fix(modbus): transport self-heals with one throttled in-gate connect; PDU length check
P3: a torn-down socket is re-dialled inside the gate (one attempt per backoff window) instead of failing fast with 'Transport not connected' until the 90 s comms-loss watchdog re-initialised the driver. L3: ConnectAsync disposes the previous/failed TcpClient, DNS runs under the per-op timeout, and the reconnect never sleeps inside the gate (the old 10-attempt backoff held it for ~81 s). H9: an MBAP response with no PDU, or an exception PDU with no code, is a TruncatedFrame desync rather than an IndexOutOfRangeException. Refs #741 Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
This commit is contained in:
Joseph Doherty
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using System.Net;
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using System.Net.Sockets;
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namespace ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests;
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/// <summary>
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/// Loopback Modbus-TCP stub whose reply to each request is scripted by the test, and whose listener can
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/// be stopped (connection refused) and restarted on the SAME port. Used by the transport self-heal tests
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/// that need a device which disappears and comes back.
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/// </summary>
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internal sealed class ScriptedModbusTcpServer : IAsyncDisposable
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{
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/// <summary>What the server does with one request.</summary>
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public enum Action
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{
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/// <summary>Answer with an FC03-shaped response of zeroed registers.</summary>
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Answer,
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/// <summary>Read the request and never answer it (the client's per-op timeout fires).</summary>
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Stall,
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/// <summary>Close the socket without answering.</summary>
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Drop,
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}
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private TcpListener? _listener;
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private CancellationTokenSource _stop = new();
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private int _requests;
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private int _accepted;
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/// <summary>Optional scripted reply. Receives the 0-based request index and the request PDU; returns
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/// the raw bytes to write AFTER the MBAP TxId (i.e. the full ADU is built by the test), or null to fall
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/// through to <see cref="Script"/>.</summary>
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public Func<int, byte[], byte[], byte[]?>? RawReply { get; set; }
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/// <summary>Per-request action; defaults to <see cref="Action.Answer"/>.</summary>
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public Func<int, Action> Script { get; set; } = _ => Action.Answer;
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/// <summary>Gets the port the server listens on (stable across <see cref="Stop"/>/<see cref="Start"/>).</summary>
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public int Port { get; }
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/// <summary>Gets the number of accepted TCP connections.</summary>
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public int AcceptedConnections => Volatile.Read(ref _accepted);
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/// <summary>Gets the number of requests received.</summary>
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public int Requests => Volatile.Read(ref _requests);
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public ScriptedModbusTcpServer()
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{
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var probe = new TcpListener(IPAddress.Loopback, 0);
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probe.Start();
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Port = ((IPEndPoint)probe.LocalEndpoint).Port;
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probe.Stop();
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Start();
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}
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/// <summary>(Re)starts listening on <see cref="Port"/>.</summary>
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public void Start()
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{
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_stop = new CancellationTokenSource();
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_listener = new TcpListener(IPAddress.Loopback, Port);
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_listener.Server.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
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_listener.Start();
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var listener = _listener;
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var token = _stop.Token;
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_ = Task.Run(() => AcceptLoopAsync(listener, token));
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}
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/// <summary>Stops listening and closes every live connection — new connects are refused.</summary>
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public void Stop()
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{
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try { _stop.Cancel(); } catch { }
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try { _listener?.Stop(); } catch { }
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_listener = null;
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}
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private async Task AcceptLoopAsync(TcpListener listener, CancellationToken token)
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{
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while (!token.IsCancellationRequested)
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{
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TcpClient client;
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try { client = await listener.AcceptTcpClientAsync(token); }
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catch { return; }
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Interlocked.Increment(ref _accepted);
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_ = Task.Run(() => ServeAsync(client, token));
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}
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}
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private async Task ServeAsync(TcpClient client, CancellationToken token)
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{
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using var _ = client;
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using var reg = token.Register(() => { try { client.Close(); } catch { } });
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try
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{
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var stream = client.GetStream();
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while (!token.IsCancellationRequested)
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{
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var header = new byte[7];
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if (!await ReadExactly(stream, header, token)) return;
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var len = (ushort)((header[4] << 8) | header[5]);
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var pdu = new byte[len - 1];
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if (!await ReadExactly(stream, pdu, token)) return;
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var idx = Interlocked.Increment(ref _requests) - 1;
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var raw = RawReply?.Invoke(idx, header, pdu);
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if (raw is not null)
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{
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await stream.WriteAsync(raw, token);
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await stream.FlushAsync(token);
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continue;
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}
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switch (Script(idx))
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{
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case Action.Stall:
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continue;
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case Action.Drop:
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client.Client.Shutdown(SocketShutdown.Both);
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return;
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}
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var qty = (ushort)((pdu[3] << 8) | pdu[4]);
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var respPdu = new byte[2 + qty * 2];
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respPdu[0] = pdu[0];
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respPdu[1] = (byte)(qty * 2);
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await stream.WriteAsync(Adu(header, respPdu), token);
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await stream.FlushAsync(token);
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}
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}
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catch { /* connection torn down */ }
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}
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/// <summary>Builds a reply ADU echoing the request's TxId/unit around <paramref name="respPdu"/>.</summary>
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public static byte[] Adu(byte[] requestHeader, byte[] respPdu)
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{
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var respLen = (ushort)(1 + respPdu.Length);
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var adu = new byte[7 + respPdu.Length];
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adu[0] = requestHeader[0]; adu[1] = requestHeader[1];
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adu[4] = (byte)(respLen >> 8); adu[5] = (byte)(respLen & 0xFF);
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adu[6] = requestHeader[6];
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Buffer.BlockCopy(respPdu, 0, adu, 7, respPdu.Length);
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return adu;
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}
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private static async Task<bool> ReadExactly(Stream s, byte[] buf, CancellationToken token)
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{
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var read = 0;
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while (read < buf.Length)
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{
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var n = await s.ReadAsync(buf.AsMemory(read), token);
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if (n == 0) return false;
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read += n;
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}
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return true;
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}
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public ValueTask DisposeAsync()
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{
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Stop();
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return ValueTask.CompletedTask;
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}
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}
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