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Merged
bdraco merged 2 commits into from
Aug 13, 2021
Merged

Reduce chance of accidental synchronization of ServiceInfo requests #955

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b5037f4
Reduce chance of accidental synchronization of ServiceInfo requests
bdraco Aug 13, 2021
40bb735
test for four queries with default timeout
bdraco Aug 13, 2021
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22 changes: 22 additions & 0 deletions tests/services/test_info.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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
12 changes: 12 additions & 0 deletions zeroconf/_services/info.py
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Original file line number Diff line number Diff line change
Expand Up @@ -21,6 +21,7 @@
"""

import ipaddress
import random
import socket
from typing import Any, Dict, List, Optional, TYPE_CHECKING, Union, cast

Expand Down Expand Up @@ -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

Expand Down Expand Up @@ -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()
Expand Down