Adjust the MqttClientPendingMessagesQueue and the MqttPacketReader to use asynchronous methods for long-running/blocking operations.

Otherwise, Worker Threads from the .NET Threadpool are blocked while waiting for the next client message, and while waiting for the next message to send.

Frameworks/MQTTnet.NetStandard/Adapter/MqttChannelAdapter.cs

@@ -120,7 +120,7 @@ namespace MQTTnet.Adapter
 
         private static async Task<ReceivedMqttPacket> ReceiveAsync(Stream stream, CancellationToken cancellationToken)
         {
-            var header = MqttPacketReader.ReadHeaderFromSource(stream, cancellationToken);
+            var header = await MqttPacketReader.ReadHeaderFromSourceAsync(stream, cancellationToken);
             if (header == null)
             {
                 return null;

Frameworks/MQTTnet.NetStandard/Serializer/MqttPacketReader.cs

@@ -22,17 +22,18 @@ namespace MQTTnet.Serializer
 
         public bool EndOfRemainingData => BaseStream.Position == _header.BodyLength;
 
-        public static MqttPacketHeader ReadHeaderFromSource(Stream stream, CancellationToken cancellationToken)
+        public static async Task<MqttPacketHeader> ReadHeaderFromSourceAsync(Stream stream, CancellationToken cancellationToken)
         {
-            var buffer = stream.ReadByte();
-            if (buffer == -1)
+            byte[] singleByteBuf = new byte[1];
+            var readCount = await stream.ReadAsync(singleByteBuf, 0, singleByteBuf.Length).ConfigureAwait(false);
+            if (readCount <= 0)
             {
                 return null;
             }
-            
-            var fixedHeader = (byte)buffer;
+
+            var fixedHeader = singleByteBuf[0];
             var controlPacketType = (MqttControlPacketType)(fixedHeader >> 4);
-            var bodyLength = ReadBodyLengthFromSource(stream, cancellationToken);
+            var bodyLength = await ReadBodyLengthFromSourceAsync(stream, cancellationToken).ConfigureAwait(false);
 
             return new MqttPacketHeader
             {
@@ -80,13 +81,15 @@ namespace MQTTnet.Serializer
             return ReadBytes(_header.BodyLength - (int)BaseStream.Position);
         }
 
-        private static int ReadBodyLengthFromSource(Stream stream, CancellationToken cancellationToken)
+        private static async Task<int> ReadBodyLengthFromSourceAsync(Stream stream, CancellationToken cancellationToken)
         {
             // Alorithm taken from http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html.
             var multiplier = 1;
             var value = 0;
             byte encodedByte;
 
+            byte[] singleByteBuf = new byte[1];
+
             var readBytes = new List<byte>();
             do
             {
@@ -95,13 +98,13 @@ namespace MQTTnet.Serializer
                     throw new TaskCanceledException();
                 }
 
-                var buffer = stream.ReadByte();
-                if (buffer == -1)
+                int readCount = await stream.ReadAsync(singleByteBuf, 0, singleByteBuf.Length).ConfigureAwait(false);
+                if (readCount <= 0)
                 {
                     throw new MqttCommunicationException("Connection closed while reading remaining length data.");
                 }
 
-                encodedByte = (byte)buffer;
+                encodedByte = singleByteBuf[0];
                 readBytes.Add(encodedByte);
 
                 value += (byte)(encodedByte & 127) * multiplier;

Frameworks/MQTTnet.NetStandard/Server/MqttClientPendingMessagesQueue.cs

@@ -12,7 +12,8 @@ namespace MQTTnet.Server
 {
     public sealed class MqttClientPendingMessagesQueue : IDisposable
     {
-        private readonly BlockingCollection<MqttBasePacket> _queue = new BlockingCollection<MqttBasePacket>();
+        private readonly ConcurrentQueue<MqttBasePacket> _queue = new ConcurrentQueue<MqttBasePacket>();
+        private readonly SemaphoreSlim _queueWaitSemaphore = new SemaphoreSlim(0);
         private readonly IMqttServerOptions _options;
         private readonly MqttClientSession _session;
         private readonly IMqttNetLogger _logger;
@@ -40,7 +41,8 @@ namespace MQTTnet.Server
         {
             if (packet == null) throw new ArgumentNullException(nameof(packet));
 
-            _queue.Add(packet);
+            _queue.Enqueue(packet);
+            _queueWaitSemaphore.Release();
             _logger.Trace<MqttClientPendingMessagesQueue>("Enqueued packet (ClientId: {0}).", _session.ClientId);
         }
 
@@ -67,7 +69,10 @@ namespace MQTTnet.Server
             MqttBasePacket packet = null;
             try
             {
-                packet = _queue.Take(cancellationToken);
+                await _queueWaitSemaphore.WaitAsync(cancellationToken).ConfigureAwait(false);
+                if (!_queue.TryDequeue(out packet)) {
+                    throw new InvalidOperationException(); // should not happen
+                }
                 await adapter.SendPacketsAsync(_options.DefaultCommunicationTimeout, cancellationToken, packet).ConfigureAwait(false);
 
                 _logger.Trace<MqttClientPendingMessagesQueue>("Enqueued packet sent (ClientId: {0}).", _session.ClientId);
@@ -95,7 +100,8 @@ namespace MQTTnet.Server
                     if (publishPacket.QualityOfServiceLevel > MqttQualityOfServiceLevel.AtMostOnce)
                     {
                         publishPacket.Dup = true;
-                        _queue.Add(packet, CancellationToken.None);
+                        _queue.Enqueue(packet);
+                        _queueWaitSemaphore.Release();
                     }
                 }
 
@@ -105,7 +111,7 @@ namespace MQTTnet.Server
 
         public void Dispose()
         {
-            _queue?.Dispose();
+            _queueWaitSemaphore?.Dispose();
         }
     }
 }