ExcInfo: don't set traceback to None instead use NULL

This makes it more similar to cpythons and without this PyTraceBack_Here does not work correctly.

from_cpython/Python/traceback.c

@@ -126,8 +126,6 @@ PyTraceBack_Here(PyFrameObject *frame)
 int
 PyTraceBack_Here_Tb(PyFrameObject *frame, PyTracebackObject** tb)
 {
-    if ((PyObject*)*tb == Py_None)
-        *tb = NULL;
     *tb = newtracebackobject(*tb, frame);
     if (*tb == NULL)
         return -1;

src/codegen/unwinding.cpp

@@ -719,12 +719,12 @@ ExcInfo* getFrameExcInfo() {
 
     if (!copy_from_exc->type) {
         // No exceptions found:
-        *copy_from_exc = ExcInfo(None, None, None);
+        *copy_from_exc = ExcInfo(None, None, NULL);
     }
 
     assert(gc::isValidGCObject(copy_from_exc->type));
     assert(gc::isValidGCObject(copy_from_exc->value));
-    assert(gc::isValidGCObject(copy_from_exc->traceback));
+    assert(!copy_from_exc->traceback || gc::isValidGCObject(copy_from_exc->traceback));
 
     for (auto* ex : to_update) {
         *ex = *copy_from_exc;

src/runtime/builtin_modules/sys.cpp

@@ -48,19 +48,18 @@ Box* sysExcInfo() {
     ExcInfo* exc = getFrameExcInfo();
     assert(exc->type);
     assert(exc->value);
-    assert(exc->traceback);
-    return BoxedTuple::create({ exc->type, exc->value, exc->traceback });
+    Box* tb = exc->traceback ? exc->traceback : None;
+    return BoxedTuple::create({ exc->type, exc->value, tb });
 }
 
 Box* sysExcClear() {
     ExcInfo* exc = getFrameExcInfo();
     assert(exc->type);
     assert(exc->value);
-    assert(exc->traceback);
 
     exc->type = None;
     exc->value = None;
-    exc->traceback = None;
+    exc->traceback = NULL;
 
     return None;
 }

src/runtime/capi.cpp

@@ -715,8 +715,6 @@ void checkAndThrowCAPIException() {
             value = None;
 
         Box* tb = cur_thread_state.curexc_traceback;
-        if (!tb)
-            tb = None;
 
         // Make sure to call PyErr_Clear() *before* normalizing the exception, since otherwise
         // the normalization can think that it had raised an exception, resulting to a call
@@ -736,7 +734,7 @@ void checkAndThrowCAPIException() {
 
         RELEASE_ASSERT(value->cls == type, "unsupported");
 
-        if (tb != None)
+        if (tb)
             throw ExcInfo(value->cls, value, tb);
         raiseExc(value);
     }
@@ -760,7 +758,7 @@ extern "C" void PyErr_SetExcInfo(PyObject* type, PyObject* value, PyObject* trac
     ExcInfo* exc = getFrameExcInfo();
     exc->type = type ? type : None;
     exc->value = value ? value : None;
-    exc->traceback = traceback ? traceback : None;
+    exc->traceback = traceback;
 }
 
 extern "C" const char* PyExceptionClass_Name(PyObject* o) noexcept {

src/runtime/cxx_unwind.cpp

@@ -73,10 +73,10 @@ namespace pyston {
 
 void checkExcInfo(const ExcInfo* exc) {
     assert(exc);
-    assert(exc->type && exc->value && exc->traceback);
+    assert(exc->type && exc->value);
     ASSERT(gc::isValidGCObject(exc->type), "%p", exc->type);
     ASSERT(gc::isValidGCObject(exc->value), "%p", exc->value);
-    ASSERT(gc::isValidGCObject(exc->traceback), "%p", exc->traceback);
+    ASSERT(!exc->traceback || gc::isValidGCObject(exc->traceback), "%p", exc->traceback);
 }
 
 static StatCounter us_unwind_loop("us_unwind_loop");

src/runtime/exceptions.cpp

@@ -27,7 +27,7 @@ namespace pyston {
 
 void raiseExc(Box* exc_obj) {
     assert(!PyErr_Occurred());
-    throw ExcInfo(exc_obj->cls, exc_obj, None);
+    throw ExcInfo(exc_obj->cls, exc_obj, NULL);
 }
 
 // Have a special helper function for syntax errors, since we want to include the location
@@ -46,7 +46,7 @@ void raiseSyntaxError(const char* msg, int lineno, int col_offset, llvm::StringR
         auto args = BoxedTuple::create({ boxString(file), boxInt(lineno), None, loc });
         Box* exc = runtimeCall(SyntaxError, ArgPassSpec(2), boxString(msg), args, NULL, NULL, NULL);
         assert(!PyErr_Occurred());
-        throw ExcInfo(exc->cls, exc, None);
+        throw ExcInfo(exc->cls, exc, NULL);
     } else {
         PyErr_SetString(SyntaxError, msg);
         PyErr_SyntaxLocation(file.str().c_str(), lineno);
@@ -140,10 +140,6 @@ ExcInfo excInfoForRaise(Box* type, Box* value, Box* tb) {
 
     assert(PyExceptionClass_Check(type));
 
-    if (tb == NULL) {
-        tb = None;
-    }
-
     return ExcInfo(type, value, tb);
 }
 
@@ -169,7 +165,7 @@ extern "C" void raise0_capi(ExcInfo* frame_exc_info) noexcept {
         frame_exc_info->type = TypeError;
         frame_exc_info->value
             = boxString("exceptions must be old-style classes or derived from BaseException, not NoneType");
-        frame_exc_info->traceback = None;
+        frame_exc_info->traceback = NULL;
         PyErr_NormalizeException(&frame_exc_info->type, &frame_exc_info->value, &frame_exc_info->traceback);
     }