Build fake worker test harness

docs/GatewayTesting.md

@@ -0,0 +1,51 @@
+# Gateway Testing
+
+Gateway tests run without installed MXAccess by using fake workers, fake
+transports, and in-process gRPC service fakes. Live MXAccess verification belongs
+in opt-in integration tests because it depends on installed COM components and
+provider state.
+
+## Fake Worker Harness
+
+`FakeWorkerHarness` in `src/MxGateway.Tests/Gateway/Workers/Fakes/` provides an
+in-process worker side for named-pipe IPC tests. It uses the same
+`WorkerFrameReader`, `WorkerFrameWriter`, and `WorkerEnvelope` contract as the
+gateway so tests exercise real frame validation and worker-client state changes.
+
+Use the harness when a gateway or session test needs worker behavior without
+starting `MxGateway.Worker.exe` or loading MXAccess COM. The harness scripts:
+
+- `WorkerHello` and `WorkerReady` startup,
+- command replies with matching correlation ids,
+- ordered `WorkerEvent` frames,
+- `WorkerFault` frames,
+- shutdown acknowledgements,
+- malformed protobuf payloads and oversized frame headers,
+- slow or hung workers by withholding a reply.
+
+Session-level tests can connect the harness to the pipe created by
+`SessionWorkerClientFactory` with `ConnectToGatewayPipeAsync`. Lower-level
+`WorkerClient` tests can use `CreateConnectedPairAsync` to create both pipe ends
+inside the test.
+
+## Focused Commands
+
+Run the fake worker tests after changing gateway worker IPC, session startup, or
+event streaming behavior:
+
+```bash
+dotnet test src/MxGateway.Tests/MxGateway.Tests.csproj --filter FullyQualifiedName~FakeWorkerHarnessTests
+dotnet test src/MxGateway.Tests/MxGateway.Tests.csproj --filter FullyQualifiedName~SessionWorkerClientFactoryFakeWorkerTests
+```
+
+Run the gateway test project after shared gateway test infrastructure changes:
+
+```bash
+dotnet test src/MxGateway.Tests/MxGateway.Tests.csproj
+```
+
+## Related Documentation
+
+- [Gateway Process Design](./gateway-process-design.md)
+- [Worker Frame Protocol](./WorkerFrameProtocol.md)
+- [MXAccess Worker Instance Detailed Design](./mxaccess-worker-instance-design.md)

docs/gateway-process-design.md

@@ -891,6 +891,11 @@ behavior unless an explicit non-parity backend is designed.
 Gateway tests should be able to run without installed MXAccess by using fake
 workers and fake transports.
 
+Use `FakeWorkerHarness` for tests that need real gateway-to-worker framing,
+handshake, command, event, fault, or malformed-protocol behavior without loading
+MXAccess COM. See [Gateway Testing](./GatewayTesting.md) for the harness scope
+and focused test commands.
+
 Focused tests:
 
 - session state transitions,

src/MxGateway.Tests/Gateway/Sessions/SessionWorkerClientFactoryFakeWorkerTests.cs

@@ -0,0 +1,216 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+using MxGateway.Contracts;
+using MxGateway.Contracts.Proto;
+using MxGateway.Server.Configuration;
+using MxGateway.Server.Metrics;
+using MxGateway.Server.Sessions;
+using MxGateway.Server.Workers;
+using MxGateway.Tests.Gateway.Workers.Fakes;
+
+namespace MxGateway.Tests.Gateway.Sessions;
+
+public sealed class SessionWorkerClientFactoryFakeWorkerTests
+{
+    private static readonly TimeSpan TestTimeout = TimeSpan.FromSeconds(5);
+
+    [Fact]
+    public async Task CreateAsync_WithScriptedFakeWorker_ReturnsReadyClient()
+    {
+        ScriptedFakeWorkerProcessLauncher launcher = new();
+        using GatewayMetrics metrics = new();
+        SessionWorkerClientFactory factory = new(
+            launcher,
+            Options.Create(CreateOptions()),
+            metrics,
+            NullLoggerFactory.Instance);
+        GatewaySession session = CreateSession();
+
+        await using IWorkerClient workerClient = await factory.CreateAsync(
+            session,
+            CancellationToken.None);
+
+        Assert.Equal(WorkerClientState.Ready, workerClient.State);
+        Assert.Equal(ScriptedFakeWorkerProcessLauncher.ProcessId, workerClient.ProcessId);
+        Assert.NotNull(launcher.Harness);
+
+        Task<WorkerCommandReply> invokeTask = workerClient.InvokeAsync(
