package main

import (
	"net"
	"os"
	"strings"
)

// NotifyReady tells systemd this process is ready to serve, for a unit
// declared Type=notify.
//
// It exists so that "the process was exec'd" and "the process is serving"
// stop being the same fact to systemd. With Type=simple, `systemctl restart`
// succeeds the instant the binary is exec'd — so a console that starts and
// then dies on its first request, or half-applies a migration, restarts
// SUCCESSFULLY as far as the adopt pass can tell, and the pass then stamps a
// version nothing is really running. Signalled after the listener is open,
// a broken build fails its restart instead, and adoption's existing rollback
// takes over.
//
// No dependency: the protocol is one datagram on the socket systemd names in
// NOTIFY_SOCKET. An unset variable means "not under systemd" — a laptop, a
// test, `go run` — and is not an error.
//
// An "@" prefix denotes a Linux abstract socket, which net.Dial expects as a
// leading NUL.
//
// This is the code half only: it is INERT until console.service is declared
// Type=notify, a pinfra change outside this repo — see the runbook in
// .superpowers/sdd/2026-08-23-console-as-an-app/.
func NotifyReady() error {
	sock := os.Getenv("NOTIFY_SOCKET")
	if sock == "" {
		return nil
	}
	if strings.HasPrefix(sock, "@") {
		sock = "\x00" + sock[1:]
	}
	conn, err := net.Dial("unixgram", sock)
	if err != nil {
		return err
	}
	defer conn.Close()
	_, err = conn.Write([]byte("READY=1"))
	return err
}