bpo-30579: Allow TracebackType creation and tb_next mutation from Python

njsmith · njsmith · commit 72115387f645 · 2018-01-06T19:21:33.000-08:00
This is admittedly an obscure use case, but currently there are projects like Jinja2 and Trio that are forced to work around this with horrible ctypes hacks: https://github.com/pallets/jinja/blob/fe3dadacdf4cf411d0a5b6bbd4d5234697a28af2/jinja2/debug.py#L345 https://github.com/python-trio/trio/blob/1e86b1aee8c0c759f6f239ae53a05d0d3963c629/trio/_core/_multierror.py#L296
diff --git a/Lib/test/test_raise.py b/Lib/test/test_raise.py
@@ -228,6 +228,72 @@ def test_accepts_traceback(self):
             self.fail("No exception raised")
 
 
+class TestTracebackType(unittest.TestCase):
+
+    def raiser(self):
+        raise ValueError
+
+    def test_attrs(self):
+        try:
+            self.raiser()
+        except Exception as exc:
+            tb = exc.__traceback__
+
+        self.assertIsInstance(tb.tb_next, types.TracebackType)
+        self.assertIs(tb.tb_frame, sys._getframe())
+        self.assertIsInstance(tb.tb_lasti, int)
+        self.assertIsInstance(tb.tb_lineno, int)
+
+        self.assertIs(tb.tb_next.tb_next, None)
+
+        # Invalid assignments
+        with self.assertRaises(TypeError):
+            del tb.tb_next
+
+        with self.assertRaises(TypeError):
+            tb.tb_next = "asdf"
+
+        # Loops
+        with self.assertRaises(ValueError):
+            tb.tb_next = tb
+
+        with self.assertRaises(ValueError):
+            tb.tb_next.tb_next = tb
+
+        # Valid assignments
+        tb.tb_next = None
+        self.assertIs(tb.tb_next, None)
+
+        new_tb = get_tb()
+        tb.tb_next = new_tb
+        self.assertIs(tb.tb_next, new_tb)
+
+    def test_constructor(self):
+        other_tb = get_tb()
+        frame = sys._getframe()
+
+        tb = types.TracebackType(other_tb, frame, 1, 2)
+        self.assertEqual(tb.tb_next, other_tb)
+        self.assertEqual(tb.tb_frame, frame)
+        self.assertEqual(tb.tb_lasti, 1)
+        self.assertEqual(tb.tb_lineno, 2)
+
+        tb = types.TracebackType(None, frame, 1, 2)
+        self.assertEqual(tb.tb_next, None)
+
+        with self.assertRaises(TypeError):
+            types.TracebackType("no", frame, 1, 2)
+
+        with self.assertRaises(TypeError):
+            types.TracebackType(other_tb, "no", 1, 2)
+
+        with self.assertRaises(TypeError):
+            types.TracebackType(other_tb, frame, "no", 2)
+
+        with self.assertRaises(TypeError):
+            types.TracebackType(other_tb, frame, 1, "nuh-uh")
+
+
 class TestContext(unittest.TestCase):
     def test_instance_context_instance_raise(self):
         context = IndexError()
diff --git a/Misc/NEWS.d/next/Core and Builtins/2017-12-11-01-52-42.bpo-30579.X6cEzf.rst b/Misc/NEWS.d/next/Core and Builtins/2017-12-11-01-52-42.bpo-30579.X6cEzf.rst
@@ -0,0 +1,2 @@
+Implement TracebackType.__new__ to allow Python-level creation of
+traceback objects, and make TracebackType.tb_next mutable.
diff --git a/Python/clinic/traceback.c.h b/Python/clinic/traceback.c.h
@@ -0,0 +1,35 @@
+/*[clinic input]
+preserve
+[clinic start generated code]*/
+
+PyDoc_STRVAR(tb_new__doc__,
+"TracebackType(tb_next, tb_frame, tb_lasti, tb_lineno)\n"
+"--\n"
+"\n"
+"Create a new traceback object.");
+
+static PyObject *
+tb_new_impl(PyTypeObject *type, PyObject *tb_next, PyFrameObject *tb_frame,
+            int tb_lasti, int tb_lineno);
+
+static PyObject *
+tb_new(PyTypeObject *type, PyObject *args, PyObject *kwargs)
+{
+    PyObject *return_value = NULL;
+    static const char * const _keywords[] = {"tb_next", "tb_frame", "tb_lasti", "tb_lineno", NULL};
+    static _PyArg_Parser _parser = {"OO!ii:TracebackType", _keywords, 0};
+    PyObject *tb_next;
+    PyFrameObject *tb_frame;
+    int tb_lasti;
+    int tb_lineno;
+
+    if (!_PyArg_ParseTupleAndKeywordsFast(args, kwargs, &_parser,
+        &tb_next, &PyFrame_Type, &tb_frame, &tb_lasti, &tb_lineno)) {
+        goto exit;
+    }
+    return_value = tb_new_impl(type, tb_next, tb_frame, tb_lasti, tb_lineno);
+
+exit:
+    return return_value;
+}
+/*[clinic end generated code: output=0133130d7d19556f input=a9049054013a1b77]*/
diff --git a/Python/traceback.c b/Python/traceback.c
@@ -27,26 +27,138 @@ _Py_IDENTIFIER(close);
 _Py_IDENTIFIER(open);
 _Py_IDENTIFIER(path);
 
