From 5a574b8417839b3867b3a5a728db2744056cd232 Mon Sep 17 00:00:00 2001
From: Anselm Kruis <anselm.kruis@atos.net>
Date: Sat, 27 Oct 2018 23:07:53 +0200
Subject: [PATCH] Stackless issue #177: Fix prickelpit.c function
 slp_cannot_execute()

I missed the C-fame case, when I changed the frame-function ref counting
in Stackless issue #117.
---
 Include/frameobject.h           | 18 ++++++++++++++++
 Stackless/changelog.txt         |  6 ++++++
 Stackless/pickling/prickelpit.c | 38 ++++++++++++++++-----------------
 Stackless/pickling/prickelpit.h |  2 +-
 4 files changed, 44 insertions(+), 20 deletions(-)

diff --git a/Include/frameobject.h b/Include/frameobject.h
index 42f6586dd9637a..e541890c24b114 100644
--- a/Include/frameobject.h
+++ b/Include/frameobject.h
@@ -10,6 +10,24 @@ extern "C" {
 
 #ifdef STACKLESS
 typedef PyObject *(PyFrame_ExecFunc) (struct _frame *, int, PyObject *);
+/*
+ * How to write frame execution functions:
+ *
+ * Special rule for frame execution functions: the function owns a reference to retval!
+ *
+ *  PyObject * example(PyFrameObject *f, int exc, PyObject *retval)
+ *  {
+ *     PyThreadState *ts = PyThreadState_GET();
+ *
+ *     do something ....
+ *     if you change retval, use Py_SETREF(retval, new_value) or
+ *     Py_CLEAR(retval)
+ *
+ *     SLP_STORE_NEXT_FRAME(ts, f->f_back);
+ *     return retval;
+ *  }
+ *
+ */
 #endif
 
 typedef struct {
diff --git a/Stackless/changelog.txt b/Stackless/changelog.txt
index c62e5f70e21e26..e843066b0fffa4 100644
--- a/Stackless/changelog.txt
+++ b/Stackless/changelog.txt
@@ -9,6 +9,12 @@ What's New in Stackless 3.X.X?
 
 *Release date: 20XX-XX-XX*
 
+- https://github.com/stackless-dev/stackless/issues/177
+  Fix a crasher in prickelpit.c slp_cannot_execute().
+  Stackless versions released after March 2017 can crash, if a program executes
+  an unpickled tasklet, that lost its C-state. Now Stackless always raises
+  RuntimeError.
+
 - https://github.com/stackless-dev/stackless/issues/152
   C-Python bpo30860 consolidates stateful C globals under a single struct.
   Stackless also moves its stateful C globals to the appropriate structures
diff --git a/Stackless/pickling/prickelpit.c b/Stackless/pickling/prickelpit.c
index b46c08a2064982..52a5b15fec7201 100644
--- a/Stackless/pickling/prickelpit.c
+++ b/Stackless/pickling/prickelpit.c
@@ -365,34 +365,34 @@ run_script(char *src, char *retname)
  */
 
 PyObject *
-slp_cannot_execute(PyFrameObject *f, char *exec_name, PyObject *retval)
+slp_cannot_execute(PyFrameObject *f, const char *exec_name, PyObject *retval)
 {
+    /*
+     * Special rule for frame execution functions: we now own a reference to retval!
+     */
+
     /*
      * show an error message and raise exception.
      */
-    PyObject *message;
     PyThreadState *tstate = PyThreadState_GET();
 
     /* if we already have an exception, we keep it */
-    if (retval == NULL)
-        goto err_exit;
-
-    message = PyUnicode_FromFormat("cannot execute invalid frame with "
-                                  "'%.100s': frame had a C state that"
-                                  " can't be restored.",
-                                  exec_name);
-    if (message == NULL) {
-        /* try at least something */
-        PyErr_SetString(PyExc_RuntimeError,
-                        "invalid frame, cannot build error message.");
-        goto err_exit;
+    if (retval != NULL) {
+        Py_DECREF(retval);
+        PyErr_Format(PyExc_RuntimeError, "cannot execute invalid frame with "
+                "'%.100s': frame had a C state that"
+                " can't be restored.",
+                exec_name);
     }
-    PyErr_SetObject(PyExc_RuntimeError, message);
-    Py_DECREF(message);
-err_exit:
+
     SLP_STORE_NEXT_FRAME(tstate, f->f_back);
-    --tstate->recursion_depth;
-    Py_XDECREF(retval);
+
+    if(PyFrame_Check(f)) {
+        /* Only real frames contribute to the recursion count.
+         * C-frames don't contribute, see tasklet_setstate(...) */
+        --tstate->recursion_depth;
+    }
+
     return NULL;
 }
 
diff --git a/Stackless/pickling/prickelpit.h b/Stackless/pickling/prickelpit.h
index d544f00a9a3a2a..1a9b3912ade2ce 100644
--- a/Stackless/pickling/prickelpit.h
+++ b/Stackless/pickling/prickelpit.h
@@ -20,7 +20,7 @@ int slp_find_execfuncs(PyTypeObject *type, PyObject *exec_name,
 
 PyObject * slp_find_execname(PyFrameObject *f, int *valid);
 
-PyObject * slp_cannot_execute(PyFrameObject *f, char *exec_name, PyObject *retval);
+PyObject * slp_cannot_execute(PyFrameObject *f, const char *exec_name, PyObject *retval);
 
 /* macros to define and use an invalid frame executor */