send udp message to feeder

inform the history feeder in case of a SECoP server without
UDP mechanism

devices.py

@@ -27,6 +27,8 @@ connected to a SEC node
 """
 
 import threading
+import socket
+import json
 from nicos import config, session
 from nicos.core import Override, Param, Moveable, status, POLLER, SIMULATION, DeviceAlias, \
     Device, anytype, listof, MASTER
@@ -38,6 +40,8 @@ from nicos.commands.basic import AddSetup, CreateAllDevices, CreateDevice
 from nicos.utils import loggers
 from servicemanager import FrappyManager, SeaManager, Reconnect, Keep
 
+
+SECOP_UDP_PORT = 10767
 SERVICES = FrappyManager.services
 
 
@@ -113,6 +117,21 @@ def get_frappy_config():
         return None
 
 
+def send_other_udp(uri, instrument, device=None):
+    """inform the feeder about the start of a frappy server"""
+    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+    sock.setsockopt(socket.SOL_SOCKET, socket.SO_BROADCAST, 1)
+    msg = {
+        'SECoP': 'for_other_node',
+        'uri': uri,
+        'instrument': instrument,
+    }
+    if device:
+        msg['device'] = device
+    msg = json.dumps(msg, ensure_ascii=False, separators=(',', ':')).encode('utf-8')
+    sock.sendto(msg, ('255.255.255.255', SECOP_UDP_PORT))
+
+
 class FrappyConfig(Device):
     # respect the order: e.g. temperature_regulation must be after temperature
     # because it will not be added to envlist when temperature is the same device
@@ -830,8 +849,10 @@ class FrappyNode(SecNodeDevice, Moveable):
                     if available_cfg is not None:
                         raise ValueError('use "frappy_list()" to get a list of valid frappy configurations')
                 uri = 'localhost:%d' % info[self.service]
+                send_other_udp(uri, config.instrument, device=cfg)
             else:
                 uri = cfg
+                send_other_udp(uri, config.instrument)
             if uri != self.uri:
                 self._disconnect(keeptarget=True)
             if uri: