Fix #4013 _resolve_host race conditions

CHANGES/4013.bugfix

@@ -0,0 +1 @@
+Fixed race conditions in _resolve_host caching and throttling when tracing is enabled.

CONTRIBUTORS.txt

@@ -132,6 +132,7 @@ Jian Zeng
 Jinkyu Yi
 Joel Watts
 Jon Nabozny
+Jonas Obrist
 Joongi Kim
 Josep Cugat
 Joshu Coats

aiohttp/connector.py

@@ -778,24 +778,28 @@ async def _resolve_host(self,
 
         if (key in self._cached_hosts) and \
                 (not self._cached_hosts.expired(key)):
+            # get result early, before any await (#4014)
+            result = self._cached_hosts.next_addrs(key)
 
             if traces:
                 for trace in traces:
                     await trace.send_dns_cache_hit(host)
-
-            return self._cached_hosts.next_addrs(key)
+            return result
 
         if key in self._throttle_dns_events:
+            # get event early, before any await (#4014)
+            event = self._throttle_dns_events[key]
             if traces:
                 for trace in traces:
                     await trace.send_dns_cache_hit(host)
-            await self._throttle_dns_events[key].wait()
+            await event.wait()
         else:
+            # update dict early, before any await (#4014)
+            self._throttle_dns_events[key] = \
+                EventResultOrError(self._loop)
             if traces:
                 for trace in traces:
                     await trace.send_dns_cache_miss(host)
-            self._throttle_dns_events[key] = \
-                EventResultOrError(self._loop)
             try:
 
                 if traces:

tests/test_connector.py

@@ -18,8 +18,9 @@
 from aiohttp import client, web
 from aiohttp.client import ClientRequest, ClientTimeout
 from aiohttp.client_reqrep import ConnectionKey
-from aiohttp.connector import Connection, _DNSCacheTable
+from aiohttp.connector import Connection, TCPConnector, _DNSCacheTable
 from aiohttp.helpers import PY_37
+from aiohttp.locks import EventResultOrError
 from aiohttp.test_utils import make_mocked_coro, unused_port
 from aiohttp.tracing import Trace
 from conftest import needs_unix
@@ -2257,3 +2258,32 @@ def test_next_addrs_single(self, dns_cache_table) -> None:
 
         addrs = dns_cache_table.next_addrs('foo')
         assert addrs == ['127.0.0.1']
+
+
+async def test_connector_cache_trace_race():
+    class DummyTracer:
+        async def send_dns_cache_hit(self, *args, **kwargs):
+            connector._cached_hosts.remove(("", 0))
+
+    token = object()
+    connector = TCPConnector()
+    connector._cached_hosts.add(("", 0), [token])
+
+    traces = [DummyTracer()]
+    assert await connector._resolve_host("", 0, traces) == [token]
+
+
+async def test_connector_throttle_trace_race(loop):
+    key = ("", 0)
+    token = object()
+
+    class DummyTracer:
+        async def send_dns_cache_hit(self, *args, **kwargs):
+            event = connector._throttle_dns_events.pop(key)
+            event.set()
+            connector._cached_hosts.add(key, [token])
+
+    connector = TCPConnector()
+    connector._throttle_dns_events[key] = EventResultOrError(loop)
+    traces = [DummyTracer()]
+    assert await connector._resolve_host("", 0, traces) == [token]