+            CreateCommand(MxCommandKind.Ping),
+            TestTimeout,
+            CancellationToken.None);
+        WorkerEnvelope commandEnvelope = await launcher.Harness.ReadCommandAsync();
+        await launcher.Harness.ReplyToCommandAsync(commandEnvelope);
+        WorkerCommandReply reply = await invokeTask.WaitAsync(TestTimeout);
+
+        Assert.Equal(MxCommandKind.Ping, reply.Reply.Kind);
+        Assert.Equal(ProtocolStatusCode.Ok, reply.Reply.ProtocolStatus.Code);
+    }
+
+    [Fact]
+    public async Task CreateAsync_WhenFakeWorkerStartupFails_ThrowsWorkerClientException()
+    {
+        FailingStartupWorkerProcessLauncher launcher = new();
+        using GatewayMetrics metrics = new();
+        SessionWorkerClientFactory factory = new(
+            launcher,
+            Options.Create(CreateOptions()),
+            metrics,
+            NullLoggerFactory.Instance);
+        GatewaySession session = CreateSession();
+
+        WorkerClientException exception = await Assert.ThrowsAsync<WorkerClientException>(
+            async () => await factory.CreateAsync(session, CancellationToken.None).WaitAsync(TestTimeout));
+
+        Assert.Equal(WorkerClientErrorCode.ProtocolViolation, exception.ErrorCode);
+        Assert.True(launcher.Process.IsDisposed);
+    }
+
+    private static GatewayOptions CreateOptions()
+    {
+        return new GatewayOptions
+        {
+            Worker = new WorkerOptions
+            {
+                StartupTimeoutSeconds = 5,
+                ShutdownTimeoutSeconds = 5,
+                HeartbeatIntervalSeconds = 30,
+                HeartbeatGraceSeconds = 30,
+                MaxMessageBytes = WorkerFrameProtocolOptions.DefaultMaxMessageBytes,
+            },
+            Events = new EventOptions
+            {
+                QueueCapacity = 16,
+            },
+        };
+    }
+
+    private static GatewaySession CreateSession()
+    {
+        return new GatewaySession(
+            FakeWorkerHarness.DefaultSessionId,
+            GatewayContractInfo.DefaultBackendName,
+            $"mxaccessgw-session-fake-worker-{Guid.NewGuid():N}",
+            FakeWorkerHarness.DefaultNonce,
+            "test-client",
+            "fake-worker-session-test",
+            "client-correlation-1",
+            TestTimeout,
+            TestTimeout,
+            TestTimeout,
+            DateTimeOffset.UtcNow);
+    }
+
+    private static WorkerCommand CreateCommand(MxCommandKind kind)
+    {
+        return new WorkerCommand
+        {
+            Command = new MxCommand
+            {
+                Kind = kind,
+            },
+        };
+    }
+
+    private sealed class ScriptedFakeWorkerProcessLauncher : IWorkerProcessLauncher
+    {
+        public const int ProcessId = 2468;
+        private readonly FakeWorkerProcess _process = new(ProcessId);
+
+        public FakeWorkerHarness? Harness { get; private set; }
+
+        public Task<WorkerProcessHandle> LaunchAsync(
+            WorkerProcessLaunchRequest request,
+            CancellationToken cancellationToken = default)
+        {
+            _ = RunWorkerAsync(request, cancellationToken);
+
+            return Task.FromResult(CreateHandle(_process));
+        }
+
+        private async Task RunWorkerAsync(
+            WorkerProcessLaunchRequest request,
+            CancellationToken cancellationToken)
+        {
+            Harness = await FakeWorkerHarness.ConnectToGatewayPipeAsync(
+                request.SessionId,
+                request.Nonce,
+                request.PipeName,
+                request.ProtocolVersion,
+                cancellationToken: cancellationToken).ConfigureAwait(false);
+            await Harness.CompleteStartupAsync(ProcessId, cancellationToken: cancellationToken).ConfigureAwait(false);
+        }
+    }
+
+    private sealed class FailingStartupWorkerProcessLauncher : IWorkerProcessLauncher
+    {
+        public FakeWorkerProcess Process { get; } = new(processId: 3579);
+
+        public Task<WorkerProcessHandle> LaunchAsync(
+            WorkerProcessLaunchRequest request,
