gh-121058: Warn if PyThreadState_Clear is called with an exception set (gh-121343)

Author: ZeroIntensity

Misc/NEWS.d/next/C API/2024-07-03-13-39-13.gh-issue-121058.MKi1MV.rst

@@ -0,0 +1,2 @@
+``PyThreadState_Clear()`` now warns (and calls ``sys.excepthook``) if the
+thread state still has an active exception.

Python/pystate.c

@@ -1649,6 +1649,11 @@ PyThreadState_Clear(PyThreadState *tstate)
           "PyThreadState_Clear: warning: thread still has a frame\n");
     }
 
+    if (verbose && tstate->current_exception != NULL) {
+        fprintf(stderr, "PyThreadState_Clear: warning: thread has an exception set\n");
+        _PyErr_Print(tstate);
+    }
+
     /* At this point tstate shouldn't be used any more,
        neither to run Python code nor for other uses.