feat: add InternalEventSystem with Channel-based send/receive loops

src/NATS.Server/Events/InternalEventSystem.cs

@@ -0,0 +1,238 @@
+using System.Collections.Concurrent;
+using System.Security.Cryptography;
+using System.Text;
+using System.Text.Json;
+using System.Threading.Channels;
+using Microsoft.Extensions.Logging;
+using NATS.Server.Auth;
+using NATS.Server.Subscriptions;
+
+namespace NATS.Server.Events;
+
+/// <summary>
+/// Internal publish message queued for the send loop.
+/// </summary>
+public sealed class PublishMessage
+{
+    public InternalClient? Client { get; init; }
+    public required string Subject { get; init; }
+    public string? Reply { get; init; }
+    public byte[]? Headers { get; init; }
+    public object? Body { get; init; }
+    public bool Echo { get; init; }
+    public bool IsLast { get; init; }
+}
+
+/// <summary>
+/// Internal received message queued for the receive loop.
+/// </summary>
+public sealed class InternalSystemMessage
+{
+    public required Subscription? Sub { get; init; }
+    public required INatsClient? Client { get; init; }
+    public required Account? Account { get; init; }
+    public required string Subject { get; init; }
+    public required string? Reply { get; init; }
+    public required ReadOnlyMemory<byte> Headers { get; init; }
+    public required ReadOnlyMemory<byte> Message { get; init; }
+    public required SystemMessageHandler Callback { get; init; }
+}
+
+/// <summary>
+/// Manages the server's internal event system with Channel-based send/receive loops.
+/// Maps to Go's internal struct in events.go:124-147 and the goroutines
+/// internalSendLoop (events.go:495) and internalReceiveLoop (events.go:476).
+/// </summary>
+public sealed class InternalEventSystem : IAsyncDisposable
+{
+    private readonly ILogger _logger;
+    private readonly Channel<PublishMessage> _sendQueue;
+    private readonly Channel<InternalSystemMessage> _receiveQueue;
+    private readonly Channel<InternalSystemMessage> _receiveQueuePings;
+    private readonly CancellationTokenSource _cts = new();
+
+    private Task? _sendLoop;
+    private Task? _receiveLoop;
+    private Task? _receiveLoopPings;
+    private NatsServer? _server;
+
+    private ulong _sequence;
+    private int _subscriptionId;
+
+    public Account SystemAccount { get; }
+    public InternalClient SystemClient { get; }
+    public string ServerHash { get; }
+
+    public InternalEventSystem(Account systemAccount, InternalClient systemClient, string serverName, ILogger logger)
+    {
+        _logger = logger;
+        SystemAccount = systemAccount;
+        SystemClient = systemClient;
+
+        // Hash server name for inbox routing (matches Go's shash)
+        ServerHash = Convert.ToHexString(SHA256.HashData(Encoding.UTF8.GetBytes(serverName)))[..8].ToLowerInvariant();
+
+        _sendQueue = Channel.CreateUnbounded<PublishMessage>(new UnboundedChannelOptions { SingleReader = true });
+        _receiveQueue = Channel.CreateUnbounded<InternalSystemMessage>(new UnboundedChannelOptions { SingleReader = true });
+        _receiveQueuePings = Channel.CreateUnbounded<InternalSystemMessage>(new UnboundedChannelOptions { SingleReader = true });
+    }
+
+    public void Start(NatsServer server)
+    {
+        _server = server;
+        var ct = _cts.Token;
+        _sendLoop = Task.Run(() => InternalSendLoopAsync(ct), ct);
+        _receiveLoop = Task.Run(() => InternalReceiveLoopAsync(_receiveQueue, ct), ct);
+        _receiveLoopPings = Task.Run(() => InternalReceiveLoopAsync(_receiveQueuePings, ct), ct);
+    }
+
+    /// <summary>
+    /// Creates a system subscription in the system account's SubList.
+    /// Maps to Go's sysSubscribe in events.go:2796.
+    /// </summary>
+    public Subscription SysSubscribe(string subject, SystemMessageHandler callback)
+    {
+        var sid = Interlocked.Increment(ref _subscriptionId).ToString();
+        var sub = new Subscription
+        {
+            Subject = subject,
+            Sid = sid,
+            Client = SystemClient,
+        };
+
+        // Wrap callback in noInlineCallback pattern: enqueue to receive loop
+        SystemClient.MessageCallback = (subj, s, reply, hdr, msg) =>
+        {
+            _receiveQueue.Writer.TryWrite(new InternalSystemMessage
+            {
+                Sub = sub,
+                Client = SystemClient,
+                Account = SystemAccount,
+                Subject = subj,
+                Reply = reply,
+                Headers = hdr,
+                Message = msg,
+                Callback = callback,
+            });