+            CancellationToken cancellationToken = default)
+        {
+            _ = RunWorkerAsync(request, cancellationToken);
+
+            return Task.FromResult(CreateHandle(Process));
+        }
+
+        private async Task RunWorkerAsync(
+            WorkerProcessLaunchRequest request,
+            CancellationToken cancellationToken)
+        {
+            await using FakeWorkerHarness harness = await FakeWorkerHarness.ConnectToGatewayPipeAsync(
+                request.SessionId,
+                request.Nonce,
+                request.PipeName,
+                request.ProtocolVersion,
+                cancellationToken: cancellationToken).ConfigureAwait(false);
+            _ = await harness.ReadGatewayEnvelopeAsync(cancellationToken).ConfigureAwait(false);
+            await harness.SendWorkerHelloAsync(
+                workerProcessId: Process.Id,
+                workerProtocolVersion: request.ProtocolVersion + 1,
+                cancellationToken: cancellationToken).ConfigureAwait(false);
+        }
+    }
+
+    private static WorkerProcessHandle CreateHandle(IWorkerProcess process)
+    {
+        return new WorkerProcessHandle(
+            process,
+            new WorkerProcessCommandLine("fake-worker.exe", []),
+            DateTimeOffset.UtcNow);
+    }
+
+    private sealed class FakeWorkerProcess(int processId) : IWorkerProcess
+    {
+        private bool _disposed;
+
+        public int Id { get; } = processId;
+
+        public bool HasExited { get; private set; }
+
+        public int? ExitCode { get; private set; }
+
+        public int KillCount { get; private set; }
+
+        public ValueTask WaitForExitAsync(CancellationToken cancellationToken)
+        {
+            HasExited = true;
+            ExitCode = 0;
+            return ValueTask.CompletedTask;
+        }
+
+        public void Kill(bool entireProcessTree)
+        {
+            KillCount++;
+            HasExited = true;
+            ExitCode = -1;
+        }
+
+        public void Dispose()
+        {
+            _disposed = true;
+        }
+
+        public bool IsDisposed => _disposed;
+    }
+}

src/MxGateway.Tests/Gateway/Workers/FakeWorkerHarnessTests.cs

@@ -0,0 +1,190 @@
+using MxGateway.Contracts;
+using MxGateway.Contracts.Proto;
+using MxGateway.Server.Workers;
+using MxGateway.Tests.Gateway.Workers.Fakes;
+
+namespace MxGateway.Tests.Gateway.Workers;
+
+public sealed class FakeWorkerHarnessTests
+{
+    private static readonly TimeSpan TestTimeout = TimeSpan.FromSeconds(5);
+
+    [Fact]
+    public async Task CompleteStartupAsync_WithHelloAndReady_TransitionsClientToReady()
+    {
+        await using FakeWorkerHarness fakeWorker = await FakeWorkerHarness.CreateConnectedPairAsync();
+        await using WorkerClient client = fakeWorker.CreateClient();
+
+        Task startTask = client.StartAsync(CancellationToken.None);
+        WorkerEnvelope gatewayHello = await fakeWorker.CompleteStartupAsync();
+        await startTask.WaitAsync(TestTimeout);
+
+        Assert.Equal(WorkerEnvelope.BodyOneofCase.GatewayHello, gatewayHello.BodyCase);
+        Assert.Equal(FakeWorkerHarness.DefaultNonce, gatewayHello.GatewayHello.Nonce);
+        Assert.Equal(WorkerClientState.Ready, client.State);
+        Assert.Equal(FakeWorkerHarness.DefaultWorkerProcessId, client.ProcessId);
+    }
+
+    [Fact]
+    public async Task StartAsync_WithProtocolMismatch_FailsStartup()
+    {
+        await using FakeWorkerHarness fakeWorker = await FakeWorkerHarness.CreateConnectedPairAsync();
+        await using WorkerClient client = fakeWorker.CreateClient();
+
+        Task startTask = client.StartAsync(CancellationToken.None);
+        WorkerEnvelope gatewayHello = await fakeWorker.ReadGatewayEnvelopeAsync();
+        Assert.Equal(WorkerEnvelope.BodyOneofCase.GatewayHello, gatewayHello.BodyCase);
+        await fakeWorker.SendWorkerHelloAsync(
+            workerProtocolVersion: GatewayContractInfo.WorkerProtocolVersion + 1);
+
+        WorkerClientException exception = await Assert.ThrowsAsync<WorkerClientException>(
