Speed up socketcan capture_message by ~32%

`socket.MSG_DONTROUTE` is an `enum.IntFlag`, so masking it built a
`MsgFlag` per frame; it is now a module-level int. `Message` is built
positionally to skip keyword binding, with a new test pinning the order.

Live 500 kbit/s bus (aarch64, CPython 3.13.15): 107.8 -> 73.3 us CPU per
frame; output identical over 3000 real and 1995 synthetic frames.

Refs #1135

Author: TheTrueAI

can/interfaces/socketcan/socketcan.py

@@ -47,6 +47,8 @@
     CMSG_SPACE(RECEIVED_TIMESTAMP_STRUCT.size) if CMSG_SPACE_available else 0
 )
 
+MSG_DONTROUTE = int(socket.MSG_DONTROUTE)
+
 
 # Setup BCM struct
 def bcm_header_factory(
@@ -656,7 +658,7 @@ def capture_message(sock: socket.socket, get_channel: bool = False) -> Message |
     error_state_indicator = bool(flags & constants.CANFD_ESI)
 
     # Section 4.7.1: MSG_DONTROUTE: set when the received frame was created on the local host.
-    is_rx = not bool(msg_flags & socket.MSG_DONTROUTE)
+    is_rx = not msg_flags & MSG_DONTROUTE
 
     if is_extended_frame_format:
         # log.debug("CAN: Extended")
@@ -666,23 +668,22 @@ def capture_message(sock: socket.socket, get_channel: bool = False) -> Message |
         # log.debug("CAN: Standard")
         arbitration_id = can_id & 0x000007FF
 
-    msg = Message(
-        timestamp=timestamp,
-        channel=channel,
-        arbitration_id=arbitration_id,
-        is_extended_id=is_extended_frame_format,
-        is_remote_frame=is_remote_transmission_request,
-        is_error_frame=is_error_frame,
-        is_fd=is_fd,
-        is_rx=is_rx,
-        bitrate_switch=bitrate_switch,
-        error_state_indicator=error_state_indicator,
-        dlc=can_dlc,
-        data=data,
+    # Built positionally: binding twelve keyword arguments is slower.
+    return Message(
+        timestamp,
+        arbitration_id,
+        is_extended_frame_format,
+        is_remote_transmission_request,
+        is_error_frame,
+        channel,
+        can_dlc,
+        data,
+        is_fd,
+        is_rx,
+        bitrate_switch,
+        error_state_indicator,
     )
 
-    return msg
-
 
 class SocketcanBus(BusABC):  # pylint: disable=abstract-method
     """A SocketCAN interface to CAN.

doc/changelog.d/2097.changed.rst

@@ -0,0 +1 @@
+Improved performance of the ``socketcan`` receive path: ``capture_message`` now uses roughly a third less CPU per frame.

test/test_socketcan.py

@@ -5,6 +5,8 @@
 """
 
 import ctypes
+import inspect
+import socket
 import struct
 import sys
 import unittest
@@ -20,6 +22,7 @@
     TX_COUNTEVT,
 )
 from can.interfaces.socketcan.socketcan import (
+    MSG_DONTROUTE,
     BcmMsgHead,
     bcm_header_factory,
     build_bcm_header,
@@ -391,5 +394,43 @@ def test_pypy_socketcan_support(self):
                 )
 
 
+class SocketCANHotPathTest(unittest.TestCase):
+    """Guard the per-frame optimisations in :func:`capture_message`."""
+
+    def test_msg_dontroute_is_plain_int(self):
+        """``socket.MSG_DONTROUTE`` is an ``enum.IntFlag`` member on CPython, and
+        evaluating ``msg_flags & <IntFlag>`` once per received frame constructs a new
+        flag instance each time. ``capture_message`` therefore tests against a plain
+        int, which must still carry the same value.
+        """
+        self.assertEqual(MSG_DONTROUTE, socket.MSG_DONTROUTE)
+        self.assertIs(type(MSG_DONTROUTE), int)
+
+    def test_capture_message_message_args(self):
+        """``capture_message`` builds its :class:`~can.Message` positionally to avoid
+        per-frame keyword binding, so a reordering of ``Message.__init__`` would
+        silently corrupt every received frame. Pin the expected order.
+        """
+        params = list(inspect.signature(can.Message.__init__).parameters)
+        self.assertEqual(
+            params[:13],
+            [
+                "self",
+                "timestamp",
+                "arbitration_id",
+                "is_extended_id",
+                "is_remote_frame",
+                "is_error_frame",
+                "channel",
+                "dlc",
+                "data",
+                "is_fd",
+                "is_rx",
+                "bitrate_switch",
+                "error_state_indicator",
+            ],
+        )
+
+
 if __name__ == "__main__":
     unittest.main()