708 Implement ReadFileRecordRequest.get_response_pdu_size() to ensure complete record (#709)

* Add get_response_pdu_size() to ensure complete file records are received correctly

Co-authored-by: jan Iversen <jancasacondor@gmail.com>

Author: theobarkerh

pymodbus/file_message.py

@@ -91,6 +91,7 @@ def __init__(self, records=None, **kwargs):
         """
         ModbusRequest.__init__(self, **kwargs)
         self.records = records or []
+        self.count = 0
 
     def encode(self):
         """Encode the request packet.
@@ -137,6 +138,19 @@ def execute(self, context):  # NOSONAR pylint: disable=unused-argument,no-self-u
         files = []
         return ReadFileRecordResponse(files)
 
+    def get_response_pdu_size(self):
+        """Get response pdu size.
+
+        Func_code (1 byte) + Response Byte Count(1 byte) +
+            N * (File Response Byte Count (1 byte) + Reference Type (1 byte) + 2 * registers to read for file record)
+            where N = record sub-request count.
+        """
+        self.count = 0
+        for record in self.records:
+            self.count += record.record_length * 2 + 2 if record.record_length else 0
+
+        return 2 + self.count
+
 
 class ReadFileRecordResponse(ModbusResponse):
     """Read file record response.