interception-tools: init at 0.1.1

The latest release of libyamlcpp in nixpkgs does not build because it
uses an older version of boost than the one in nixpkgs and therefore
expects a particular header file which does not exist in the latest
boost anymore. For this reason, a later (git) version of libyamlcpp is
used here (which actually doesn't even require boost).

The substituteInPlace in the prePatch phase is needed because libevdev
places its headers in non-standard places, meaning Nix cannot normally
find them. The `cut` command removes the first two "-I" characters from
the output of `pkg-config`. This needs to be in the prePatch phase
because otherwise Nix will patch these lines to `/var/empty`, meaning
you would have less specific replacement (in case other lines are also
patched to `/var/empty`).

I wrote the patch. (I believe it is NixOS specific.)

nixos/modules/services/hardware/interception-tools.nix

@@ -0,0 +1,61 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.services.interception-tools;
+in {
+  options.services.interception-tools = {
+    enable = mkOption {
+      type = types.bool;
+      default = false;
+      description = "Whether to enable the interception tools service.";
+    };
+
+    plugins = mkOption {
+      type = types.listOf types.package;
+      default = [ pkgs.interception-tools-plugins.caps2esc ];
+      description = ''
+        A list of interception tools plugins that will be made available to use
+        inside the udevmon configuration.
+      '';
+    };
+
+    udevmonConfig = mkOption {
+      type = types.either types.str types.path;
+      default = ''
+        - JOB: "intercept -g $DEVNODE | caps2esc | uinput -d $DEVNODE"
+          DEVICE:
+            EVENTS:
+              EV_KEY: [KEY_CAPSLOCK, KEY_ESC]
+      '';
+      example = ''
+        - JOB: "intercept -g $DEVNODE | y2z | x2y | uinput -d $DEVNODE"
+          DEVICE:
+            EVENTS:
+              EV_KEY: [KEY_X, KEY_Y]
+      '';
+      description = ''
+        String of udevmon YAML configuration, or path to a udevmon YAML
+        configuration file.
+      '';
+    };
+  };
+
+  config = mkIf cfg.enable {
+    systemd.services.interception-tools = {
+      description = "Interception tools";
+      path = [ pkgs.bash pkgs.interception-tools ] ++ cfg.plugins;
+      serviceConfig = {
+        ExecStart = ''
+          ${pkgs.interception-tools}/bin/udevmon -c \
+          ${if builtins.typeOf cfg.udevmonConfig == "path"
+          then cfg.udevmonConfig
+          else pkgs.writeText "udevmon.yaml" cfg.udevmonConfig}
+        '';
+        Nice = -20;
+      };
+      wantedBy = [ "multi-user.target" ];
+    };
+  };
+}