natsdotnet/tests/NATS.Server.Tests/Gateways/GatewayConnectionTests.cs

using System.Net;
using System.Net.Sockets;
using System.Text;
using Microsoft.Extensions.Logging.Abstractions;
using NATS.Client.Core;
using NATS.Server.Configuration;
using NATS.Server.Gateways;
using NATS.Server.Subscriptions;

namespace NATS.Server.Tests.Gateways;

/// <summary>
/// Gateway connection establishment, handshake, lifecycle, and reconnection tests.
/// Ported from golang/nats-server/server/gateway_test.go.
/// </summary>
public class GatewayConnectionTests
{
    // ── Handshake and Connection Establishment ──────────────────────────

    // Go: TestGatewayBasic server/gateway_test.go:399
    [Fact]
    public async Task Gateway_outbound_handshake_sets_remote_id()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;

        using var clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await clientSocket.ConnectAsync(IPAddress.Loopback, port);
        using var serverSocket = await listener.AcceptSocketAsync();

        await using var gw = new GatewayConnection(serverSocket);
        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshake = gw.PerformOutboundHandshakeAsync("LOCAL-SERVER", cts.Token);
        var line = await ReadLineAsync(clientSocket, cts.Token);
        line.ShouldBe("GATEWAY LOCAL-SERVER");
        await WriteLineAsync(clientSocket, "GATEWAY REMOTE-SERVER", cts.Token);
        await handshake;

        gw.RemoteId.ShouldBe("REMOTE-SERVER");
    }

    // Go: TestGatewayBasic server/gateway_test.go:399
    [Fact]
    public async Task Gateway_inbound_handshake_sets_remote_id()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;

        using var clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await clientSocket.ConnectAsync(IPAddress.Loopback, port);
        using var serverSocket = await listener.AcceptSocketAsync();

        await using var gw = new GatewayConnection(serverSocket);
        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshake = gw.PerformInboundHandshakeAsync("LOCAL-SERVER", cts.Token);
        await WriteLineAsync(clientSocket, "GATEWAY REMOTE-CLIENT", cts.Token);
        var line = await ReadLineAsync(clientSocket, cts.Token);
        line.ShouldBe("GATEWAY LOCAL-SERVER");
        await handshake;

        gw.RemoteId.ShouldBe("REMOTE-CLIENT");
    }

    // Go: TestGatewayBasic server/gateway_test.go:399
    [Fact]
    public async Task Gateway_handshake_rejects_invalid_protocol()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;

        using var clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await clientSocket.ConnectAsync(IPAddress.Loopback, port);
        using var serverSocket = await listener.AcceptSocketAsync();

        await using var gw = new GatewayConnection(serverSocket);
        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshake = gw.PerformInboundHandshakeAsync("LOCAL", cts.Token);
        await WriteLineAsync(clientSocket, "INVALID protocol", cts.Token);

        await Should.ThrowAsync<InvalidOperationException>(async () => await handshake);
    }

    // Go: TestGatewayBasic server/gateway_test.go:399
    [Fact]
    public async Task Gateway_handshake_rejects_empty_id()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;

        using var clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await clientSocket.ConnectAsync(IPAddress.Loopback, port);
        using var serverSocket = await listener.AcceptSocketAsync();

        await using var gw = new GatewayConnection(serverSocket);
        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshake = gw.PerformInboundHandshakeAsync("LOCAL", cts.Token);
        await WriteLineAsync(clientSocket, "GATEWAY ", cts.Token);

        await Should.ThrowAsync<InvalidOperationException>(async () => await handshake);
    }

    // Go: TestGatewayBasic server/gateway_test.go:399
    [Fact]
    public async Task Two_clusters_establish_gateway_connections()
    {
        await using var fixture = await GatewayConnectionFixture.StartAsync();

        fixture.Local.Stats.Gateways.ShouldBeGreaterThan(0);
        fixture.Remote.Stats.Gateways.ShouldBeGreaterThan(0);
    }

