Fix DNSCache race-condition (#2620)

aiohttp/connector.py

@@ -8,6 +8,7 @@
 from hashlib import md5, sha1, sha256
 from http.cookies import SimpleCookie
 from itertools import cycle, islice
+from random import shuffle
 from time import monotonic
 from types import MappingProxyType
 
@@ -547,7 +548,6 @@ class _DNSCacheTable:
 
     def __init__(self, ttl=None):
         self._addrs = {}
-        self._addrs_rr = {}
         self._timestamps = {}
         self._ttl = ttl
 
@@ -560,28 +560,24 @@ def addrs(self):
 
     def add(self, host, addrs):
         self._addrs[host] = addrs
-        self._addrs_rr[host] = cycle(addrs)
 
         if self._ttl:
             self._timestamps[host] = monotonic()
 
     def remove(self, host):
         self._addrs.pop(host, None)
-        self._addrs_rr.pop(host, None)
 
         if self._ttl:
             self._timestamps.pop(host, None)
 
     def clear(self):
         self._addrs.clear()
-        self._addrs_rr.clear()
         self._timestamps.clear()
 
     def next_addrs(self, host):
-        # Return an iterator that will get at maximum as many addrs
-        # there are for the specific host starting from the last
-        # not itereated addr.
-        return islice(self._addrs_rr[host], len(self._addrs[host]))
+        addrs = self._addrs[host].copy()
+        shuffle(addrs)
+        return addrs
 
     def expired(self, host):
         if self._ttl is None: