Add support for NixOS inside containers

examples/full-nixos/arion-compose.nix

@@ -0,0 +1,11 @@
+{
+  docker-compose.services.webserver = { pkgs, ... }: {
+    nixos.useInit = true;
+    nixos.configuration.services.nginx.enable = true;
+    nixos.configuration.services.nginx.virtualHosts.localhost.root = "${pkgs.nix.doc}/share/doc/nix/manual";
+    service.useHostStore = true;
+    service.ports = [
+      "8000:80" # host:container
+    ];
+  };
+}

examples/full-nixos/arion-pkgs.nix

@@ -0,0 +1,2 @@
+# Instead of pinning Nixpkgs, we can opt to use the one in NIX_PATH
+import <nixpkgs> {}

examples/nixos-unit/arion-compose.nix

@@ -0,0 +1,40 @@
+
+/*
+
+  DISCLAIMER
+
+  This uses a somewhat hidden feature in NixOS, which is the
+  "runner". It's a script that's available on systemd services
+  that lets you run the service independently from systemd.
+  However, it was clearly not intended for public consumption
+  so please use it with care.
+  It does not support all features of systemd so you are on
+  your own if you use it in production.
+
+  One known issue is that the script does not respond to docker's
+  SIGTERM shutdown signal.
+
+ */
+
+{
+  docker-compose.services.webserver = { config, pkgs, ... }: {
+
+    nixos.configuration = {config, pkgs, ...}: {
+      boot.isContainer = true;
+      services.nginx.enable = true;
+      services.nginx.virtualHosts.localhost.root = "${pkgs.nix.doc}/share/doc/nix/manual";
+      system.build.run-nginx = pkgs.writeScript "run-nginx" ''
+        #!${pkgs.bash}/bin/bash
+        PATH='${config.systemd.services.nginx.environment.PATH}'
+        echo nginx:x:${toString config.users.users.nginx.uid}:${toString config.users.groups.nginx.gid}:nginx web server user:/var/empty:/bin/sh >>/etc/passwd
+        echo nginx:x:${toString config.users.groups.nginx.gid}:nginx >>/etc/group
+        ${config.systemd.services.nginx.runner}
+      '';
+    };
+    service.command = [ config.nixos.build.run-nginx ];
+    service.useHostStore = true;
+    service.ports = [
+      "8000:80" # host:container
+    ];
+  };
+}

examples/nixos-unit/arion-pkgs.nix

@@ -0,0 +1,2 @@
+# Instead of pinning Nixpkgs, we can opt to use the one in NIX_PATH
+import <nixpkgs> {}

src/nix/eval-service.nix

@@ -12,6 +12,8 @@ let
     ./modules/service/docker-compose-service.nix
     ./modules/service/host-store.nix
     ./modules/service/host.nix
+    ./modules/service/nixos.nix
+    ./modules/service/nixos-init.nix
   ];
 
   argsModule = {

src/nix/modules/nixos/container-systemd.nix

@@ -0,0 +1,24 @@
+/*
+  NixOS configuration to for running a mostly normal systemd-based
+  NixOS in Docker.
+ */
+{ pkgs, lib, ...}: {
+
+  # imports = [
+  #   # This profile doesn't seem to work well.
+  #   (pkgs.path + "/nixos/modules/profiles/docker-container.nix")
+  #   This one works, but can not be imported here due because imports can not depend on pkgs.
+  #   (pkgs.path + "/nixos/modules/profiles/minimal.nix")
+  # ];
+  boot.isContainer = true;
+  boot.specialFileSystems = lib.mkForce {};
+  networking.hostName = "";
+
+  services.journald.console = "/dev/console";
+
+  systemd.services.systemd-logind.enable = false;
+  systemd.services.console-getty.enable = false;
+
+  systemd.sockets.nix-daemon.enable = false;
+  systemd.services.nix-daemon.enable = false;
+}

src/nix/modules/service/docker-compose-service.nix

@@ -36,7 +36,12 @@ in
     service.volumes = mkOption {
       type = listOf types.unspecified;
       default = [];
-      description = "";
+      description = dockerComposeRef "volumes";
+    };
+    service.tmpfs = mkOption {
+      type = listOf types.str;
+      default = [];
+      description = dockerComposeRef "tmpfs";
     };
     service.build.context = mkOption {
       type = nullOr str;
@@ -52,6 +57,11 @@ in
       default = null;
       description = dockerComposeKitchenSink;
     };
+    service.tty = mkOption {
+      type = nullOr bool;
+      default = null;
+      description = dockerComposeKitchenSink;
+    };
     service.environment = mkOption {
       type = attrsOf (either str int);
       default = {};
@@ -130,6 +140,11 @@ in
       default = null;
       description = dockerComposeRef "network_mode";
     };
+    service.stop_signal = mkOption {
+      type = nullOr str;
+      default = null;
+      description = dockerComposeRef "stop_signal";
+    };
   };
 
   config.build.service = {
@@ -166,6 +181,12 @@ in
     inherit (config.service) network_mode;
   } // lib.optionalAttrs (config.service.restart != null) {
     inherit (config.service) restart;
+  } // lib.optionalAttrs (config.service.stop_signal != null) {
+    inherit (config.service) stop_signal;
+  } // lib.optionalAttrs (config.service.tmpfs != []) {
+    inherit (config.service) tmpfs;
+  } // lib.optionalAttrs (config.service.tty != null) {
+    inherit (config.service) tty;
   } // lib.optionalAttrs (config.service.working_dir != null) {
     inherit (config.service) working_dir;
   };

