natsdotnet/tests/NATS.Server.Tests/LeafNodes/LeafNodeConnectionTests.cs

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

namespace NATS.Server.Tests.LeafNodes;

/// <summary>
/// Tests for leaf node connection establishment, authentication, and lifecycle.
/// Reference: golang/nats-server/server/leafnode_test.go
/// </summary>
public class LeafNodeConnectionTests
{
    // Go: TestLeafNodeBasicAuthSingleton server/leafnode_test.go:602
    [Fact]
    public async Task Leaf_node_connects_with_basic_hub_spoke_setup()
    {
        await using var fixture = await LeafFixture.StartAsync();
        fixture.Hub.Stats.Leafs.ShouldBeGreaterThan(0);
        fixture.Spoke.Stats.Leafs.ShouldBeGreaterThan(0);
    }

    // Go: TestLeafNodesBasicTokenAuth server/leafnode_test.go:10862
    [Fact]
    public async Task Leaf_node_connects_with_token_auth_on_hub()
    {
        var hubOptions = new NatsOptions
        {
            Host = "127.0.0.1",
            Port = 0,
            Authorization = "secret-token",
            LeafNode = new LeafNodeOptions { Host = "127.0.0.1", Port = 0 },
        };

        var hub = new NatsServer(hubOptions, NullLoggerFactory.Instance);
        var hubCts = new CancellationTokenSource();
        _ = hub.StartAsync(hubCts.Token);
        await hub.WaitForReadyAsync();

        var spokeOptions = new NatsOptions
        {
            Host = "127.0.0.1",
            Port = 0,
            LeafNode = new LeafNodeOptions
            {
                Host = "127.0.0.1",
                Port = 0,
                Remotes = [hub.LeafListen!],
            },
        };

        var spoke = new NatsServer(spokeOptions, NullLoggerFactory.Instance);
        var spokeCts = new CancellationTokenSource();
        _ = spoke.StartAsync(spokeCts.Token);
        await spoke.WaitForReadyAsync();

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

        hub.Stats.Leafs.ShouldBeGreaterThan(0);
        spoke.Stats.Leafs.ShouldBeGreaterThan(0);

        await spokeCts.CancelAsync();
        await hubCts.CancelAsync();
        spoke.Dispose();
        hub.Dispose();
        spokeCts.Dispose();
        hubCts.Dispose();
    }

    // Go: TestLeafNodeBasicAuthSingleton server/leafnode_test.go:602
    [Fact]
    public async Task Leaf_node_connects_with_user_password_auth()
    {
        var users = new User[] { new() { Username = "leafuser", Password = "leafpass" } };

        var hubOptions = new NatsOptions
        {
            Host = "127.0.0.1", Port = 0, Users = users,
            LeafNode = new LeafNodeOptions { Host = "127.0.0.1", Port = 0 },
        };

        var hub = new NatsServer(hubOptions, NullLoggerFactory.Instance);
        var hubCts = new CancellationTokenSource();
        _ = hub.StartAsync(hubCts.Token);
        await hub.WaitForReadyAsync();

        var spokeOptions = new NatsOptions
        {
            Host = "127.0.0.1", Port = 0,
            LeafNode = new LeafNodeOptions { Host = "127.0.0.1", Port = 0, Remotes = [hub.LeafListen!] },
        };

        var spoke = new NatsServer(spokeOptions, NullLoggerFactory.Instance);
        var spokeCts = new CancellationTokenSource();
        _ = spoke.StartAsync(spokeCts.Token);
        await spoke.WaitForReadyAsync();

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

        hub.Stats.Leafs.ShouldBeGreaterThan(0);

        await spokeCts.CancelAsync();
        await hubCts.CancelAsync();
        spoke.Dispose();
        hub.Dispose();
        spokeCts.Dispose();
        hubCts.Dispose();
    }

    // Go: TestLeafNodeRTT server/leafnode_test.go:488
    [Fact]
    public async Task Hub_and_spoke_both_report_leaf_connection_count()
    {
        await using var fixture = await LeafFixture.StartAsync();
        Interlocked.Read(ref fixture.Hub.Stats.Leafs).ShouldBe(1);
        Interlocked.Read(ref fixture.Spoke.Stats.Leafs).ShouldBe(1);
    }

