bpo-20526: Fix PyThreadState_Clear(): don't decref frame (GH-19120) (GH-19136) (GH-19137)

PyThreadState.frame is a borrowed reference, not a strong reference:
PyThreadState_Clear() must not call Py_CLEAR(tstate->frame).

Remove test_threading.test_warnings_at_exit(): we cannot warranty
that the Python thread state of daemon threads is cleared in a
reliable way during Python shutdown.

(cherry picked from commit 5804f878e7)
(cherry picked from commit e97c8b0688)

Include/pystate.h

@@ -216,6 +216,7 @@ typedef struct _ts {
     struct _ts *next;
     PyInterpreterState *interp;
 
+    /* Borrowed reference to the current frame (it can be NULL) */
     struct _frame *frame;
     int recursion_depth;
     char overflowed; /* The stack has overflowed. Allow 50 more calls

Misc/NEWS.d/next/Core and Builtins/2020-03-23-18-08-34.bpo-20526.NHNZIv.rst

@@ -0,0 +1,3 @@
+Fix :c:func:`PyThreadState_Clear()`. ``PyThreadState.frame`` is a borrowed
+reference, not a strong reference: ``PyThreadState_Clear()`` must not call
+``Py_CLEAR(tstate->frame)``.

Python/pystate.c

@@ -566,11 +566,19 @@ _PyState_ClearModules(void)
 void
 PyThreadState_Clear(PyThreadState *tstate)
 {
-    if (Py_VerboseFlag && tstate->frame != NULL)
+    if (Py_VerboseFlag && tstate->frame != NULL) {
+        /* bpo-20526: After the main thread calls
+           _PyRuntimeState_SetFinalizing() in Py_FinalizeEx(), threads must
+           exit when trying to take the GIL. If a thread exit in the middle of
+           _PyEval_EvalFrameDefault(), tstate->frame is not reset to its
+           previous value. It is more likely with daemon threads, but it can
+           happen with regular threads if threading._shutdown() fails
+           (ex: interrupted by CTRL+C). */
         fprintf(stderr,
           "PyThreadState_Clear: warning: thread still has a frame\n");
+    }
 
-    Py_CLEAR(tstate->frame);
+    /* Don't clear tstate->frame: it is a borrowed reference */
 
     Py_CLEAR(tstate->dict);
     Py_CLEAR(tstate->async_exc);