GH-93316: socketmodule: remove constraint on queue size for macOS and FreeBSD.

Doc/library/socket.rst

@@ -1738,16 +1738,23 @@ Socket Objects
 
    .. method:: listen([backlog])
 
-      Enable a server to accept connections.  If *backlog* is specified, it must
-      be at least 0 (if it is lower, it is set to 0); it specifies the number of
-      unaccepted connections that the system will allow before refusing new
-      connections. If not specified, a default reasonable value is chosen.
+      Enable a server to accept connections.  If *backlog* is specified, it
+      specifies the number of unaccepted connections that the system will allow
+      before refusing new connections.  If not specified, a default reasonable
+      value is chosen.  The value is passed on to the system, which may adjust
+      it; the meaning of a value less than or equal to 0 is system dependent.
+      POSIX specifies that a negative value is treated as 0, but on Linux,
+      macOS and FreeBSD it selects the system maximum instead.
 
       .. availability:: not WASI.
 
       .. versionchanged:: 3.5
          The *backlog* parameter is now optional.
 
+      .. versionchanged:: next
+         A negative *backlog* is now passed on to the system instead of being
+         clamped to 0.
+
 
    .. method:: makefile(mode='r', buffering=None, *, encoding=None, \
                                errors=None, newline=None)

Lib/test/test_socket.py

@@ -1997,6 +1997,21 @@ def test_listen_backlog(self):
             srv.bind((HOST, 0))
             srv.listen()
 
+    def test_listen_backlog_connect(self):
+        # gh-93316: a negative backlog is passed on to listen(2) instead of
+        # being clamped to 0, and the socket can still accept a connection.
+        # backlog=0 is deliberately not tested here: a system is then free
+        # to not queue any incoming connection at all.
+        with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as srv:
+            srv.bind((HOST, 0))
+            srv.listen(-1)
+            srv.settimeout(support.LOOPBACK_TIMEOUT)
+            with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as cli:
+                cli.settimeout(support.LOOPBACK_TIMEOUT)
+                cli.connect(srv.getsockname())
+                conn, _ = srv.accept()
+                conn.close()
+
     @support.cpython_only
     @unittest.skipIf(_testcapi is None, "requires _testcapi")
     def test_listen_backlog_overflow(self):

Misc/NEWS.d/next/Library/2022-05-28-09-40-49.gh-issue-93316.1r1Lo_.rst

@@ -0,0 +1,2 @@
+:meth:`socket.socket.listen` no longer clamps a negative *backlog* to 0, but
+passes it on to the system, whose interpretation of it is system dependent.

Modules/socketmodule.c

@@ -3917,10 +3917,6 @@ sock_listen(PyObject *self, PyObject *args)
         return NULL;
 
     Py_BEGIN_ALLOW_THREADS
-    /* To avoid problems on systems that don't allow a negative backlog
-     * (which doesn't make sense anyway) we force a minimum value of 0. */
-    if (backlog < 0)
-        backlog = 0;
     res = listen(get_sock_fd(s), backlog);
     Py_END_ALLOW_THREADS
     if (res < 0)