nixos: Add nixos.channel.enable

For those who wish to get rid of nix-channel.

nixos/modules/config/nix-channel.nix

···
 {
   options = {
     nix = {
+      channel = {
+        enable = mkOption {
+          description = lib.mdDoc ''
+            Whether the `nix-channel` command and state files are made available on the machine.
+
+            The following files are initialized when enabled:
+             - `/nix/var/nix/profiles/per-user/root/channels`
+             - `/root/.nix-channels`
+             - `$HOME/.nix-defexpr/channels` (on login)
+
+            Disabling this option will not remove the state files from the system.
+          '';
+          type = types.bool;
+          default = true;
+        };
+      };
+
       nixPath = mkOption {
         type = types.listOf types.str;
-        default = [
-          "nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos"
-          "nixos-config=/etc/nixos/configuration.nix"
-          "/nix/var/nix/profiles/per-user/root/channels"
-        ];
+        default =
+          if cfg.channel.enable
+          then [
+            "nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos"
+            "nixos-config=/etc/nixos/configuration.nix"
+            "/nix/var/nix/profiles/per-user/root/channels"
+          ]
+          else [];
+        defaultText = ''
+          if nix.channel.enable
+          then [
+            "nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos"
+            "nixos-config=/etc/nixos/configuration.nix"
+            "/nix/var/nix/profiles/per-user/root/channels"
+          ]
+          else [];
+        '';
         description = lib.mdDoc ''
           The default Nix expression search path, used by the Nix
           evaluator to look up paths enclosed in angle brackets
···
   config = mkIf cfg.enable {
 
     environment.extraInit =
-      ''
+      mkIf cfg.channel.enable ''
         if [ -e "$HOME/.nix-defexpr/channels" ]; then
           export NIX_PATH="$HOME/.nix-defexpr/channels''${NIX_PATH:+:$NIX_PATH}"
         fi
       '';
 
-    environment.sessionVariables = {
+    environment.extraSetup = mkIf (! cfg.channel.enable) ''
+      rm $out/bin/nix-channel
+    '';
+
+    environment.sessionVariables = mkIf (cfg.nixPath != []) {
       NIX_PATH = cfg.nixPath;
     };
 
-    system.activationScripts.nix-channel = stringAfter [ "etc" "users" ]
-      ''
+    system.activationScripts.nix-channel = mkIf cfg.channel.enable
+      (stringAfter [ "etc" "users" ] ''
         # Subscribe the root user to the NixOS channel by default.
         if [ ! -e "/root/.nix-channels" ]; then
             echo "${config.system.defaultChannel} nixos" > "/root/.nix-channels"
         fi
-      '';
+      '');
   };
 }

nixos/tests/installer.nix

···
 
   # The configuration to install.
   makeConfig = { bootLoader, grubDevice, grubIdentifier, grubUseEfi
-               , extraConfig, forceGrubReinstallCount ? 0
+               , extraConfig, forceGrubReinstallCount ? 0, flake ? false
                }:
     pkgs.writeText "configuration.nix" ''
       { config, lib, pkgs, modulesPath, ... }:
 
       { imports =
           [ ./hardware-configuration.nix
-            <nixpkgs/nixos/modules/testing/test-instrumentation.nix>
+            ${if flake
+              then "" # Still included, but via installer/flake.nix
+              else "<nixpkgs/nixos/modules/testing/test-instrumentation.nix>"}
           ];
 
         networking.hostName = "thatworked";
···
   # partitions and filesystems.
   testScriptFun = { bootLoader, createPartitions, grubDevice, grubUseEfi
                   , grubIdentifier, preBootCommands, postBootCommands, extraConfig
-                  , testSpecialisationConfig
+                  , testSpecialisationConfig, testFlakeSwitch
                   }:
     let iface = "virtio";
         isEfi = bootLoader == "systemd-boot" || (bootLoader == "grub" && grubUseEfi);
···
 
