wwtools/mbproxy/tests/Mbproxy.Tests/Proxy/Multiplexing/UpstreamPipeTests.cs

using System.Net;
using System.Net.Sockets;
using Mbproxy.Proxy.Multiplexing;
using Microsoft.Extensions.Logging.Abstractions;
using Shouldly;
using Xunit;

namespace Mbproxy.Tests.Proxy.Multiplexing;

/// <summary>
/// Unit tests for <see cref="UpstreamPipe"/>'s response-channel contract — particularly
/// the Phase 12 (W1.3) <see cref="UpstreamPipe.TrySendResponse"/> non-blocking enqueue
/// added so the per-PLC backend reader cannot be stalled by one slow upstream client.
/// </summary>
[Trait("Category", "Unit")]
public sealed class UpstreamPipeTests
{
    // ── Helpers ───────────────────────────────────────────────────────────────

    private static async Task<(Socket clientSide, Socket serverSide)> AcceptedSocketPairAsync()
    {
        // Build a loopback listener and connect a client to get a real socket pair.
        var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        try
        {
            int port = ((IPEndPoint)listener.LocalEndpoint).Port;
            var clientSide = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            var connectTask = clientSide.ConnectAsync(IPAddress.Loopback, port);
            var serverSide = await listener.AcceptSocketAsync();
            await connectTask;
            return (clientSide, serverSide);
        }
        finally
        {
            listener.Stop();
        }
    }

    // ── Tests ─────────────────────────────────────────────────────────────────

    /// <summary>
    /// W1.3 — when no write-loop is draining the response channel, repeated
    /// <see cref="UpstreamPipe.TrySendResponse"/> calls must succeed up to the channel's
    /// bounded capacity and return <c>false</c> on every subsequent call without blocking.
    /// This is the non-blocking contract the per-PLC backend reader relies on.
    /// </summary>
    [Fact]
    public async Task TrySendResponse_WhenChannelFull_ReturnsFalse_WithoutBlocking()
    {
        var (client, server) = await AcceptedSocketPairAsync();
        try
        {
            // Construct the pipe but do NOT call RunWriteLoopAsync — the channel will not
            // be drained, so it fills after `ResponseChannelCapacity` (= 16) writes.
            var pipe = new UpstreamPipe(server, "TEST", NullLogger.Instance);

            int successes = 0;
            int failures = 0;

            for (int i = 0; i < 100; i++)
            {
                bool ok = pipe.TrySendResponse(new byte[] { 0, 0 });
                if (ok) successes++;
                else failures++;
            }

            successes.ShouldBe(16,
                "the channel's bounded capacity is 16; first 16 writes must succeed");
            failures.ShouldBe(84,
                "after capacity is reached, every further TrySendResponse must return false (not block)");

            await pipe.DisposeAsync();
        }
        finally
        {
            try { client.Dispose(); } catch { }
            try { server.Dispose(); } catch { }
        }
    }

    /// <summary>
    /// W1.3 — once the pipe has been disposed, <see cref="UpstreamPipe.TrySendResponse"/>
    /// returns <c>false</c> regardless of channel state, never throws.
    /// </summary>
    [Fact]
    public async Task TrySendResponse_AfterDispose_ReturnsFalse()
    {
        var (client, server) = await AcceptedSocketPairAsync();
        try
        {
            var pipe = new UpstreamPipe(server, "TEST", NullLogger.Instance);
            await pipe.DisposeAsync();

            bool ok = pipe.TrySendResponse(new byte[] { 0, 0 });
            ok.ShouldBeFalse("a disposed pipe must reject sends without throwing");
        }
        finally
        {
            try { client.Dispose(); } catch { }
            try { server.Dispose(); } catch { }
        }
    }
}