+        };
+
+        SystemAccount.SubList.Insert(sub);
+        return sub;
+    }
+
+    /// <summary>
+    /// Enqueue an internal message for publishing through the send loop.
+    /// </summary>
+    public void Enqueue(PublishMessage message)
+    {
+        _sendQueue.Writer.TryWrite(message);
+    }
+
+    /// <summary>
+    /// The send loop: serializes messages and delivers them via the server's routing.
+    /// Maps to Go's internalSendLoop in events.go:495-668.
+    /// </summary>
+    private async Task InternalSendLoopAsync(CancellationToken ct)
+    {
+        try
+        {
+            await foreach (var pm in _sendQueue.Reader.ReadAllAsync(ct))
+            {
+                try
+                {
+                    var seq = Interlocked.Increment(ref _sequence);
+
+                    // Serialize body to JSON
+                    byte[] payload;
+                    if (pm.Body is byte[] raw)
+                    {
+                        payload = raw;
+                    }
+                    else if (pm.Body != null)
+                    {
+                        // Try source-generated context first, fall back to reflection-based for unknown types
+                        var bodyType = pm.Body.GetType();
+                        var typeInfo = EventJsonContext.Default.GetTypeInfo(bodyType);
+                        payload = typeInfo != null
+                            ? JsonSerializer.SerializeToUtf8Bytes(pm.Body, typeInfo)
+                            : JsonSerializer.SerializeToUtf8Bytes(pm.Body, bodyType);
+                    }
+                    else
+                    {
+                        payload = [];
+                    }
+
+                    // Deliver via the system account's SubList matching
+                    var result = SystemAccount.SubList.Match(pm.Subject);
+
+                    foreach (var sub in result.PlainSubs)
+                    {
+                        sub.Client?.SendMessage(pm.Subject, sub.Sid, pm.Reply,
+                            pm.Headers ?? ReadOnlyMemory<byte>.Empty,
+                            payload);
+                    }
+
+                    foreach (var queueGroup in result.QueueSubs)
+                    {
+                        if (queueGroup.Length == 0) continue;
+                        var sub = queueGroup[0]; // Simple pick for internal
+                        sub.Client?.SendMessage(pm.Subject, sub.Sid, pm.Reply,
+                            pm.Headers ?? ReadOnlyMemory<byte>.Empty,
+                            payload);
+                    }
+
+                    if (pm.IsLast)
+                        break;
+                }
+                catch (Exception ex)
+                {
+                    _logger.LogWarning(ex, "Error in internal send loop processing message on {Subject}", pm.Subject);
+                }
+            }
+        }
+        catch (OperationCanceledException)
+        {
+            // Normal shutdown
+        }
+    }
+
+    /// <summary>
+    /// The receive loop: dispatches callbacks for internally-received messages.
+    /// Maps to Go's internalReceiveLoop in events.go:476-491.
+    /// </summary>
+    private async Task InternalReceiveLoopAsync(Channel<InternalSystemMessage> queue, CancellationToken ct)
+    {
+        try
+        {
+            await foreach (var msg in queue.Reader.ReadAllAsync(ct))
+            {
+                try
+                {
+                    msg.Callback(msg.Sub, msg.Client, msg.Account, msg.Subject, msg.Reply, msg.Headers, msg.Message);
+                }
+                catch (Exception ex)
+                {
+                    _logger.LogWarning(ex, "Error in internal receive loop processing {Subject}", msg.Subject);
+                }
+            }
+        }
+        catch (OperationCanceledException)
+        {
+            // Normal shutdown
+        }
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        await _cts.CancelAsync();
+        _sendQueue.Writer.TryComplete();
+        _receiveQueue.Writer.TryComplete();
+        _receiveQueuePings.Writer.TryComplete();
+
+        if (_sendLoop != null) await _sendLoop.WaitAsync(TimeSpan.FromSeconds(2)).ConfigureAwait(ConfigureAwaitOptions.SuppressThrowing);
+        if (_receiveLoop != null) await _receiveLoop.WaitAsync(TimeSpan.FromSeconds(2)).ConfigureAwait(ConfigureAwaitOptions.SuppressThrowing);
+        if (_receiveLoopPings != null) await _receiveLoopPings.WaitAsync(TimeSpan.FromSeconds(2)).ConfigureAwait(ConfigureAwaitOptions.SuppressThrowing);
+
+        _cts.Dispose();
+    }
+}

