Add basic NixOS modules for starting ROS nodes.

modules/core.nix

@@ -0,0 +1,48 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  rosCfg = config.services.ros;
+  cfg = rosCfg.core;
+in {
+  # Interface
+
+  options.services.ros.core = {
+    enable = mkEnableOption "roscore";
+
+    port = mkOption {
+      type = types.ints.unsigned;
+      default = 11311;
+      description = ''
+        Port the ROS master will bind to.
+      '';
+    };
+  };
+
+  # Implementation
+
+  config = mkIf cfg.enable {
+    systemd.services.roscore = {
+      description = "ROS core";
+      serviceConfig = {
+        Type = "exec";
+        ExecStart = let
+          env = with rosCfg.pkgs; buildEnv {
+            name = "roscore-env";
+            paths = [ roslaunch ];
+          };
+        in "${env}/bin/roscore -p ${toString cfg.port}";
+        User = "ros";
+        Group = "ros";
+        StateDirectory = "ros";
+      };
+      wantedBy = [ "multi-user.target" ];
+      environment = {
+        ROS_HOSTNAME = rosCfg.hostname;
+        ROS_MASTER_URI = rosCfg.masterUri;
+        ROS_HOME = "/var/lib/ros";
+      };
+    };
+  };
+}

modules/default.nix

@@ -0,0 +1,6 @@
+{ ... }: {
+  imports = [
+    ./ros.nix
+    ./core.nix
+  ];
+}

modules/ros.nix

@@ -0,0 +1,175 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.services.ros;
+
+  pkgsType = mkOptionType {
+    name = "ros-packages";
+    description = "ROS package set";
+    check = p: isAttrs p && hasAttr "roslaunch" p;
+  };
+  overlayType = mkOptionType {
+    name = "ros-overlay";
+    description = "ROS package set overlay";
+    check = isFunction;
+  };
+in {
+  # Interface
+
+  options.services.ros = {
+    enable = mkEnableOption "Robot Operating System";
+
+    distro = mkOption {
+      type = types.str;
+      default = "noetic";
+      description = ''
+        ROS distro to use. Must be defined in distros/default.nix.
+      '';
+    };
+
+    pkgs = mkOption {
+      type = pkgsType;
+      description = ''
+        ROS package set for the selected distro.
+      '';
+    };
+
+    overlays = mkOption {
+      type = types.listOf overlayType;
+      default = [];
+      apply = foldr composeExtensions (_: _: {});
+      description = ''
+        Set of package overlays to apply to ROS package set for the configured
+        distro.
+      '';
+    };
+
+    hostname = mkOption {
+      type = types.str;
+      example = "localhost";
+      description = ''
+        Value of the ROS_HOSTNAME environment variable. Defaults to
+        <option>networking.hostName</option>.
+      '';
+    };
+
+    masterUri = mkOption {
+      type = types.str;
+      example = "https://localhost:11311/";
+      description = ''
+        Value of the ROS_MASTER_URI environment variable.
+      '';
+    };
+
+    nodes = mkOption {
+      type = types.attrsOf (types.submodule ({ name, config, ... }: {
+        options = {
+          package = mkOption {
+            type = types.str;
+            description = ''
+              ROS package name containing this node. This is the ROS name for
+              the package, rather than the attribute name, meaning it uses
+              underscores rather than dashes.
+            '';
+          };
+
+          node = mkOption {
+            type = types.str;
+            description = ''
+              Name of the node to launch.
+            '';
+          };
+
+          args = mkOption {
+            type = types.listOf types.str;
+            default = [];
+            description = ''
+              Arguments to pass to the node.
+            '';
+          };
+
+          paths = mkOption {
+            type = types.listOf types.package;
+            description = ''
+              Additional paths to add to the environment of this node. The
+              <option>package</option> option will be turned into an attribute
+              name and automatically added to this option if valid.
+            '';
+          };
+
+          env = mkOption {
+            type = types.package;
+            description = ''
+              Environment created with the ROS specific buildEnv function for
+              this node.
+            '';
+          };
+        };
+
+        config = {
+          # Try to convert package name to attribute and add it to the
+          # environment
+          paths = [ cfg.pkgs.rosbash ] ++ (let
+            packageAttr = replaceStrings ["_"] ["-"] config.package;
+          in optional (hasAttr packageAttr cfg.pkgs) cfg.pkgs."${packageAttr}");
+
+          env = mkDefault (cfg.pkgs.buildEnv {
+            name = "ros-node-${name}-env";
+            inherit (config) paths;
+          });
+        };
+      }));
+      default = {};
+      description = ''
+        ROS nodes to launch at boot as systemd services.
+      '';
+    };
+  };
+
+  # Implementation
+
+  config = mkIf cfg.enable {
+    # FIXME: mkAfter is used to make sure the Python overlay is applied. That
+    # means all other user configured Python overlays are ignored. This needs a
+    # fix in nixpkgs: https://github.com/NixOS/nixpkgs/issues/44426
+    nixpkgs.overlays = mkAfter (singleton (import ../overlay.nix));
+
+    services.ros = {
+      pkgs = mkDefault (pkgs.rosPackages."${cfg.distro}".extend cfg.overlays);
+
+      hostname = mkDefault config.networking.hostName;
+      masterUri = mkDefault "http://${cfg.hostname}:11311/";
+    };
+
+    environment.variables = {
+      ROS_HOSTNAME = cfg.hostname;
+      ROS_MASTER_URI = cfg.masterUri;
+    };
+
+    users = {
+      users.ros = {
+        group = "ros";
+        isSystemUser = true;
+      };
+      groups.ros = { };
+    };
+
+    systemd.services = mapAttrs' (name: config: nameValuePair name {
+      serviceConfig = {
+        Type = "exec";
+        StateDirectory = "ros";
+        User = "ros";
+        Group = "ros";
+        ExecStart = escapeShellArgs ([ "${config.env}/bin/rosrun" config.package config.node ] ++ config.args);
+      };
+      wantedBy = [ "multi-user.target" ];
+      environment = {
+        ROS_HOSTNAME = cfg.hostname;
+        ROS_MASTER_URI = cfg.masterUri;
+        ROS_HOME = "/var/lib/ros";
+      };
+    }) cfg.nodes;
+  };
+}