Avoid truncating unknown types in SDO upload (fixes #436) (#591)

When an OD entry is found in SdoClient.upload(), the truncation is
currently skipped only for those types explicitly listed as variable
length data (domain, strings).  That causes unknown types to be
truncated to whatever their length indicates, which is usually one
byte.

Invert the condition to check all types with an explicitly known size,
a.k.a. those listed in STRUCT_TYPES where the required length can be
deduced from the structure format.

Re-enable the unit tests which were skipped based on the previously
buggy behavior.

Author: acolomb

canopen/objectdictionary/__init__.py

@@ -419,6 +419,12 @@ def add_bit_definition(self, name: str, bits: List[int]) -> None:
         """
         self.bit_definitions[name] = bits
 
+    @property
+    def fixed_size(self) -> bool:
+        """Indicate whether the amount of needed data is known in advance."""
+        # Only for types which we parse using a structure.
+        return self.data_type in self.STRUCT_TYPES
+
     def decode_raw(self, data: bytes) -> Union[int, float, str, bytes, bytearray]:
         if self.data_type == VISIBLE_STRING:
             # Strip any trailing NUL characters from C-based systems

canopen/sdo/client.py

@@ -126,9 +126,7 @@ def upload(self, index: int, subindex: int) -> bytes:
         var = self.od.get_variable(index, subindex)
         if var is not None:
             # Found a matching variable in OD
-            # If this is a data type (string, domain etc) the size is
-            # unknown anyway so keep the data as is
-            if var.data_type not in objectdictionary.DATA_TYPES:
+            if var.fixed_size:
                 # Get the size in bytes for this variable
                 var_size = len(var) // 8
                 if response_size is None or var_size < response_size:

test/test_sdo.py

@@ -814,7 +814,6 @@ def test_unknown_od_112(self):
 
     def test_unknown_datatype32(self):
         """Test an unknown datatype, but known OD, of 32 bits (4 bytes)."""
-        return  # FIXME: Disabled temporarily until datatype conditionals are fixed, see #436
         # Add fake entry 0x2100 to OD, using fake datatype 0xFF
         if 0x2100 not in self.node.object_dictionary:
             fake_var = ODVariable("Fake", 0x2100)
@@ -829,7 +828,6 @@ def test_unknown_datatype32(self):
 
     def test_unknown_datatype112(self):
         """Test an unknown datatype, but known OD, of 112 bits (14 bytes)."""
-        return  # FIXME: Disabled temporarily until datatype conditionals are fixed, see #436
         # Add fake entry 0x2100 to OD, using fake datatype 0xFF
         if 0x2100 not in self.node.object_dictionary:
             fake_var = ODVariable("Fake", 0x2100)