Fix ROS 2 message equality check (#964)

### Public-Facing Changes

Fix ROS 2 message equality check

### Description
Use a custom comparison function that compares messages by their
`__slots__` fields.

Fixes #959
Resolves FG-4775

python/mcap-ros2-support/mcap_ros2/__init__.py

@@ -1 +1 @@
-__version__ = "0.5.1"
+__version__ = "0.5.2"

python/mcap-ros2-support/mcap_ros2/_dynamic.py

@@ -259,6 +259,8 @@ def _read_complex_type(
             "__slots__": [field.name for field in msgdef.fields],
             "__repr__": __repr__,
             "__str__": __repr__,
+            "__eq__": __eq__,
+            "__ne__": __ne__,
             "_type": str(msgdef.base_type),
             "_full_text": str(msgdef),
         },
@@ -605,3 +607,25 @@ def _coerce_values(
 def __repr__(self: Any) -> str:
     fields = ", ".join(f"{field}={getattr(self, field)}" for field in self.__slots__)
     return f"{self.__name__}({fields})"
+
+
+def __eq__(self: Any, other: Any) -> bool:
+    if not isinstance(other, type(self)):
+        return False
+
+    if (
+        not hasattr(self, "__slots__")
+        or not hasattr(other, "__slots__")
+        or len(self.__slots__) != len(other.__slots__)
+    ):
+        return False
+
+    for attr in self.__slots__:
+        if getattr(self, attr) != getattr(other, attr):
+            return False
+
+    return True
+
+
+def __ne__(self: Any, other: Any) -> bool:
+    return not __eq__(self, other)

python/mcap-ros2-support/tests/test_ros2_decoder.py

@@ -24,3 +24,16 @@ def test_ros2_decoder():
             assert ros_msg._full_text == "# std_msgs/Empty"
             count += 1
         assert count == 10
+
+
+def test_ros2_decoder_msg_eq():
+    with generate_sample_data() as m:
+        reader = make_reader(m, decoder_factories=[DecoderFactory()])
+
+        decoded_messages = reader.iter_decoded_messages("/chatter")
+        _, _, _, msg0 = next(decoded_messages)
+        _, _, _, msg1 = next(decoded_messages)
+        assert msg0.data == "string message 0"
+        assert msg1.data == "string message 1"
+        assert msg0 == msg0 and msg1 == msg1
+        assert msg0 != msg1 and msg1 != msg0