feat: add ephemeral port (port=0) support

src/NATS.Server/NatsServer.cs

@@ -38,6 +38,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     public string ServerId => _serverInfo.ServerId;
     public string ServerName => _serverInfo.ServerName;
     public int ClientCount => _clients.Count;
+    public int Port => _options.Port;
     public IEnumerable<NatsClient> GetClients() => _clients.Values;
 
     public Task WaitForReadyAsync() => _listeningStarted.Task;
@@ -82,6 +83,15 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
             _options.Port));
         Interlocked.Exchange(ref _startTimeTicks, DateTime.UtcNow.Ticks);
         _listener.Listen(128);
+
+        // Resolve ephemeral port if port=0
+        if (_options.Port == 0)
+        {
+            var actualPort = ((IPEndPoint)_listener.LocalEndPoint!).Port;
+            _options.Port = actualPort;
+            _serverInfo.Port = actualPort;
+        }
+
         _listeningStarted.TrySetResult();
 
         _logger.LogInformation("Listening on {Host}:{Port}", _options.Host, _options.Port);

tests/NATS.Server.Tests/ServerTests.cs

@@ -213,6 +213,39 @@ public class ServerTests : IAsyncLifetime
     }
 }
 
+public class EphemeralPortTests
+{
+    [Fact]
+    public async Task Server_resolves_ephemeral_port()
+    {
+        using var cts = new CancellationTokenSource();
+        var server = new NatsServer(new NatsOptions { Port = 0 }, NullLoggerFactory.Instance);
+
+        _ = server.StartAsync(cts.Token);
+        await server.WaitForReadyAsync();
+
+        try
+        {
+            // Port should have been resolved to a real port
+            server.Port.ShouldBeGreaterThan(0);
+
+            // Connect a raw socket to prove the port actually works
+            using var client = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+            await client.ConnectAsync(IPAddress.Loopback, server.Port);
+
+            var buf = new byte[4096];
+            var n = await client.ReceiveAsync(buf, SocketFlags.None);
+            var response = Encoding.ASCII.GetString(buf, 0, n);
+            response.ShouldStartWith("INFO ");
+        }
+        finally
+        {
+            await cts.CancelAsync();
+            server.Dispose();
+        }
+    }
+}
+
 public class MaxConnectionsTests : IAsyncLifetime
 {
     private readonly NatsServer _server;
@@ -423,3 +456,72 @@ public class PingKeepaliveTests : IAsyncLifetime
         client.Dispose();
     }
 }
+
+public class CloseReasonTests : IAsyncLifetime
+{
+    private readonly NatsServer _server;
+    private readonly int _port;
+    private readonly CancellationTokenSource _cts = new();
+
+    public CloseReasonTests()
+    {
+        _port = GetFreePort();
+        _server = new NatsServer(new NatsOptions { Port = _port }, NullLoggerFactory.Instance);
+    }
+
+    public async Task InitializeAsync()
+    {
+        _ = _server.StartAsync(_cts.Token);
+        await _server.WaitForReadyAsync();
+    }
+
+    public async Task DisposeAsync()
+    {
+        await _cts.CancelAsync();
+        _server.Dispose();
+    }
+
+    private static int GetFreePort()
+    {
+        using var sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+        sock.Bind(new IPEndPoint(IPAddress.Loopback, 0));
+        return ((IPEndPoint)sock.LocalEndPoint!).Port;
+    }
+
+    [Fact]
+    public async Task Client_close_reason_set_on_normal_disconnect()
+    {
+        // Connect a raw TCP client
+        using var client = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+        await client.ConnectAsync(IPAddress.Loopback, _port);
+
+        // Read INFO
+        var buf = new byte[4096];
+        await client.ReceiveAsync(buf, SocketFlags.None);
+
+        // Send CONNECT + PING, wait for PONG
+        await client.SendAsync(Encoding.ASCII.GetBytes("CONNECT {}\r\nPING\r\n"));
+        var sb = new StringBuilder();
+        using var readCts = new CancellationTokenSource(TimeSpan.FromSeconds(5));
+        while (!sb.ToString().Contains("PONG"))
+        {
+            var n = await client.ReceiveAsync(buf, SocketFlags.None, readCts.Token);
+            if (n == 0) break;
+            sb.Append(Encoding.ASCII.GetString(buf, 0, n));
+        }
+        sb.ToString().ShouldContain("PONG");
+
+        // Get the NatsClient from the server
+        var natsClient = _server.GetClients().First();
+
+        // Close the TCP socket (normal client disconnect)
+        client.Shutdown(SocketShutdown.Both);
+        client.Close();
+
+        // Wait for the server to detect the disconnect
+        await Task.Delay(500);
+
+        // The close reason should be ClientClosed (normal disconnect falls through to finally)
+        natsClient.CloseReason.ShouldBe(ClosedState.ClientClosed);
+    }
+}