acdream/src/AcDream.Core.Net/NetClient.cs

using System.Net;
using System.Net.Sockets;

namespace AcDream.Core.Net;

/// <summary>
/// Minimum-viable UDP transport for acdream. Wraps a <see cref="UdpClient"/>
/// with synchronous send + timeout-based receive — good enough for the
/// Phase 4.6 handshake smoke test and early state-machine bring-up.
///
/// <para>
/// <b>Not yet provided</b> (deferred to a later phase once the handshake
/// actually works): background receive thread, outbound queue, ack/retransmit
/// window, heartbeat timer, concurrent send/receive. The acdream game loop
/// will need a real async pump eventually but building that now would be
/// debugging two things at once when we hit the first protocol mismatch.
/// </para>
/// </summary>
public sealed class NetClient : IDisposable
{
    private readonly UdpClient _udp;
    private readonly IPEndPoint _remote;

    /// <summary>Last <see cref="Socket.ReceiveTimeout"/> value applied to the
    /// underlying socket, so repeated <see cref="Receive"/> calls with the
    /// same timeout (every 250ms heartbeat tick) skip the setsockopt syscall.</summary>
    private int _lastAppliedReceiveTimeoutMs = int.MinValue;

    public NetClient(IPEndPoint remote)
    {
        _remote = remote;
        // Bind to an OS-assigned local port; server will reply to it.
        _udp = new UdpClient(new IPEndPoint(IPAddress.Any, 0));

        if (OperatingSystem.IsWindows())
        {
            // SIO_UDP_CONNRESET: Windows' UDP stack otherwise surfaces an
            // ICMP "port unreachable" reply to an EARLIER Send (e.g. a stale
            // server session that already tore down its socket) as a
            // WSAECONNRESET SocketException on this socket's NEXT unrelated
            // Receive call. An ICMP error from one send must not poison
            // receiving for the rest of the session — disable that reporting.
            const int SioUdpConnReset = -1744830452; // 0x9800000C
            _udp.Client.IOControl((IOControlCode)SioUdpConnReset, new byte[] { 0 }, null);
        }
    }

    /// <summary>The local endpoint the OS assigned us.</summary>
    public IPEndPoint LocalEndPoint => (IPEndPoint)_udp.Client.LocalEndPoint!;

    /// <summary>The remote endpoint we're talking to.</summary>
    public IPEndPoint RemoteEndPoint => _remote;

    /// <summary>
    /// Send a datagram to the configured default remote. Blocks until the
    /// OS has accepted the bytes (fast — just a kernel buffer copy on loopback).
    /// </summary>
    public void Send(ReadOnlySpan<byte> datagram)
    {
        _udp.Send(datagram.ToArray(), datagram.Length, _remote);
    }

    /// <summary>
    /// Send a datagram to an arbitrary remote endpoint. Needed for the AC
    /// handshake because the server binds separate listeners on port 9000
    /// (LoginRequest) and port 9001 (ConnectResponse), so the second
    /// handshake leg targets a different port than the first.
    /// </summary>
    public void Send(IPEndPoint remote, ReadOnlySpan<byte> datagram)
    {
        _udp.Send(datagram.ToArray(), datagram.Length, remote);
    }

    /// <summary>
    /// Block until a datagram arrives or <paramref name="timeout"/> elapses.
    /// Returns the raw bytes, or <c>null</c> on timeout. The sender's
    /// endpoint is recorded in <paramref name="from"/> so the caller can
    /// verify it matches the expected remote.
    /// </summary>
    public byte[]? Receive(TimeSpan timeout, out IPEndPoint? from)
    {
        int timeoutMs = (int)timeout.TotalMilliseconds;
        if (timeoutMs != _lastAppliedReceiveTimeoutMs)
        {
            _udp.Client.ReceiveTimeout = timeoutMs;
            _lastAppliedReceiveTimeoutMs = timeoutMs;
        }
        try
        {
            IPEndPoint any = new(IPAddress.Any, 0);
            var bytes = _udp.Receive(ref any);
            from = any;
            return bytes;
        }
        catch (SocketException ex) when (ex.SocketErrorCode == SocketError.TimedOut)
        {
            from = null;
            return null;
        }
    }

    /// <summary>
    /// Non-blocking receive: returns the next pending datagram if one is
    /// already in the kernel buffer, or <c>null</c> if not. Never blocks,
    /// so it's safe to call once per game-loop frame. Uses
    /// <see cref="UdpClient.Available"/> under the hood.
    /// </summary>
    public byte[]? TryReceive(out IPEndPoint? from)
    {
        if (_udp.Available == 0)
        {
            from = null;
            return null;
        }

        try
        {
            IPEndPoint any = new(IPAddress.Any, 0);
            var bytes = _udp.Receive(ref any);
            from = any;
            return bytes;
        }
        catch (SocketException)
        {
            from = null;
            return null;
        }
    }

    public void Dispose() => _udp.Dispose();
}