    // Go: TestGatewayBasic server/gateway_test.go:399
    [Fact]
    public async Task Gateway_connection_count_tracked_in_stats()
    {
        await using var fixture = await GatewayConnectionFixture.StartAsync();

        fixture.Local.Stats.Gateways.ShouldBeGreaterThanOrEqualTo(1);
        fixture.Remote.Stats.Gateways.ShouldBeGreaterThanOrEqualTo(1);
    }

    // Go: TestGatewayDoesntSendBackToItself server/gateway_test.go:2150
    [Fact]
    public async Task Gateway_does_not_create_echo_cycle()
    {
        await using var fixture = await GatewayConnectionFixture.StartAsync();

        await using var remoteSub = new NatsConnection(new NatsOpts
        {
            Url = $"nats://127.0.0.1:{fixture.Remote.Port}",
        });
        await remoteSub.ConnectAsync();

        await using var localConn = new NatsConnection(new NatsOpts
        {
            Url = $"nats://127.0.0.1:{fixture.Local.Port}",
        });
        await localConn.ConnectAsync();

        await using var sub = await remoteSub.SubscribeCoreAsync<string>("cycle.test");
        await remoteSub.PingAsync();
        await fixture.WaitForRemoteInterestOnLocalAsync("cycle.test");

        await localConn.PublishAsync("cycle.test", "ping");
        await localConn.PingAsync();

        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(3));
        var msg = await sub.Msgs.ReadAsync(timeout.Token);
        msg.Data.ShouldBe("ping");

        // Verify no additional cycle messages arrive
        await Task.Delay(200);
        using var noMoreTimeout = new CancellationTokenSource(TimeSpan.FromMilliseconds(300));
        await Should.ThrowAsync<OperationCanceledException>(async () =>
            await sub.Msgs.ReadAsync(noMoreTimeout.Token));
    }

    // Go: TestGatewaySolicitShutdown server/gateway_test.go:784
    [Fact]
    public async Task Gateway_manager_shutdown_does_not_hang()
    {
        var options = new GatewayOptions
        {
            Name = "TEST",
            Host = "127.0.0.1",
            Port = 0,
            Remotes = ["127.0.0.1:19999"], // Non-existent host
        };
        var manager = new GatewayManager(
            options,
            new ServerStats(),
            "S1",
            _ => { },
            _ => { },
            NullLogger<GatewayManager>.Instance);

        await manager.StartAsync(CancellationToken.None);
        // Dispose should complete promptly even with pending reconnect attempts
        var disposeTask = manager.DisposeAsync().AsTask();
        var completed = await Task.WhenAny(disposeTask, Task.Delay(TimeSpan.FromSeconds(5)));
        completed.ShouldBe(disposeTask, "DisposeAsync should complete within timeout");
    }

    // Go: TestGatewayBasic server/gateway_test.go:399 (reconnection part)
    [Fact]
    public async Task Gateway_reconnects_after_remote_shutdown()
    {
        var localOptions = new NatsOptions
        {
            Host = "127.0.0.1",
            Port = 0,
            Gateway = new GatewayOptions
            {
                Name = "LOCAL",
                Host = "127.0.0.1",
                Port = 0,
            },
        };

        var local = new NatsServer(localOptions, NullLoggerFactory.Instance);
        var localCts = new CancellationTokenSource();
        _ = local.StartAsync(localCts.Token);
        await local.WaitForReadyAsync();

        // Start remote
        var remoteOptions = new NatsOptions
        {
            Host = "127.0.0.1",
            Port = 0,
            Gateway = new GatewayOptions
            {
                Name = "REMOTE",
                Host = "127.0.0.1",
                Port = 0,
                Remotes = [local.GatewayListen!],
            },
        };
        var remote = new NatsServer(remoteOptions, NullLoggerFactory.Instance);
        var remoteCts = new CancellationTokenSource();
        _ = remote.StartAsync(remoteCts.Token);
        await remote.WaitForReadyAsync();

