May 21, 2025 · May 21, 2025
diff --git a/Lib/asyncio/selector_events.py b/Lib/asyncio/selector_events.py
        # 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:
diff --git a/Lib/test/test_asyncio/test_selector_events.py b/Lib/test/test_asyncio/test_selector_events.py
              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())
            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()
        # 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):

diff --git a/Misc/NEWS.d/next/Library/2025-05-20-21-45-58.gh-issue-90871.Gkvtp6.rst b/Misc/NEWS.d/next/Library/2025-05-20-21-45-58.gh-issue-90871.Gkvtp6.rst
 Fixed an off by one error concerning the backlog parameter in
 :meth:`~asyncio.loop.create_unix_server`. Contributed by Christian Harries.
Original file line number	Diff line number	Diff line change
Expand Up		@@ -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:
Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -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())
Expand All		@@ -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()
Expand All		@@ -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):

Expand Down
Original file line number	Diff line number	Diff line change
		@@ -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.