Add attrwrapper_cls->tp_as_mapping->mp_subscript

This is an optimization but also fixes an lxml issue:
it calls AttrWrapper::getitem even though an error is currenly set. (which would assert if we do a callFunc call for the getitem)

src/runtime/dict.cpp

@@ -322,18 +322,20 @@ extern "C" PyObject* PyDict_GetItem(PyObject* dict, PyObject* key) noexcept {
         return d->getOrNull(key);
     }
 
-    // XXX this would be easy to make much faster.
-
-    // This path doesn't exist in CPython; we have it to support extension modules that do
-    // something along the lines of PyDict_GetItem(PyModule_GetDict()):
-    try {
-        return getitem(dict, key);
-    } catch (ExcInfo e) {
-        // PyDict_GetItem has special error behavior in CPython for backwards-compatibility reasons,
-        // and apparently it's important enough that we have to follow that.
-        // The behavior is that all errors get suppressed, and in fact I think it's supposed to
-        // restore the previous exception afterwards (we don't do that yet).
-        return NULL;
+    auto&& tstate = _PyThreadState_Current;
+    if (tstate != NULL && tstate->curexc_type != NULL) {
+        /* preserve the existing exception */
+        PyObject* err_type, *err_value, *err_tb;
+        PyErr_Fetch(&err_type, &err_value, &err_tb);
+        Box* b = getitemInternal<CAPI>(dict, key, NULL);
+        /* ignore errors */
+        PyErr_Restore(err_type, err_value, err_tb);
+        return b;
+    } else {
+        Box* b = getitemInternal<CAPI>(dict, key, NULL);
+        if (b == NULL)
+            PyErr_Clear();
+        return b;
     }
 }
 

src/runtime/types.cpp

@@ -2289,7 +2289,7 @@ public:
         return r;
     }
 
-    static Box* getitem(Box* _self, Box* _key) {
+    template <ExceptionStyle S> static Box* getitem(Box* _self, Box* _key) noexcept(S == CAPI) {
         RELEASE_ASSERT(_self->cls == attrwrapper_cls, "");
         AttrWrapper* self = static_cast<AttrWrapper*>(_self);
 
@@ -2300,8 +2300,12 @@ public:
         internStringMortalInplace(key);
 
         Box* r = self->b->getattr(key);
-        if (!r)
-            raiseExcHelper(KeyError, "'%s'", key->data());
+        if (!r) {
+            if (S == CXX)
+                raiseExcHelper(KeyError, "'%s'", key->data());
+            else
+                PyErr_Format(KeyError, "'%s'", key->data());
+        }
         return r;
     }
 
@@ -3842,7 +3846,8 @@ void setupRuntime() {
     attrwrapper_cls->giveAttr("__setitem__", new BoxedFunction(boxRTFunction((void*)AttrWrapper::setitem, UNKNOWN, 3)));
     attrwrapper_cls->giveAttr(
         "pop", new BoxedFunction(boxRTFunction((void*)AttrWrapper::pop, UNKNOWN, 3, false, false), { NULL }));
-    attrwrapper_cls->giveAttr("__getitem__", new BoxedFunction(boxRTFunction((void*)AttrWrapper::getitem, UNKNOWN, 2)));
+    attrwrapper_cls->giveAttr("__getitem__",
+                              new BoxedFunction(boxRTFunction((void*)AttrWrapper::getitem<CXX>, UNKNOWN, 2)));
     attrwrapper_cls->giveAttr("__delitem__", new BoxedFunction(boxRTFunction((void*)AttrWrapper::delitem, UNKNOWN, 2)));
     attrwrapper_cls->giveAttr("setdefault",
                               new BoxedFunction(boxRTFunction((void*)AttrWrapper::setdefault, UNKNOWN, 3)));
@@ -3868,6 +3873,7 @@ void setupRuntime() {
                               new BoxedFunction(boxRTFunction((void*)AttrWrapper::update, NONE, 1, true, true)));
     attrwrapper_cls->freeze();
     attrwrapper_cls->tp_iter = AttrWrapper::iter;
+    attrwrapper_cls->tp_as_mapping->mp_subscript = (binaryfunc)AttrWrapper::getitem<CAPI>;
 
     attrwrapperiter_cls->giveAttr("__hasnext__",
                                   new BoxedFunction(boxRTFunction((void*)AttrWrapperIter::hasnext, UNKNOWN, 1)));