src/nix/modules/service/nixos-init.nix

@@ -0,0 +1,39 @@
+/*
+  Invokes the NixOS init system in the container.
+ */
+{ config, lib, pkgs, ... }:
+let
+  inherit (lib) types;
+in
+{
+  options = {
+    nixos.useInit = lib.mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        When enabled, call the NixOS init system.
+
+        Configure NixOS with <code>nixos.configuration</code>.
+      '';
+    };
+  };
+
+  config = lib.mkIf (config.nixos.useInit) {
+    nixos.configuration.imports = [
+      ../nixos/container-systemd.nix
+      (pkgs.path + "/nixos/modules/profiles/minimal.nix")
+    ];
+    service.command = [ "${config.nixos.build.toplevel}/init" ];
+    service.environment.container = "docker";
+    service.volumes = [
+      "/sys/fs/cgroup:/sys/fs/cgroup:ro"
+    ];
+    service.tmpfs = [
+      "/tmp"
+      "/run"
+      "/run/wrappers"
+    ];
+    service.stop_signal = "SIGRTMIN+3";
+    service.tty = true;
+  };
+}

src/nix/modules/service/nixos.nix

@@ -0,0 +1,60 @@
+/*
+  Generic options for adding NixOS modules and evaluating the NixOS
+  configuration. The methods for installing NixOS in the image are
+  defined elsewhere.
+ */
+{ pkgs, lib, config, ... }:
+let
+  inherit (lib) types;
+  inherit (types) attrs;
+in
+{
+  options = {
+    nixos.configuration = lib.mkOption {
+      type = with types; coercedTo unspecified (a: [a]) (listOf unspecified);
+      default = {};
+      description = ''
+        Modules to add to the NixOS configuration.
+
+        This option is unused by default, because not all images use NixOS.
+
+        One way to use this is to enable <code>nixos.useInit</code>, but the
+        NixOS configuration can be used in other ways.
+      '';
+    };
+
+    nixos.build = lib.mkOption {
+      type = attrs;
+      readOnly = true;
+      description = ''
+        NixOS build products from <code>config.system.build</code>, such as <code>toplevel</code> and <code>etc</code>.
+
+        This option is unused by default, because not all images use NixOS.
+
+        One way to use this is to enable <code>nixos.useInit</code>, but the
+        NixOS configuration can be used in other ways.
+      '';
+    };
+
+    nixos.evaluatedConfig = lib.mkOption {
+      type = attrs;
+      readOnly = true;
+      description = ''
+        Evaluated NixOS configuration, to be read by service-level modules.
+
+        This option is unused by default, because not all images use NixOS.
+
+        One way to use this is to enable <code>nixos.useInit</code>, but the
+        NixOS configuration can be used in other ways.
+      '';
+    };
+  };
+
+  config = {
+    nixos.build = builtins.addErrorContext "while evaluating the service nixos configuration" (
+      pkgs.nixos config.nixos.configuration
+    );
+    nixos.configuration = { config, ... }: { system.build.theConfig = config; };
+    nixos.evaluatedConfig = config.nixos.build.theConfig;
+  };
+}

tests/arion-test/default.nix

@@ -2,8 +2,8 @@
 
 let
   # To make some prebuilt derivations available in the vm
-  preEval = import ../../src/nix/eval-composition.nix {
-    modules = [ ../../examples/minimal/arion-compose.nix ];
+  preEval = modules: import ../../src/nix/eval-composition.nix {
+    inherit modules;
     inherit pkgs;
   };
 in
@@ -31,14 +31,35 @@ in
     virtualisation.pathsInNixDB = [
       # Pre-build the image because we don't want to build the world
       # in the vm.
-      preEval.config.build.dockerComposeYaml
+      (preEval [ ../../examples/minimal/arion-compose.nix ]).config.build.dockerComposeYaml
+      (preEval [ ../../examples/full-nixos/arion-compose.nix ]).config.build.dockerComposeYaml
+      (preEval [ ../../examples/nixos-unit/arion-compose.nix ]).config.build.dockerComposeYaml
       pkgs.stdenv
     ];
   };
   testScript = ''
     $machine->fail("curl localhost:8000");
     $machine->succeed("docker --version");
-    $machine->succeed("cp -r ${../../examples/minimal} work && cd work && NIX_PATH=nixpkgs='${pkgs.path}' arion up -d");
-    $machine->waitUntilSucceeds("curl localhost:8000");
+
+    subtest "minimal", sub {
+      $machine->succeed("cp -r ${../../examples/minimal} work && cd work && NIX_PATH=nixpkgs='${pkgs.path}' arion up -d");
+      $machine->waitUntilSucceeds("curl localhost:8000");
+      $machine->succeed("cd work && NIX_PATH=nixpkgs='${pkgs.path}' arion down && rm -rf work");
+      $machine->waitUntilFails("curl localhost:8000");
+    };
+
+    subtest "full-nixos", sub {
+      $machine->succeed("cp -r ${../../examples/full-nixos} work && cd work && NIX_PATH=nixpkgs='${pkgs.path}' arion up -d");
+      $machine->waitUntilSucceeds("curl localhost:8000");
+      $machine->succeed("cd work && NIX_PATH=nixpkgs='${pkgs.path}' arion down && rm -rf work");
+      $machine->waitUntilFails("curl localhost:8000");
+    };
+
+    subtest "nixos-unit", sub {
+      $machine->succeed("cp -r ${../../examples/nixos-unit} work && cd work && NIX_PATH=nixpkgs='${pkgs.path}' arion up -d");
+      $machine->waitUntilSucceeds("curl localhost:8000");
+      $machine->succeed("cd work && NIX_PATH=nixpkgs='${pkgs.path}' arion down && rm -rf work");
+      $machine->waitUntilFails("curl localhost:8000");
+    };
   '';
 }