bpo-28134: Auto-detect socket values from file descriptor (#1349)

Fix socket(fileno=fd) by auto-detecting the socket's family, type,
and proto from the file descriptor. The auto-detection can be overruled
by passing in family, type, and proto explicitly.

Without the fix, all socket except for TCP/IP over IPv4 are basically broken:

>>> s = socket.create_connection(('www.python.org', 443))
>>> s
<socket.socket fd=3, family=AddressFamily.AF_INET6, type=SocketKind.SOCK_STREAM, proto=6, laddr=('2003:58:bc4a:3b00:56ee:75ff:fe47:ca7b', 59730, 0, 0), raddr=('2a04:4e42:1b::223', 443, 0, 0)>
>>> socket.socket(fileno=s.fileno())
<socket.socket fd=3, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=0, laddr=('2003:58:bc4a:3b00::%2550471192', 59730, 0, 2550471192), raddr=('2a04:4e42:1b:0:700c:e70b:ff7f:0%2550471192', 443, 0, 2550471192)>

Signed-off-by: Christian Heimes <christian@python.org>

Doc/library/socket.rst

@@ -461,10 +461,15 @@ The following functions all create :ref:`socket objects <socket-objects>`.
    :const:`SOCK_DGRAM`, :const:`SOCK_RAW` or perhaps one of the other ``SOCK_``
    constants. The protocol number is usually zero and may be omitted or in the
    case where the address family is :const:`AF_CAN` the protocol should be one
-   of :const:`CAN_RAW`, :const:`CAN_BCM` or :const:`CAN_ISOTP`.  If *fileno* is specified, the other
-   arguments are ignored, causing the socket with the specified file descriptor
-   to return.  Unlike :func:`socket.fromfd`, *fileno* will return the same
-   socket and not a duplicate. This may help close a detached socket using
+   of :const:`CAN_RAW`, :const:`CAN_BCM` or :const:`CAN_ISOTP`
+
+   If *fileno* is specified, the values for *family*, *type*, and *proto* are
+   auto-detected from the specified file descriptor.  Auto-detection can be
+   overruled by calling the function with explicit *family*, *type*, or *proto*
+   arguments.  This only affects how Python represents e.g. the return value
+   of :meth:`socket.getpeername` but not the actual OS resource.  Unlike
+   :func:`socket.fromfd`, *fileno* will return the same socket and not a
+   duplicate. This may help close a detached socket using
    :meth:`socket.close()`.
 
    The newly created socket is :ref:`non-inheritable <fd_inheritance>`.

Lib/socket.py

@@ -136,11 +136,18 @@ class socket(_socket.socket):
 
     __slots__ = ["__weakref__", "_io_refs", "_closed"]
 
-    def __init__(self, family=AF_INET, type=SOCK_STREAM, proto=0, fileno=None):
+    def __init__(self, family=-1, type=-1, proto=-1, fileno=None):
         # For user code address family and type values are IntEnum members, but
         # for the underlying _socket.socket they're just integers. The
         # constructor of _socket.socket converts the given argument to an
         # integer automatically.
+        if fileno is None:
+            if family == -1:
+                family = AF_INET
+            if type == -1:
+                type = SOCK_STREAM
+            if proto == -1:
+                proto = 0
         _socket.socket.__init__(self, family, type, proto, fileno)
         self._io_refs = 0
         self._closed = False

Lib/test/_test_multiprocessing.py

@@ -4204,7 +4204,7 @@ class TestCloseFds(unittest.TestCase):
 
     def close(self, fd):
         if WIN32:
-            socket.socket(fileno=fd).close()
+            socket.socket(socket.AF_INET, socket.SOCK_STREAM, fileno=fd).close()
         else:
             os.close(fd)
 

Lib/test/test_socket.py

@@ -20,6 +20,7 @@ import math
 import pickle
 import struct
 import random
+import shutil
 import string
 import _thread as thread
 import threading
@@ -1284,6 +1285,7 @@ class GeneralModuleTests(unittest.TestCase):
 
     def testCloseException(self):
         sock = socket.socket()
+        sock.bind((socket._LOCALHOST, 0))
         socket.socket(fileno=sock.fileno()).close()
         try:
             sock.close()
@@ -1636,9 +1638,11 @@ class GeneralModuleTests(unittest.TestCase):
         ) + 1
 
         with socket.socket(
-                family=unknown_family, type=unknown_type, fileno=fd) as s:
+                family=unknown_family, type=unknown_type, proto=23,
+                fileno=fd) as s:
             self.assertEqual(s.family, unknown_family)
             self.assertEqual(s.type, unknown_type)
+            self.assertEqual(s.proto, 23)
 
     @unittest.skipUnless(hasattr(os, 'sendfile'), 'test needs os.sendfile()')
     def test__sendfile_use_sendfile(self):
@@ -1658,6 +1662,45 @@ class GeneralModuleTests(unittest.TestCase):
             with self.assertRaises(TypeError):
                 sock._sendfile_use_sendfile(File(None))
 
