fix(client_reqrep): clear _traces in close/release to break session reference cycle

CHANGES/11043.bugfix

@@ -0,0 +1 @@
+Fixed a slow memory leak where ``ClientResponse`` retained a circular reference to ``ClientSession`` via ``_traces`` after the response was closed.

aiohttp/client_reqrep.py

@@ -525,6 +525,7 @@ def close(self) -> None:
             return
 
         self._cleanup_writer()
+        self._traces = []
         if self._connection is not None:
             self._connection.close()
             self._connection = None
@@ -536,6 +537,7 @@ def release(self) -> None:
         self._closed = True
 
         self._cleanup_writer()
+        self._traces = []
         self._release_connection()
 
     @property

tests/test_client_response.py

@@ -3,6 +3,7 @@
 import asyncio
 import gc
 import sys
+import weakref
 from http.cookies import SimpleCookie
 from json import JSONDecodeError
 from unittest import mock
@@ -1834,3 +1835,124 @@ def test_response_cookies_setter_updates_raw_headers(
     response.cookies = empty_cookies
     # Should not set _raw_cookie_headers for empty cookies
     assert response._raw_cookie_headers is None
+
+
+def test_close_clears_traces_and_session(
+    event_loop: asyncio.AbstractEventLoop, session: ClientSession
+) -> None:
+    url = URL("http://def-cl-resp.org")
+    trace = mock.Mock()
+    response = ClientResponse(
+        "get",
+        url,
+        writer=WriterMock(),
+        continue100=None,
+        timer=TimerNoop(),
+        traces=[trace],
+        loop=event_loop,
+        session=session,
+        request_headers=CIMultiDict[str](),
+        original_url=url,
+        stream_writer=mock.create_autospec(
+            AbstractStreamWriter, spec_set=True, instance=True
+        ),
+    )
+    response._closed = False
+    response._connection = mock.Mock()
+    response.close()
+    assert response._traces == []
+    assert response._session is None
+
+
+def test_release_clears_traces_and_session(
+    event_loop: asyncio.AbstractEventLoop, session: ClientSession
+) -> None:
+    url = URL("http://def-cl-resp.org")
+    trace = mock.Mock()
+    response = ClientResponse(
+        "get",
+        url,
+        writer=WriterMock(),
+        continue100=None,
+        timer=TimerNoop(),
+        traces=[trace],
+        loop=event_loop,
+        session=session,
+        request_headers=CIMultiDict[str](),
+        original_url=url,
+        stream_writer=mock.create_autospec(
+            AbstractStreamWriter, spec_set=True, instance=True
+        ),
+    )
+    response._closed = False
+    response._connection = mock.Mock()
+    response.release()
+    assert response._traces == []
+    assert response._session is None
+
+
+def test_response_eof_clears_session_but_not_traces(
+    event_loop: asyncio.AbstractEventLoop, session: ClientSession
+) -> None:
+    # _response_eof() must NOT clear _traces: it fires during payload EOF which
+    # happens inside read(), before read() iterates _traces to send callbacks.
+    # Clearing _traces here would silently drop on_response_chunk_received hooks.
+    url = URL("http://def-cl-resp.org")
+    trace = mock.Mock()
+    response = ClientResponse(
+        "get",
+        url,
+        writer=None,
+        continue100=None,
+        timer=TimerNoop(),
+        traces=[trace],
+        loop=event_loop,
+        session=session,
+        request_headers=CIMultiDict[str](),
+        original_url=url,
+        stream_writer=mock.create_autospec(
+            AbstractStreamWriter, spec_set=True, instance=True
+        ),
+    )
+    response._closed = False
+    conn = response._connection = mock.Mock()
+    conn.protocol.upgraded = False
+
+    response._response_eof()
+    assert response._traces == [trace]  # preserved so read() callbacks can still fire
+    assert response._session is None
+
+
+@pytest.mark.skipif(
+    sys.implementation.name != "cpython",
+    reason="Other implementations have different GC strategies",
+)
+def test_no_circular_ref_after_close(
+    event_loop: asyncio.AbstractEventLoop, session: ClientSession
+) -> None:
+    url = URL("http://def-cl-resp.org")
+    trace = mock.Mock()
+    response = ClientResponse(
+        "get",
+        url,
+        writer=WriterMock(),
+        continue100=None,
+        timer=TimerNoop(),
+        traces=[trace],
+        loop=event_loop,
+        session=session,
+        request_headers=CIMultiDict[str](),
+        original_url=url,
+        stream_writer=mock.create_autospec(
+            AbstractStreamWriter, spec_set=True, instance=True
+        ),
+    )
+    response._closed = False
+    response._connection = mock.Mock()
+
+    ref = weakref.ref(response)
+    response.close()
+    del response
+    gc.collect()
+
+    assert ref() is None