Fix for race condition on connections in BaseConnector (#4937)

(cherry picked from commit ad00c2e)

CHANGES/4936.bugfix

@@ -0,0 +1 @@
+Fix for race condition on connections in BaseConnector that leads to exceeding the connection limit.

CONTRIBUTORS.txt

@@ -269,6 +269,7 @@ Weiwei Wang
 Will McGugan
 Willem de Groot
 William Grzybowski
+William S.
 Wilson Ong
 Yang Zhou
 Yannick Koechlin

aiohttp/connector.py

@@ -474,8 +474,9 @@ async def connect(self, req: 'ClientRequest',
         key = req.connection_key
         available = self._available_connections(key)
 
-        # Wait if there are no available connections.
-        if available <= 0:
+        # Wait if there are no available connections or if there are/were
+        # waiters (i.e. don't steal connection from a waiter about to wake up)
+        if available <= 0 or key in self._waiters:
             fut = self._loop.create_future()
 
             # This connection will now count towards the limit.

tests/test_connector.py

@@ -1713,7 +1713,7 @@ async def create_connection(req, traces, timeout):
     # with multiple concurrent requests and stops when it hits a
     # predefined maximum number of requests.
 
-    max_requests = 10
+    max_requests = 50
     num_requests = 0
     start_requests = max_connections + 1
 
@@ -1726,6 +1726,7 @@ async def f(start=True):
             connection = await conn.connect(req, None, ClientTimeout())
             await asyncio.sleep(0)
             connection.release()
+            await asyncio.sleep(0)
         tasks = [
             loop.create_task(f(start=False))
             for i in range(start_requests)