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>

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):