        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(5));
        while (!timeout.IsCancellationRequested && (local.Stats.Gateways == 0 || remote.Stats.Gateways == 0))
            await Task.Delay(50, timeout.Token).ContinueWith(_ => { }, TaskScheduler.Default);

        local.Stats.Gateways.ShouldBeGreaterThan(0);
        remote.Stats.Gateways.ShouldBeGreaterThan(0);

        // Shutdown remote
        await remoteCts.CancelAsync();
        remote.Dispose();

        // Wait for gateway count to drop
        using var dropTimeout = new CancellationTokenSource(TimeSpan.FromSeconds(5));
        while (!dropTimeout.IsCancellationRequested && local.Stats.Gateways > 0)
            await Task.Delay(50, dropTimeout.Token).ContinueWith(_ => { }, TaskScheduler.Default);

        // Restart remote connecting to local
        var remote2Options = new NatsOptions
        {
            Host = "127.0.0.1",
            Port = 0,
            Gateway = new GatewayOptions
            {
                Name = "REMOTE2",
                Host = "127.0.0.1",
                Port = 0,
                Remotes = [local.GatewayListen!],
            },
        };
        var remote2 = new NatsServer(remote2Options, NullLoggerFactory.Instance);
        var remote2Cts = new CancellationTokenSource();
        _ = remote2.StartAsync(remote2Cts.Token);
        await remote2.WaitForReadyAsync();

        // Wait for new gateway link
        using var reconTimeout = new CancellationTokenSource(TimeSpan.FromSeconds(5));
        while (!reconTimeout.IsCancellationRequested && (local.Stats.Gateways == 0 || remote2.Stats.Gateways == 0))
            await Task.Delay(50, reconTimeout.Token).ContinueWith(_ => { }, TaskScheduler.Default);

        local.Stats.Gateways.ShouldBeGreaterThan(0);
        remote2.Stats.Gateways.ShouldBeGreaterThan(0);

        await localCts.CancelAsync();
        await remote2Cts.CancelAsync();
        local.Dispose();
        remote2.Dispose();
        localCts.Dispose();
        remote2Cts.Dispose();
        remoteCts.Dispose();
    }

    // Go: TestGatewayNoReconnectOnClose server/gateway_test.go:1735
    [Fact]
    public async Task Connection_read_loop_starts_and_processes_messages()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;

        using var clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await clientSocket.ConnectAsync(IPAddress.Loopback, port);
        using var serverSocket = await listener.AcceptSocketAsync();

        await using var gw = new GatewayConnection(serverSocket);
        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        // Perform handshake
        var handshake = gw.PerformOutboundHandshakeAsync("LOCAL", cts.Token);
        await ReadLineAsync(clientSocket, cts.Token);
        await WriteLineAsync(clientSocket, "GATEWAY REMOTE", cts.Token);
        await handshake;

        var receivedMessage = new TaskCompletionSource<GatewayMessage>();
        gw.MessageReceived = msg =>
        {
            receivedMessage.TrySetResult(msg);
            return Task.CompletedTask;
        };
        gw.StartLoop(cts.Token);

        // Send a GMSG message
        var payload = "hello-gateway"u8.ToArray();
        var line = $"GMSG $G test.subject - {payload.Length}\r\n";
        await clientSocket.SendAsync(Encoding.ASCII.GetBytes(line), SocketFlags.None, cts.Token);
        await clientSocket.SendAsync(payload, SocketFlags.None, cts.Token);
        await clientSocket.SendAsync("\r\n"u8.ToArray(), SocketFlags.None, cts.Token);

        var msg = await receivedMessage.Task.WaitAsync(cts.Token);
        msg.Subject.ShouldBe("test.subject");
        msg.ReplyTo.ShouldBeNull();
        Encoding.UTF8.GetString(msg.Payload.Span).ShouldBe("hello-gateway");
    }

