Drop SLCAN standard-ID data frames instead of aliasing them as extended

Review finding #8 (Medium). The SLCAN 't' command (11-bit standard-ID data
frame) was decoded into an ordinary Frame, indistinguishable from an
extended frame with a small ID because Frame carries no IDE flag - in
violation of the extended-only Interface contract that the SocketCAN and
python-can backends enforce. On a mixed bus a standard frame could parse
as a bogus transfer or even trigger a spurious node-ID collision re-roll.
Standard-ID 't' lines are now dropped alongside the already-dropped 'r'/'R'
remote frames; the reference consumes only CAN_EFF_FLAG frames.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CabiCDQ5DNBYq8WDKRzG4Z

Author: pavel-kirienko

src/pycyphal2/can/_media_slcan.py

@@ -6,7 +6,7 @@
 
 import logging
 
-from ._interface import CAN_EXT_ID_MASK, CAN_STD_ID_MASK, Frame
+from ._interface import CAN_EXT_ID_MASK, Frame
 from ._wire import DLC_TO_LENGTH, MTU_CAN_CLASSIC
 
 _logger = logging.getLogger(__name__)
@@ -126,12 +126,12 @@ def _parse_line(line: bytes) -> Frame | None:
     command = line[:1]
     if command in (b"T", b"x"):
         return _parse_data_frame(line, id_length=8, max_payload_length=MTU_CAN_CLASSIC)
-    if command == b"t":
-        return _parse_data_frame(line, id_length=3, max_payload_length=MTU_CAN_CLASSIC)
     if command == b"D":
         return _parse_data_frame(line, id_length=8, max_payload_length=64)
-    if command in (b"r", b"R"):
-        _logger.debug("SLCAN drop unsupported frame type cmd=%r", command)
+    if command in (b"t", b"r", b"R"):
+        # Standard-ID (11-bit) frames: the Interface contract is extended-only and Frame carries no IDE
+        # discriminator, so forwarding a 't' data frame would alias an extended frame with a small ID.
+        _logger.debug("SLCAN drop standard-id frame cmd=%r", command)
         return None
     _logger.debug("SLCAN drop unknown line=%r", line)
     return None
@@ -155,9 +155,6 @@ def _parse_data_frame(line: bytes, *, id_length: int, max_payload_length: int) -
     if len(line) < expected:
         _logger.debug("SLCAN drop data dlc mismatch len=%d expected=%d", len(line), expected)
         return None
-    if id_length == 3 and identifier > CAN_STD_ID_MASK:
-        _logger.debug("SLCAN drop invalid standard id=%x", identifier)
-        return None
     data = _parse_hex_bytes(line[header_length:expected])
     if data is None:
         _logger.debug("SLCAN drop malformed data id=%08x", identifier)

tests/can/test_media_slcan.py

@@ -53,9 +53,11 @@ def test_parse_classic_extended_frames() -> None:
 
     assert parser.feed(b"T000001232ABCD\r") == [Frame(id=0x123, data=b"\xab\xcd")]
     assert parser.feed(b"T000001232abCd\r") == [Frame(id=0x123, data=b"\xab\xcd")]
-    assert parser.feed(b"t1231AA\r") == [Frame(id=0x123, data=b"\xaa")]
-    assert parser.feed(b"t7FF1AA\r") == [Frame(id=0x7FF, data=b"\xaa")]
-    assert parser.feed(b"t7FF0\r") == [Frame(id=0x7FF, data=b"")]
+    # Standard-ID 't' data frames are dropped: the Interface contract is extended-only, and Frame has
+    # no IDE discriminator, so forwarding them would alias extended frames with small IDs.
+    assert parser.feed(b"t1231AA\r") == []
+    assert parser.feed(b"t7FF1AA\r") == []
+    assert parser.feed(b"t7FF0\r") == []
     assert parser.feed(b"T000001") == []
     assert parser.feed(b"230\r") == [Frame(id=0x123, data=b"")]
     assert parser.feed(b"x1BADC0DE201AB\r") == [Frame(id=0x1BADC0DE, data=b"\x01\xab")]
@@ -68,7 +70,7 @@ def test_parse_ignores_optional_frame_suffix() -> None:
     assert parser.feed(b"T000001232ABCDL\r") == [Frame(id=0x123, data=b"\xab\xcd")]
     assert parser.feed(b"T000001232ABCD1234L\r") == [Frame(id=0x123, data=b"\xab\xcd")]
     assert parser.feed(b"T000001232ABCDzzzz\r") == [Frame(id=0x123, data=b"\xab\xcd")]
-    assert parser.feed(b"t1231AAL\r") == [Frame(id=0x123, data=b"\xaa")]
+    assert parser.feed(b"t1231AAL\r") == []  # Standard-ID frames are dropped regardless of suffix.
     assert parser.feed(b"T000001232ABCD1234\x03\r") == [Frame(id=0x123, data=b"\xab\xcd")]
     assert parser.feed(b"T10AE6EFF8000000FF000000A07071\r") == [
         Frame(id=0x10AE6EFF, data=b"\x00\x00\x00\xff\x00\x00\x00\xa0"),