Reduce chance of accidental synchronization of ServiceInfo requests

bdraco · bdraco · commit b47fef305f8f · 2021-08-13T08:29:51.000-05:00
- ServiceInfo requests are frequently triggered by multicast responses
  to ServiceBrowser requests.  When multiple instances of zeroconf are
  present on the same network, they can all end up sending ServiceInfo
  queries at the same time.  We now use a random delay as described in
  rfc6762 sec 5.2 after the first request.  Ideally we would add the
  delay before the first query as well, however that may break existing
  workflows so it was not done at this time.
diff --git a/zeroconf/_services/info.py b/zeroconf/_services/info.py
@@ -21,6 +21,7 @@
 """
 
 import ipaddress
+import random
 import socket
 from typing import Any, Dict, List, Optional, TYPE_CHECKING, Union, cast
 
@@ -52,6 +53,16 @@
 )
 
 
+# https://datatracker.ietf.org/doc/html/rfc6762#section-5.2
+# The most common case for calling ServiceInfo is from a
+# ServiceBrowser. After the first request we add a few random
+# milliseconds to the delay between requests to reduce the chance
+# that there are multiple ServiceBrowser callbacks running on
+# the network that are firing at the same time when they
+# see the same multicast response and decide to refresh
+# the A/AAAA/SRV records for a host.
+_AVOID_SYNC_DELAY_RANDOM_INTERVAL = (20, 120)
+
 if TYPE_CHECKING:
     from .._core import Zeroconf
 
@@ -446,15 +457,17 @@ async def async_request(
                 if last <= now:
                     return False
                 if next_ <= now:
-                    out = self.generate_request_query(
-                        zc, now, question_type or DNSQuestionType.QU if first_request else DNSQuestionType.QM
+                    this_question_type = (
+                        question_type or DNSQuestionType.QU if first_request else DNSQuestionType.QM
                     )
+                    out = self.generate_request_query(zc, now, this_question_type)
                     first_request = False
                     if not out.questions:
                         return self.load_from_cache(zc)
                     zc.async_send(out)
                     next_ = now + delay
                     delay *= 2
+                    next_ += random.randint(*_AVOID_SYNC_DELAY_RANDOM_INTERVAL)
 
                 await zc.async_wait(min(next_, last) - now)
                 now = current_time_millis()
