// SPDX-License-Identifier: GPL-3.0-or-later

// Package sdnotify implements the small systemd notification subset needed by
// the service binary without adding a runtime dependency.
package sdnotify

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

// Notifier sends state records to the socket inherited from systemd. An empty
// socket makes Notify a no-op so the agent behaves identically outside a unit.
type Notifier struct {
	socket string
}

// FromEnvironment returns a notifier for systemd's NOTIFY_SOCKET contract.
func FromEnvironment() Notifier {
	return New(os.Getenv("NOTIFY_SOCKET"))
}

// New constructs a notifier for an explicit socket, primarily for tests.
func New(socket string) Notifier {
	return Notifier{socket: socket}
}

// Notify sends one newline-delimited systemd state record.
func (n Notifier) Notify(state string) error {
	if n.socket == "" {
		return nil
	}
	if state == "" || strings.ContainsRune(state, '\x00') {
		return fmt.Errorf("invalid empty or NUL-containing notification")
	}
	connection, err := net.DialUnix(
		"unixgram", nil, &net.UnixAddr{Name: n.socket, Net: "unixgram"},
	)
	if err != nil {
		return fmt.Errorf("connect notify socket: %w", err)
	}
	defer connection.Close()
	if _, err := connection.Write([]byte(state)); err != nil {
		return fmt.Errorf("write notify socket: %w", err)
	}
	return nil
}