Allow CAN unicast to remote node-ID 0

Review finding #12 (Medium). Node-ID 0 is a valid regular node in
Cyphal/CAN v1 (only v0/DroneCAN treated 0 as anonymous; libcanard permits
a remote 0 and a service destination 0), and the wire encoder already
accepts it, but unicast() guarded 1 <= remote_id, so the ACK path could
not answer a node-0 peer and reliable delivery from such a peer never
completed. The destination range is now 0..127.

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

Author: pavel-kirienko

src/pycyphal2/can/_transport.py

@@ -250,7 +250,9 @@ def unicast_listen(self, handler: Callable[[TransportArrival], None]) -> None:
     async def unicast(self, deadline: Instant, priority: Priority, remote_id: int, message: bytes | memoryview) -> None:
         if self._closed:
             raise ClosedError("CAN transport closed")
-        if not (1 <= remote_id <= NODE_ID_MAX):
+        # Node-ID 0 is a valid regular node in Cyphal/CAN v1 (only v0 treated it as anonymous), so it is
+        # a legal unicast destination; rejecting it would make the ACK path unable to answer a node-0 peer.
+        if not (0 <= remote_id <= NODE_ID_MAX):
             raise ValueError(f"Invalid remote node-ID: {remote_id}")
         transfer_id = self._unicast_tid[remote_id]
         self._unicast_tid[remote_id] = (transfer_id + 1) % TRANSFER_ID_MODULO

tests/can/test_transport_internal.py

@@ -10,7 +10,7 @@
 from pycyphal2._transport import SUBJECT_ID_MODULUS_16bit, TransportArrival
 from pycyphal2.can import CANTransport, TimestampedFrame
 from pycyphal2.can._transport import _CANTransportImpl, _PinnedSubjectState
-from pycyphal2.can._wire import NODE_ID_ANONYMOUS, TransferKind, serialize_transfer
+from pycyphal2.can._wire import NODE_ID_ANONYMOUS, TransferKind, parse_frame, serialize_transfer
 from tests.can._support import MockCANBus, MockCANInterface, wait_for
 
 
@@ -99,9 +99,18 @@ async def test_writer_unicast_and_send_transfer_error_paths() -> None:
     with pytest.raises(ClosedError, match="CAN transport closed"):
         await writer2(Instant.now() + 1.0, Priority.NOMINAL, b"x")
 
-    live = CANTransport.new(MockCANInterface(bus, "if1"))
-    with pytest.raises(ValueError, match="Invalid remote node-ID"):
-        await live.unicast(Instant.now() + 1.0, Priority.NOMINAL, 0, b"x")
+    live_if = MockCANInterface(bus, "if1")
+    live = CANTransport.new(live_if)
+    # Node-ID 0 is a valid regular Cyphal/CAN v1 node, hence a legal unicast destination;
+    # the wire encoding must carry destination 0.
+    await live.unicast(Instant.now() + 1.0, Priority.NOMINAL, 0, b"x")
+    uni_id, uni_frames, _ = live_if.enqueue_history[-1]
+    parsed = parse_frame(uni_id, uni_frames[0])
+    assert parsed is not None
+    assert parsed.destination_id == 0
+    for bad_remote in (-1, 128):
+        with pytest.raises(ValueError, match="Invalid remote node-ID"):
+            await live.unicast(Instant.now() + 1.0, Priority.NOMINAL, bad_remote, b"x")
 
     live_impl = cast(_CANTransportImpl, live)
     with pytest.raises(SendError, match="Deadline exceeded"):