Fix DNSCache race-condition

CHANGES/2620.bugfix

@@ -0,0 +1 @@
+Fixed race-condition for iterating addresses from the DNSCache.

aiohttp/connector.py

@@ -578,10 +578,11 @@ def clear(self):
         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]))
+        loop = self._addrs_rr[host]
+        addrs = list(islice(loop, len(self._addrs[host])))
+        # Consume one more element to shift internal state of `cycle`
+        next(loop)
+        return addrs
 
     def expired(self, host):
         if self._ttl is None:

tests/test_connector.py

@@ -1959,15 +1959,27 @@ async def test_expired_ttl(self, loop):
         assert dns_cache_table.expired('localhost')
 
     def test_next_addrs(self, dns_cache_table):
-        dns_cache_table.add('foo', ['127.0.0.1', '127.0.0.2'])
+        dns_cache_table.add('foo', ['127.0.0.1', '127.0.0.2', '127.0.0.3'])
 
-        # max elements returned are the full list of addrs
-        addrs = list(dns_cache_table.next_addrs('foo'))
-        assert addrs == ['127.0.0.1', '127.0.0.2']
-
-        # different calls to next_addrs return the hosts using
+        # Each calls to next_addrs return the hosts using
         # a round robin strategy.
         addrs = dns_cache_table.next_addrs('foo')
-        assert next(addrs) == '127.0.0.1'
+        assert addrs == ['127.0.0.1', '127.0.0.2', '127.0.0.3']
+
+        addrs = dns_cache_table.next_addrs('foo')
+        assert addrs == ['127.0.0.2', '127.0.0.3', '127.0.0.1']
+
+        addrs = dns_cache_table.next_addrs('foo')
+        assert addrs == ['127.0.0.3', '127.0.0.1', '127.0.0.2']
+
+        addrs = dns_cache_table.next_addrs('foo')
+        assert addrs == ['127.0.0.1', '127.0.0.2', '127.0.0.3']
+
+    def test_next_addrs_single(self, dns_cache_table):
+        dns_cache_table.add('foo', ['127.0.0.1'])
+
+        addrs = dns_cache_table.next_addrs('foo')
+        assert addrs == ['127.0.0.1']
+
         addrs = dns_cache_table.next_addrs('foo')
-        assert next(addrs) == '127.0.0.2'
+        assert addrs == ['127.0.0.1']