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

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 5804f878e779712e803be927ca8a6df389d82cdf)

diff --git a/Include/cpython/pystate.h b/Include/cpython/pystate.h
index 94b0809cd4f0158d456134f270c531a67bf73d3f..e22b0539136083eea918f7ea42906c7419aa0863 100644 (file)
--- a/Include/cpython/pystate.h
+++ b/Include/cpython/pystate.h
@@ -55,6 +55,7 @@ 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

diff --git a/Misc/NEWS.d/next/Core and Builtins/2020-03-23-18-08-34.bpo-20526.NHNZIv.rst b/Misc/NEWS.d/next/Core and Builtins/2020-03-23-18-08-34.bpo-20526.NHNZIv.rst
new file mode 100644 (file)
index 0000000..c808b76
--- /dev/null
+++ b/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)``.

diff --git a/Python/pystate.c b/Python/pystate.c
index 9f99060feceb2180f7f9eaf774925c5b7c4a76e1..3e1085568b61a4220f6eced52ac563442fc4d929 100644 (file)
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -762,11 +762,19 @@ 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);