Avoid scheduling the writer if it can finish synchronously (#8510)

CHANGES/8510.misc.rst

@@ -0,0 +1 @@
+When using Python 3.12 or later, the writer is no longer scheduled on the event loop if it can finish synchronously. Avoiding event loop scheduling reduces latency and improves performance. -- by :user:`bdraco`.

aiohttp/client_reqrep.py

@@ -283,7 +283,12 @@ def _writer(self, writer: Optional["asyncio.Task[None]"]) -> None:
         if self.__writer is not None:
             self.__writer.remove_done_callback(self.__reset_writer)
         self.__writer = writer
-        if writer is not None:
+        if writer is None:
+            return
+        if writer.done():
+            # The writer is already done, so we can reset it immediately.
+            self.__reset_writer()
+        else:
             writer.add_done_callback(self.__reset_writer)
 
     def is_ssl(self) -> bool:
@@ -657,9 +662,17 @@ async def send(self, conn: "Connection") -> "ClientResponse":
             self.method, path, v=self.version
         )
         await writer.write_headers(status_line, self.headers)
+        coro = self.write_bytes(writer, conn)
 
-        self._writer = self.loop.create_task(self.write_bytes(writer, conn))
+        if sys.version_info >= (3, 12):
+            # Optimization for Python 3.12, try to write
+            # bytes immediately to avoid having to schedule
+            # the task on the event loop.
+            task = asyncio.Task(coro, loop=self.loop, eager_start=True)
+        else:
+            task = self.loop.create_task(coro)
 
+        self._writer = task
         response_class = self.response_class
         assert response_class is not None
         self.response = response_class(
@@ -776,7 +789,12 @@ def _writer(self, writer: Optional["asyncio.Task[None]"]) -> None:
         if self.__writer is not None:
             self.__writer.remove_done_callback(self.__reset_writer)
         self.__writer = writer
-        if writer is not None:
+        if writer is None:
+            return
+        if writer.done():
+            # The writer is already done, so we can reset it immediately.
+            self.__reset_writer()
+        else:
             writer.add_done_callback(self.__reset_writer)
 
     @reify

tests/test_client_request.py

@@ -972,8 +972,15 @@ async def gen():
     req = ClientRequest("POST", URL("http://python.org/"), data=gen(), loop=loop)
     assert req.chunked
     assert req.headers["TRANSFER-ENCODING"] == "chunked"
+    original_write_bytes = req.write_bytes
 
-    resp = await req.send(conn)
+    async def _mock_write_bytes(*args, **kwargs):
+        # Ensure the task is scheduled
+        await asyncio.sleep(0)
+        return await original_write_bytes(*args, **kwargs)
+
+    with mock.patch.object(req, "write_bytes", _mock_write_bytes):
+        resp = await req.send(conn)
     assert asyncio.isfuture(req._writer)
     await resp.wait_for_close()
     assert req._writer is None
@@ -1155,14 +1162,28 @@ async def test_oserror_on_write_bytes(loop: Any, conn: Any) -> None:
 
 async def test_terminate(loop: Any, conn: Any) -> None:
     req = ClientRequest("get", URL("http://python.org"), loop=loop)
-    resp = await req.send(conn)
+
+    async def _mock_write_bytes(*args, **kwargs):
+        # Ensure the task is scheduled
+        await asyncio.sleep(0)
+
+    with mock.patch.object(req, "write_bytes", _mock_write_bytes):
+        resp = await req.send(conn)
+
     assert req._writer is not None
-    writer = req._writer = WriterMock()
+    assert resp._writer is not None
+    await resp._writer
+    writer = WriterMock()
+    writer.done = mock.Mock(return_value=False)
     writer.cancel = mock.Mock()
+    req._writer = writer
+    resp._writer = writer
 
+    assert req._writer is not None
+    assert resp._writer is not None
     req.terminate()
-    assert req._writer is None
     writer.cancel.assert_called_with()
+    writer.done.assert_called_with()
     resp.close()
 
 
@@ -1172,9 +1193,19 @@ def test_terminate_with_closed_loop(loop: Any, conn: Any) -> None:
     async def go():
         nonlocal req, resp, writer
         req = ClientRequest("get", URL("http://python.org"), loop=loop)
-        resp = await req.send(conn)
+
+        async def _mock_write_bytes(*args, **kwargs):
+            # Ensure the task is scheduled
+            await asyncio.sleep(0)
+
+        with mock.patch.object(req, "write_bytes", _mock_write_bytes):
+            resp = await req.send(conn)
+
         assert req._writer is not None
-        writer = req._writer = WriterMock()
+        writer = WriterMock()
+        writer.done = mock.Mock(return_value=False)
+        req._writer = writer
+        resp._writer = writer
 
         await asyncio.sleep(0.05)