src/NATS.Server/NatsServer.cs

@@ -9,6 +9,7 @@ using Microsoft.Extensions.Logging;
 using NATS.NKeys;
 using NATS.Server.Auth;
 using NATS.Server.Configuration;
+using NATS.Server.Events;
 using NATS.Server.Monitoring;
 using NATS.Server.Protocol;
 using NATS.Server.Subscriptions;
@@ -35,6 +36,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     private string? _configDigest;
     private readonly Account _globalAccount;
     private readonly Account _systemAccount;
+    private InternalEventSystem? _eventSystem;
     private readonly SslServerAuthenticationOptions? _sslOptions;
     private readonly TlsRateLimiter? _tlsRateLimiter;
     private readonly SubjectTransform[] _subjectTransforms;
@@ -70,6 +72,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     public int Port => _options.Port;
     public Account SystemAccount => _systemAccount;
     public string ServerNKey { get; }
+    public InternalEventSystem? EventSystem => _eventSystem;
     public bool IsShuttingDown => Volatile.Read(ref _shutdown) != 0;
     public bool IsLameDuckMode => Volatile.Read(ref _lameDuck) != 0;
     public Action? ReOpenLogFile { get; set; }
@@ -90,6 +93,10 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
 
         _logger.LogInformation("Initiating Shutdown...");
 
+        // Dispose event system before tearing down clients
+        if (_eventSystem != null)
+            await _eventSystem.DisposeAsync();
+
         // Signal all internal loops to stop
         await _quitCts.CancelAsync();
 
@@ -265,6 +272,14 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
         _systemAccount = new Account("$SYS");
         _accounts["$SYS"] = _systemAccount;
 
+        // Create system internal client and event system
+        var sysClientId = Interlocked.Increment(ref _nextClientId);
+        var sysClient = new InternalClient(sysClientId, ClientKind.System, _systemAccount);
+        _eventSystem = new InternalEventSystem(
+            _systemAccount, sysClient,
+            options.ServerName ?? $"nats-dotnet-{Environment.MachineName}",
+            _loggerFactory.CreateLogger<InternalEventSystem>());
+
         // Generate Ed25519 server NKey identity
         using var serverKeyPair = KeyPair.CreatePair(PrefixByte.Server);
         ServerNKey = serverKeyPair.GetPublicKey();
@@ -371,6 +386,8 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
 
         _listeningStarted.TrySetResult();
 
+        _eventSystem?.Start(this);
+
         _logger.LogInformation("Listening for client connections on {Host}:{Port}", _options.Host, _options.Port);
 
         // Warn about stub features
@@ -686,6 +703,16 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
         });
     }
 
+    public void SendInternalMsg(string subject, string? reply, object? msg)
+    {
+        _eventSystem?.Enqueue(new PublishMessage { Subject = subject, Reply = reply, Body = msg });
+    }
+
+    public void SendInternalAccountMsg(Account account, string subject, object? msg)
+    {
+        _eventSystem?.Enqueue(new PublishMessage { Subject = subject, Body = msg });
+    }
+
     public void RemoveClient(NatsClient client)
     {
         _clients.TryRemove(client.Id, out _);

tests/NATS.Server.Tests/EventSystemTests.cs

@@ -1,4 +1,5 @@
 using System.Text.Json;
+using Microsoft.Extensions.Logging.Abstractions;
 using NATS.Server.Events;
 
 namespace NATS.Server.Tests;
@@ -65,4 +66,56 @@ public class EventSystemTests
         json.ShouldContain("\"connections\":10");
         json.ShouldContain("\"in_msgs\":5000");
     }
+
+    [Fact]
+    public async Task InternalEventSystem_start_and_stop_lifecycle()
+    {
+        using var server = CreateTestServer();
+        _ = server.StartAsync(CancellationToken.None);
+        await server.WaitForReadyAsync();
+
+        var eventSystem = server.EventSystem;
+        eventSystem.ShouldNotBeNull();
+        eventSystem.SystemClient.ShouldNotBeNull();
+        eventSystem.SystemClient.Kind.ShouldBe(ClientKind.System);
+
+        await server.ShutdownAsync();
+    }
+
+    [Fact]
+    public async Task SendInternalMsg_delivers_to_system_subscriber()
+    {
+        using var server = CreateTestServer();
+        _ = server.StartAsync(CancellationToken.None);
+        await server.WaitForReadyAsync();
+
+        var received = new TaskCompletionSource<string>();
+        server.EventSystem!.SysSubscribe("test.subject", (sub, client, acc, subject, reply, hdr, msg) =>
+        {
+            received.TrySetResult(subject);
+        });
+
+        server.SendInternalMsg("test.subject", null, new { Value = "hello" });
+
+        var result = await received.Task.WaitAsync(TimeSpan.FromSeconds(5));
+        result.ShouldBe("test.subject");
+
+        await server.ShutdownAsync();
+    }
+
+    private static NatsServer CreateTestServer()
+    {
+        var port = GetFreePort();
+        return new NatsServer(new NatsOptions { Port = port }, NullLoggerFactory.Instance);
+    }
+
+    private static int GetFreePort()
+    {
+        using var sock = new System.Net.Sockets.Socket(
+            System.Net.Sockets.AddressFamily.InterNetwork,
+            System.Net.Sockets.SocketType.Stream,
+            System.Net.Sockets.ProtocolType.Tcp);
+        sock.Bind(new System.Net.IPEndPoint(System.Net.IPAddress.Loopback, 0));
+        return ((System.Net.IPEndPoint)sock.LocalEndPoint!).Port;
+    }
 }