Rendezvous/src/FinalFactory.Rendezvous.Server/Transport/UdpMediatorService.cs

using System.Net;
using System.Net.Sockets;
using Microsoft.Extensions.Options;

namespace FinalFactory.Rendezvous.Server.Transport;

/// <summary>
/// Owns the cancellable UDP socket used by the future NAT mediator.
/// </summary>
public sealed partial class UdpMediatorService : BackgroundService
{
    private readonly ILogger<UdpMediatorService> _logger;
    private readonly UdpMediatorOptions _options;
    private UdpClient? _udpClient;

    /// <summary>
    /// Initializes a new UDP mediator service.
    /// </summary>
    public UdpMediatorService(
        IOptions<UdpMediatorOptions> options,
        ILogger<UdpMediatorService> logger)
    {
        _options = options.Value;
        _logger = logger;
    }

    /// <summary>
    /// Gets the bound endpoint after startup completes.
    /// </summary>
    public IPEndPoint? LocalEndpoint { get; private set; }

    /// <inheritdoc />
    public override Task StartAsync(CancellationToken cancellationToken)
    {
        cancellationToken.ThrowIfCancellationRequested();

        if (_udpClient is not null)
        {
            throw new InvalidOperationException("The UDP mediator is already running.");
        }

        IPAddress listenAddress = IPAddress.Parse(_options.ListenAddress);
        UdpClient udpClient = new(new IPEndPoint(listenAddress, _options.Port));
        _udpClient = udpClient;
        IPEndPoint localEndpoint =
            (IPEndPoint?)udpClient.Client.LocalEndPoint
            ?? throw new InvalidOperationException("The UDP socket did not expose its bound endpoint.");
        LocalEndpoint = localEndpoint;

        LogMediatorListening(_logger, localEndpoint.Address, localEndpoint.Port);

        return base.StartAsync(cancellationToken);
    }

    /// <inheritdoc />
    public override async Task StopAsync(CancellationToken cancellationToken)
    {
        await base.StopAsync(cancellationToken).ConfigureAwait(false);
        _udpClient?.Dispose();
        _udpClient = null;
        LocalEndpoint = null;
        LogMediatorStopped(_logger);
    }

    /// <inheritdoc />
    public override void Dispose()
    {
        _udpClient?.Dispose();
        _udpClient = null;
        LocalEndpoint = null;
        base.Dispose();
    }

    /// <inheritdoc />
    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        UdpClient udpClient = _udpClient
            ?? throw new InvalidOperationException("The UDP mediator socket was not initialized.");

        try
        {
            while (!stoppingToken.IsCancellationRequested)
            {
                _ = await udpClient.ReceiveAsync(stoppingToken).ConfigureAwait(false);
                // Bootstrap deliberately emits no UDP response. Protocol handling lands in #11.
            }
        }
        catch (OperationCanceledException) when (stoppingToken.IsCancellationRequested)
        {
            // Expected during normal shutdown.
        }
        catch (ObjectDisposedException) when (stoppingToken.IsCancellationRequested)
        {
            // Disposing the socket is the fallback that releases a blocked receive.
        }
        finally
        {
            LocalEndpoint = null;
        }
    }

    [LoggerMessage(
        EventId = 1,
        Level = LogLevel.Information,
        Message = "UDP mediator listening on {ListenAddress}:{ListenPort}")]
    private static partial void LogMediatorListening(
        ILogger logger,
        IPAddress listenAddress,
        int listenPort);

    [LoggerMessage(
        EventId = 2,
        Level = LogLevel.Information,
        Message = "UDP mediator stopped")]
    private static partial void LogMediatorStopped(ILogger logger);
}