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

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.

1 files changed, 10 insertions, 2 deletions

diff --git a/Python/pystate.c b/Python/pystate.c
index 6331a85..8f0b6b8 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -765,11 +765,19 @@ PyThreadState_Clear(PyThreadState *tstate)
 {
     int verbose = tstate->interp->config.verbose;
 
-    if (verbose && tstate->frame != NULL)
+    if (verbose && 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);