Issue #8498: In socket.accept(), allow to specify 0 as a backlog value in

order to accept exactly one connection.  Patch by Daniel Evers.

Doc/library/socket.rst

@@ -662,8 +662,8 @@ correspond to Unix system calls applicable to sockets.
 .. method:: socket.listen(backlog)
 
    Listen for connections made to the socket.  The *backlog* argument specifies the
-   maximum number of queued connections and should be at least 1; the maximum value
-   is system-dependent (usually 5).
+   maximum number of queued connections and should be at least 0; the maximum value
+   is system-dependent (usually 5), the minimum value is forced to 0.
 
 
 .. method:: socket.makefile(mode='r', buffering=None, *, encoding=None, \

Lib/test/test_socket.py

@@ -797,6 +797,13 @@ class GeneralModuleTests(unittest.TestCase):
             for protocol in range(pickle.HIGHEST_PROTOCOL + 1):
                 self.assertRaises(TypeError, pickle.dumps, sock, protocol)
 
+    def test_listen_backlog0(self):
+        srv = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+        srv.bind((HOST, 0))
+        # backlog = 0
+        srv.listen(0)
+        srv.close()
+
 
 @unittest.skipUnless(thread, 'Threading required for this test.')
 class BasicTCPTest(SocketConnectedTest):

Misc/ACKS

@@ -270,6 +270,7 @@ Carey Evans
 Tim Everett
 Paul Everitt
 David Everly
+Daniel Evers
 Winston Ewert
 Greg Ewing
 Martijn Faassen

Misc/NEWS

@@ -143,6 +143,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #8498: In socket.accept(), allow to specify 0 as a backlog value in
+  order to accept exactly one connection.  Patch by Daniel Evers.
+
 - Issue #12011: signal.signal() and signal.siginterrupt() raise an OSError,
   instead of a RuntimeError: OSError has an errno attribute.
 

Modules/socketmodule.c

@@ -2220,8 +2220,10 @@ sock_listen(PySocketSockObject *s, PyObject *arg)
     if (backlog == -1 && PyErr_Occurred())
         return NULL;
     Py_BEGIN_ALLOW_THREADS
-    if (backlog < 1)
-        backlog = 1;
+    /* 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(s->sock_fd, backlog);
     Py_END_ALLOW_THREADS
     if (res < 0)
@@ -2234,8 +2236,9 @@ PyDoc_STRVAR(listen_doc,
 "listen(backlog)\n\
 \n\
 Enable a server to accept connections.  The backlog argument must be at\n\
-least 1; it specifies the number of unaccepted connection that the system\n\
-will allow before refusing new connections.");
+least 0 (if it is lower, it is set to 0); it specifies the number of\n\
+unaccepted connections that the system will allow before refusing new\n\
+connections.");
 
 
 /*