NixOS module: dispatch PE execution to either WINE or Mono based on whether it is a CLR executable

binfmt_dispatch.go

package main

import (
	"debug/pe"
	"errors"
	"fmt"
	"os"
	"syscall"
)

func main() {
	args := os.Args[1:]

	f, err := pe.Open(args[0])
	if err == nil {
		err = f.Close()
	}
	if err != nil {
		fmt.Fprintln(os.Stderr, err)
		os.Exit(-1)
	}

	var dataDirectories *[16]pe.DataDirectory
	switch hdr := f.OptionalHeader.(type) {
	default:
		panic(fmt.Errorf("unexpected OptionalHeader type: %T", hdr))
	case nil:
	case *pe.OptionalHeader32:
		dataDirectories = &hdr.DataDirectory
	case *pe.OptionalHeader64:
		dataDirectories = &hdr.DataDirectory
	}

	isCLR := dataDirectories != nil && dataDirectories[14].Size != 0
	execute := "@executorWIN@"
	if isCLR {
		execute = "@executorCLR@"
	}

	err = syscall.Exec(execute, append([]string{execute}, args...), os.Environ())
	if err == nil {
		err = errors.New("unexpected return from syscall.Exec")
	}
	fmt.Fprintln(os.Stderr, err)
	os.Exit(-1)
}

default.nix

{ pkgs, ... }:
let
  executorWIN = "${pkgs.wine}/bin/wine";
  executorCLR = "${pkgs.mono}/bin/mono";

  dispatch = (pkgs.goPackages.buildGoPackage rec {
    name = "binfmt_dispatch";
    goPackagePath = name;
    preferLocalBuild = true;
    src = pkgs.substituteAll {
      name = "binfmt_dispatch.go";
      src = ./binfmt_dispatch.go;
      dir = ".";
      inherit executorWIN executorCLR;
    };
  }).bin;
in {
  environment.systemPackages = [ dispatch pkgs.wine pkgs.mono ];
  environment.etc."binfmt.d/pe.conf" = {
    text = ":DOSWin:M::MZ::${dispatch}/bin/binfmt_dispatch:\n";
  };

  systemd.additionalUpstreamSystemUnits = [ "systemd-binfmt.service" ];
  systemd.services.systemd-binfmt.wants = [ "proc-sys-fs-binfmt_misc.automount" ];
}