Add feature to detect socketcand beacon

can/interfaces/socketcand/__init__.py

@@ -8,7 +8,8 @@
 
 __all__ = [
     "SocketCanDaemonBus",
+    "detect_beacon",
     "socketcand",
 ]
 
-from .socketcand import SocketCanDaemonBus
+from .socketcand import SocketCanDaemonBus, detect_beacon

can/interfaces/socketcand/socketcand.py

@@ -13,12 +13,115 @@
 import socket
 import time
 import traceback
+import urllib.parse as urlparselib
+import xml.etree.ElementTree as ET
 from collections import deque
+from typing import List
 
 import can
 
 log = logging.getLogger(__name__)
 
+DEFAULT_SOCKETCAND_DISCOVERY_ADDRESS = ""
+DEFAULT_SOCKETCAND_DISCOVERY_PORT = 42000
+
+
+def detect_beacon(timeout_ms) -> List[can.typechecking.AutoDetectedConfig]:
+    """
+    Detects socketcand servers
+
+        This is what :meth:`can.SocketCanDaemonBus._detect_available_configs`
+        uses to search for available socketcand servers with fixed timeout of
+        3.1 seconds (socketcand sends out a beacon packet every 3 seconds).
+
+        Using this method directly allows for adjusting the timeout. Extending
+        the timeout beyond the default time period could be useful if UDP
+        packet loss is a concern.
+
+        :param timeout_ms:
+            Timeout in milliseconds to wait for socketcand beacon packets
+
+        :return:
+            See :meth:`can.BusABC._detect_available_configs`
+    """
+    with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as sock:
+        sock.bind(
+            (DEFAULT_SOCKETCAND_DISCOVERY_ADDRESS, DEFAULT_SOCKETCAND_DISCOVERY_PORT)
+        )
+        log.info(
+            "Listening on for socketcand UDP advertisement on %s:%s",
+            DEFAULT_SOCKETCAND_DISCOVERY_ADDRESS,
+            DEFAULT_SOCKETCAND_DISCOVERY_PORT,
+        )
+
+        now = time.time() * 1000
+        end_time = now + timeout_ms
+        while (time.time() * 1000) < end_time:
+            try:
+                # get all sockets that are ready (can be a list with a single value
+                # being self.socket or an empty list if self.socket is not ready)
+                ready_receive_sockets, _, _ = select.select([sock], [], [], 1)
+
+                if not ready_receive_sockets:
+                    log.debug("No advertisement received")
+                    continue
+
+                msg = sock.recv(1024).decode("utf-8")
+                root = ET.fromstring(msg)
+                if root.tag != "CANBeacon":
+                    log.debug("Unexpected message received over UDP")
+                    continue
+
+                det_devs = []
+                det_host = None
+                det_port = None
+                for child in root:
+                    if child.tag == "Bus":
+                        bus_name = child.attrib["name"]
+                        det_devs.append(bus_name)
+                    elif child.tag == "URL":
+                        url = urlparselib.urlparse(child.text)
+                        det_host = url.hostname
+                        det_port = url.port
+
+                if not det_devs:
+                    log.debug(
+                        "Got advertisement, but no SocketCAN devices advertised by socketcand"
+                    )
+                    continue
+
+                if (det_host is None) or (det_port is None):
+                    det_host = None
+                    det_port = None
+                    log.debug(
+                        "Got advertisement, but no SocketCAN URL advertised by socketcand"
+                    )
+                    continue
+
+                log.info(f"Found SocketCAN devices: {det_devs}")
+                return [
+                    {
+                        "interface": "socketcand",
+                        "host": det_host,
+                        "port": det_port,
+                        "channel": channel,
+                    }
+                    for channel in det_devs
+                ]
+
+            except ET.ParseError:
+                log.debug("Unexpected message received over UDP")
+                continue
+
+            except Exception as exc:
+                # something bad happened (e.g. the interface went down)
+                log.error(f"Failed to detect beacon: {exc}  {traceback.format_exc()}")
+                raise OSError(
+                    f"Failed to detect beacon: {exc} {traceback.format_exc()}"
+                )
+
+        return []
+
 
 def convert_ascii_message_to_can_message(ascii_msg: str) -> can.Message:
     if not ascii_msg.startswith("< frame ") or not ascii_msg.endswith(" >"):
@@ -79,6 +182,9 @@ class SocketCanDaemonBus(can.BusABC):
     def __init__(self, channel, host, port, tcp_tune=False, can_filters=None, **kwargs):
         """Connects to a CAN bus served by socketcand.
 
+        It implements :meth:`can.BusABC._detect_available_configs` to search for
+        available interfaces.
+
         It will attempt to connect to the server for up to 10s, after which a
         TimeoutError exception will be thrown.
 
@@ -231,3 +337,13 @@ def shutdown(self):
         """Stops all active periodic tasks and closes the socket."""
         super().shutdown()
         self.__socket.close()
+
+    @staticmethod
+    def _detect_available_configs() -> List[can.typechecking.AutoDetectedConfig]:
+        try:
+            # Time between beacons no more than 3 seconds. Allow for a little
+            # more than 3s
+            return detect_beacon(3.1)
+        except Exception as e:
+            log.warning(f"Could not detect socketcand beacon: {e}")
+            return []

doc/interfaces/socketcand.rst

@@ -2,16 +2,16 @@
 
 socketcand Interface
 ====================
-`Socketcand <https://github.com/linux-can/socketcand>`__ is part of the 
-`Linux-CAN <https://github.com/linux-can>`__ project, providing a 
+`Socketcand <https://github.com/linux-can/socketcand>`__ is part of the
+`Linux-CAN <https://github.com/linux-can>`__ project, providing a
 Network-to-CAN bridge as a Linux damon. It implements a specific
 `TCP/IP based communication protocol <https://github.com/linux-can/socketcand/blob/master/doc/protocol.md>`__
 to transfer CAN frames and control commands.
 
 The main advantage compared to UDP-based protocols (e.g. virtual interface)
 is, that TCP guarantees delivery and that the message order is kept.
 
-Here is a small example dumping all can messages received by a socketcand 
+Here is a small example dumping all can messages received by a socketcand
 daemon running on a remote Raspberry Pi:
 
 .. code-block:: python
@@ -37,6 +37,33 @@ The output may look like this::
     Timestamp: 1637791111.609763    ID: 0000031d    X Rx                DLC:  8    16 27 d8 3d fe d8 31 24
     Timestamp: 1637791111.634630    ID: 00000587    X Rx                DLC:  8    4e 06 85 23 6f 81 2b 65
 
+
+This interface also supports :meth:`can.BusABC._detect_available_configs`.
+
+.. code-block:: python
+
+    import can
+    import can.interfaces.socketcand
+
+    cfg = can.interfaces.socketcand._detect_available_configs()
+    if cfg:
+        bus = can.Bus(**cfg[0])
+
+The socketcand daemon broadcasts UDP beacons every 3 seconds. The default
+detection method waits for slightly more than 3 seconds to receive the beacon
+packet. If you want to increase the timeout, you can use
+:meth:`can.interfaces.socketcand.detect_beacon` directly. Below is an example
+which detects the beacon and uses the configuration to create a socketcand bus.
+
+.. code-block:: python
+
+    import can
+    import can.interfaces.socketcand
+
+    cfg = can.interfaces.socketcand.detect_beacon(6000)
+    if cfg:
+        bus = can.Bus(**cfg[0])
+
 Bus
 ---