Recall socket recv until get a complete response. (#143)

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: ccatterina

pymodbus/transaction.py

@@ -68,7 +68,7 @@ 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
 
@@ -79,21 +79,26 @@ def execute(self, 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:
                 self.client.connect()
                 self.client._send(self.client.framer.buildPacket(request))
-                result = self.client._recv(expected_response_length)
+
+                result = self.client._recv(expected_response_length or 1024)
+                while result and expected_response_length and len(result) < expected_response_length:
+                    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)