gh-141044: Require 6 stack margins on debug builds too

Py_DEBUG (and UBSan) share the larger stack margin with ASan/TSan, but still used a 3-margin minimum. threading.Thread bootstrap then leaked references at that floor; the new refcount test failed on Ubuntu/macOS CI. Align the minimum with ASan/TSan. Release builds stay at 3 margins.
KingLizard1020 · Sep 17, 2026 · Sep 21, 2026 · Sep 21, 2026 · Sep 21, 2026 · Sep 21, 2026
commit 3aef3107dabcdade64a9567dd6b707598a90632f
@@ -62,12 +62,18 @@ extern PyObject* _PyRun_SimpleString(
 #  define _PyOS_STACK_MARGIN_SHIFT (_PyOS_LOG2_STACK_MARGIN + 2)
 #endif
 
-#if defined(_Py_THREAD_SANITIZER) || defined(_Py_ADDRESS_SANITIZER)
-/* Sanitizer builds need more than the default 3 margins:
+#if (defined(Py_DEBUG) \
+     || defined(_Py_ADDRESS_SANITIZER) \
+     || defined(_Py_THREAD_SANITIZER) \
+     || defined(_Py_UNDEFINED_BEHAVIOR_SANITIZER))
+/* Debug and sanitizer builds need more than the default 3 margins:
  * - TSan: tstate_set_stack() only uses half the stack.
- * - ASan (gh-141044): instrumentation consumes extra C stack, so 3
- *   margins leave threading.Thread bootstrap with no working space
- *   above the soft recursion limit and leak thread objects. */
+ * - ASan (gh-141044): instrumentation consumes extra C stack.
+ * - Py_DEBUG / UBSan: larger C frames leave threading.Thread bootstrap
+ *   with no working space above the soft recursion limit at 3 margins,
+ *   leaking thread objects (same failure mode as ASan).
+ * Require 6 margins, matching the builds that already use a larger
+ * _PyOS_LOG2_STACK_MARGIN above. Release builds stay at 3. */
 #  define _PyOS_MIN_STACK_SIZE (_PyOS_STACK_MARGIN_BYTES * 6)
 #else
 #  define _PyOS_MIN_STACK_SIZE (_PyOS_STACK_MARGIN_BYTES * 3)
@@ -78,4 +84,3 @@ extern PyObject* _PyRun_SimpleString(
 }
 #endif
 #endif  // !Py_INTERNAL_PYTHONRUN_H
-