bpo-44584: Deprecate PYTHONTHREADDEBUG env var (GH-27065)


The threading debug (PYTHONTHREADDEBUG environment variable) is
deprecated in Python 3.10 and will be removed in Python 3.12. This
feature requires a debug build of Python.
(cherry picked from commit 4d77691172aae81bdcbb0ea75839d0e896c43781)

Co-authored-by: Victor Stinner <vstinner@python.org>
diff --git a/Lib/test/test_threading.py b/Lib/test/test_threading.py
index 23fe2d3..c51de6f 100644
--- a/Lib/test/test_threading.py
+++ b/Lib/test/test_threading.py
@@ -32,6 +32,9 @@
 # on platforms known to behave badly.
 platforms_to_skip = ('netbsd5', 'hp-ux11')
 
+# Is Python built with Py_DEBUG macro defined?
+Py_DEBUG = hasattr(sys, 'gettotalrefcount')
+
 
 def restore_default_excepthook(testcase):
     testcase.addCleanup(setattr, threading, 'excepthook', threading.excepthook)
@@ -915,6 +918,16 @@ def noop(): pass
             threading.Thread(target=noop).start()
             # Thread.join() is not called
 
+    @unittest.skipUnless(Py_DEBUG, 'need debug build (Py_DEBUG)')
+    def test_debug_deprecation(self):
+        # bpo-44584: The PYTHONTHREADDEBUG environment variable is deprecated
+        rc, out, err = assert_python_ok("-Wdefault", "-c", "pass",
+                                        PYTHONTHREADDEBUG="1")
+        msg = (b'DeprecationWarning: The threading debug '
+               b'(PYTHONTHREADDEBUG environment variable) '
+               b'is deprecated and will be removed in Python 3.12')
+        self.assertIn(msg, err)
+
 
 class ThreadJoinOnShutdown(BaseTestCase):