Abort SDO server upload when the object has zero length (#587)

A data size of zero bytes cannot be encoded in the "number of bytes
that do not contain data" semantics, as that would require n=4, which
is however limited to two bits (maximum value 3).  So the SDO
expedited upload protocol simply cannot convey that condition.  The
current implementation however still tries and thereby corrupts the
the reserved, always-zero field value.

Avoid that protocol violation by responding with an SDO abort code of
0800 0024h, No data available.

Add a test case for this condition, verifying that the client raises
the appropriate SdoAbortedError exception.

Author: acolomb

canopen/sdo/server.py

@@ -66,7 +66,11 @@ def init_upload(self, request):
 
         data = self._node.get_data(index, subindex, check_readable=True)
         size = len(data)
-        if size <= 4:
+        if size == 0:
+            logger.info("No content to upload for 0x%04X:%02X", index, subindex)
+            self.abort(0x0800_0024)
+            return
+        elif size <= 4:
             logger.info("Expedited upload for 0x%04X:%02X", index, subindex)
             res_command |= EXPEDITED
             res_command |= (4 - size) << 2

test/test_local.py

@@ -62,6 +62,13 @@ def test_expedited_upload_default_value_real(self):
         sampling_rate = self.remote_node.sdo["Sensor Sampling Rate (Hz)"].raw
         self.assertAlmostEqual(sampling_rate, 5.2, places=2)
 
+    def test_upload_zero_length(self):
+        self.local_node.sdo["Manufacturer device name"].raw = b""
+        with self.assertRaises(canopen.SdoAbortedError) as error:
+            self.remote_node.sdo["Manufacturer device name"].data
+        # Should be No data available
+        self.assertEqual(error.exception.code, 0x0800_0024)
+
     def test_segmented_upload(self):
         self.local_node.sdo["Manufacturer device name"].raw = "Some cool device"
         device_name = self.remote_node.sdo["Manufacturer device name"].data