Stackless issue python#177: Fix prickelpit.c function slp_cannot_exec…

…ute()

I missed the C-fame case, when I changed the frame-function ref counting
in Stackless issue python#117.

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 {

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

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;
 }
 

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 */