test_socket: add regression test for sendmsg re-entrancy

Author: priyanshu2282-cyber

Lib/test/test_socket.py

@@ -3572,6 +3572,33 @@ def _testSendmsgAfterClose(self):
 class SendmsgStreamTests(SendmsgTests):
     # Tests for sendmsg() which require a stream socket and do not
     # involve recvmsg() or recvmsg_into().
+    @unittest.skipUnless(hasattr(socket.socket, "sendmsg"),
+                     "sendmsg not supported")
+    def _test_sendmsg_reentrant_ancillary_mutation(self):
+        import socket
+
+        seq = []
+
+        class Mut:
+            def __init__(self):
+                self.tripped = False
+            def __index__(self):
+                if not self.tripped:
+                    self.tripped = True
+                    seq.clear()
+                return 0
+
+        seq[:] = [
+            (socket.SOL_SOCKET, Mut(), b'x'),
+            (socket.SOL_SOCKET, 0, b'x'),
+        ]
+
+        left, right = socket.socketpair()
+        self.addCleanup(left.close)
+        self.addCleanup(right.close)
+        
+        with self.assertRaises(Exception):
+            left.sendmsg([b'x'], seq)
 
     def testSendmsgExplicitNoneAddr(self):
         # Check that peer address can be specified as None.