Fixes #1276 timestamp check in server keep-alive monitor (#1277)

* Fix timestamp check in server keep-alive monitor

* Retain the original client statistics property name
- add comments to clarify meaning
- add separate public "received" timestamp property

* fix comment

Source/MQTTnet/Server/Internal/MqttClientConnectionStatistics.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Threading;
 using MQTTnet.Packets;
 using MQTTnet.Server.Status;
@@ -32,8 +32,17 @@ public MqttClientConnectionStatistics()
             _lastNonKeepAlivePacketReceivedTimestamp = _connectedTimestamp;
         }
 
+        /// <summary>
+        /// Timestamp of the last package that has been received from the client ("sent" from the client's perspective)
+        /// </summary>
+        public DateTime LastPacketSentTimestamp => _lastPacketSentTimestamp;
+
+        /// <summary>
+        /// Timestamp of the last package that has been sent to the client ("received" from the client's perspective)
+        /// </summary>
         public DateTime LastPacketReceivedTimestamp => _lastPacketReceivedTimestamp;
 
+
         public void HandleReceivedPacket(MqttBasePacket packet)
         {
             if (packet == null) throw new ArgumentNullException(nameof(packet));

Source/MQTTnet/Server/Internal/MqttServerKeepAliveMonitor.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Client.Disconnecting;
@@ -95,7 +95,7 @@ void TryMaintainConnection(MqttClientConnection connection, DateTime now)
                 // If the client sends 1 sec. the server will allow up to 1.5 seconds.
                 var maxDurationWithoutPacket = connection.KeepAlivePeriod * 1.5D;
 
-                var secondsWithoutPackage = (now - connection.Statistics.LastPacketReceivedTimestamp).TotalSeconds;
+                var secondsWithoutPackage = (now - connection.Statistics.LastPacketSentTimestamp).TotalSeconds;
                 if (secondsWithoutPackage < maxDurationWithoutPacket)
                 {
                     // A packet was received before the timeout is affected.

Tests/MQTTnet.Core.Tests/Client/MqttClient_Tests.cs

@@ -647,6 +647,39 @@ public async Task Publish_QoS_1_In_ApplicationMessageReceiveHandler()
             }
         }
 
+        [TestMethod]
+        public async Task Publish_QoS_0_Over_Period_Exceeding_KeepAlive()
+        {
+            using (var testEnvironment = new TestEnvironment(TestContext))
+            {
+                const int KeepAlivePeriodSecs = 3;
+
+                await testEnvironment.StartServer();
+
+                var options = new MqttClientOptionsBuilder().WithKeepAlivePeriod(TimeSpan.FromSeconds(KeepAlivePeriodSecs));
+                var client = await testEnvironment.ConnectClient(options);
+                var message = new MqttApplicationMessageBuilder().WithTopic("a").Build();
+
+                try
+                {
+                    // Publish messages over a time period exceeding the keep alive period.
+                    // This should not cause an exception because of, i.e. "Client disconnected".
+
+                    for (var count = 0; count < KeepAlivePeriodSecs * 3; ++count)
+                    {
+                        // Send Publish requests well before the keep alive period expires
+                        await client.PublishAsync(message);
+                        await Task.Delay(1000);
+                    }
+                }
+                catch (Exception ex)
+                {
+                    Assert.Fail(ex.Message);
+                }
+            }
+        }
+
+
         [TestMethod]
         public async Task Subscribe_In_Callback_Events()
         {