Unregister a Zeroconf instance if it already exists before publishing a backend service.

`mdns` connection are another culprit for the increasing number of open
files in the process.

platypush/backend/__init__.py

@@ -402,6 +402,13 @@ class Backend(Thread, EventGenerator, ExtensionWithManifest):
             )
             return
 
+        if self.zeroconf:
+            self.logger.info(
+                'Zeroconf service already registered for %s, removing the previous instance',
+                self.__class__.__name__,
+            )
+            self.unregister_service()
+
         self.zeroconf = Zeroconf()
         srv_desc = {
             'name': 'Platypush',

platypush/plugins/zeroconf/__init__.py

@@ -46,9 +46,7 @@ class ZeroconfListener(ServiceListener):
             'properties': {
                 k.decode()
                 if isinstance(k, bytes)
-                else k: v.decode()
-                if isinstance(v, bytes)
-                else v
+                else k: (v.decode() if isinstance(v, bytes) else v)
                 for k, v in info.properties.items()
             },
             'server': info.server,
@@ -166,9 +164,7 @@ class ZeroconfPlugin(Plugin):
                     get_bus().post(evt)
                 except queue.Empty:
                     if not services:
-                        self.logger.warning(
-                            'No such service discovered: {}'.format(service)
-                        )
+                        self.logger.warning('No such service discovered: %s', service)
         finally:
             if browser:
                 browser.cancel()