    // Go: TestLeafNodeTwoRemotesToSameHubAccount server/leafnode_test.go:8758
    [Fact]
    public async Task Two_spoke_servers_can_connect_to_same_hub()
    {
        await using var fixture = await TwoSpokeFixture.StartAsync();
        Interlocked.Read(ref fixture.Hub.Stats.Leafs).ShouldBeGreaterThanOrEqualTo(2);
        Interlocked.Read(ref fixture.Spoke1.Stats.Leafs).ShouldBeGreaterThan(0);
        Interlocked.Read(ref fixture.Spoke2.Stats.Leafs).ShouldBeGreaterThan(0);
    }

    // Go: TestLeafNodeRemoteWrongPort server/leafnode_test.go:1095
    [Fact]
    public async Task Outbound_handshake_completes_between_raw_sockets()
    {
        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 acceptedSocket = await listener.AcceptSocketAsync();

        await using var leaf = new LeafConnection(acceptedSocket);
        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshakeTask = leaf.PerformOutboundHandshakeAsync("LOCAL", timeout.Token);
        (await ReadLineAsync(clientSocket, timeout.Token)).ShouldBe("LEAF LOCAL");
        await WriteLineAsync(clientSocket, "LEAF REMOTE", timeout.Token);
        await handshakeTask;

        leaf.RemoteId.ShouldBe("REMOTE");
    }

    // Go: TestLeafNodeCloseTLSConnection server/leafnode_test.go:968
    [Fact]
    public async Task Inbound_handshake_completes_between_raw_sockets()
    {
        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 acceptedSocket = await listener.AcceptSocketAsync();

        await using var leaf = new LeafConnection(acceptedSocket);
        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshakeTask = leaf.PerformInboundHandshakeAsync("SERVER", timeout.Token);
        await WriteLineAsync(clientSocket, "LEAF REMOTE_CLIENT", timeout.Token);
        (await ReadLineAsync(clientSocket, timeout.Token)).ShouldBe("LEAF SERVER");
        await handshakeTask;

        leaf.RemoteId.ShouldBe("REMOTE_CLIENT");
    }

    // Go: TestLeafNodeNoPingBeforeConnect server/leafnode_test.go:3713
    [Fact]
    public async Task Leaf_connection_disposes_cleanly_without_starting_loop()
    {
        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 acceptedSocket = await listener.AcceptSocketAsync();

        var leaf = new LeafConnection(acceptedSocket);
        await leaf.DisposeAsync();

        var buffer = new byte[1];
        var read = await clientSocket.ReceiveAsync(buffer, SocketFlags.None);
        read.ShouldBe(0);
    }

    // Go: TestLeafNodeBannerNoClusterNameIfNoCluster server/leafnode_test.go:9803
    [Fact]
    public async Task Leaf_connection_sends_LS_plus_and_LS_minus()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;
        using var remoteSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await remoteSocket.ConnectAsync(IPAddress.Loopback, port);
        using var leafSocket = await listener.AcceptSocketAsync();

        await using var leaf = new LeafConnection(leafSocket);
        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshakeTask = leaf.PerformOutboundHandshakeAsync("LOCAL", timeout.Token);
        (await ReadLineAsync(remoteSocket, timeout.Token)).ShouldBe("LEAF LOCAL");
        await WriteLineAsync(remoteSocket, "LEAF REMOTE", timeout.Token);
        await handshakeTask;

        await leaf.SendLsPlusAsync("$G", "foo.bar", null, timeout.Token);
        (await ReadLineAsync(remoteSocket, timeout.Token)).ShouldBe("LS+ $G foo.bar");

        await leaf.SendLsPlusAsync("$G", "foo.baz", "queue1", timeout.Token);
        (await ReadLineAsync(remoteSocket, timeout.Token)).ShouldBe("LS+ $G foo.baz queue1");

        await leaf.SendLsMinusAsync("$G", "foo.bar", null, timeout.Token);
        (await ReadLineAsync(remoteSocket, timeout.Token)).ShouldBe("LS- $G foo.bar");
    }

