This is roughly socket2.diff from issue 1378, with a few changes applied

to ssl.py (no need to test whether we can dup any more).
Regular sockets no longer have a _base, but we still have explicit
reference counting of socket objects for the benefit of makefile();
using duplicate sockets won't work for SSLSocket.

Include/longobject.h

@@ -26,6 +26,15 @@ PyAPI_FUNC(size_t) PyLong_AsSize_t(PyObject *);
 PyAPI_FUNC(unsigned long) PyLong_AsUnsignedLong(PyObject *);
 PyAPI_FUNC(unsigned long) PyLong_AsUnsignedLongMask(PyObject *);
 
+/* Used by socketmodule.c */
+#if SIZEOF_SOCKET_T <= SIZEOF_LONG
+#define PyLong_FromSocket_t(fd) PyLong_FromLong((SOCKET_T)(fd))
+#define PyLong_AsSocket_t(fd) (SOCKET_T)PyLong_AsLong(fd)
+#else
+#define PyLong_FromSocket_t(fd) PyLong_FromLongLong(((SOCKET_T)(fd));
+#define PyLong_AsSocket_t(fd) (SOCKET_T)PyLong_AsLongLong(fd)
+#endif
+
 /* For use by intobject.c only */
 PyAPI_DATA(int) _PyLong_DigitValue[256];
 

Lib/socket.py

@@ -79,27 +79,13 @@
     __all__.append("errorTab")
 
 
-# True if os.dup() can duplicate socket descriptors.
-# (On Windows at least, os.dup only works on files)
-_can_dup_socket = hasattr(_socket.socket, "dup")
-
-if _can_dup_socket:
-    def fromfd(fd, family=AF_INET, type=SOCK_STREAM, proto=0):
-        nfd = os.dup(fd)
-        return socket(family, type, proto, fileno=nfd)
-
 class socket(_socket.socket):
 
     """A subclass of _socket.socket adding the makefile() method."""
 
     __slots__ = ["__weakref__", "_io_refs", "_closed"]
-    if not _can_dup_socket:
-        __slots__.append("_base")
 
     def __init__(self, family=AF_INET, type=SOCK_STREAM, proto=0, fileno=None):
-        if fileno is None:
-            _socket.socket.__init__(self, family, type, proto)
-        else:
         _socket.socket.__init__(self, family, type, proto, fileno)
         self._io_refs = 0
         self._closed = False
@@ -114,23 +100,29 @@ def __repr__(self):
                                 s[7:])
         return s
 
+    def dup(self):
+        """dup() -> socket object
+
+        Return a new socket object connected to the same system resource.
+        """
+        fd = dup(self.fileno())
+        sock = self.__class__(self.family, self.type, self.proto, fileno=fd)
+        sock.settimeout(self.gettimeout())
+        return sock
+
     def accept(self):
-        """Wrap accept() to give the connection the right type."""
+        """accept() -> (socket object, address info)
-        conn, addr = _socket.socket.accept(self)
+
-        fd = conn.fileno()
+        Wait for an incoming connection.  Return a new socket
-        nfd = fd
+        representing the connection, and the address of the client.
-        if _can_dup_socket:
+        For IP sockets, the address info is a pair (hostaddr, port).
-            nfd = os.dup(fd)
+        """
-        wrapper = socket(self.family, self.type, self.proto, fileno=nfd)
+        fd, addr = self._accept()
-        if fd == nfd:
+        return socket(self.family, self.type, self.proto, fileno=fd), addr
-            wrapper._base = conn  # Keep the base alive
-        else:
-            conn.close()
-        return wrapper, addr
 
     def makefile(self, mode="r", buffering=None, *,
                  encoding=None, newline=None):
-        """Return an I/O stream connected to the socket.
+        """makefile(...) -> an I/O stream connected to the socket
 
         The arguments are as for io.open() after the filename,
         except the only mode characters supported are 'r', 'w' and 'b'.
@@ -184,23 +176,20 @@ def _decref_socketios(self):
 
     def close(self):
         self._closed = True
-        if self._io_refs < 1:
+        if self._io_refs <= 0:
-            self._real_close()
-
-    # _real_close calls close on the _socket.socket base class.
-
-    if not _can_dup_socket:
-        def _real_close(self):
-            _socket.socket.close(self)
-            base = getattr(self, "_base", None)
-            if base is not None:
-                self._base = None
-                base.close()
-    else:
-        def _real_close(self):
             _socket.socket.close(self)
 
 
+def fromfd(fd, family, type, proto=0):
+    """ fromfd(fd, family, type[, proto]) -> socket object
+
+    Create a socket object from a duplicate of the given file
+    descriptor.  The remaining arguments are the same as for socket().
+    """
+    nfd = dup(fd)
+    return socket(family, type, proto, nfd)
+
+
 class SocketIO(io.RawIOBase):
 
     """Raw I/O implementation for stream sockets.

