Optimize _WS_EXT_RE backtracking on Python 3.11+

CHANGES/12346.misc.rst

@@ -0,0 +1,3 @@
+Improved performance of ``_WS_EXT_RE`` regular expression on Python 3.11+
+by using atomic grouping when parsing ``Sec-WebSocket-Extensions`` headers
+-- by :user:`HarshithReddy01`.

CONTRIBUTORS.txt

@@ -160,6 +160,7 @@ Günther Jena
 Hans Adema
 Harmon Y.
 Harry Liu
+Harshith Reddy
 Hiroshi Ogawa
 Hrishikesh Paranjape
 Hu Bo

aiohttp/_websocket/helpers.py

@@ -2,6 +2,7 @@
 
 import functools
 import re
+import sys
 from re import Pattern
 from struct import Struct
 from typing import TYPE_CHECKING, Final
@@ -70,6 +71,12 @@ def _websocket_mask_python(mask: bytes, data: bytearray) -> None:
     r"(client_no_context_takeover)|"
     r"(server_max_window_bits(?:=(\d+))?)|"
     r"(client_max_window_bits(?:=(\d+))?)))*$"
+    if sys.version_info < (3, 11)
+    else r"^(?>;\s*(?:"
+    r"(server_no_context_takeover)|"
+    r"(client_no_context_takeover)|"
+    r"(server_max_window_bits(?:=(\d+))?)|"
+    r"(client_max_window_bits(?:=(\d+))?)))*$"
 )
 
 _WS_EXT_RE_SPLIT: Final[Pattern[str]] = re.compile(r"permessage-deflate([^,]+)?")

tests/test_websocket_helpers.py

@@ -0,0 +1,52 @@
+import time
+
+import pytest
+
+from aiohttp._websocket.helpers import ws_ext_parse
+from aiohttp.http_websocket import WSHandshakeError
+
+
+@pytest.mark.parametrize(
+    ("msg", "server", "expected"),
+    (
+        ("permessage-deflate", False, (15, False)),
+        ("permessage-deflate; server_no_context_takeover", True, (15, True)),
+        ("permessage-deflate; client_no_context_takeover", False, (15, True)),
+        ("permessage-deflate; server_max_window_bits=12", True, (12, False)),
+        ("permessage-deflate; client_max_window_bits=10", False, (10, False)),
+        # out-of-range wbits on server side → skip rather than fail
+        ("permessage-deflate; server_max_window_bits=8", True, (0, False)),
+        # unknown param on server side → no match, return zero
+        ("permessage-deflate; unknown_param", True, (0, False)),
+    ),
+)
+def test_ws_ext_parse(msg: str, server: bool, expected: tuple[int, bool]) -> None:
+    assert ws_ext_parse(msg, isserver=server) == expected
+
+
+@pytest.mark.parametrize(
+    ("msg", "server"),
+    (
+        ("permessage-deflate; client_max_window_bits=8", False),
+        ("permessage-deflate; unknown_param", False),
+    ),
+)
+def test_ws_ext_parse_raises(msg: str, server: bool) -> None:
+    with pytest.raises(WSHandshakeError):
+        ws_ext_parse(msg, isserver=server)
+
+
+def test_ws_ext_parse_empty() -> None:
+    assert ws_ext_parse(None) == (0, False)
+    assert ws_ext_parse("") == (0, False)
+
+
+def test_ws_ext_parse_backtracking_performance() -> None:
+    # Many valid tokens followed by an invalid suffix — the classic input that
+    # triggers exponential backtracking in the outer repeating group.
+    evil = "permessage-deflate" + ("; server_no_context_takeover" * 30) + ";INVALID"
+    start = time.perf_counter()
+    with pytest.raises(WSHandshakeError):
+        ws_ext_parse(evil, isserver=False)
+    elapsed = time.perf_counter() - start
+    assert elapsed < 1.0, f"backtracking regression: took {elapsed:.3f}s"