    // Go: TestGatewayBasic server/gateway_test.go:399
    [Fact]
    public async Task Connection_read_loop_processes_gmsg_with_reply()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;

        using var clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await clientSocket.ConnectAsync(IPAddress.Loopback, port);
        using var serverSocket = await listener.AcceptSocketAsync();

        await using var gw = new GatewayConnection(serverSocket);
        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshake = gw.PerformOutboundHandshakeAsync("LOCAL", cts.Token);
        await ReadLineAsync(clientSocket, cts.Token);
        await WriteLineAsync(clientSocket, "GATEWAY REMOTE", cts.Token);
        await handshake;

        var receivedMessage = new TaskCompletionSource<GatewayMessage>();
        gw.MessageReceived = msg =>
        {
            receivedMessage.TrySetResult(msg);
            return Task.CompletedTask;
        };
        gw.StartLoop(cts.Token);

        var payload = "data"u8.ToArray();
        var line = $"GMSG $G test.subject _INBOX.abc {payload.Length}\r\n";
        await clientSocket.SendAsync(Encoding.ASCII.GetBytes(line), SocketFlags.None, cts.Token);
        await clientSocket.SendAsync(payload, SocketFlags.None, cts.Token);
        await clientSocket.SendAsync("\r\n"u8.ToArray(), SocketFlags.None, cts.Token);

        var msg = await receivedMessage.Task.WaitAsync(cts.Token);
        msg.Subject.ShouldBe("test.subject");
        msg.ReplyTo.ShouldBe("_INBOX.abc");
        msg.Account.ShouldBe("$G");
    }

    // Go: TestGatewayBasic server/gateway_test.go:399
    [Fact]
    public async Task Connection_read_loop_processes_account_scoped_gmsg()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;

        using var clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await clientSocket.ConnectAsync(IPAddress.Loopback, port);
        using var serverSocket = await listener.AcceptSocketAsync();

        await using var gw = new GatewayConnection(serverSocket);
        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshake = gw.PerformOutboundHandshakeAsync("LOCAL", cts.Token);
        await ReadLineAsync(clientSocket, cts.Token);
        await WriteLineAsync(clientSocket, "GATEWAY REMOTE", cts.Token);
        await handshake;

        var receivedMessage = new TaskCompletionSource<GatewayMessage>();
        gw.MessageReceived = msg =>
        {
            receivedMessage.TrySetResult(msg);
            return Task.CompletedTask;
        };
        gw.StartLoop(cts.Token);

        var payload = "msg"u8.ToArray();
        var line = $"GMSG ACCT test.subject - {payload.Length}\r\n";
        await clientSocket.SendAsync(Encoding.ASCII.GetBytes(line), SocketFlags.None, cts.Token);
        await clientSocket.SendAsync(payload, SocketFlags.None, cts.Token);
        await clientSocket.SendAsync("\r\n"u8.ToArray(), SocketFlags.None, cts.Token);

        var msg = await receivedMessage.Task.WaitAsync(cts.Token);
        msg.Account.ShouldBe("ACCT");
        msg.Subject.ShouldBe("test.subject");
    }

    // Go: TestGatewayDontSendSubInterest server/gateway_test.go:1755
    [Fact]
    public async Task Connection_read_loop_processes_aplus_interest()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;

        using var clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await clientSocket.ConnectAsync(IPAddress.Loopback, port);
        using var serverSocket = await listener.AcceptSocketAsync();

        await using var gw = new GatewayConnection(serverSocket);
        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshake = gw.PerformOutboundHandshakeAsync("LOCAL", cts.Token);
        await ReadLineAsync(clientSocket, cts.Token);
        await WriteLineAsync(clientSocket, "GATEWAY REMOTE", cts.Token);
        await handshake;

        var receivedSub = new TaskCompletionSource<RemoteSubscription>();
        gw.RemoteSubscriptionReceived = sub =>
        {
            receivedSub.TrySetResult(sub);
            return Task.CompletedTask;
        };
        gw.StartLoop(cts.Token);

        await WriteLineAsync(clientSocket, "A+ MYACC orders.>", cts.Token);

        var sub = await receivedSub.Task.WaitAsync(cts.Token);
        sub.Subject.ShouldBe("orders.>");
        sub.Account.ShouldBe("MYACC");
        sub.IsRemoval.ShouldBeFalse();
    }

