From cfda592de91d11ed0d0ce9e70845766921c97f4e Mon Sep 17 00:00:00 2001
From: Timofey Ivankov <playboicartina@gmail.com>
Date: Sat, 30 May 2026 20:16:11 +0300
Subject: [PATCH 1/3] gh-150621: Avoid O(N^2) bytes slicing in asyncio
 _feed_data_to_buffered_proto

---
 Lib/asyncio/protocols.py | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/Lib/asyncio/protocols.py b/Lib/asyncio/protocols.py
index 09987b164c66e57..1ead5b15539ba0f 100644
--- a/Lib/asyncio/protocols.py
+++ b/Lib/asyncio/protocols.py
@@ -199,6 +199,7 @@ 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)
@@ -206,11 +207,11 @@ def _feed_data_to_buffered_proto(proto, data):
             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

From 9e684bb30f772712b6ce72a48f133bd0f764b595 Mon Sep 17 00:00:00 2001
From: Timofey Ivankov <playboicartina@gmail.com>
Date: Sat, 30 May 2026 22:35:22 +0300
Subject: [PATCH 2/3] add NEWS entry

---
 .../Library/2026-05-30-22-32-49.gh-issue-150621.6RHpsO.rst  | 6 ++++++
 1 file changed, 6 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2026-05-30-22-32-49.gh-issue-150621.6RHpsO.rst

diff --git a/Misc/NEWS.d/next/Library/2026-05-30-22-32-49.gh-issue-150621.6RHpsO.rst b/Misc/NEWS.d/next/Library/2026-05-30-22-32-49.gh-issue-150621.6RHpsO.rst
new file mode 100644
index 000000000000000..a266ca4dcadeac6
--- /dev/null
+++ b/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.

From 05989d36ae3e957187dcd0ec917fde0c89aa45f3 Mon Sep 17 00:00:00 2001
From: Timofey Ivankov <playboicartina@gmail.com>
Date: Sat, 30 May 2026 23:01:53 +0300
Subject: [PATCH 3/3] Add tests for _feed_data_to_buffered_proto

---
 Lib/test/test_asyncio/test_protocols.py | 30 ++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/Lib/test/test_asyncio/test_protocols.py b/Lib/test/test_asyncio/test_protocols.py
index 29d3bd22705c6af..9e4c1b390be6c9d 100644
--- a/Lib/test/test_asyncio/test_protocols.py
+++ b/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()