From c0511d2599478c72cc69f197602c7f43b494e58b Mon Sep 17 00:00:00 2001
From: Joaquin Hui Gomez <132194176+joaquinhuigomez@users.noreply.github.com>
Date: Sat, 28 Mar 2026 20:27:51 +0000
Subject: [PATCH 1/3] Remove asyncio.shield in _drain_helper to prevent
 unhandled future warnings

Replace asyncio.shield(waiter) with direct await on waiter in
_drain_helper. HTTP/1.1 has a single consumer per drain waiter,
so shield is unnecessary and causes "Future exception was never
retrieved" warnings on cancellation.

Closes #12281
---
 CHANGES/12281.bugfix.rst    |  4 ++++
 aiohttp/base_protocol.py    |  2 +-
 tests/test_base_protocol.py | 44 +++++++++++++++++++++++++++++++++++++
 3 files changed, 49 insertions(+), 1 deletion(-)
 create mode 100644 CHANGES/12281.bugfix.rst

diff --git a/CHANGES/12281.bugfix.rst b/CHANGES/12281.bugfix.rst
new file mode 100644
index 00000000000..217dc224e67
--- /dev/null
+++ b/CHANGES/12281.bugfix.rst
@@ -0,0 +1,4 @@
+Fixed "Future exception was never retrieved" warning when a request handler
+is cancelled during TCP write backpressure. ``_drain_helper`` now awaits the
+drain waiter directly instead of wrapping it in :func:`asyncio.shield`
+-- by :user:`joaquinhuigomez`.
diff --git a/aiohttp/base_protocol.py b/aiohttp/base_protocol.py
index 7f01830f4e9..d7d83425b88 100644
--- a/aiohttp/base_protocol.py
+++ b/aiohttp/base_protocol.py
@@ -97,4 +97,4 @@ async def _drain_helper(self) -> None:
         if waiter is None:
             waiter = self._loop.create_future()
             self._drain_waiter = waiter
-        await asyncio.shield(waiter)
+        await waiter
diff --git a/tests/test_base_protocol.py b/tests/test_base_protocol.py
index 713dba2d0c2..989957da79d 100644
--- a/tests/test_base_protocol.py
+++ b/tests/test_base_protocol.py
@@ -242,6 +242,50 @@ async def wait() -> None:
     assert pr._drain_waiter is None
 
 
+async def test_cancelled_drain_no_unhandled_future_warning() -> None:
+    """Cancelling a task during backpressure must not leave an orphaned future.
+
+    When the handler task is cancelled while awaiting _drain_helper and
+    connection_lost fires with an exception afterward, the waiter should
+    already be done (cancelled) so set_exception is skipped. No "Future
+    exception was never retrieved" warning should appear.
+
+    Regression test for https://github.com/aio-libs/aiohttp/issues/12281
+    """
+    loop = asyncio.get_event_loop()
+    pr = BaseProtocol(loop=loop)
+    tr = mock.Mock()
+    pr.connection_made(tr)
+    pr.pause_writing()
+
+    fut = loop.create_future()
+
+    async def wait() -> None:
+        fut.set_result(None)
+        await pr._drain_helper()
+
+    t = loop.create_task(wait())
+    await fut
+    t.cancel()
+    with suppress(asyncio.CancelledError):
+        await t
+
+    # After cancellation the waiter should be done (cancelled), so
+    # connection_lost with an exception must not call set_exception.
+    assert pr._drain_waiter is not None
+    waiter = pr._drain_waiter
+    assert waiter.done(), "waiter must be cancelled when task is cancelled"
+
+    # This previously left an orphaned future with an unhandled exception
+    # because asyncio.shield kept the original waiter alive and uncancelled.
+    exc = RuntimeError("connection died")
+    pr.connection_lost(exc)
+    assert pr._drain_waiter is None
+
+    # Verify the waiter is cancelled, not set with an exception.
+    assert waiter.cancelled()
+
+
 async def test_parallel_drain_race_condition() -> None:
     loop = asyncio.get_event_loop()
     pr = BaseProtocol(loop=loop)

From 59ed244c1a85ffd4324169b7adef89b5ca25b66f Mon Sep 17 00:00:00 2001
From: Joaquin Hui Gomez <132194176+joaquinhuigomez@users.noreply.github.com>
Date: Sun, 29 Mar 2026 13:02:17 +0100
Subject: [PATCH 2/3] Fix mypy unreachable false positive in cancellation test
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

mypy flags the final assertion as unreachable after the suppress(CancelledError)
context manager. The code IS reachable at runtime — the suppress catches the
CancelledError from the cancelled task, and execution continues normally.
---
 tests/test_base_protocol.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/test_base_protocol.py b/tests/test_base_protocol.py
index 989957da79d..12d9466dbd0 100644
--- a/tests/test_base_protocol.py
+++ b/tests/test_base_protocol.py
@@ -283,7 +283,7 @@ async def wait() -> None:
     assert pr._drain_waiter is None
 
     # Verify the waiter is cancelled, not set with an exception.
-    assert waiter.cancelled()
+    assert waiter.cancelled()  # type: ignore[unreachable]
 
 
 async def test_parallel_drain_race_condition() -> None:

From 5cf4a25f149412d8573b7ce5a720c57a188f73a4 Mon Sep 17 00:00:00 2001
From: Joaquin Hui Gomez <132194176+joaquinhuigomez@users.noreply.github.com>
Date: Sun, 29 Mar 2026 19:24:40 +0100
Subject: [PATCH 3/3] Fix doc-spelling: use hyphenated back-pressure

---
 CHANGES/12281.bugfix.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/CHANGES/12281.bugfix.rst b/CHANGES/12281.bugfix.rst
index 217dc224e67..8edde0ad3de 100644
--- a/CHANGES/12281.bugfix.rst
+++ b/CHANGES/12281.bugfix.rst
@@ -1,4 +1,4 @@
 Fixed "Future exception was never retrieved" warning when a request handler
-is cancelled during TCP write backpressure. ``_drain_helper`` now awaits the
+is cancelled during TCP write back-pressure. ``_drain_helper`` now awaits the
 drain waiter directly instead of wrapping it in :func:`asyncio.shield`
 -- by :user:`joaquinhuigomez`.