fix: don't modify msg.data when padding frame for GsUsbFrame

can/interfaces/gs_usb.py

@@ -142,14 +142,14 @@ def send(self, msg: can.Message, timeout: float | None = None) -> None:
         if msg.is_error_frame:
             can_id = can_id | CAN_ERR_FLAG
 
-        # Pad message data
-        msg.data.extend([0x00] * (CAN_MAX_DLC - len(msg.data)))
+        # Copy and pad message data without mutating msg.data
+        msg_data = msg.data + bytearray([0x00] * (CAN_MAX_DLC - len(msg.data)))
 
         frame = GsUsbFrame()
         frame.can_id = can_id
         frame.can_dlc = msg.dlc
         frame.timestamp_us = 0  # timestamp frame field is only useful on receive
-        frame.data = list(msg.data)
+        frame.data = list(msg_data)
 
         try:
             self.gs_usb.send(frame)

doc/changelog.d/2085.fixed.rst

@@ -0,0 +1 @@
+Fixed gs_usb.py modyfying the data member of the can.Message msg passed to GsUsbBus send method.

test/test_interface_gs_usb.py

@@ -2,6 +2,7 @@
 
 from unittest.mock import MagicMock, patch
 
+import can
 import pytest
 
 from can.interfaces.gs_usb import (
@@ -65,3 +66,22 @@ def test_find_devices_returns_empty_list_when_no_devices(mock_find):
     devices = _find_gs_usb_devices()
 
     assert devices == []
+
+
+@patch("can.interfaces.gs_usb.GsUsb")
+@patch("can.interfaces.gs_usb._find_gs_usb_devices")
+def test_send_dlc_zero_does_not_modify_message_data(mock_find_devices, mock_gs_usb_cls):
+    """Verify that send() does not mutate msg.data when DLC < CAN_MAX_DLC."""
+    mock_find_devices.return_value = [MagicMock()]
+    mock_gs_usb = MagicMock()
+    mock_gs_usb_cls.return_value = mock_gs_usb
+    mock_gs_usb.device_capability.fclk_can = 48_000_000
+
+    bus = GsUsbBus(channel=0, bitrate=500_000)
+    msg = can.Message(arbitration_id=0x123, data=bytearray(), dlc=0)
+    msg_data_copy = msg.data.copy()
+
+    bus.send(msg)
+
+    assert msg.data == msg_data_copy
+    mock_gs_usb.send.assert_called_once()