[3.14] gh-90871: fix connection backlog offset in asyncio (gh-134392) (gh-134421)

(cherry picked from commit 109f7597d2)

Co-authored-by: Christian Harries <68507104+ChristianHrs@users.noreply.github.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>
Co-authored-by: Kumar Aditya <kumaraditya@python.org>

Lib/asyncio/selector_events.py

@@ -173,7 +173,7 @@ def _accept_connection(
         # listening socket has triggered an EVENT_READ. There may be multiple
         # connections waiting for an .accept() so it is called in a loop.
         # See https://bugs.python.org/issue27906 for more details.
-        for _ in range(backlog):
+        for _ in range(backlog + 1):
             try:
                 conn, addr = sock.accept()
                 if self._debug:

Lib/test/test_asyncio/test_selector_events.py

@@ -347,6 +347,18 @@ def test_process_events_write_cancelled(self):
               selectors.EVENT_WRITE)])
         self.loop._remove_writer.assert_called_with(1)
 
+    def test_accept_connection_zero_one(self):
+        for backlog in [0, 1]:
+            sock = mock.Mock()
+            sock.accept.return_value = (mock.Mock(), mock.Mock())
+            with self.subTest(backlog):
+                mock_obj = mock.patch.object
+                with mock_obj(self.loop, '_accept_connection2') as accept2_mock:
+                    self.loop._accept_connection(
+                        mock.Mock(), sock, backlog=backlog)
+                self.loop.run_until_complete(asyncio.sleep(0))
+                self.assertEqual(sock.accept.call_count, backlog + 1)
+
     def test_accept_connection_multiple(self):
         sock = mock.Mock()
         sock.accept.return_value = (mock.Mock(), mock.Mock())
@@ -362,7 +374,7 @@ def test_accept_connection_multiple(self):
             self.loop._accept_connection(
                 mock.Mock(), sock, backlog=backlog)
         self.loop.run_until_complete(asyncio.sleep(0))
-        self.assertEqual(sock.accept.call_count, backlog)
+        self.assertEqual(sock.accept.call_count, backlog + 1)
 
     def test_accept_connection_skip_connectionabortederror(self):
         sock = mock.Mock()
@@ -388,7 +400,7 @@ def mock_sock_accept():
         # as in test_accept_connection_multiple avoid task pending
         # warnings by using asyncio.sleep(0)
         self.loop.run_until_complete(asyncio.sleep(0))
-        self.assertEqual(sock.accept.call_count, backlog)
+        self.assertEqual(sock.accept.call_count, backlog + 1)
 
 class SelectorTransportTests(test_utils.TestCase):
 

Misc/NEWS.d/next/Library/2025-05-20-21-45-58.gh-issue-90871.Gkvtp6.rst

@@ -0,0 +1,2 @@
+Fixed an off by one error concerning the backlog parameter in
+:meth:`~asyncio.loop.create_unix_server`. Contributed by Christian Harries.