    // Go: TestGatewayAccountUnsub server/gateway_test.go:1912
    [Fact]
    public async Task Connection_read_loop_processes_aminus_interest()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;

        using var clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await clientSocket.ConnectAsync(IPAddress.Loopback, port);
        using var serverSocket = await listener.AcceptSocketAsync();

        await using var gw = new GatewayConnection(serverSocket);
        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshake = gw.PerformOutboundHandshakeAsync("LOCAL", cts.Token);
        await ReadLineAsync(clientSocket, cts.Token);
        await WriteLineAsync(clientSocket, "GATEWAY REMOTE", cts.Token);
        await handshake;

        var receivedSubs = new List<RemoteSubscription>();
        var tcs = new TaskCompletionSource();
        gw.RemoteSubscriptionReceived = sub =>
        {
            receivedSubs.Add(sub);
            if (receivedSubs.Count >= 2)
                tcs.TrySetResult();
            return Task.CompletedTask;
        };
        gw.StartLoop(cts.Token);

        await WriteLineAsync(clientSocket, "A+ ACC foo.*", cts.Token);
        await WriteLineAsync(clientSocket, "A- ACC foo.*", cts.Token);

        await tcs.Task.WaitAsync(cts.Token);
        receivedSubs[0].IsRemoval.ShouldBeFalse();
        receivedSubs[1].IsRemoval.ShouldBeTrue();
    }

    // Go: TestGatewayQueueSub server/gateway_test.go:2265
    [Fact]
    public async Task Connection_read_loop_processes_aplus_with_queue()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;

        using var clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await clientSocket.ConnectAsync(IPAddress.Loopback, port);
        using var serverSocket = await listener.AcceptSocketAsync();

        await using var gw = new GatewayConnection(serverSocket);
        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshake = gw.PerformOutboundHandshakeAsync("LOCAL", cts.Token);
        await ReadLineAsync(clientSocket, cts.Token);
        await WriteLineAsync(clientSocket, "GATEWAY REMOTE", cts.Token);
        await handshake;

        var receivedSub = new TaskCompletionSource<RemoteSubscription>();
        gw.RemoteSubscriptionReceived = sub =>
        {
            receivedSub.TrySetResult(sub);
            return Task.CompletedTask;
        };
        gw.StartLoop(cts.Token);

        await WriteLineAsync(clientSocket, "A+ $G foo.bar workers", cts.Token);

        var sub = await receivedSub.Task.WaitAsync(cts.Token);
        sub.Subject.ShouldBe("foo.bar");
        sub.Queue.ShouldBe("workers");
        sub.Account.ShouldBe("$G");
    }

    // Go: TestGatewayBasic server/gateway_test.go:399
    [Fact]
    public async Task Send_message_writes_gmsg_protocol()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;

        using var clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await clientSocket.ConnectAsync(IPAddress.Loopback, port);
        using var serverSocket = await listener.AcceptSocketAsync();

        await using var gw = new GatewayConnection(serverSocket);
        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshake = gw.PerformOutboundHandshakeAsync("LOCAL", cts.Token);
        await ReadLineAsync(clientSocket, cts.Token);
        await WriteLineAsync(clientSocket, "GATEWAY REMOTE", cts.Token);
        await handshake;

        var payload = Encoding.UTF8.GetBytes("payload-data");
        await gw.SendMessageAsync("$G", "test.subject", "_INBOX.reply", payload, cts.Token);