+            async () => await startTask.WaitAsync(TestTimeout));
+
+        Assert.Equal(WorkerClientErrorCode.ProtocolViolation, exception.ErrorCode);
+    }
+
+    [Fact]
+    public async Task InvokeAsync_WithScriptedReply_CompletesCommand()
+    {
+        await using FakeWorkerHarness fakeWorker = await FakeWorkerHarness.CreateConnectedPairAsync();
+        await using WorkerClient client = fakeWorker.CreateClient();
+        await StartClientAsync(fakeWorker, client);
+
+        Task<WorkerCommandReply> invokeTask = client.InvokeAsync(
+            CreateCommand(MxCommandKind.Ping),
+            TestTimeout,
+            CancellationToken.None);
+        WorkerEnvelope commandEnvelope = await fakeWorker.ReadCommandAsync();
+        await fakeWorker.ReplyToCommandAsync(commandEnvelope);
+
+        WorkerCommandReply reply = await invokeTask.WaitAsync(TestTimeout);
+
+        Assert.Equal(commandEnvelope.CorrelationId, reply.Reply.CorrelationId);
+        Assert.Equal(MxCommandKind.Ping, reply.Reply.Kind);
+        Assert.Equal(ProtocolStatusCode.Ok, reply.Reply.ProtocolStatus.Code);
+    }
+
+    [Fact]
+    public async Task ReadEventsAsync_WithScriptedEvents_YieldsOrderedEvents()
+    {
+        await using FakeWorkerHarness fakeWorker = await FakeWorkerHarness.CreateConnectedPairAsync();
+        await using WorkerClient client = fakeWorker.CreateClient();
+        await StartClientAsync(fakeWorker, client);
+        using CancellationTokenSource cancellationTokenSource = new(TestTimeout);
+
+        await using IAsyncEnumerator<WorkerEvent> events =
+            client.ReadEventsAsync(cancellationTokenSource.Token).GetAsyncEnumerator(cancellationTokenSource.Token);
+
+        await fakeWorker.EmitEventAsync(MxEventFamily.OnDataChange, cancellationTokenSource.Token);
+        await fakeWorker.EmitEventAsync(MxEventFamily.OperationComplete, cancellationTokenSource.Token);
+
+        Assert.True(await events.MoveNextAsync());
+        Assert.Equal((ulong)3, events.Current.Event.WorkerSequence);
+        Assert.Equal(MxEventFamily.OnDataChange, events.Current.Event.Family);
+
+        Assert.True(await events.MoveNextAsync());
+        Assert.Equal((ulong)4, events.Current.Event.WorkerSequence);
+        Assert.Equal(MxEventFamily.OperationComplete, events.Current.Event.Family);
+    }
+
+    [Fact]
+    public async Task ReadLoop_WithScriptedFault_FaultsClient()
+    {
+        await using FakeWorkerHarness fakeWorker = await FakeWorkerHarness.CreateConnectedPairAsync();
+        await using WorkerClient client = fakeWorker.CreateClient();
+        await StartClientAsync(fakeWorker, client);
+
+        await fakeWorker.EmitFaultAsync(
+            WorkerFaultCategory.MxaccessCommandFailed,
+            "scripted MXAccess command fault");
+
+        await WaitUntilAsync(
+            () => client.State == WorkerClientState.Faulted,
+            TestTimeout);
+
+        Assert.Equal(WorkerClientState.Faulted, client.State);
+    }
+
+    [Fact]
+    public async Task InvokeAsync_WithHungWorker_TimesOutPendingCommand()
+    {
+        await using FakeWorkerHarness fakeWorker = await FakeWorkerHarness.CreateConnectedPairAsync();
+        await using WorkerClient client = fakeWorker.CreateClient();
+        await StartClientAsync(fakeWorker, client);
+
+        Task<WorkerCommandReply> invokeTask = client.InvokeAsync(
+            CreateCommand(MxCommandKind.Ping),
+            TimeSpan.FromMilliseconds(50),
+            CancellationToken.None);
+        WorkerEnvelope commandEnvelope = await fakeWorker.ReadCommandAsync();
+
+        WorkerClientException exception = await Assert.ThrowsAsync<WorkerClientException>(
+            async () => await invokeTask.WaitAsync(TestTimeout));
+
+        Assert.Equal(WorkerEnvelope.BodyOneofCase.WorkerCommand, commandEnvelope.BodyCase);
+        Assert.Equal(WorkerClientErrorCode.CommandTimeout, exception.ErrorCode);
+    }
+
+    [Fact]
+    public async Task ReadLoop_WithMalformedFrame_FaultsClient()
+    {
+        await using FakeWorkerHarness fakeWorker = await FakeWorkerHarness.CreateConnectedPairAsync();
