unfifed timeout handling and fixed memory usage due to Task.Delay tasks for each send package are present for the duration of the timeout resulting in memory usage. new approach uses cancellationtoken that will be cleaned up directly if operation completes before timeout

MQTTnet.Core/Internal/TaskExtensions.cs

@@ -0,0 +1,55 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using MQTTnet.Core.Exceptions;
+
+namespace MQTTnet.Core.Internal
+{
+    public static class TaskExtensions
+    {
+        public static Task TimeoutAfter( this Task task, TimeSpan timeout )
+        {
+            return TimeoutAfter( task.ContinueWith( t => 0 ), timeout );
+        }
+
+        public static async Task<TResult> TimeoutAfter<TResult>(this Task<TResult> task, TimeSpan timeout)
+        {
+            using (var cancellationTokenSource = new CancellationTokenSource())
+            {
+                var tcs = new TaskCompletionSource<TResult>();
+
+                cancellationTokenSource.Token.Register(() =>
+                {
+                    tcs.TrySetCanceled();
+                } );
+                
+                try
+                {
+                    cancellationTokenSource.CancelAfter(timeout);
+                    task.ContinueWith( t =>
+                    {
+                        if (t.IsFaulted)
+                        {
+                            tcs.TrySetException(t.Exception);
+                        }
+
+                        if (t.IsCompleted)
+                        {
+                            tcs.TrySetResult(t.Result);
+                        }
+                    }, cancellationTokenSource.Token );
+
+                    return await tcs.Task;
+                }
+                catch (TaskCanceledException)
+                {
+                    throw new MqttCommunicationTimedOutException();
+                }
+                catch (Exception e)
+                {
+                    throw new MqttCommunicationException(e);
+                }
+            }
+        }
+    }
+}