    // Go: TestLeafNodeLMsgSplit server/leafnode_test.go:2387
    [Fact]
    public async Task Leaf_connection_sends_LMSG()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;
        using var remoteSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await remoteSocket.ConnectAsync(IPAddress.Loopback, port);
        using var leafSocket = await listener.AcceptSocketAsync();

        await using var leaf = new LeafConnection(leafSocket);
        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshakeTask = leaf.PerformOutboundHandshakeAsync("LOCAL", timeout.Token);
        (await ReadLineAsync(remoteSocket, timeout.Token)).ShouldBe("LEAF LOCAL");
        await WriteLineAsync(remoteSocket, "LEAF REMOTE", timeout.Token);
        await handshakeTask;

        var payload = "hello world"u8.ToArray();
        await leaf.SendMessageAsync("$G", "test.subject", "reply-to", payload, timeout.Token);

        var controlLine = await ReadLineAsync(remoteSocket, timeout.Token);
        controlLine.ShouldBe($"LMSG $G test.subject reply-to {payload.Length}");
    }

    // Go: TestLeafNodeLMsgSplit server/leafnode_test.go:2387
    [Fact]
    public async Task Leaf_connection_sends_LMSG_with_no_reply()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;
        using var remoteSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await remoteSocket.ConnectAsync(IPAddress.Loopback, port);
        using var leafSocket = await listener.AcceptSocketAsync();

        await using var leaf = new LeafConnection(leafSocket);
        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshakeTask = leaf.PerformOutboundHandshakeAsync("LOCAL", timeout.Token);
        (await ReadLineAsync(remoteSocket, timeout.Token)).ShouldBe("LEAF LOCAL");
        await WriteLineAsync(remoteSocket, "LEAF REMOTE", timeout.Token);
        await handshakeTask;

        var payload = "test"u8.ToArray();
        await leaf.SendMessageAsync("ACCT", "subject", null, payload, timeout.Token);

        var controlLine = await ReadLineAsync(remoteSocket, timeout.Token);
        controlLine.ShouldBe($"LMSG ACCT subject - {payload.Length}");
    }

    // Go: TestLeafNodeLMsgSplit server/leafnode_test.go:2387
    [Fact]
    public async Task Leaf_connection_sends_LMSG_with_empty_payload()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;
        using var remoteSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await remoteSocket.ConnectAsync(IPAddress.Loopback, port);
        using var leafSocket = await listener.AcceptSocketAsync();

        await using var leaf = new LeafConnection(leafSocket);
        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshakeTask = leaf.PerformOutboundHandshakeAsync("LOCAL", timeout.Token);
        (await ReadLineAsync(remoteSocket, timeout.Token)).ShouldBe("LEAF LOCAL");
        await WriteLineAsync(remoteSocket, "LEAF REMOTE", timeout.Token);
        await handshakeTask;

        await leaf.SendMessageAsync("$G", "empty.msg", null, ReadOnlyMemory<byte>.Empty, timeout.Token);
        var controlLine = await ReadLineAsync(remoteSocket, timeout.Token);
        controlLine.ShouldBe("LMSG $G empty.msg - 0");
    }

    // Go: TestLeafNodeTmpClients server/leafnode_test.go:1663
    [Fact]
    public async Task Leaf_connection_receives_LS_plus_and_triggers_callback()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;
        using var remoteSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await remoteSocket.ConnectAsync(IPAddress.Loopback, port);
        using var leafSocket = await listener.AcceptSocketAsync();

        await using var leaf = new LeafConnection(leafSocket);
        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshakeTask = leaf.PerformOutboundHandshakeAsync("LOCAL", timeout.Token);
        (await ReadLineAsync(remoteSocket, timeout.Token)).ShouldBe("LEAF LOCAL");
        await WriteLineAsync(remoteSocket, "LEAF REMOTE", timeout.Token);
        await handshakeTask;

        var received = new List<RemoteSubscription>();
        leaf.RemoteSubscriptionReceived = sub => { received.Add(sub); return Task.CompletedTask; };
        leaf.StartLoop(timeout.Token);

        await WriteLineAsync(remoteSocket, "LS+ $G orders.>", timeout.Token);
        await WaitForAsync(() => received.Count >= 1, timeout.Token);

        received[0].Subject.ShouldBe("orders.>");
        received[0].Account.ShouldBe("$G");
        received[0].IsRemoval.ShouldBeFalse();
    }

