python · ambv · May 21, 2025 · May 20, 2025 · May 20, 2025 · May 20, 2025
@@ -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:

@@ -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):


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
@@ -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.
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.