server: service discovery over UDP.

implement RFC-005
- server broadcasts once on startup and answers to broadcasts
- small tool for listening on the port and sending broadcasts

Change-Id: I02d1184d6be62bef6f964eb9d238220aef062e94
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/34851
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Georg Brandl <g.brandl@fz-juelich.de>

frappy/protocol/discovery.py

@@ -0,0 +1,108 @@
+# *****************************************************************************
+#
+# This program is free software; you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free Software
+# Foundation; either version 2 of the License, or (at your option) any later
+# version.
+#
+# This program is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+# FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more
+# details.
+#
+# You should have received a copy of the GNU General Public License along with
+# this program; if not, write to the Free Software Foundation, Inc.,
+# 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+#
+# Module authors:
+#   Alexander Zaft <a.zaft@fz-juelich.de>
+#
+# *****************************************************************************
+"""Discovery via UDP broadcasts."""
+
+import os
+import json
+import socket
+
+from frappy.lib import closeSocket
+from frappy.protocol.interface.tcp import format_address
+from frappy.version import get_version
+
+UDP_PORT = 10767
+MAX_MESSAGE_LEN = 508
+
+
+class UDPListener:
+    def __init__(self, equipment_id, description, ifaces, logger, *,
+                 startup_broadcast=True):
+        self.equipment_id = equipment_id
+        self.log = logger
+        self.description = description or ''
+        self.firmware = 'FRAPPY ' + get_version()
+        self.ports = [int(iface.split('://')[1])
+                      for iface in ifaces if iface.startswith('tcp')]
+        self.running = False
+        self.is_enabled = True
+        self.startup_broadcast = startup_broadcast
+
+        self.sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+        if os.name == 'nt':
+            self.sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+        else:
+            self.sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEPORT, 1)
+        if startup_broadcast:
+            self.sock.setsockopt(socket.SOL_SOCKET, socket.SO_BROADCAST, 1)
+        self.sock.bind(('0.0.0.0', UDP_PORT))
+
+        available = MAX_MESSAGE_LEN - len(self._getMessage(2**16-1))
+        if available < 0:
+            desc_length = len(self.description.encode('utf-8'))
+            if available + desc_length < 0:
+                self.log.warn('Equipment id and firmware name exceed 430 byte '
+                              'limit, not answering to udp discovery')
+                self.is_enabled = False
+            else:
+                self.log.debug('truncating description for udp discovery')
+                # with errors='ignore', cutting insite a utf-8 glyph will not
+                # report an error but remove the rest of the glyph from the
+                # output.
+                self.description = self.description \
+                                       .encode('utf-8')[:available] \
+                                       .decode('utf-8', errors='ignore')
+
+    def _getMessage(self, port):
+        return json.dumps({
+            'SECoP': 'node',
+            'port': port,
+            'equipment_id': self.equipment_id,
+            'firmware': self.firmware,
+            'description': self.description,
+        }, ensure_ascii=False, separators=(',', ':')).encode('utf-8')
+
+    def run(self):
+        if self.startup_broadcast:
+            self.log.debug('Sending startup UDP broadcast.')
+            for port in self.ports:
+                self.sock.sendto(self._getMessage(port),
+                                 ('255.255.255.255', UDP_PORT))
+        self.running = True
+        while self.running and self.is_enabled:
+            try:
+                msg, addr = self.sock.recvfrom(1024)
+            except socket.error:
+                return
+            try:
+                request = json.loads(msg.decode('utf-8'))
+            except json.JSONDecodeError:
+                continue
+            if 'SECoP' not in request or request['SECoP'] != 'discover':
+                continue
+            self.log.debug('Answering UDP broadcast from: %s',
+                           format_address(addr))
+            for port in self.ports:
+                self.sock.sendto(self._getMessage(port), addr)
+
+    def shutdown(self):
+        self.log.debug('shut down of discovery listener')
+        self.running = False
+        closeSocket(self.sock)