+        await using WorkerClient client = fakeWorker.CreateClient();
+        await StartClientAsync(fakeWorker, client);
+
+        await fakeWorker.WriteMalformedPayloadAsync(new byte[] { 0x08, 0x96, 0x01 });
+
+        await WaitUntilAsync(
+            () => client.State == WorkerClientState.Faulted,
+            TestTimeout);
+
+        Assert.Equal(WorkerClientState.Faulted, client.State);
+    }
+
+    [Fact]
+    public async Task ShutdownAsync_WithShutdownAck_ClosesClient()
+    {
+        await using FakeWorkerHarness fakeWorker = await FakeWorkerHarness.CreateConnectedPairAsync();
+        await using WorkerClient client = fakeWorker.CreateClient();
+        await StartClientAsync(fakeWorker, client);
+
+        Task shutdownTask = client.ShutdownAsync(TestTimeout, CancellationToken.None);
+        WorkerEnvelope shutdownEnvelope = await fakeWorker.ReadShutdownAsync();
+        await fakeWorker.SendShutdownAckAsync();
+        await shutdownTask.WaitAsync(TestTimeout);
+
+        Assert.Equal(WorkerEnvelope.BodyOneofCase.WorkerShutdown, shutdownEnvelope.BodyCase);
+        Assert.Equal(WorkerClientState.Closed, client.State);
+    }
+
+    private static async Task StartClientAsync(
+        FakeWorkerHarness fakeWorker,
+        WorkerClient client)
+    {
+        Task startTask = client.StartAsync(CancellationToken.None);
+        await fakeWorker.CompleteStartupAsync().ConfigureAwait(false);
+        await startTask.WaitAsync(TestTimeout).ConfigureAwait(false);
+    }
+
+    private static WorkerCommand CreateCommand(MxCommandKind kind)
+    {
+        return new WorkerCommand
+        {
+            Command = new MxCommand
+            {
+                Kind = kind,
+            },
+        };
+    }
+
+    private static async Task WaitUntilAsync(
+        Func<bool> predicate,
+        TimeSpan timeout)
+    {
+        using CancellationTokenSource cancellationTokenSource = new(timeout);
+        while (!predicate())
+        {
+            await Task.Delay(TimeSpan.FromMilliseconds(10), cancellationTokenSource.Token);
+        }
+    }
+}

src/MxGateway.Tests/Gateway/Workers/Fakes/FakeWorkerHarness.cs

@@ -0,0 +1,378 @@
+using System.Buffers.Binary;
+using System.IO.Pipes;
+using Google.Protobuf.WellKnownTypes;
+using MxGateway.Contracts;
+using MxGateway.Contracts.Proto;
+using MxGateway.Server.Metrics;
+using MxGateway.Server.Workers;
+
+namespace MxGateway.Tests.Gateway.Workers.Fakes;
+
+public sealed class FakeWorkerHarness : IAsyncDisposable
+{
+    public const string DefaultSessionId = "session-fake-worker";
+    public const string DefaultNonce = "nonce-fake-worker";
+    public const int DefaultWorkerProcessId = 9321;
+
+    private readonly NamedPipeServerStream? _gatewayStream;
+    private readonly NamedPipeClientStream _workerStream;
+    private readonly WorkerFrameProtocolOptions _frameOptions;
+    private readonly WorkerFrameReader _reader;
+    private readonly WorkerFrameWriter _writer;
+    private bool _workerSideDisposed;
+
+    private FakeWorkerHarness(
+        string sessionId,
+        string nonce,
+        NamedPipeServerStream? gatewayStream,
+        NamedPipeClientStream workerStream,
+        WorkerFrameProtocolOptions frameOptions)
+    {
+        SessionId = sessionId;
+        Nonce = nonce;
+        _gatewayStream = gatewayStream;
+        _workerStream = workerStream;
+        _frameOptions = frameOptions;
+        _reader = new WorkerFrameReader(_workerStream, frameOptions);
+        _writer = new WorkerFrameWriter(_workerStream, frameOptions);
+    }
+
+    public string SessionId { get; }
+
+    public string Nonce { get; }
+
+    public ulong NextWorkerSequence { get; private set; }
+
+    public static async Task<FakeWorkerHarness> CreateConnectedPairAsync(
+        string sessionId = DefaultSessionId,
+        string nonce = DefaultNonce,
+        uint protocolVersion = GatewayContractInfo.WorkerProtocolVersion,
+        int maxMessageBytes = WorkerFrameProtocolOptions.DefaultMaxMessageBytes,
+        CancellationToken cancellationToken = default)
+    {
+        string pipeName = $"mxaccessgw-fake-worker-{Guid.NewGuid():N}";
+        NamedPipeServerStream gatewayStream = new(
+            pipeName,
+            PipeDirection.InOut,
+            maxNumberOfServerInstances: 1,
+            PipeTransmissionMode.Byte,
+            PipeOptions.Asynchronous);
+        NamedPipeClientStream workerStream = CreateWorkerStream(pipeName);
+
+        Task waitForConnectionTask = gatewayStream.WaitForConnectionAsync(cancellationToken);
+        await workerStream.ConnectAsync(cancellationToken).ConfigureAwait(false);
+        await waitForConnectionTask.ConfigureAwait(false);
+
+        return new FakeWorkerHarness(
+            sessionId,
+            nonce,
+            gatewayStream,
+            workerStream,
+            new WorkerFrameProtocolOptions(sessionId, protocolVersion, maxMessageBytes));
+    }
+
+    public static async Task<FakeWorkerHarness> ConnectToGatewayPipeAsync(
+        string sessionId,
+        string nonce,
+        string pipeName,
+        uint protocolVersion = GatewayContractInfo.WorkerProtocolVersion,
+        int maxMessageBytes = WorkerFrameProtocolOptions.DefaultMaxMessageBytes,
+        CancellationToken cancellationToken = default)
+    {
+        NamedPipeClientStream workerStream = CreateWorkerStream(pipeName);
+        await workerStream.ConnectAsync(cancellationToken).ConfigureAwait(false);
+
+        return new FakeWorkerHarness(
+            sessionId,
+            nonce,
+            gatewayStream: null,
+            workerStream,
+            new WorkerFrameProtocolOptions(sessionId, protocolVersion, maxMessageBytes));
+    }
+
+    public WorkerClient CreateClient(
+        WorkerClientOptions? options = null,
+        GatewayMetrics? metrics = null,
+        TimeProvider? timeProvider = null)
+    {
+        if (_gatewayStream is null)
+        {
+            throw new InvalidOperationException("This fake worker is connected to a gateway-owned pipe.");
+        }
+
+        WorkerClientConnection connection = new(
+            SessionId,
+            Nonce,
+            _gatewayStream,
+            _frameOptions);
+
+        return new WorkerClient(connection, options, metrics, timeProvider);
+    }
+
+    public async Task<WorkerEnvelope> CompleteStartupAsync(
+        int workerProcessId = DefaultWorkerProcessId,
+        string workerVersion = "fake-worker",
+        string mxaccessProgid = "LMXProxy.LMXProxyServer.1",
+        string mxaccessClsid = "{C30B52F5-2CB5-4760-AF0A-3A344A7EB5DC}",
+        CancellationToken cancellationToken = default)
+    {
+        WorkerEnvelope gatewayHello = await ReadGatewayEnvelopeAsync(cancellationToken).ConfigureAwait(false);
+        if (gatewayHello.BodyCase != WorkerEnvelope.BodyOneofCase.GatewayHello)
+        {
+            throw new InvalidOperationException($"Expected GatewayHello but received {gatewayHello.BodyCase}.");
+        }
+
+        await SendWorkerHelloAsync(
+            workerProcessId,
+            workerVersion,
+            cancellationToken: cancellationToken).ConfigureAwait(false);
+        await SendWorkerReadyAsync(
+            workerProcessId,
+            mxaccessProgid,
+            mxaccessClsid,
+            cancellationToken).ConfigureAwait(false);
+
+        return gatewayHello;
+    }
+
+    public async Task<WorkerEnvelope> ReadGatewayEnvelopeAsync(CancellationToken cancellationToken = default)
+    {
+        return await _reader.ReadAsync(cancellationToken).ConfigureAwait(false);
+    }
+
+    public async Task<WorkerEnvelope> ReadCommandAsync(CancellationToken cancellationToken = default)
+    {
+        WorkerEnvelope envelope = await ReadGatewayEnvelopeAsync(cancellationToken).ConfigureAwait(false);
+        if (envelope.BodyCase != WorkerEnvelope.BodyOneofCase.WorkerCommand)
+        {
+            throw new InvalidOperationException($"Expected WorkerCommand but received {envelope.BodyCase}.");
+        }
+
+        return envelope;
+    }
+
+    public async Task<WorkerEnvelope> ReadShutdownAsync(CancellationToken cancellationToken = default)
+    {
+        WorkerEnvelope envelope = await ReadGatewayEnvelopeAsync(cancellationToken).ConfigureAwait(false);
