Remove oldest channel if not used by session, when number of channels reaches MaxChannelCount - 1 (#2986)

* Remove older channel if not used by session, when number of channels reaches MaxChannelCount - 1

* Corrected channelCount and renamings

* Operate on snapshot

* Snapshot the entire concurentdictionary

Author: mrsuciu

Stack/Opc.Ua.Core/Stack/Tcp/TcpListenerChannel.cs

@@ -161,6 +161,21 @@ public void IdleCleanup()
         /// or received a keep alive.
         /// </summary>
         public int ElapsedSinceLastActiveTime => (HiResClock.TickCount - LastActiveTickCount);
+
+        /// <summary>
+        /// Has the channel been used in a session
+        /// </summary>
+        public bool UsedBySession
+        {
+            get
+            {
+                return m_usedBySession;
+            }
+            protected set
+            {
+                m_usedBySession = value;
+            }
+        }
         #endregion
 
         #region Socket Event Handlers
@@ -562,6 +577,7 @@ protected uint GetNewTokenId()
         private ReportAuditCloseSecureChannelEventHandler m_reportAuditCloseSecureChannelEvent;
         private ReportAuditCertificateEventHandler m_reportAuditCertificateEvent;
         private long m_lastTokenId;
+        private bool m_usedBySession;
         #endregion
     }
 

Stack/Opc.Ua.Core/Stack/Tcp/TcpServerChannel.cs

@@ -1090,6 +1090,11 @@ public void SendResponse(uint requestId, IServiceResponse response)
                     m_queuedResponses[requestId] = response;
                     return;
                 }
+
+                if (response is ActivateSessionResponse activateSessionResponse)
+                {
+                    UsedBySession = StatusCode.IsGood(activateSessionResponse.ResponseHeader.ServiceResult);
+                }
             }
         }
 

Stack/Opc.Ua.Core/Stack/Tcp/TcpTransportListener.cs

@@ -801,6 +801,31 @@ private void OnAccept(object sender, SocketAsyncEventArgs e)
                     {
                         // TODO: .Count is flagged as hotpath, implement separate counter
                         int channelCount = channels.Count;
+
+                        // Remove oldest channel that does not have a session attached to it
+                        // before reaching m_maxChannelCount
+                        if (m_maxChannelCount > 0 && m_maxChannelCount == channelCount)
+                        {
+                            var snapshot = channels.ToArray();
+
+                            // Identify channels without established sessions
+                            var nonSessionChannels = snapshot.Where(ch => !ch.Value.UsedBySession).ToArray();
+
+                            if (nonSessionChannels.Any())
+                            {
+                                var oldestIdChannel = nonSessionChannels.Aggregate((max, current) =>
+                                    current.Value.ElapsedSinceLastActiveTime > max.Value.ElapsedSinceLastActiveTime ? current : max);
+
+                                Utils.LogInfo("TCPLISTENER: Channel Id {0} scheduled for IdleCleanup - Oldest without established session.",
+                                    oldestIdChannel.Value.Id);
+                                oldestIdChannel.Value.IdleCleanup();
+                                Utils.LogInfo("TCPLISTENER: Channel Id {0} finished IdleCleanup - Oldest without established session.",
+                                    oldestIdChannel.Value.Id);
+
+                                channelCount--;
+                            }
+                        }
+
                         bool serveChannel = !(m_maxChannelCount > 0 && m_maxChannelCount < channelCount);
                         if (!serveChannel)
                         {