+/*[clinic input]
+class TracebackType "PyTracebackObject *" "&PyTraceback_Type"
+[clinic start generated code]*/
+/*[clinic end generated code: output=da39a3ee5e6b4b0d input=928fa06c10151120]*/
+
+#include "clinic/traceback.c.h"
+
+static PyObject *
+tb_create_raw(PyTracebackObject *next, PyFrameObject *frame, int lasti,
+              int lineno)
+{
+    PyTracebackObject *tb;
+    if ((next != NULL && !PyTraceBack_Check(next)) ||
+                    frame == NULL || !PyFrame_Check(frame)) {
+        PyErr_BadInternalCall();
+        return NULL;
+    }
+    tb = PyObject_GC_New(PyTracebackObject, &PyTraceBack_Type);
+    if (tb != NULL) {
+        Py_XINCREF(next);
+        tb->tb_next = next;
+        Py_XINCREF(frame);
+        tb->tb_frame = frame;
+        tb->tb_lasti = lasti;
+        tb->tb_lineno = lineno;
+        PyObject_GC_Track(tb);
+    }
+    return (PyObject *)tb;
+}
+
+/*[clinic input]
+@classmethod
+TracebackType.__new__ as tb_new
+
+  tb_next: object
+  tb_frame: object(type='PyFrameObject *', subclass_of='&PyFrame_Type')
+  tb_lasti: int
+  tb_lineno: int
+
+Create a new traceback object.
+[clinic start generated code]*/
+
+static PyObject *
+tb_new_impl(PyTypeObject *type, PyObject *tb_next, PyFrameObject *tb_frame,
+            int tb_lasti, int tb_lineno)
+/*[clinic end generated code: output=fa077debd72d861a input=01cbe8ec8783fca7]*/
+{
+    if (tb_next == Py_None) {
+        tb_next = NULL;
+    } else if (!PyTraceBack_Check(tb_next)) {
+        return PyErr_Format(PyExc_TypeError,
+                            "expected traceback object or None, got '%s'",
+                            Py_TYPE(tb_next)->tp_name);
+    }
+
+    return tb_create_raw((PyTracebackObject *)tb_next, tb_frame, tb_lasti,
+                         tb_lineno);
+}
+
 static PyObject *
 tb_dir(PyTracebackObject *self)
 {
     return Py_BuildValue("[ssss]", "tb_frame", "tb_next",
                                    "tb_lasti", "tb_lineno");
 }
 