+        if (envelope.BodyCase != WorkerEnvelope.BodyOneofCase.WorkerShutdown)
+        {
+            throw new InvalidOperationException($"Expected WorkerShutdown but received {envelope.BodyCase}.");
+        }
+
+        return envelope;
+    }
+
+    public async Task SendWorkerHelloAsync(
+        int workerProcessId = DefaultWorkerProcessId,
+        string workerVersion = "fake-worker",
+        uint? workerProtocolVersion = null,
+        string? nonce = null,
+        CancellationToken cancellationToken = default)
+    {
+        await _writer.WriteAsync(
+            CreateEnvelope(
+                correlationId: string.Empty,
+                envelope => envelope.WorkerHello = new WorkerHello
+                {
+                    ProtocolVersion = workerProtocolVersion ?? _frameOptions.ProtocolVersion,
+                    Nonce = nonce ?? Nonce,
+                    WorkerProcessId = workerProcessId,
+                    WorkerVersion = workerVersion,
+                }),
+            cancellationToken).ConfigureAwait(false);
+    }
+
+    public async Task SendWorkerReadyAsync(
+        int workerProcessId = DefaultWorkerProcessId,
+        string mxaccessProgid = "LMXProxy.LMXProxyServer.1",
+        string mxaccessClsid = "{C30B52F5-2CB5-4760-AF0A-3A344A7EB5DC}",
+        CancellationToken cancellationToken = default)
+    {
+        await _writer.WriteAsync(
+            CreateEnvelope(
+                correlationId: string.Empty,
+                envelope => envelope.WorkerReady = new WorkerReady
+                {
+                    WorkerProcessId = workerProcessId,
+                    MxaccessProgid = mxaccessProgid,
+                    MxaccessClsid = mxaccessClsid,
+                    ReadyTimestamp = Timestamp.FromDateTimeOffset(DateTimeOffset.UtcNow),
+                }),
+            cancellationToken).ConfigureAwait(false);
+    }
+
+    public async Task ReplyToCommandAsync(
+        WorkerEnvelope commandEnvelope,
+        ProtocolStatusCode statusCode = ProtocolStatusCode.Ok,
+        string statusMessage = "OK",
+        CancellationToken cancellationToken = default)
+    {
+        if (commandEnvelope.BodyCase != WorkerEnvelope.BodyOneofCase.WorkerCommand)
+        {
+            throw new ArgumentException("Command envelope must contain WorkerCommand.", nameof(commandEnvelope));
+        }
+
+        MxCommandKind kind = commandEnvelope.WorkerCommand.Command?.Kind ?? MxCommandKind.Unspecified;
+        await _writer.WriteAsync(
+            CreateEnvelope(
+                commandEnvelope.CorrelationId,
+                envelope => envelope.WorkerCommandReply = new WorkerCommandReply
+                {
+                    Reply = new MxCommandReply
+                    {
+                        SessionId = SessionId,
+                        CorrelationId = commandEnvelope.CorrelationId,
+                        Kind = kind,
+                        ProtocolStatus = new ProtocolStatus
+                        {
+                            Code = statusCode,
+                            Message = statusMessage,
+                        },
+                    },
+                    CompletedTimestamp = Timestamp.FromDateTimeOffset(DateTimeOffset.UtcNow),
+                }),
+            cancellationToken).ConfigureAwait(false);
+    }
+
+    public async Task EmitEventAsync(
+        MxEventFamily family,
+        CancellationToken cancellationToken = default)
+    {
+        ulong sequence = NextWorkerSequence + 1;
+        await _writer.WriteAsync(
+            CreateEnvelope(
+                correlationId: string.Empty,
+                envelope => envelope.WorkerEvent = new WorkerEvent
+                {
+                    Event = new MxEvent
+                    {
+                        SessionId = SessionId,
+                        Family = family,
+                        WorkerSequence = sequence,
+                        WorkerTimestamp = Timestamp.FromDateTimeOffset(DateTimeOffset.UtcNow),
+                    },
+                }),
