-        defaultText = lib.literalExpression ''!(config.environment.etc ? "resolv.conf")'';
+        default = true;
         description = ''
           Whether DNS configuration is managed by resolvconf.
         '';
@@ -170,6 +169,17 @@ in
     }
 
     (lib.mkIf cfg.enable {
+      assertions = [
+        {
+          assertion = !(config.environment.etc ? "resolv.conf");
+          message = ''
+            networking.resolvconf.enable is true but environment.etc."resolv.conf"
+            is also set. Set networking.resolvconf.enable = false if another
+            service manages /etc/resolv.conf.
+          '';
+        }
+      ];
+
       users.groups.resolvconf = { };
 
       networking.resolvconf.subscriberFiles = [ "/etc/resolv.conf" ];

nixos/modules/services/networking/tetrd.nix

@@ -14,6 +14,9 @@
       etc."resolv.conf".source = "/etc/tetrd/resolv.conf";
     };
 
+    # Our resolv.conf will override resolvconf's version.
+    networking.resolvconf.enable = false;
+
     systemd = {
       tmpfiles.rules = [ "f /etc/tetrd/resolv.conf - - -" ];
 

nixos/modules/system/boot/resolved.nix

@@ -219,6 +219,10 @@ in
       # If networkmanager is enabled, ask it to interface with resolved.
       networking.networkmanager.dns = "systemd-resolved";
 
+      # Since we explicitly provide a resolv.conf, disable resolvconf
+      networking.resolvconf.enable = false;
+
+      # ... but we still set the package for correct compatibility.
       networking.resolvconf.package = config.systemd.package;
 
       nix.firewall.extraNftablesRules = [