+static PyObject *
+tb_next_get(PyTracebackObject *self, void *Py_UNUSED(_))
+{
+    PyObject* ret = (PyObject*)self->tb_next;
+    if (!ret) {
+        ret = Py_None;
+    }
+    Py_INCREF(ret);
+    return ret;
+}
+
+static int
+tb_next_set(PyTracebackObject *self, PyObject *new_next, void *Py_UNUSED(_))
+{
+    if (!new_next) {
+        PyErr_Format(PyExc_TypeError, "can't delete tb_next attribute");
+        return -1;
+    }
+
+    /* We accept None or a traceback object, and map None -> NULL (inverse of
+       tb_next_get) */
+    if (new_next == Py_None) {
+        new_next = NULL;
+    } else if (!PyTraceBack_Check(new_next)) {
+        PyErr_Format(PyExc_TypeError,
+                     "expected traceback object, got '%s'",
+                     Py_TYPE(new_next)->tp_name);
+        return -1;
+    }
+
+    /* Check for loops */
+    PyTracebackObject *cursor = (PyTracebackObject *)new_next;
+    while (cursor) {
+        if (cursor == self) {
+            PyErr_Format(PyExc_ValueError, "traceback loop detected");
+            return -1;
+        }
+        cursor = cursor->tb_next;
+    }
+
+    PyObject *old_next = (PyObject*)self->tb_next;
+    Py_XINCREF(new_next);
+    self->tb_next = (PyTracebackObject *)new_next;
+    Py_XDECREF(old_next);
+
+    return 0;
+}
+
+
 static PyMethodDef tb_methods[] = {
    {"__dir__", (PyCFunction)tb_dir, METH_NOARGS},
    {NULL, NULL, 0, NULL},
 };
 
 static PyMemberDef tb_memberlist[] = {
-    {"tb_next",         T_OBJECT,       OFF(tb_next),   READONLY},
     {"tb_frame",        T_OBJECT,       OFF(tb_frame),  READONLY},
     {"tb_lasti",        T_INT,          OFF(tb_lasti),  READONLY},
     {"tb_lineno",       T_INT,          OFF(tb_lineno), READONLY},
     {NULL}      /* Sentinel */
 };
 
+static PyGetSetDef tb_getsetters[] = {
+    {"tb_next", (getter)tb_next_get, (setter)tb_next_set, NULL, NULL},
+    {NULL}      /* Sentinel */
+};
+
 static void
 tb_dealloc(PyTracebackObject *tb)
 {
@@ -94,7 +206,7 @@ PyTypeObject PyTraceBack_Type = {
     0,                  /* tp_setattro */
     0,                                          /* tp_as_buffer */
     Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,/* tp_flags */
-    0,                                          /* tp_doc */
+    tb_new__doc__,                              /* tp_doc */
     (traverseproc)tb_traverse,                  /* tp_traverse */
     (inquiry)tb_clear,                          /* tp_clear */
     0,                                          /* tp_richcompare */
@@ -103,39 +215,24 @@ PyTypeObject PyTraceBack_Type = {
     0,                                          /* tp_iternext */
     tb_methods,         /* tp_methods */
     tb_memberlist,      /* tp_members */
-    0,                                          /* tp_getset */
+    tb_getsetters,                              /* tp_getset */
     0,                                          /* tp_base */
     0,                                          /* tp_dict */
+    0,                                          /* tp_descr_get */
+    0,                                          /* tp_descr_set */
+    0,                                          /* tp_dictoffset */
+    0,                                          /* tp_init */
+    0,                                          /* tp_alloc */
+    tb_new,                                     /* tp_new */
 };
 
-static PyTracebackObject *
-newtracebackobject(PyTracebackObject *next, PyFrameObject *frame)
-{
-    PyTracebackObject *tb;
-    if ((next != NULL && !PyTraceBack_Check(next)) ||
-                    frame == NULL || !PyFrame_Check(frame)) {
-        PyErr_BadInternalCall();
-        return NULL;
-    }
-    tb = PyObject_GC_New(PyTracebackObject, &PyTraceBack_Type);
-    if (tb != NULL) {
-        Py_XINCREF(next);
-        tb->tb_next = next;
-        Py_XINCREF(frame);
-        tb->tb_frame = frame;
-        tb->tb_lasti = frame->f_lasti;
-        tb->tb_lineno = PyFrame_GetLineNumber(frame);
-        PyObject_GC_Track(tb);
-    }
-    return tb;
-}
-
 int
 PyTraceBack_Here(PyFrameObject *frame)
 {
     PyObject *exc, *val, *tb, *newtb;
     PyErr_Fetch(&exc, &val, &tb);
-    newtb = (PyObject *)newtracebackobject((PyTracebackObject *)tb, frame);
+    newtb = tb_create_raw((PyTracebackObject *)tb, frame, frame->f_lasti,
+                          PyFrame_GetLineNumber(frame));
     if (newtb == NULL) {
         _PyErr_ChainExceptions(exc, val, tb);
         return -1;

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	`+Implement TracebackType.__new__ to allow Python-level creation of`
	`2`	`+traceback objects, and make TracebackType.tb_next mutable.`