gh-108342: Break ref cycle in test_ssl preauth

In preauth tests of test_ssl, explicitly break reference cycles
invoving SingleConnectionTestServerThread to make sure that the
thread is deleted. Otherwise, the test marks the environment as
altered because the threading module sees a "dangling thread"
(SingleConnectionTestServerThread).

This test leak was introduced by the test added for the fix of issue
gh-108310.

Author: vstinner

Lib/test/test_ssl.py

@@ -4734,8 +4734,13 @@ def non_linux_skip_if_other_okay_error(self, err):
             # we're specifically trying to test. The way this test is written
             # is known to work on Linux. We'll skip it anywhere else that it
             # does not present as doing so.
-            self.skipTest(f"Could not recreate conditions on {sys.platform}:"
-                          f" {err=}")
+            try:
+                self.skipTest(f"Could not recreate conditions on {sys.platform}:"
+                              f" {err=}")
+            finally:
+                # gh-108342: Explicitly break the reference cycle
+                err = None
+
         # If maintaining this conditional winds up being a problem.
         # just turn this into an unconditional skip anything but Linux.
         # The important thing is that our CI has the logic covered.
@@ -4767,15 +4772,22 @@ def call_after_accept(unused):
 
         ready_for_server_wrap_socket.set()
         server.join()
+
         wrap_error = server.wrap_error
-        self.assertEqual(b"", server.received_data)
-        self.assertIsInstance(wrap_error, OSError)  # All platforms.
-        self.non_linux_skip_if_other_okay_error(wrap_error)
-        self.assertIsInstance(wrap_error, ssl.SSLError)
-        self.assertIn("before TLS handshake with data", wrap_error.args[1])
-        self.assertIn("before TLS handshake with data", wrap_error.reason)
-        self.assertNotEqual(0, wrap_error.args[0])
-        self.assertIsNone(wrap_error.library, msg="attr must exist")
+        server.wrap_error = None
+        try:
+            self.assertEqual(b"", server.received_data)
+            self.assertIsInstance(wrap_error, OSError)  # All platforms.
+            self.non_linux_skip_if_other_okay_error(wrap_error)
+            self.assertIsInstance(wrap_error, ssl.SSLError)
+            self.assertIn("before TLS handshake with data", wrap_error.args[1])
+            self.assertIn("before TLS handshake with data", wrap_error.reason)
+            self.assertNotEqual(0, wrap_error.args[0])
+            self.assertIsNone(wrap_error.library, msg="attr must exist")
+        finally:
+            # gh-108342: Explicitly break the reference cycle
+            wrap_error = None
+            server = None
 
     def test_preauth_data_to_tls_client(self):
         client_can_continue_with_wrap_socket = threading.Event()
@@ -4815,14 +4827,19 @@ def call_after_accept(conn_to_client):
                 tls_client.close()
 
         server.join()
-        self.assertEqual(b"", received_data)
-        self.assertIsInstance(wrap_error, OSError)  # All platforms.
-        self.non_linux_skip_if_other_okay_error(wrap_error)
-        self.assertIsInstance(wrap_error, ssl.SSLError)
-        self.assertIn("before TLS handshake with data", wrap_error.args[1])
-        self.assertIn("before TLS handshake with data", wrap_error.reason)
-        self.assertNotEqual(0, wrap_error.args[0])
-        self.assertIsNone(wrap_error.library, msg="attr must exist")
+        try:
+            self.assertEqual(b"", received_data)
+            self.assertIsInstance(wrap_error, OSError)  # All platforms.
+            self.non_linux_skip_if_other_okay_error(wrap_error)
+            self.assertIsInstance(wrap_error, ssl.SSLError)
+            self.assertIn("before TLS handshake with data", wrap_error.args[1])
+            self.assertIn("before TLS handshake with data", wrap_error.reason)
+            self.assertNotEqual(0, wrap_error.args[0])
+            self.assertIsNone(wrap_error.library, msg="attr must exist")
+        finally:
+            # gh-108342: Explicitly break the reference cycle
+            wrap_error = None
+            server = None
 
     def test_https_client_non_tls_response_ignored(self):