From 9e1c95a75fafb773143ab42e59dbec269157a61b Mon Sep 17 00:00:00 2001
From: Will <PandaWill@users.noreply.github.com>
Date: Thu, 15 Oct 2020 14:04:14 +0100
Subject: [PATCH] Fix for race condition on connections in BaseConnector
 (#4937)

(cherry picked from commit ad00c2e44b97e4e69827a8166d299d5ab171f7b9)
---
 CHANGES/4936.bugfix     | 1 +
 CONTRIBUTORS.txt        | 1 +
 aiohttp/connector.py    | 5 +++--
 tests/test_connector.py | 3 ++-
 4 files changed, 7 insertions(+), 3 deletions(-)
 create mode 100644 CHANGES/4936.bugfix

diff --git a/CHANGES/4936.bugfix b/CHANGES/4936.bugfix
new file mode 100644
index 00000000000..b3a0c6d8e80
--- /dev/null
+++ b/CHANGES/4936.bugfix
@@ -0,0 +1 @@
+Fix for race condition on connections in BaseConnector that leads to exceeding the connection limit.
diff --git a/CONTRIBUTORS.txt b/CONTRIBUTORS.txt
index 4c85d255b72..79db9e68b3e 100644
--- a/CONTRIBUTORS.txt
+++ b/CONTRIBUTORS.txt
@@ -269,6 +269,7 @@ Weiwei Wang
 Will McGugan
 Willem de Groot
 William Grzybowski
+William S.
 Wilson Ong
 Yang Zhou
 Yannick Koechlin
diff --git a/aiohttp/connector.py b/aiohttp/connector.py
index 3efea213b2f..1a4d12d6b1e 100644
--- a/aiohttp/connector.py
+++ b/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.
diff --git a/tests/test_connector.py b/tests/test_connector.py
index d854890dd4d..b3522f17f81 100644
--- a/tests/test_connector.py
+++ b/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)