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>

bin/frappy-scan

@@ -0,0 +1,128 @@
+#!/usr/bin/env python3
+# *****************************************************************************
+#
+# 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>
+#
+# *****************************************************************************
+
+"""SEC node autodiscovery tool."""
+
+import argparse
+import json
+import os
+import select
+import socket
+import sys
+from collections import namedtuple
+from time import time as currenttime
+
+UDP_PORT = 10767
+
+Answer = namedtuple('Answer',
+                    'address, port, equipment_id, firmware, description')
+
+
+def decode(msg, addr):
+    msg = msg.decode('utf-8')
+    try:
+        data = json.loads(msg)
+    except Exception:
+        return None
+    if not isinstance(data, dict):
+        return None
+    if data.get('SECoP') != 'node':
+        return None
+    try:
+        eq_id = data['equipment_id']
+        fw = data['firmware']
+        desc = data['description']
+        port = data['port']
+    except KeyError:
+        return None
+    addr, _scanport = addr
+    return Answer(addr, port, eq_id, fw, desc)
+
+
+def print_answer(answer, *, short=False):
+    if short:
+        # NOTE: keep this easily parseable!
+        print(f'{answer.equipment_id} {answer.address}:{answer.port}')
+        return
+    print(f'Found {answer.equipment_id} at {answer.address}:')
+    print(f'  Port: {answer.port}')
+    print(f'  Firmware: {answer.firmware}')
+    desc = answer.description.replace('\n', '\n    ')
+    print(f'  Node description: {desc}')
+    print()
+
+
+def scan(max_wait=1.0):
+    s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+    s.setsockopt(socket.SOL_SOCKET, socket.SO_BROADCAST, 1)
+    # send a general broadcast
+    try:
+        s.sendto(json.dumps(dict(SECoP='discover')).encode('utf-8'),
+                 ('255.255.255.255', UDP_PORT))
+    except OSError as e:
+        print('could not send the broadcast:', e)
+        # we still keep listening for self-announcements
+    start = currenttime()
+    seen = set()
+    while currenttime() < start + max_wait:
+        res = select.select([s], [], [], 0.1)
+        if res[0]:
+            try:
+                msg, addr = s.recvfrom(1024)
+            except socket.error:  # pragma: no cover
+                continue
+            answer = decode(msg, addr)
+            if answer is None:
+                continue
+            if (answer.address, answer.equipment_id, answer.port) in seen:
+                continue
+            seen.add((answer.address, answer.equipment_id, answer.port))
+            yield answer
+
+
+def listen(*, short=False):
+    s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+    if os.name == 'nt':
+        s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+    else:
+        s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEPORT, 1)
+    s.bind(('0.0.0.0', UDP_PORT))
+    while True:
+        try:
+            msg, addr = s.recvfrom(1024)
+        except KeyboardInterrupt:
+            break
+        answer = decode(msg, addr)
+        if answer:
+            print_answer(answer, short=short)
+
+
+if __name__ == '__main__':
+    parser = argparse.ArgumentParser()
+    parser.add_argument('-l', '--listen', action='store_true',
+                        help='Print short info. '
+                        'Keep listening after the broadcast.')
+    args = parser.parse_args(sys.argv[1:])
+    for answer in scan():
+        print_answer(answer, short=args.listen)
+    if args.listen:
+        listen(short=args.listen)