Close connection on no/erroneous message received

Always close connection on no response/error response from modbus
unit. This attempts to resolve issue #538, where the response of a
previously timed out request over synchronized TCP is being returned
as the response to the current request, by leaving the connection open
only if there is no unanswered response.

This commit only touches the code for synchronized clients, and only
seeks to resolve it for TCP based clients, though the situation for
UDP clients should improve as well.

Author: thormick

pymodbus/transaction.py

@@ -200,6 +200,7 @@ def execute(self, request):
                                 "/Unable to decode response")
                             response = ModbusIOException(last_exception,
                                                          request.function_code)
+                        self.client.close()
                     if hasattr(self.client, "state"):
                         _logger.debug("Changing transaction state from "
                                       "'PROCESSING REPLY' to "
@@ -209,6 +210,7 @@ def execute(self, request):
                 return response
             except ModbusIOException as ex:
                 # Handle decode errors in processIncomingPacket method
+                self.client.close()
                 _logger.exception(ex)
                 self.client.state = ModbusTransactionState.TRANSACTION_COMPLETE
                 return ex