gh-150621: avoid quadratic bytes slicing in asyncio.protocols._feed_data_to_buffered_proto

Lib/asyncio/protocols.py

@@ -199,18 +199,19 @@ def process_exited(self):
 
 def _feed_data_to_buffered_proto(proto, data):
     data_len = len(data)
+    start = 0
     while data_len:
         buf = proto.get_buffer(data_len)
         buf_len = len(buf)
         if not buf_len:
             raise RuntimeError('get_buffer() returned an empty buffer')
 
         if buf_len >= data_len:
-            buf[:data_len] = data
+            buf[:data_len] = data[start:start + data_len] if start else data
             proto.buffer_updated(data_len)
             return
         else:
-            buf[:buf_len] = data[:buf_len]
+            buf[:buf_len] = data[start:start + buf_len]
             proto.buffer_updated(buf_len)
-            data = data[buf_len:]
-            data_len = len(data)
+            start += buf_len
+            data_len -= buf_len

Lib/test/test_asyncio/test_protocols.py

@@ -2,6 +2,7 @@
 from unittest import mock
 
 import asyncio
+from asyncio import protocols
 
 
 def tearDownModule():
@@ -63,5 +64,32 @@ def test_subprocess_protocol(self):
         self.assertNotHasAttr(sp, '__dict__')
 
 
-if __name__ == '__main__':
+class FeedDataToBufferedProtoTests(unittest.TestCase):
+    def _make_proto(self, bufsize):
+        received = bytearray()
+        buf = bytearray(bufsize)
+
+        class P(asyncio.BufferedProtocol):
+            def get_buffer(self, sizehint):
+                return buf
+
+            def buffer_updated(self, nbytes):
+                received.extend(buf[:nbytes])
+
+        return P(), received
+
+    def test_large_multi_iteration(self):
+        proto, received = self._make_proto(64)
+        data = bytes(range(256)) * 16
+        protocols._feed_data_to_buffered_proto(proto, data)
+        self.assertEqual(bytes(received), data)
+
+    def test_memoryview_input(self):
+        proto, received = self._make_proto(64)
+        payload = b"y" * 200
+        protocols._feed_data_to_buffered_proto(proto, memoryview(payload))
+        self.assertEqual(bytes(received), payload)
+
+
+if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Library/2026-05-30-22-32-49.gh-issue-150621.6RHpsO.rst

@@ -0,0 +1,6 @@
+Improve performance of
+:func:`!asyncio.protocols._feed_data_to_buffered_proto`, used by the
+proactor event loop's read transports (sockets, pipes, subprocess
+stdout/stderr) when the protocol is a :class:`~asyncio.BufferedProtocol`.
+Total work drops from ``O(N²)`` to ``O(N)`` when the payload exceeds a
+single buffer.