//go:build linux

package http

import (
	"fmt"

	"net"
	"os"
	"strconv"
)

func getSystemdListeners() (Listeners, error) {
	pidStr := os.Getenv("LISTEN_PID")
	fdStr := os.Getenv("LISTEN_FDS")

	if pidStr == "" || fdStr == "" {
		return nil, nil // Not running under systemd
	}

	pid, err := strconv.Atoi(pidStr)
	if err != nil {
		// Not our activation — another process might have inherited the env.
		return nil, nil
	}
	if pid != os.Getpid() {
		return nil, fmt.Errorf("LISTEN_PID %d does not match current PID %d", pid, os.Getpid())
	}

	defer func() {
		_ = os.Unsetenv("LISTEN_PID")
		_ = os.Unsetenv("LISTEN_FDS")
	}()

	nfds, err := strconv.ParseUint(fdStr, 10, 64)
	if err != nil {
		return nil, fmt.Errorf("invalid LISTEN_FDS: %w", err)
	}
	if nfds == 0 {
		return nil, nil // Nothing to do
	}

	lns := make(Listeners, 0, nfds)
	for i := range nfds {
		fd := i + 3
		file := os.NewFile(uintptr(fd), fmt.Sprintf("fd_%d", fd))
		if file == nil {
			return nil, fmt.Errorf("fd %d was nil", fd)
		}
		ln, err := net.FileListener(file)
		if err != nil {
			file.Close()
			return nil, fmt.Errorf("failed to create listener from fd %d: %w", fd, err)
		}
		lns = append(lns, ln)
	}

	return lns, nil
}