Allow SSDP discovery msg sending to fail on almost all address families

Some machines cause a PermissionError when sending to IPv6, but if IPv4
still works fine we should just use that instead. This change counts the
number of failed address families, and only if all of them fail we stop
the discovery.

CHANGELOG.md

@@ -8,6 +8,7 @@ changed functionality, fixed bugs).
 - Fixed parsing of `--config` CLI param on Python 3.5
 - Added `--debug` CLI parameter to easily enable debug logging without having
   to edit `flamenco-worker.cfg`.
+- Only fail UPnP/SSDP discovery when it fails to send on both IPv4 and IPv6.
 
 
 ## Version 2.0.7 (released 2017-07-04)

flamenco_worker/ssdp_discover.py

@@ -57,6 +57,8 @@ def find_flamenco_manager(timeout=1, retries=5):
     families_and_addresses = list(unique(interface_addresses()))
 
     for _ in range(retries):
+        failed_families = 0
+
         for family, addr in families_and_addresses:
             try:
                 dest = DESTINATIONS[family]
@@ -64,17 +66,23 @@ def find_flamenco_manager(timeout=1, retries=5):
                 log.warning('Unknown address family %s, skipping', family)
                 continue
 
-            log.debug('Sending to %s %s, dest=%s' % (family, addr, dest))
+            log.debug('Sending to %s %s, dest=%s', family, addr, dest)
 
             sock = socket.socket(family, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
             sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
             sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, 2)
             sock.bind((addr, 0))
 
-            for _ in range(2):
-                # sending it more than once will
-                # decrease the probability of a timeout
-                sock.sendto(DISCOVERY_MSG, (dest, 1900))
+            try:
+                for _ in range(2):
+                    # sending it more than once will
+                    # decrease the probability of a timeout
+                    sock.sendto(DISCOVERY_MSG, (dest, 1900))
+            except PermissionError:
+                log.info('Failed sending UPnP/SSDP discovery message to %s %s, dest=%s',
+                         family, addr, dest)
+                failed_families += 1
+                continue
 
             try:
                 data = sock.recv(1024)
@@ -84,6 +92,11 @@ def find_flamenco_manager(timeout=1, retries=5):
                 response = Response(data)
                 return response.getheader('Location')
 
+        if failed_families >= len(families_and_addresses):
+            log.error('Failed to send UPnP/SSDP discovery message '
+                      'to every address family (IPv4/IPv6)')
+            break
+
     raise DiscoveryFailed('Unable to find Flamenco Manager after %i tries' % retries)