Lib/ssl.py

@@ -78,8 +78,8 @@
 from socket import socket, AF_INET, SOCK_STREAM, error
 from socket import getnameinfo as _getnameinfo
 from socket import error as socket_error
+from socket import dup as _dup
 import base64        # for DER-to-PEM translation
-_can_dup_socket = hasattr(socket, "dup")
 
 class SSLSocket(socket):
 
@@ -99,20 +99,11 @@ def __init__(self, sock=None, keyfile=None, certfile=None,
         if sock is not None:
             # copied this code from socket.accept()
             fd = sock.fileno()
-            nfd = fd
+            nfd = _dup(fd)
-            if _can_dup_socket:
-                nfd = os.dup(fd)
-            try:
             socket.__init__(self, family=sock.family, type=sock.type,
                             proto=sock.proto, fileno=nfd)
-            except:
-                if nfd != fd:
-                    os.close(nfd)
-            else:
-                if fd != nfd:
             sock.close()
             sock = None
-
         elif fileno is not None:
             socket.__init__(self, fileno=fileno)
         else:

Lib/test/test_socket.py

@@ -575,6 +575,15 @@ def testFromFd(self):
     def _testFromFd(self):
         self.serv_conn.send(MSG)
 
+    def testDup(self):
+        # Testing dup()
+        sock = self.cli_conn.dup()
+        msg = sock.recv(1024)
+        self.assertEqual(msg, MSG)
+
+    def _testDup(self):
+        self.serv_conn.send(MSG)
+
     def testShutdown(self):
         # Testing shutdown()
         msg = self.cli_conn.recv(1024)

Modules/socketmodule.c

@@ -89,12 +89,12 @@ A socket object represents one endpoint of a network connection.\n\
 \n\
 Methods of socket objects (keyword arguments not allowed):\n\
 \n\
-accept() -- accept a connection, returning new socket and client address\n\
+_accept() -- accept connection, returning new socket fd and client address\n\
 bind(addr) -- bind the socket to a local address\n\
 close() -- close the socket\n\
 connect(addr) -- connect the socket to a remote address\n\
 connect_ex(addr) -- connect, return an error code instead of an exception\n\
-dup() -- return a new socket object identical to the current one [*]\n\
+_dup() -- return a new socket fd duplicated from fileno()\n\
 fileno() -- return underlying file descriptor\n\
 getpeername() -- return remote address [*]\n\
 getsockname() -- return local address\n\
@@ -327,10 +327,26 @@ const char *inet_ntop(int af, const void *src, char *dst, socklen_t size);
 #include "getnameinfo.c"
 #endif
 
-#if defined(MS_WINDOWS)
+#ifdef MS_WINDOWS
-/* seem to be a few differences in the API */
+/* On Windows a socket is really a handle not an fd */
+static SOCKET
+dup_socket(SOCKET handle)
+{
+	HANDLE newhandle;
+
+	if (!DuplicateHandle(GetCurrentProcess(), (HANDLE)handle,
+			     GetCurrentProcess(), &newhandle,
+			     0, FALSE, DUPLICATE_SAME_ACCESS))
+	{
+		WSASetLastError(GetLastError());
+		return INVALID_SOCKET;
+	}
+	return (SOCKET)newhandle;
+}
 #define SOCKETCLOSE closesocket
-#define NO_DUP /* Actually it exists on NT 3.5, but what the heck... */
+#else
+/* On Unix we can use dup to duplicate the file descriptor of a socket*/
+#define dup_socket(fd) dup(fd)
 #endif
 
 #ifdef MS_WIN32
@@ -1423,7 +1439,7 @@ getsockaddrlen(PySocketSockObject *s, socklen_t *len_ret)
 }
 
 
-/* s.accept() method */
+/* s._accept() -> (fd, address) */
 
 static PyObject *
 sock_accept(PySocketSockObject *s)
@@ -1457,17 +1473,12 @@ sock_accept(PySocketSockObject *s)
 	if (newfd == INVALID_SOCKET)
 		return s->errorhandler();
 
-	/* Create the new object with unspecified family,
+	sock = PyLong_FromSocket_t(newfd);
-	   to avoid calls to bind() etc. on it. */
-	sock = (PyObject *) new_sockobject(newfd,
-					   s->sock_family,
-					   s->sock_type,
-					   s->sock_proto);
-
 	if (sock == NULL) {
 		SOCKETCLOSE(newfd);
 		goto finally;
 	}
+
 	addr = makesockaddr(s->sock_fd, SAS2SA(&addrbuf),
 			    addrlen, s->sock_proto);
 	if (addr == NULL)
@@ -1482,11 +1493,11 @@ sock_accept(PySocketSockObject *s)
 }
 
 PyDoc_STRVAR(accept_doc,
-"accept() -> (socket object, address info)\n\
+"_accept() -> (integer, address info)\n\
 \n\
-Wait for an incoming connection.  Return a new socket representing the\n\
+Wait for an incoming connection.  Return a new socket file descriptor\n\
-connection, and the address of the client.  For IP sockets, the address\n\
+representing the connection, and the address of the client.\n\
-info is a pair (hostaddr, port).");
+For IP sockets, the address info is a pair (hostaddr, port).");
 
