// Copyright (C) 2023-2024  Umorpha Systems
// SPDX-License-Identifier: AGPL-3.0-or-later

package main

import (
	"fmt"
	"net"
	"os"
	"strconv"
	"strings"

	sd "git.lukeshu.com/go/libsystemd/sd_daemon"
)

// netListen is like [net.Listen], but with support for our special
// "fd" address family.
//
// The purpose of doing this is that for development I really want to
// be able to have it listen on a TCP port, but for actual deployment
// I want it to be able to accept an already-opened socket from
// systemd, and so I wrote a reasonably genereral solution.
//
// Did I avoid unnecessary complexity writing special-purpose code, or
// did I introduce it by failing to remember that YAGNI?
func netListen(stype, saddr string) (net.Listener, error) {
	switch stype {
	case "fd":
		switch saddr {
		case "stdin", "0":
			return net.FileListener(os.Stdin)
		case "stdout", "1":
			return net.FileListener(os.Stdout)
		case "stderr", "2":
			return net.FileListener(os.Stderr)
		case "systemd":
			sdFds := sd.ListenFds(true)
			if len(sdFds) == 0 {
				return nil, fmt.Errorf("fd:systemd given, but no systemd file descriptors passed in")
			}
			return net.FileListener(sdFds[0])
		default:
			if fd, _ := strconv.Atoi(saddr); fd > 0 {
				return net.FileListener(os.NewFile(uintptr(fd), fmt.Sprintf("/dev/fd/%d", fd)))
			}
			if name, ok := strings.CutPrefix(saddr, "systemd:"); ok {
				sdFiles := sd.ListenFds(true)
				for _, file := range sdFiles {
					if file.Name() == name {
						return net.FileListener(file)
					}
				}
				if n, err := strconv.Atoi(name); err == nil && n >= 0 && n < len(sdFiles) {
					return net.FileListener(sdFiles[n])
				}
				return nil, fmt.Errorf("does not match any systemd file descriptor: %q", name)
			}
			return nil, fmt.Errorf("invalid file descriptor name: %q", saddr)
		}
	default:
		return net.Listen(stype, saddr)
	}
}