+    def _test_socket_fileno(self, s, family, stype):
+        self.assertEqual(s.family, family)
+        self.assertEqual(s.type, stype)
+
+        fd = s.fileno()
+        s2 = socket.socket(fileno=fd)
+        self.addCleanup(s2.close)
+        # detach old fd to avoid double close
+        s.detach()
+        self.assertEqual(s2.family, family)
+        self.assertEqual(s2.type, stype)
+        self.assertEqual(s2.fileno(), fd)
+
+    def test_socket_fileno(self):
+        s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+        self.addCleanup(s.close)
+        s.bind((support.HOST, 0))
+        self._test_socket_fileno(s, socket.AF_INET, socket.SOCK_STREAM)
+
+        if hasattr(socket, "SOCK_DGRAM"):
+            s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+            self.addCleanup(s.close)
+            s.bind((support.HOST, 0))
+            self._test_socket_fileno(s, socket.AF_INET, socket.SOCK_DGRAM)
+
+        if support.IPV6_ENABLED:
+            s = socket.socket(socket.AF_INET6, socket.SOCK_STREAM)
+            self.addCleanup(s.close)
+            s.bind((support.HOSTv6, 0, 0, 0))
+            self._test_socket_fileno(s, socket.AF_INET6, socket.SOCK_STREAM)
+
+        if hasattr(socket, "AF_UNIX"):
+            tmpdir = tempfile.mkdtemp()
+            self.addCleanup(shutil.rmtree, tmpdir)
+            s = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
+            self.addCleanup(s.close)
+            s.bind(os.path.join(tmpdir, 'socket'))
+            self._test_socket_fileno(s, socket.AF_UNIX, socket.SOCK_STREAM)
+
 
 @unittest.skipUnless(HAVE_SOCKET_CAN, 'SocketCan required for this test.')
 class BasicCANTest(unittest.TestCase):

Misc/NEWS.d/next/Library/2017-12-24-20-01-09.bpo-28134.HJ8Beb.rst

@@ -0,0 +1,2 @@
+Sockets now auto-detect family, type and protocol from file descriptor by
+default.

Modules/socketmodule.c

@@ -102,7 +102,8 @@ Local naming conventions:
 
 /* Socket object documentation */
 PyDoc_STRVAR(sock_doc,
-"socket(family=AF_INET, type=SOCK_STREAM, proto=0, fileno=None) -> socket object\n\
+"socket(family=AF_INET, type=SOCK_STREAM, proto=0) -> socket object\n\
+socket(family=-1, type=-1, proto=-1, fileno=None) -> socket object\n\
 \n\
 Open a socket of the given type.  The family argument specifies the\n\
 address family; it defaults to AF_INET.  The type argument specifies\n\
@@ -111,6 +112,9 @@ or datagram (SOCK_DGRAM) socket.  The protocol argument defaults to 0,\n\
 specifying the default protocol.  Keyword arguments are accepted.\n\
 The socket is created as non-inheritable.\n\
 \n\
+When a fileno is passed in, family, type and proto are auto-detected,\n\
+unless they are explicitly set.\n\
+\n\
 A socket object represents one endpoint of a network connection.\n\
 \n\
 Methods of socket objects (keyword arguments not allowed):\n\
@@ -4792,7 +4796,7 @@ sock_initobj(PyObject *self, PyObject *args, PyObject *kwds)
     PySocketSockObject *s = (PySocketSockObject *)self;
     PyObject *fdobj = NULL;
     SOCKET_T fd = INVALID_SOCKET;
-    int family = AF_INET, type = SOCK_STREAM, proto = 0;
+    int family = -1, type = -1, proto = -1;
     static char *keywords[] = {"family", "type", "proto", "fileno", 0};
 #ifndef MS_WINDOWS
 #ifdef SOCK_CLOEXEC
@@ -4842,9 +4846,72 @@ sock_initobj(PyObject *self, PyObject *args, PyObject *kwds)
                                 "can't use invalid socket value");
                 return -1;
             }
+
+            if (family == -1) {
+                sock_addr_t addrbuf;
+                socklen_t addrlen = sizeof(sock_addr_t);
+
+                memset(&addrbuf, 0, addrlen);
+                if (getsockname(fd, SAS2SA(&addrbuf), &addrlen) == 0) {
+                    family = SAS2SA(&addrbuf)->sa_family;
+                } else {
+#ifdef MS_WINDOWS
+                    PyErr_SetFromWindowsErrWithFilename(0, "family");
+#else
+                    PyErr_SetFromErrnoWithFilename(PyExc_OSError, "family");
+#endif
+                    return -1;
+                }
+            }
+#ifdef SO_TYPE
+            if (type == -1) {
+                int tmp;
+                socklen_t slen = sizeof(tmp);
+                if (getsockopt(fd, SOL_SOCKET, SO_TYPE, &tmp, &slen) == 0) {
+                    type = tmp;
+                } else {
+#ifdef MS_WINDOWS
+                    PyErr_SetFromWindowsErrWithFilename(0, "type");
+#else
+                    PyErr_SetFromErrnoWithFilename(PyExc_OSError, "type");
+#endif
+                    return -1;
+                }
+            }
+#else
+            type = SOCK_STREAM;
+#endif
+#ifdef SO_PROTOCOL
+            if (proto == -1) {
+                int tmp;
+                socklen_t slen = sizeof(tmp);
+                if (getsockopt(fd, SOL_SOCKET, SO_PROTOCOL, &tmp, &slen) == 0) {
+                    proto = tmp;
+                } else {
+#ifdef MS_WINDOWS
+                    PyErr_SetFromWindowsErrWithFilename(0, "protocol");
+#else
+                    PyErr_SetFromErrnoWithFilename(PyExc_OSError, "protocol");
+#endif
+                    return -1;
+                }
+            }
+#else
+            proto = 0;
+#endif
         }
     }
     else {
+        /* No fd, default to AF_INET and SOCK_STREAM */
+        if (family == -1) {
+            family = AF_INET;
+        }
+        if (type == -1) {
+            type = SOCK_STREAM;
+        }
+        if (proto == -1) {
+            proto = 0;
+        }
 #ifdef MS_WINDOWS
         /* Windows implementation */
 #ifndef WSA_FLAG_NO_HANDLE_INHERIT