systemd.nspawn: add definition (#18320)

this adds the option to manage systemd.nspawn files via
config.systemd.nspawn. The files are placed in "/etc/systemd/nspawn".

nixos/modules/module-list.nix

@@ -559,6 +559,7 @@
   ./system/boot/stage-1.nix
   ./system/boot/stage-2.nix
   ./system/boot/systemd.nix
+  ./system/boot/systemd-nspawn.nix
   ./system/boot/timesyncd.nix
   ./system/boot/tmp.nix
   ./system/etc/etc.nix

nixos/modules/system/boot/systemd-nspawn.nix

@@ -0,0 +1,121 @@
+{ config, lib , pkgs, ...}:
+
+with lib;
+with import ./systemd-unit-options.nix { inherit config lib; };
+with import ./systemd-lib.nix { inherit config lib pkgs; };
+
+let
+  cfg = config.systemd.nspawn;
+  assertions = [
+    # boot = true -> processtwo != true
+  ];
+
+  checkExec = checkUnitConfig "Exec" [
+    (assertOnlyFields [
+      "Boot" "ProcessTwo" "Parameters" "Environment" "User" "WorkingDirectory"
+      "Capability" "DropCapability" "KillSignal" "Personality" "MachineId"
+      "PrivateUsers"
+    ])
+    (assertValueOneOf "Boot" boolValues)
+    (assertValueOneOf "ProcessTwo" boolValues)
+    (assertValueOneOf "PrivateUsers" (boolValues ++ [ "pick" ]))
+  ];
+
+  checkFiles = checkUnitConfig "Files" [
+    (assertOnlyFields [
+      "ReadOnly" "Volatile" "Bind" "BindReadOnly" "TemporaryFileSystems"
+      "PrivateUsersChown"
+    ])
+    (assertValueOneOf "ReadOnly" boolValues)
+    (assertValueOneOf "Volatile" (boolValues ++ [ "state" ]))
+    (assertValueOneOf "PrivateUsersChown" boolValues)
+  ];
+
+  checkNetwork = checkUnitConfig "Network" [
+    (assertOnlyFields [
+      "Private" "VirtualEthernet" "VirtualEthernetExtra" "Interface" "MACVLAN"
+      "IPVLAN" "Bridge" "Zone" "Port"
+    ])
+    (assertValueOneOf "Private" boolValues)
+    (assertValueOneOf "VirtualEthernet" boolValues)
+  ];
+
+  instanceOptions = {
+
+    execConfig = mkOption {
+      default = {};
+      example = { Parameters = "/bin/sh"; };
+      type = types.addCheck (types.attrsOf unitOption) checkExec;
+      description = ''
+        Each attribute in this set specifies an option in the
+        <literal>[Exec]</literal> section of this unit. See
+        <citerefentry><refentrytitle>systemd.nspawn</refentrytitle>
+        <manvolnum>5</manvolnum></citerefentry> for details.
+      '';
+    };
+
+    filesConfig = mkOption {
+      default = {};
+      example = { Bind = [ "/home/alice" ]; };
+      type = types.addCheck (types.attrsOf unitOption) checkFiles;
+      description = ''
+        Each attribute in this set specifies an option in the
+        <literal>[Files]</literal> section of this unit. See
+        <citerefentry><refentrytitle>systemd.nspawn</refentrytitle>
+        <manvolnum>5</manvolnum></citerefentry> for details.
+      '';
+    };
+
+    networkConfig = mkOption {
+      default = {};
+      example = { Private = false; };
+      type = types.addCheck (types.attrsOf unitOption) checkNetwork;
+      description = ''
+        Each attribute in this set specifies an option in the
+        <literal>[Network]</literal> section of this unit. See
+        <citerefentry><refentrytitle>systemd.nspawn</refentrytitle>
+        <manvolnum>5</manvolnum></citerefentry> for details.
+      '';
+    };
+
+  };
+
+  instanceToUnit = name: def: 
+    { text = ''
+      [Exec]
+      ${attrsToSection def.execConfig}
+
+      [Files]
+      ${attrsToSection def.filesConfig}
+
+      [Network]
+      ${attrsToSection def.networkConfig}
+    '';
+    };
+
+in {
+
+  options = {
+
+    systemd.nspawn = mkOption {
+      default = {};
+      type = types.attrsOf types.optionSet;
+      options = [ instanceOptions ];
+      description = "Definition of systemd-nspawn configurations.";
+    };
+
+  };
+
+  config =
+    let
+      units = mapAttrs' (n: v: nameValuePair "${n}.nspawn" (instanceToUnit n v)) cfg.instances;
+    in mkIf (cfg != {}) {
+
+      environment.etc."systemd/nspawn".source = generateUnits "nspawn" units [] [];
+
+      systemd.services."systemd-nspawn@" = {
+        wantedBy = [ "machine.target" ];
+      };
+  };
+
+}