From 1be815aac49bc0dc3937eb3a618fc9f1a6deb51a Mon Sep 17 00:00:00 2001
From: Antoine Pitrou <solipsis@pitrou.net>
Date: Tue, 10 May 2011 19:16:29 +0200
Subject: [PATCH] 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  |  4 ++--
 Lib/test/test_socket.py |  7 +++++++
 Misc/ACKS               |  1 +
 Misc/NEWS               |  3 +++
 Modules/socketmodule.c  | 11 +++++++----
 5 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/Doc/library/socket.rst b/Doc/library/socket.rst
index bfb8ae94146..14cd4ff5ce6 100644
--- a/Doc/library/socket.rst
+++ b/Doc/library/socket.rst
@@ -652,8 +652,8 @@ 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, \
diff --git a/Lib/test/test_socket.py b/Lib/test/test_socket.py
index a948541825a..4100c34452c 100644
--- a/Lib/test/test_socket.py
+++ b/Lib/test/test_socket.py
@@ -788,6 +788,13 @@ class GeneralModuleTests(unittest.TestCase):
             fp.close()
             self.assertEqual(repr(fp), "<_io.BufferedReader name=-1>")
 
+    def testListenBacklog0(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):
diff --git a/Misc/ACKS b/Misc/ACKS
index 5c33b2e0a2b..399de8cba9a 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -268,6 +268,7 @@ Carey Evans
 Tim Everett
 Paul Everitt
 David Everly
+Daniel Evers
 Greg Ewing
 Martijn Faassen
 Clovis Fabricio
diff --git a/Misc/NEWS b/Misc/NEWS
index 4309858ec5d..ac9ac79da25 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -86,6 +86,9 @@ 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 #11164: Stop trying to use _xmlplus in the xml module.
 
 - Issue #11927: SMTP_SSL now uses port 465 by default as documented.  Patch
diff --git a/Modules/socketmodule.c b/Modules/socketmodule.c
index e2fdb5e12eb..bc11b23c567 100644
--- a/Modules/socketmodule.c
+++ b/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.");
 
 
 /*