    // Go: TestLeafNodeRouteParseLSUnsub server/leafnode_test.go:2486
    [Fact]
    public async Task Leaf_connection_receives_LS_minus_and_triggers_removal()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;
        using var remoteSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await remoteSocket.ConnectAsync(IPAddress.Loopback, port);
        using var leafSocket = await listener.AcceptSocketAsync();

        await using var leaf = new LeafConnection(leafSocket);
        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshakeTask = leaf.PerformOutboundHandshakeAsync("LOCAL", timeout.Token);
        (await ReadLineAsync(remoteSocket, timeout.Token)).ShouldBe("LEAF LOCAL");
        await WriteLineAsync(remoteSocket, "LEAF REMOTE", timeout.Token);
        await handshakeTask;

        var received = new List<RemoteSubscription>();
        leaf.RemoteSubscriptionReceived = sub => { received.Add(sub); return Task.CompletedTask; };
        leaf.StartLoop(timeout.Token);

        await WriteLineAsync(remoteSocket, "LS+ $G foo.bar", timeout.Token);
        await WaitForAsync(() => received.Count >= 1, timeout.Token);

        await WriteLineAsync(remoteSocket, "LS- $G foo.bar", timeout.Token);
        await WaitForAsync(() => received.Count >= 2, timeout.Token);

        received[1].Subject.ShouldBe("foo.bar");
        received[1].IsRemoval.ShouldBeTrue();
    }

    // Go: TestLeafNodeLMsgSplit server/leafnode_test.go:2387
    [Fact]
    public async Task Leaf_connection_receives_LMSG_and_triggers_message_callback()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;
        using var remoteSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await remoteSocket.ConnectAsync(IPAddress.Loopback, port);
        using var leafSocket = await listener.AcceptSocketAsync();

        await using var leaf = new LeafConnection(leafSocket);
        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshakeTask = leaf.PerformOutboundHandshakeAsync("LOCAL", timeout.Token);
        (await ReadLineAsync(remoteSocket, timeout.Token)).ShouldBe("LEAF LOCAL");
        await WriteLineAsync(remoteSocket, "LEAF REMOTE", timeout.Token);
        await handshakeTask;

        var messages = new List<LeafMessage>();
        leaf.MessageReceived = msg => { messages.Add(msg); return Task.CompletedTask; };
        leaf.StartLoop(timeout.Token);

        var payload = "hello from remote"u8.ToArray();
        await WriteLineAsync(remoteSocket, $"LMSG $G test.subject reply-to {payload.Length}", timeout.Token);
        await remoteSocket.SendAsync(payload, SocketFlags.None, timeout.Token);
        await remoteSocket.SendAsync("\r\n"u8.ToArray(), SocketFlags.None, timeout.Token);

        await WaitForAsync(() => messages.Count >= 1, timeout.Token);

        messages[0].Subject.ShouldBe("test.subject");
        messages[0].ReplyTo.ShouldBe("reply-to");
        messages[0].Account.ShouldBe("$G");
        Encoding.ASCII.GetString(messages[0].Payload.Span).ShouldBe("hello from remote");
    }

    // Go: TestLeafNodeLMsgSplit server/leafnode_test.go:2387
    [Fact]
    public async Task Leaf_connection_receives_LMSG_with_account_scoped_format()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;
        using var remoteSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await remoteSocket.ConnectAsync(IPAddress.Loopback, port);
        using var leafSocket = await listener.AcceptSocketAsync();

        await using var leaf = new LeafConnection(leafSocket);
        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshakeTask = leaf.PerformOutboundHandshakeAsync("LOCAL", timeout.Token);
        (await ReadLineAsync(remoteSocket, timeout.Token)).ShouldBe("LEAF LOCAL");
        await WriteLineAsync(remoteSocket, "LEAF REMOTE", timeout.Token);
        await handshakeTask;

        var messages = new List<LeafMessage>();
        leaf.MessageReceived = msg => { messages.Add(msg); return Task.CompletedTask; };
        leaf.StartLoop(timeout.Token);