        var buf = new byte[4096];
        var total = new StringBuilder();
        using var readCts = new CancellationTokenSource(TimeSpan.FromSeconds(2));
        while (true)
        {
            var n = await clientSocket.ReceiveAsync(buf, SocketFlags.None, readCts.Token);
            if (n == 0) break;
            total.Append(Encoding.ASCII.GetString(buf, 0, n));
            if (total.ToString().Contains("payload-data", StringComparison.Ordinal))
                break;
        }

        var received = total.ToString();
        received.ShouldContain("GMSG $G test.subject _INBOX.reply");
        received.ShouldContain("payload-data");
    }

    // Go: TestGatewayBasic server/gateway_test.go:399
    [Fact]
    public async Task Send_aplus_writes_interest_protocol()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;

        using var clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await clientSocket.ConnectAsync(IPAddress.Loopback, port);
        using var serverSocket = await listener.AcceptSocketAsync();

        await using var gw = new GatewayConnection(serverSocket);
        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshake = gw.PerformOutboundHandshakeAsync("LOCAL", cts.Token);
        await ReadLineAsync(clientSocket, cts.Token);
        await WriteLineAsync(clientSocket, "GATEWAY REMOTE", cts.Token);
        await handshake;

        await gw.SendAPlusAsync("$G", "orders.>", null, cts.Token);

        var line = await ReadLineAsync(clientSocket, cts.Token);
        line.ShouldBe("A+ $G orders.>");
    }

    // Go: TestGatewayQueueSub server/gateway_test.go:2265
    [Fact]
    public async Task Send_aplus_with_queue_writes_interest_protocol()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;

        using var clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await clientSocket.ConnectAsync(IPAddress.Loopback, port);
        using var serverSocket = await listener.AcceptSocketAsync();

        await using var gw = new GatewayConnection(serverSocket);
        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshake = gw.PerformOutboundHandshakeAsync("LOCAL", cts.Token);
        await ReadLineAsync(clientSocket, cts.Token);
        await WriteLineAsync(clientSocket, "GATEWAY REMOTE", cts.Token);
        await handshake;

        await gw.SendAPlusAsync("$G", "foo", "workers", cts.Token);

        var line = await ReadLineAsync(clientSocket, cts.Token);
        line.ShouldBe("A+ $G foo workers");
    }

    // Go: TestGatewayAccountUnsub server/gateway_test.go:1912
    [Fact]
    public async Task Send_aminus_writes_unsubscribe_interest_protocol()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;

        using var clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await clientSocket.ConnectAsync(IPAddress.Loopback, port);
        using var serverSocket = await listener.AcceptSocketAsync();

        await using var gw = new GatewayConnection(serverSocket);
        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshake = gw.PerformOutboundHandshakeAsync("LOCAL", cts.Token);
        await ReadLineAsync(clientSocket, cts.Token);
        await WriteLineAsync(clientSocket, "GATEWAY REMOTE", cts.Token);
        await handshake;

        await gw.SendAMinusAsync("$G", "orders.>", null, cts.Token);

        var line = await ReadLineAsync(clientSocket, cts.Token);
        line.ShouldBe("A- $G orders.>");
    }

    // Go: TestGatewayBasic server/gateway_test.go:399
    [Fact]
    public async Task Send_message_with_no_reply_uses_dash()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;

        using var clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await clientSocket.ConnectAsync(IPAddress.Loopback, port);
        using var serverSocket = await listener.AcceptSocketAsync();

        await using var gw = new GatewayConnection(serverSocket);
        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshake = gw.PerformOutboundHandshakeAsync("LOCAL", cts.Token);
        await ReadLineAsync(clientSocket, cts.Token);
        await WriteLineAsync(clientSocket, "GATEWAY REMOTE", cts.Token);
        await handshake;

        await gw.SendMessageAsync("$G", "test.subject", null, new byte[] { 0x41 }, cts.Token);

