Partial fix for CSHARP-302. The default action now when there is an exception on a connection is to discard only that connection instead of the whole connection pool, preventing a Connect/Disconnect storm. Also made Disconnect synchronous so it should no longer be possible to open new connections faster than old ones are being closed.

Driver/Internal/MongoConnection.cs

@@ -460,21 +460,45 @@ namespace MongoDB.Driver.Internal {
         private void HandleException(
             Exception ex
         ) {
-            // TODO: figure out which exceptions are more serious than others
             // there are three possible situations:
             // 1. we can keep using the connection
             // 2. just this one connection needs to be discarded
             // 3. the whole connection pool needs to be discarded
-            // for now the only exception we know affects only one connection is FileFormatException
-            // and there are no cases where the connection can continue to be used
 
-            state = MongoConnectionState.Damaged;
-            if (!(ex is FileFormatException)) {
-                try {
-                    serverInstance.Disconnect();
-                } catch { } // ignore any further exceptions
+            switch (DetermineAction(ex)) {
+                case HandleExceptionAction.KeepConnection:
+                    break;
+                case HandleExceptionAction.DiscardConnection:
+                    state = MongoConnectionState.Damaged;
+                    break;
+                case HandleExceptionAction.DiscardConnectionPool:
+                    state = MongoConnectionState.Damaged;
+                    try {
+                        serverInstance.Disconnect();
+                    } catch { } // ignore exceptions
+                    break;
+                default:
+                    throw new MongoInternalException("Invalid HandleExceptionAction");
             }
         }
+
+        private enum HandleExceptionAction {
+            KeepConnection,
+            DiscardConnection,
+            DiscardConnectionPool
+        }
+
+        private HandleExceptionAction DetermineAction(
+            Exception ex
+        ) {
+            // TODO: figure out when to return KeepConnection or DiscardConnectionPool (if ever)
+
+            // don't return DiscardConnectionPool unless you are *sure* it is the right action
+            // definitely don't make DiscardConnectionPool the default action
+            // returning DiscardConnectionPool frequently can result in Connect/Disconnect storms
+
+            return HandleExceptionAction.DiscardConnection; // this should always be the default action
+        }
         #endregion
 
         #region private nested classes

Driver/Internal/MongoConnectionPool.cs

@@ -114,7 +114,7 @@ namespace MongoDB.Driver.Internal {
 
                             // otherwise replace the least recently used connection with a brand new one
                             // if this happens a lot the connection pool size should be increased
-                            ThreadPool.QueueUserWorkItem(CloseConnectionWorkItem, availableConnections[0]);
+                            availableConnections[0].Close();
                             availableConnections.RemoveAt(0);
                             return new MongoConnection(this);
                         }
@@ -145,7 +145,7 @@ namespace MongoDB.Driver.Internal {
             lock (connectionPoolLock) {
                 if (!closed) {
                     foreach (var connection in availableConnections) {
-                        ThreadPool.QueueUserWorkItem(CloseConnectionWorkItem, connection);
+                        connection.Close();
                     }
                     availableConnections = null;
                     closed = true;
@@ -206,9 +206,9 @@ namespace MongoDB.Driver.Internal {
             }
 
             lock (connectionPoolLock) {
-                // if connection pool is closed just close connection on worker thread
+                // if connection pool is closed just close connection
                 if (closed) {
-                    ThreadPool.QueueUserWorkItem(CloseConnectionWorkItem, connection);
+                    connection.Close();
                     return;
                 }
 
@@ -235,23 +235,13 @@ namespace MongoDB.Driver.Internal {
         #endregion
 
         #region private methods
-        // note: this method runs on a thread from the ThreadPool
-        private void CloseConnectionWorkItem(
-            object parameters
-        ) {
-            try {
-                var connection = (MongoConnection) parameters;
-                connection.Close();
-            } catch { } // ignore exceptions
-        }
-
         private void RemoveConnection(
             MongoConnection connection
         ) {
             availableConnections.Remove(connection); // it might or might not be in availableConnections (but remove it if it is)
             poolSize -= 1;
             connectionsRemovedSinceLastTimerTick += 1;
-            ThreadPool.QueueUserWorkItem(CloseConnectionWorkItem, connection);
+            connection.Close();
             Monitor.Pulse(connectionPoolLock);
         }