fix(server): treat a client disconnect as a normal end, not an error

snap7/server/__init__.py

@@ -2606,6 +2606,12 @@ def accept_connection(self) -> bool:
             logger.debug("ISO connection established")
             return True
 
+        except (ConnectionResetError, ConnectionAbortedError, TimeoutError) as e:
+            # A peer that goes away before the ISO handshake completes is
+            # routine - port scans, health checks, a cancelled connect - and
+            # says nothing about this server.
+            logger.info(f"Peer left before the ISO connection was established: {e}")
+            return False
         except Exception as e:
             logger.error(f"Error accepting ISO connection: {e}")
             return False
@@ -2638,6 +2644,16 @@ def receive_data(self) -> bytes:
 
             pdu_len, pdu_type, eot_num = struct.unpack(">BBB", payload[:3])
 
+            if pdu_type == self.COTP_DR:
+                # The peer is closing the connection the way ISO 8073 says to;
+                # confirm it and let the caller treat this as a normal end.
+                logger.debug("Received COTP DR from client")
+                try:
+                    self.socket.sendall(self._build_tpkt(self._build_cotp_dc()))
+                except OSError:
+                    pass  # the peer may already be gone
+                raise ConnectionAbortedError("Client requested disconnect")
+
             if pdu_type != self.COTP_DT:
                 raise S7ConnectionError(f"Expected COTP DT, got {pdu_type:#02x}")
 
@@ -2715,6 +2731,16 @@ def _build_cotp_cc(self) -> bytes:
 
         return base_pdu + pdu_size_param
 
+    def _build_cotp_dc(self) -> bytes:
+        """Build COTP Disconnect Confirm."""
+        return struct.pack(
+            ">BBHH",
+            5,  # PDU length
+            self.COTP_DC,  # PDU type
+            self.dst_ref,  # Destination reference
+            self.src_ref,  # Source reference
+        )
+
     def _recv_exact(self, size: int, deadline: float | None = None) -> bytes:
         """Receive exactly the specified bytes within one absolute deadline."""
         if size < 0:

tests/test_server.py

@@ -327,6 +327,49 @@ def test_connection_confirm_has_valid_length_and_tpdu_size(self) -> None:
         assert connection_confirm == bytes.fromhex("09d0000f000100c00109")
         assert connection_confirm[0] == len(connection_confirm) - 1
 
+    def test_disconnect_confirm_has_valid_length(self) -> None:
+        client_socket = MagicMock()
+        connection = ServerISOConnection(client_socket)
+        connection.dst_ref = 0x000F
+        connection.src_ref = 0x0001
+
+        disconnect_confirm = connection._build_cotp_dc()
+
+        assert disconnect_confirm == bytes.fromhex("05c0000f0001")
+        assert disconnect_confirm[0] == len(disconnect_confirm) - 1
+
+    def test_a_disconnect_request_ends_the_connection_normally(self) -> None:
+        # The client sends a COTP DR when it disconnects; treating it as an
+        # unexpected PDU logs an error for an ordinary goodbye.
+        client_socket = MagicMock()
+        connection = ServerISOConnection(client_socket)
+        connection._recv_exact = MagicMock(
+            side_effect=[
+                b"\x03\x00\x00\x0b",
+                b"\x06\x80\x00\x00\x01\x00\x00",
+            ]
+        )
+
+        with pytest.raises(ConnectionAbortedError):
+            connection.receive_data()
+
+        sent = b"".join(call.args[0] for call in client_socket.sendall.call_args_list)
+        assert sent[5:6] == bytes([connection.COTP_DC]), "the disconnect is confirmed"
+
+    def test_a_disconnect_request_is_confirmed_even_if_the_peer_is_gone(self) -> None:
+        client_socket = MagicMock()
+        client_socket.sendall.side_effect = OSError("broken pipe")
+        connection = ServerISOConnection(client_socket)
+        connection._recv_exact = MagicMock(
+            side_effect=[
+                b"\x03\x00\x00\x0b",
+                b"\x06\x80\x00\x00\x01\x00\x00",
+            ]
+        )
+
+        with pytest.raises(ConnectionAbortedError):
+            connection.receive_data()
+
     def test_partial_frame_timeout_closes_connection(self) -> None:
         client_socket = MagicMock()
         client_socket.recv.side_effect = [b"\x03", TimeoutError()]