        var buf = new byte[4096];
        var total = new StringBuilder();
        using var readCts = new CancellationTokenSource(TimeSpan.FromSeconds(2));
        while (true)
        {
            var n = await clientSocket.ReceiveAsync(buf, SocketFlags.None, readCts.Token);
            if (n == 0) break;
            total.Append(Encoding.ASCII.GetString(buf, 0, n));
            if (total.ToString().Contains("\r\n", StringComparison.Ordinal) && total.Length > 20)
                break;
        }

        total.ToString().ShouldContain("GMSG $G test.subject - 1");
    }

    // Go: TestGatewayBasic server/gateway_test.go:399
    [Fact]
    public async Task Send_message_with_empty_payload()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;

        using var clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await clientSocket.ConnectAsync(IPAddress.Loopback, port);
        using var serverSocket = await listener.AcceptSocketAsync();

        await using var gw = new GatewayConnection(serverSocket);
        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshake = gw.PerformOutboundHandshakeAsync("LOCAL", cts.Token);
        await ReadLineAsync(clientSocket, cts.Token);
        await WriteLineAsync(clientSocket, "GATEWAY REMOTE", cts.Token);
        await handshake;

        await gw.SendMessageAsync("$G", "test.empty", null, ReadOnlyMemory<byte>.Empty, cts.Token);

        var buf = new byte[4096];
        var total = new StringBuilder();
        using var readCts = new CancellationTokenSource(TimeSpan.FromSeconds(2));
        while (true)
        {
            var n = await clientSocket.ReceiveAsync(buf, SocketFlags.None, readCts.Token);
            if (n == 0) break;
            total.Append(Encoding.ASCII.GetString(buf, 0, n));
            if (total.ToString().Contains("GMSG", StringComparison.Ordinal))
                break;
        }

        total.ToString().ShouldContain("GMSG $G test.empty - 0");
    }

    // Go: TestGatewayBasic server/gateway_test.go:399
    [Fact]
    public async Task Connection_dispose_cleans_up_gracefully()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;

        using var clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await clientSocket.ConnectAsync(IPAddress.Loopback, port);
        using var serverSocket = await listener.AcceptSocketAsync();

        var gw = new GatewayConnection(serverSocket);
        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshake = gw.PerformOutboundHandshakeAsync("LOCAL", cts.Token);
        await ReadLineAsync(clientSocket, cts.Token);
        await WriteLineAsync(clientSocket, "GATEWAY REMOTE", cts.Token);
        await handshake;

        gw.StartLoop(cts.Token);
        await gw.DisposeAsync(); // Should not throw

        // Verify the connection is no longer usable after dispose
        gw.RemoteId.ShouldBe("REMOTE");
    }

    // Go: TestGatewayBasic server/gateway_test.go:399
    [Fact]
    public async Task Multiple_concurrent_sends_are_serialized()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;

        using var clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await clientSocket.ConnectAsync(IPAddress.Loopback, port);
        using var serverSocket = await listener.AcceptSocketAsync();

        await using var gw = new GatewayConnection(serverSocket);
        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshake = gw.PerformOutboundHandshakeAsync("LOCAL", cts.Token);
        await ReadLineAsync(clientSocket, cts.Token);
        await WriteLineAsync(clientSocket, "GATEWAY REMOTE", cts.Token);
        await handshake;

        // Fire off concurrent sends
        var tasks = new List<Task>();
        for (int i = 0; i < 10; i++)
        {
            var idx = i;
            tasks.Add(gw.SendMessageAsync("$G", $"sub.{idx}", null, Encoding.UTF8.GetBytes($"msg-{idx}"), cts.Token));
        }

        await Task.WhenAll(tasks);

        // Drain all data from socket
        var buf = new byte[8192];
        var total = new StringBuilder();
        using var readCts = new CancellationTokenSource(TimeSpan.FromSeconds(2));
        while (true)
        {
            try
            {
                var n = await clientSocket.ReceiveAsync(buf, SocketFlags.None, readCts.Token);
                if (n == 0) break;
                total.Append(Encoding.ASCII.GetString(buf, 0, n));
            }
            catch (OperationCanceledException)
            {
                break;
            }
        }