 /* s.setblocking(flag) method.  Argument:
    False -- non-blocking mode; same as settimeout(0)
@@ -1882,11 +1893,7 @@ instead of raising an exception when an error occurs.");
 static PyObject *
 sock_fileno(PySocketSockObject *s)
 {
-#if SIZEOF_SOCKET_T <= SIZEOF_LONG
+	return PyLong_FromSocket_t(s->sock_fd);
-	return PyInt_FromLong((long) s->sock_fd);
-#else
-	return PyLong_FromLongLong((PY_LONG_LONG)s->sock_fd);
-#endif
 }
 
 PyDoc_STRVAR(fileno_doc,
@@ -1895,35 +1902,6 @@ PyDoc_STRVAR(fileno_doc,
 Return the integer file descriptor of the socket.");
 
 
-#ifndef NO_DUP
-/* s.dup() method */
-
-static PyObject *
-sock_dup(PySocketSockObject *s)
-{
-	SOCKET_T newfd;
-	PyObject *sock;
-
-	newfd = dup(s->sock_fd);
-	if (newfd < 0)
-		return s->errorhandler();
-	sock = (PyObject *) new_sockobject(newfd,
-					   s->sock_family,
-					   s->sock_type,
-					   s->sock_proto);
-	if (sock == NULL)
-		SOCKETCLOSE(newfd);
-	return sock;
-}
-
-PyDoc_STRVAR(dup_doc,
-"dup() -> socket object\n\
-\n\
-Return a new socket object connected to the same system resource.");
-
-#endif
-
-
 /* s.getsockname() method */
 
 static PyObject *
@@ -2542,7 +2520,7 @@ of the socket (flag == SHUT_WR), or both ends (flag == SHUT_RDWR).");
 /* List of methods for socket objects */
 
 static PyMethodDef sock_methods[] = {
-	{"accept",	  (PyCFunction)sock_accept, METH_NOARGS,
+	{"_accept",	  (PyCFunction)sock_accept, METH_NOARGS,
 			  accept_doc},
 	{"bind",	  (PyCFunction)sock_bind, METH_O,
 			  bind_doc},
@@ -2552,10 +2530,6 @@ static PyMethodDef sock_methods[] = {
 			  connect_doc},
 	{"connect_ex",	  (PyCFunction)sock_connect_ex, METH_O,
 			  connect_ex_doc},
-#ifndef NO_DUP
-	{"dup",		  (PyCFunction)sock_dup, METH_NOARGS,
-			  dup_doc},
-#endif
 	{"fileno",	  (PyCFunction)sock_fileno, METH_NOARGS,
 			  fileno_doc},
 #ifdef HAVE_GETPEERNAME
@@ -2672,8 +2646,8 @@ sock_initobj(PyObject *self, PyObject *args, PyObject *kwds)
 					 &family, &type, &proto, &fdobj))
 		return -1;
 
-	if (fdobj != NULL) {
+	if (fdobj != NULL && fdobj != Py_None) {
-		fd = PyLong_AsLongLong(fdobj);
+		fd = PyLong_AsSocket_t(fdobj);
 		if (fd == (SOCKET_T)(-1) && PyErr_Occurred())
 			return -1;
 		if (fd == INVALID_SOCKET) {
@@ -3172,6 +3146,38 @@ PyDoc_STRVAR(getprotobyname_doc,
 Return the protocol number for the named protocol.  (Rarely used.)");
 
 
+#ifndef NO_DUP
+/* dup() function for socket fds */
+
+static PyObject *
+socket_dup(PyObject *self, PyObject *fdobj)
+{
+	SOCKET_T fd, newfd;
+	PyObject *newfdobj;
+
+
+	fd = PyLong_AsSocket_t(fdobj);
+	if (fd == (SOCKET_T)(-1) && PyErr_Occurred())
+		return NULL;
+
+	newfd = dup_socket(fd);
+	if (newfd == INVALID_SOCKET)
+		return set_error();
+
+	newfdobj = PyLong_FromSocket_t(newfd);
+	if (newfdobj == NULL)
+		SOCKETCLOSE(newfd);
+	return newfdobj;
+}
+
+PyDoc_STRVAR(dup_doc,
+"dup(integer) -> integer\n\
+\n\
+Duplicate an integer socket file descriptor.  This is like os.dup(), but for\n\
+sockets; on some platforms os.dup() won't work for socket file descriptors.");
+#endif
+
+
 #ifdef HAVE_SOCKETPAIR
 /* Create a pair of sockets using the socketpair() function.
    Arguments as for socket() except the default family is AF_UNIX if
@@ -3811,6 +3817,10 @@ static PyMethodDef socket_methods[] = {
 	 METH_VARARGS, getservbyport_doc},
 	{"getprotobyname",	socket_getprotobyname,
 	 METH_VARARGS, getprotobyname_doc},
+#ifndef NO_DUP
+	{"dup",			socket_dup,
+         METH_O, dup_doc},
+#endif
 #ifdef HAVE_SOCKETPAIR
 	{"socketpair",		socket_socketpair,
 	 METH_VARARGS, socketpair_doc},