        var payload = "acct"u8.ToArray();
        await WriteLineAsync(remoteSocket, $"LMSG MYACCT test.subject - {payload.Length}", timeout.Token);
        await remoteSocket.SendAsync(payload, SocketFlags.None, timeout.Token);
        await remoteSocket.SendAsync("\r\n"u8.ToArray(), SocketFlags.None, timeout.Token);

        await WaitForAsync(() => messages.Count >= 1, timeout.Token);

        messages[0].Account.ShouldBe("MYACCT");
        messages[0].Subject.ShouldBe("test.subject");
        messages[0].ReplyTo.ShouldBeNull();
    }

    // Go: TestLeafNodeTwoRemotesToSameHubAccount server/leafnode_test.go:2210
    [Fact]
    public async Task Leaf_connection_receives_LS_plus_with_queue()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;
        using var remoteSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await remoteSocket.ConnectAsync(IPAddress.Loopback, port);
        using var leafSocket = await listener.AcceptSocketAsync();

        await using var leaf = new LeafConnection(leafSocket);
        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshakeTask = leaf.PerformOutboundHandshakeAsync("LOCAL", timeout.Token);
        (await ReadLineAsync(remoteSocket, timeout.Token)).ShouldBe("LEAF LOCAL");
        await WriteLineAsync(remoteSocket, "LEAF REMOTE", timeout.Token);
        await handshakeTask;

        var received = new List<RemoteSubscription>();
        leaf.RemoteSubscriptionReceived = sub => { received.Add(sub); return Task.CompletedTask; };
        leaf.StartLoop(timeout.Token);

        await WriteLineAsync(remoteSocket, "LS+ $G work.> workers", timeout.Token);
        await WaitForAsync(() => received.Count >= 1, timeout.Token);

        received[0].Subject.ShouldBe("work.>");
        received[0].Queue.ShouldBe("workers");
        received[0].Account.ShouldBe("$G");
    }

    // Go: TestLeafNodeSlowConsumer server/leafnode_test.go:9103
    [Fact]
    public async Task Leaf_connection_handles_multiple_rapid_LMSG_messages()
    {
        using var listener = new TcpListener(IPAddress.Loopback, 0);
        listener.Start();
        var port = ((IPEndPoint)listener.LocalEndpoint).Port;
        using var remoteSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        await remoteSocket.ConnectAsync(IPAddress.Loopback, port);
        using var leafSocket = await listener.AcceptSocketAsync();

        await using var leaf = new LeafConnection(leafSocket);
        using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(5));

        var handshakeTask = leaf.PerformOutboundHandshakeAsync("LOCAL", timeout.Token);
        (await ReadLineAsync(remoteSocket, timeout.Token)).ShouldBe("LEAF LOCAL");
        await WriteLineAsync(remoteSocket, "LEAF REMOTE", timeout.Token);
        await handshakeTask;

        var messageCount = 0;
        leaf.MessageReceived = _ => { Interlocked.Increment(ref messageCount); return Task.CompletedTask; };
        leaf.StartLoop(timeout.Token);

        const int numMessages = 20;
        for (var i = 0; i < numMessages; i++)
        {
            var payload = Encoding.ASCII.GetBytes($"msg-{i}");
            var line = $"LMSG $G test.multi - {payload.Length}\r\n";
            await remoteSocket.SendAsync(Encoding.ASCII.GetBytes(line), SocketFlags.None, timeout.Token);
            await remoteSocket.SendAsync(payload, SocketFlags.None, timeout.Token);
            await remoteSocket.SendAsync("\r\n"u8.ToArray(), SocketFlags.None, timeout.Token);
        }

        await WaitForAsync(() => Volatile.Read(ref messageCount) >= numMessages, timeout.Token);
        Volatile.Read(ref messageCount).ShouldBe(numMessages);
    }

    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();

    private static async Task WaitForAsync(Func<bool> predicate, CancellationToken ct)
    {
        while (!ct.IsCancellationRequested)
        {
            if (predicate()) return;
            await Task.Delay(20, ct);
        }

        throw new TimeoutException("Timed out waiting for condition.");
    }
}