        // All 10 messages should be present
        var received = total.ToString();
        for (int i = 0; i < 10; i++)
        {
            received.ShouldContain($"sub.{i}");
        }
    }

    // ── Helpers ─────────────────────────────────────────────────────────

    private static async Task<string> ReadLineAsync(Socket socket, CancellationToken ct)
    {
        var bytes = new List<byte>(64);
        var single = new byte[1];
        while (true)
        {
            var read = await socket.ReceiveAsync(single, SocketFlags.None, ct);
            if (read == 0)
                break;
            if (single[0] == (byte)'\n')
                break;
            if (single[0] != (byte)'\r')
                bytes.Add(single[0]);
        }

        return Encoding.ASCII.GetString([.. bytes]);
    }

    private static Task WriteLineAsync(Socket socket, string line, CancellationToken ct)
        => socket.SendAsync(Encoding.ASCII.GetBytes($"{line}\r\n"), SocketFlags.None, ct).AsTask();
}

/// <summary>
/// Shared fixture for gateway connection tests that need two running server clusters.
/// </summary>
internal sealed class GatewayConnectionFixture : IAsyncDisposable
{
    private readonly CancellationTokenSource _localCts;
    private readonly CancellationTokenSource _remoteCts;

    private GatewayConnectionFixture(NatsServer local, NatsServer remote, CancellationTokenSource localCts, CancellationTokenSource remoteCts)
    {
        Local = local;
        Remote = remote;
        _localCts = localCts;
        _remoteCts = remoteCts;
    }

    public NatsServer Local { get; }
    public NatsServer Remote { get; }

    public static async Task<GatewayConnectionFixture> StartAsync()
    {
        var localOptions = new NatsOptions
        {
            Host = "127.0.0.1",
            Port = 0,
            Gateway = new GatewayOptions
            {
                Name = "LOCAL",
                Host = "127.0.0.1",
                Port = 0,
            },
        };

        var local = new NatsServer(localOptions, NullLoggerFactory.Instance);
        var localCts = new CancellationTokenSource();
        _ = local.StartAsync(localCts.Token);
        await local.WaitForReadyAsync();

        var remoteOptions = new NatsOptions
        {
            Host = "127.0.0.1",
            Port = 0,
            Gateway = new GatewayOptions
            {
                Name = "REMOTE",
                Host = "127.0.0.1",
                Port = 0,
                Remotes = [local.GatewayListen!],
            },
        };

        var remote = new NatsServer(remoteOptions, NullLoggerFactory.Instance);
        var remoteCts = new CancellationTokenSource();
        _ = remote.StartAsync(remoteCts.Token);
        await remote.WaitForReadyAsync();

        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(5));
        while (!timeout.IsCancellationRequested && (local.Stats.Gateways == 0 || remote.Stats.Gateways == 0))
            await Task.Delay(50, timeout.Token).ContinueWith(_ => { }, TaskScheduler.Default);

        return new GatewayConnectionFixture(local, remote, localCts, remoteCts);
    }

    public async Task WaitForRemoteInterestOnLocalAsync(string subject)
    {
        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(5));
        while (!timeout.IsCancellationRequested)
        {
            if (Local.HasRemoteInterest(subject))
                return;
            await Task.Delay(50, timeout.Token).ContinueWith(_ => { }, TaskScheduler.Default);
        }

        throw new TimeoutException($"Timed out waiting for remote interest on '{subject}'.");
    }

    public async ValueTask DisposeAsync()
    {
        await _localCts.CancelAsync();
        await _remoteCts.CancelAsync();
        Local.Dispose();
        Remote.Dispose();
        _localCts.Dispose();
        _remoteCts.Dispose();
    }
}