Use shared zeroconf instance and single service browser (#252)

Co-authored-by: J. Nick Koston <nick@koston.org>

Author: balloob

netdisco/discovery.py

@@ -45,11 +45,11 @@ def __init__(self):
         self.is_discovering = False
         self.discoverables = None
 
-    def scan(self):
+    def scan(self, zeroconf_instance=None):
         """Start and tells scanners to scan."""
         self.is_discovering = True
 
-        self.mdns = MDNS()
+        self.mdns = MDNS(zeroconf_instance)
 
         # Needs to be after MDNS init
         self._load_device_support()

netdisco/mdns.py

@@ -1,17 +1,26 @@
 """Add support for discovering mDNS services."""
 from typing import List  # noqa: F401
 
-import zeroconf
+from zeroconf import Zeroconf, ServiceBrowser, ServiceInfo, DNSRecord
+
+
+class FastServiceBrowser(ServiceBrowser):
+    """ServiceBrowser that does not process record updates."""
+
+    def update_record(self, zc: Zeroconf, now: float, record: DNSRecord) -> None:
+        """Ignore record updates as we only care about the cache anyways."""
+        return
 
 
 class MDNS:
     """Base class to discover mDNS services."""
 
-    def __init__(self):
+    def __init__(self, zeroconf_instance=None):
         """Initialize the discovery."""
-        self.zeroconf = None
-        self.services = []  # type: List[zeroconf.ServiceInfo]
-        self._browsers = []  # type: List[zeroconf.ServiceBrowser]
+        self.zeroconf = zeroconf_instance
+        self._created_zeroconf = False
+        self.services = []  # type: List[ServiceInfo]
+        self._browser = None  # type: ServiceBrowser
 
     def register_service(self, service):
         """Register a mDNS service."""
@@ -20,24 +29,31 @@ def register_service(self, service):
     def start(self):
         """Start discovery."""
         try:
-            self.zeroconf = zeroconf.Zeroconf()
+            if not self.zeroconf:
+                self.zeroconf = Zeroconf()
+                self._created_zeroconf = True
+
+            def _service_update(*args, **kwargs):
+                return
 
-            for service in self.services:
-                self._browsers.append(zeroconf.ServiceBrowser(
-                    self.zeroconf, service.typ, service))
+            types = [service.typ for service in self.services]
+            self._browser = FastServiceBrowser(
+                self.zeroconf, types, handlers=[_service_update]
+            )
         except Exception:  # pylint: disable=broad-except
             self.stop()
             raise
 
     def stop(self):
         """Stop discovering."""
-        while self._browsers:
-            self._browsers.pop().cancel()
+        if self._browser:
+            self._browser.cancel()
+            self._browser = None
 
         for service in self.services:
             service.reset()
 
-        if self.zeroconf:
+        if self._created_zeroconf:
             self.zeroconf.close()
             self.zeroconf = None