Reduce chance of accidental synchronization of ServiceInfo requests (#955)

bdraco · web-flow · commit c77293692062 · 2021-08-13T09:11:51.000-05:00
diff --git a/tests/services/test_info.py b/tests/services/test_info.py
@@ -754,3 +754,25 @@ def test_request_timeout():
     # 3000ms for the default timeout
     # 1000ms for loaded systems + schedule overhead
     assert (end_time - start_time) < 3000 + 1000
+
+
+@pytest.mark.asyncio
+async def test_we_try_four_times_with_random_delay():
+    """Verify we try four times even with the random delay."""
+    type_ = "_typethatisnothere._tcp.local."
+    aiozc = AsyncZeroconf(interfaces=['127.0.0.1'])
+
+    # we are going to patch the zeroconf send to check query transmission
+    request_count = 0
+    def async_send(out, addr=const._MDNS_ADDR, port=const._MDNS_PORT):
+        """Sends an outgoing packet."""
+        nonlocal request_count
+        request_count += 1
+
+    # patch the zeroconf send
+    with patch.object(aiozc.zeroconf, "async_send", async_send):
+        await aiozc.async_get_service_info(f"willnotbefound.{type_}", type_)
+
+    await aiozc.async_close()
+
+    assert request_count == 4
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
 
@@ -455,6 +466,7 @@ async def async_request(
                     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()
