[3.10] gh-119461: Fix ThreadedVSOCKSocketStreamTest (GH-129171) (#129440)

* [3.11] gh-119461: Fix ThreadedVSOCKSocketStreamTest (GH-129171)

Fix ThreadedVSOCKSocketStreamTest: if get_cid() returns the host
address or the "any" address, use the local communication address
(loopback): VMADDR_CID_LOCAL.

On Linux 6.9, apparently, the /dev/vsock device is now available but
get_cid() returns VMADDR_CID_ANY (-1).

(cherry picked from commit 45db419)

Co-authored-by: Victor Stinner <vstinner@python.org>
(cherry picked from commit e94dbe4)
(cherry picked from commit c750061)
(cherry picked from commit cbfe302)

* gh-119461: Restore the testSocket VSOCK skipUnless removed by PR #119465 (#129561)

Restore the skipUnless removed by #119465.

This test can only pass on virtual machines, not actual machines.

actual machines see:

```
    self.cli.connect((cid, VSOCKPORT))
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
OSError: [Errno 19] No such device
```

Reproduced on (Linux) Ubuntu 24.04.1 running 6.8.0-52-generic.

---------

Co-authored-by: Victor Stinner <vstinner@python.org>
Co-authored-by: Gregory P. Smith <greg@krypto.org>

Author: 3 people

Lib/test/test_socket.py

@@ -41,6 +41,7 @@
 # test unicode string and carriage return
 MSG = 'Michael Gilfix was here\u1234\r\n'.encode('utf-8')
 
+VMADDR_CID_LOCAL = 1
 VSOCKPORT = 1234
 AIX = platform.system() == "AIX"
 
@@ -124,8 +125,8 @@ def _have_socket_qipcrtr():
 
 def _have_socket_vsock():
     """Check whether AF_VSOCK sockets are supported on this host."""
-    ret = get_cid() is not None
-    return ret
+    cid = get_cid()
+    return (cid is not None)
 
 
 def _have_socket_bluetooth():
@@ -487,8 +488,8 @@ def clientTearDown(self):
 @unittest.skipIf(fcntl is None, "need fcntl")
 @unittest.skipUnless(HAVE_SOCKET_VSOCK,
           'VSOCK sockets required for this test.')
-@unittest.skipUnless(get_cid() != 2,
-          "This test can only be run on a virtual guest.")
+@unittest.skipUnless(get_cid() != 2,  # VMADDR_CID_HOST
+                     "This test can only be run on a virtual guest.")
 class ThreadedVSOCKSocketStreamTest(unittest.TestCase, ThreadableTest):
 
     def __init__(self, methodName='runTest'):
@@ -509,10 +510,16 @@ def clientSetUp(self):
         self.cli = socket.socket(socket.AF_VSOCK, socket.SOCK_STREAM)
         self.addCleanup(self.cli.close)
         cid = get_cid()
+        if cid in (socket.VMADDR_CID_HOST, socket.VMADDR_CID_ANY):
+            # gh-119461: Use the local communication address (loopback)
+            cid = VMADDR_CID_LOCAL
         self.cli.connect((cid, VSOCKPORT))
 
     def testStream(self):
-        msg = self.conn.recv(1024)
+        try:
+            msg = self.conn.recv(1024)
+        except PermissionError as exc:
+            self.skipTest(repr(exc))
         self.assertEqual(msg, MSG)
 
     def _testStream(self):