feat: add server-side PING keepalive with stale connection detection

src/NATS.Server/NatsClient.cs

@@ -44,6 +44,10 @@ public sealed class NatsClient : IDisposable
     public long InBytes;
     public long OutBytes;
 
+    // PING keepalive state
+    private int _pingsOut;
+    private long _lastIn;
+
     public IReadOnlyDictionary<string, Subscription> Subscriptions => _subs;
 
     public NatsClient(ulong id, Socket socket, NatsOptions options, ServerInfo serverInfo, ILogger logger)
@@ -60,17 +64,19 @@ public sealed class NatsClient : IDisposable
     public async Task RunAsync(CancellationToken ct)
     {
         _clientCts = CancellationTokenSource.CreateLinkedTokenSource(ct);
+        Interlocked.Exchange(ref _lastIn, Environment.TickCount64);
         var pipe = new Pipe();
         try
         {
             // Send INFO
             await SendInfoAsync(_clientCts.Token);
 
-            // Start read pump and command processing in parallel
+            // Start read pump, command processing, and ping timer in parallel
             var fillTask = FillPipeAsync(pipe.Writer, _clientCts.Token);
             var processTask = ProcessCommandsAsync(pipe.Reader, _clientCts.Token);
+            var pingTask = RunPingTimerAsync(_clientCts.Token);
 
-            await Task.WhenAny(fillTask, processTask);
+            await Task.WhenAny(fillTask, processTask, pingTask);
         }
         catch (OperationCanceledException)
         {
@@ -82,15 +88,10 @@ public sealed class NatsClient : IDisposable
         }
         finally
         {
+            try { _socket.Shutdown(SocketShutdown.Both); }
+            catch (SocketException) { }
+            catch (ObjectDisposedException) { }
             Router?.RemoveClient(this);
-            try
-            {
-                _socket.Shutdown(SocketShutdown.Both);
-            }
-            catch (Exception ex)
-            {
-                _logger.LogDebug(ex, "Client {ClientId} socket shutdown error", Id);
-            }
         }
     }
 
@@ -128,6 +129,7 @@ public sealed class NatsClient : IDisposable
 
                 while (_parser.TryParse(ref buffer, out var cmd))
                 {
+                    Interlocked.Exchange(ref _lastIn, Environment.TickCount64);
                     await DispatchCommandAsync(cmd, ct);
                 }
 
@@ -156,7 +158,7 @@ public sealed class NatsClient : IDisposable
                 break;
 
             case CommandType.Pong:
-                // Update RTT tracking (placeholder)
+                Interlocked.Exchange(ref _pingsOut, 0);
                 break;
 
             case CommandType.Sub:
@@ -339,6 +341,48 @@ public sealed class NatsClient : IDisposable
             _socket.Close();
     }
 
+    private async Task RunPingTimerAsync(CancellationToken ct)
+    {
+        using var timer = new PeriodicTimer(_options.PingInterval);
+        try
+        {
+            while (await timer.WaitForNextTickAsync(ct))
+            {
+                var elapsed = Environment.TickCount64 - Interlocked.Read(ref _lastIn);
+                if (elapsed < (long)_options.PingInterval.TotalMilliseconds)
+                {
+                    // Client was recently active, skip ping
+                    Interlocked.Exchange(ref _pingsOut, 0);
+                    continue;
+                }
+
+                var currentPingsOut = Interlocked.Increment(ref _pingsOut);
+                if (currentPingsOut > _options.MaxPingsOut)
+                {
+                    _logger.LogDebug("Client {ClientId} stale connection — closing", Id);
+                    await SendErrAndCloseAsync(NatsProtocol.ErrStaleConnection);
+                    return;
+                }
+
+                _logger.LogDebug("Client {ClientId} sending PING ({PingsOut}/{MaxPingsOut})",
+                    Id, currentPingsOut, _options.MaxPingsOut);
+                try
+                {
+                    await WriteAsync(NatsProtocol.PingBytes, ct);
+                }
+                catch (Exception ex)
+                {
+                    _logger.LogDebug(ex, "Client {ClientId} failed to send PING", Id);
+                    return;
+                }
+            }
+        }
+        catch (OperationCanceledException)
+        {
+            // Normal shutdown
+        }
+    }
+
     public void RemoveAllSubscriptions(SubList subList)
     {
         foreach (var sub in _subs.Values)