       ${postBootCommands}
       machine.shutdown()
+    ''
+    + optionalString testFlakeSwitch ''
+      ${preBootCommands}
+      machine.start()
+
+      with subtest("Configure system with flake"):
+        # TODO: evaluate as user?
+        machine.succeed("""
+          mkdir /root/my-config
+          mv /etc/nixos/hardware-configuration.nix /root/my-config/
+          mv /etc/nixos/secret /root/my-config/
+          rm /etc/nixos/configuration.nix
+        """)
+        machine.copy_from_host_via_shell(
+          "${ makeConfig {
+                inherit bootLoader grubDevice grubIdentifier
+                        grubUseEfi extraConfig;
+                forceGrubReinstallCount = 1;
+                flake = true;
+              }
+          }",
+          "/root/my-config/configuration.nix",
+        )
+        machine.copy_from_host_via_shell(
+          "${./installer/flake.nix}",
+          "/root/my-config/flake.nix",
+        )
+        machine.succeed("""
+          # for some reason the image does not have `pkgs.path`, so
+          # we use readlink to find a Nixpkgs source.
+          pkgs=$(readlink -f /nix/var/nix/profiles/per-user/root/channels)/nixos
+          if ! [[ -e $pkgs/pkgs/top-level/default.nix ]]; then
+            echo 1>&2 "$pkgs does not seem to be a nixpkgs source. Please fix the test so that pkgs points to a nixpkgs source.";
+            exit 1;
+          fi
+          sed -e s^@nixpkgs@^$pkgs^ -i /root/my-config/flake.nix
+        """)
+
+      with subtest("Switch to flake based config"):
+        machine.succeed("nixos-rebuild switch --flake /root/my-config#xyz")
+
+      ${postBootCommands}
+      machine.shutdown()
+
+      ${preBootCommands}
+      machine.start()
+
+      machine.wait_for_unit("multi-user.target")
+
+      with subtest("nix-channel command is not available anymore"):
+        machine.succeed("! which nix-channel")
+
+      with subtest("Evaluate flake config in fresh env without nix-channel"):
+        machine.succeed("nixos-rebuild switch --flake /root/my-config#xyz")
+
+      with subtest("Evaluate flake config in fresh env without channel profiles"):
+        machine.succeed("""
+          (
+            exec 1>&2
+            rm -v /root/.nix-channels
+            rm -vrf ~/.nix-defexpr
+            rm -vrf /nix/var/nix/profiles/per-user/root/channels*
+          )
+        """)
+        machine.succeed("nixos-rebuild switch --flake /root/my-config#xyz")
+
+      ${postBootCommands}
+      machine.shutdown()
+
     '';
 
 
···
     , grubDevice ? "/dev/vda", grubIdentifier ? "uuid", grubUseEfi ? false
     , enableOCR ? false, meta ? {}
     , testSpecialisationConfig ? false
+    , testFlakeSwitch ? false
     }:
     makeTest {
       inherit enableOCR;
···
       testScript = testScriptFun {
         inherit bootLoader createPartitions preBootCommands postBootCommands
                 grubDevice grubIdentifier grubUseEfi extraConfig
-                testSpecialisationConfig;
+                testSpecialisationConfig testFlakeSwitch;
       };
     };
 
···
     '';
   };
 
+  simple-test-config-flake = simple-test-config // {
+    testFlakeSwitch = true;
+  };
+
   simple-uefi-grub-config = {
     createPartitions = ''
       machine.succeed(
···
   # The (almost) simplest partitioning scheme: a swap partition and
   # one big filesystem partition.
   simple = makeInstallerTest "simple" simple-test-config;
+
+  switchToFlake = makeInstallerTest "switch-to-flake" simple-test-config-flake;
 
   # Test cloned configurations with the simple grub configuration
   simpleSpecialised = makeInstallerTest "simpleSpecialised" (simple-test-config // specialisation-test-extraconfig);

nixos/tests/installer/flake.nix

···
+# This file gets copied into the installation
+
+{
+  # To keep things simple, we'll use an absolute path dependency here.
+  inputs.nixpkgs.url = "@nixpkgs@";
+
+  outputs = { nixpkgs, ... }: {
+
+    nixosConfigurations.xyz = nixpkgs.lib.nixosSystem {
+      modules = [
+        ./configuration.nix
+        ( nixpkgs + "/nixos/modules/testing/test-instrumentation.nix" )
+        {
+          # We don't need nix-channel anymore
+          nix.channel.enable = false;
+        }
+      ];
+    };
+  };
+}