Recall socket recv until get a complete response.

Socket recv waits until it gets some data from the host but
not necessarily the entire response that can be fragmented in
many packets. In this case it receives an incomplete frame
that is rejected by the frame handler.
This commit solves this issue by recalling socket recv until
get a complete modbus response.

Author: Claudio Catterina

pymodbus/transaction.py

@@ -68,22 +68,51 @@ def _set_adu_size(self):
 
     def _calculate_response_length(self, expected_pdu_size):
         if self.base_adu_size == -1:
-            return 1024
+            return None
         else:
             return self.base_adu_size + expected_pdu_size
 
+    def _calculate_exception_length(self):
+        ''' Returns the length of the Modbus Exception Response according to
+        the type of Framer.
+        '''
+        if isinstance(self.client.framer, ModbusSocketFramer):
+            return self.base_adu_size + 2  # Fcode(1), ExcecptionCode(1)
+        elif isinstance(self.client.framer, ModbusAsciiFramer):
+            return self.base_adu_size + 4  # Fcode(2), ExcecptionCode(2)
+        elif isinstance(self.client.framer, (ModbusRtuFramer, ModbusBinaryFramer)):
+            return self.base_adu_size + 2  # Fcode(1), ExcecptionCode(1)
+
+        return None
+
+    def _check_response(self, response):
+        ''' Checks if the response is a Modbus Exception.
+        '''
+        if isinstance(self.client.framer, ModbusSocketFramer):
+            if len(response) >= 8 and byte2int(response[7]) > 128:
+                return False
+        elif isinstance(self.client.framer, ModbusAsciiFramer):
+            if len(response) >= 5 and int(response[3:5], 16) > 128:
+                return False
+        elif isinstance(self.client.framer, (ModbusRtuFramer, ModbusBinaryFramer)):
+            if len(response) >= 2 and byte2int(response[1]) > 128:
+                return False
+
+        return True
+
     def execute(self, request):
         ''' Starts the producer to send the next request to
         consumer.write(Frame(request))
         '''
         retries = self.retries
         request.transaction_id = self.getNextTID()
         _logger.debug("Running transaction %d" % request.transaction_id)
+
+        expected_response_length = None
         if hasattr(request, "get_response_pdu_size"):
             response_pdu_size = request.get_response_pdu_size()
-            expected_response_length = self._calculate_response_length(response_pdu_size)
-        else:
-            expected_response_length = 1024
+            if response_pdu_size:
+                expected_response_length = self._calculate_response_length(response_pdu_size)
 
         while retries > 0:
             try:
@@ -92,11 +121,20 @@ def execute(self, request):
                 if _logger.isEnabledFor(logging.DEBUG):
                     _logger.debug("send: " + " ".join([hex(byte2int(x)) for x in packet]))
                 self.client._send(packet)
-                result = self.client._recv(expected_response_length)
+
+                exception = False
+                result = self.client._recv(expected_response_length or 1024)
+                while result and expected_response_length and len(result) < expected_response_length:
+                    if not exception and not self._check_response(result):
+                        exception = True
+                        expected_response_length = self._calculate_exception_length()
+                        continue
+                    result += self.client._recv(expected_response_length - len(result))
 
                 if not result and self.retry_on_empty:
                     retries -= 1
                     continue
+
                 if _logger.isEnabledFor(logging.DEBUG):
                     _logger.debug("recv: " + " ".join([hex(byte2int(x)) for x in result]))
                 self.client.framer.processIncomingPacket(result, self.addTransaction)