+            cancellationToken).ConfigureAwait(false);
+    }
+
+    public async Task EmitFaultAsync(
+        WorkerFaultCategory category,
+        string diagnosticMessage,
+        CancellationToken cancellationToken = default)
+    {
+        await _writer.WriteAsync(
+            CreateEnvelope(
+                correlationId: string.Empty,
+                envelope => envelope.WorkerFault = new WorkerFault
+                {
+                    Category = category,
+                    DiagnosticMessage = diagnosticMessage,
+                    ProtocolStatus = new ProtocolStatus
+                    {
+                        Code = ProtocolStatusCode.WorkerUnavailable,
+                        Message = diagnosticMessage,
+                    },
+                }),
+            cancellationToken).ConfigureAwait(false);
+    }
+
+    public async Task SendShutdownAckAsync(
+        ProtocolStatusCode statusCode = ProtocolStatusCode.Ok,
+        CancellationToken cancellationToken = default)
+    {
+        await _writer.WriteAsync(
+            CreateEnvelope(
+                correlationId: string.Empty,
+                envelope => envelope.WorkerShutdownAck = new WorkerShutdownAck
+                {
+                    Status = new ProtocolStatus
+                    {
+                        Code = statusCode,
+                        Message = statusCode.ToString(),
+                    },
+                }),
+            cancellationToken).ConfigureAwait(false);
+    }
+
+    public async Task WriteMalformedPayloadAsync(
+        ReadOnlyMemory<byte> payload,
+        CancellationToken cancellationToken = default)
+    {
+        if (payload.IsEmpty)
+        {
+            throw new ArgumentException("Malformed payload must include at least one byte.", nameof(payload));
+        }
+
+        byte[] lengthPrefix = new byte[sizeof(uint)];
+        BinaryPrimitives.WriteUInt32LittleEndian(lengthPrefix, (uint)payload.Length);
+        await _workerStream.WriteAsync(lengthPrefix, cancellationToken).ConfigureAwait(false);
+        await _workerStream.WriteAsync(payload, cancellationToken).ConfigureAwait(false);
+    }
+
+    public async Task WriteOversizedFrameHeaderAsync(
+        uint payloadLength,
+        CancellationToken cancellationToken = default)
+    {
+        if (payloadLength <= _frameOptions.MaxMessageBytes)
+        {
+            throw new ArgumentOutOfRangeException(
+                nameof(payloadLength),
+                payloadLength,
+                "Payload length must exceed the configured maximum.");
+        }
+
+        byte[] lengthPrefix = new byte[sizeof(uint)];
+        BinaryPrimitives.WriteUInt32LittleEndian(lengthPrefix, payloadLength);
+        await _workerStream.WriteAsync(lengthPrefix, cancellationToken).ConfigureAwait(false);
+    }
+
+    public async ValueTask DisposeWorkerSideAsync()
+    {
+        if (_workerSideDisposed)
+        {
+            return;
+        }
+
+        await _workerStream.DisposeAsync().ConfigureAwait(false);
+        _workerSideDisposed = true;
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        await DisposeWorkerSideAsync().ConfigureAwait(false);
+        if (_gatewayStream is not null)
+        {
+            await _gatewayStream.DisposeAsync().ConfigureAwait(false);
+        }
+    }
+
+    private WorkerEnvelope CreateEnvelope(
+        string correlationId,
+        Action<WorkerEnvelope> setBody)
+    {
+        WorkerEnvelope envelope = new()
+        {
+            ProtocolVersion = _frameOptions.ProtocolVersion,
+            SessionId = SessionId,
+            Sequence = AdvanceSequence(),
+            CorrelationId = correlationId,
+        };
+        setBody(envelope);
+
+        return envelope;
+    }
+
+    private ulong AdvanceSequence()
+    {
+        return ++NextWorkerSequence;
+    }
+
+    private static NamedPipeClientStream CreateWorkerStream(string pipeName)
+    {
+        return new NamedPipeClientStream(
+            ".",
+            pipeName,
+            PipeDirection.InOut,
+            PipeOptions.Asynchronous);
+    }
+}