Add support for the __del__ slot and some refactoring.

Make sure __del__ methods get assigned to tp_del in new style classes.

Go through an attribute lookup in old style classes. Also, change the
function calling structure to match CPython's a bit better.

For exceptions thrown in tp_del methods, print a warning and ignore
them, like CPython does.

src/capi/typeobject.cpp

@@ -977,6 +977,26 @@ static PyObject* slot_tp_getattr_hook(PyObject* self, PyObject* name) noexcept {
     }
 }
 
+static PyObject* slot_tp_del(PyObject* self) noexcept {
+    static BoxedString* del_str = internStringImmortal("__del__");
+    try {
+        // TODO: runtime ICs?
+        Box* del_attr = typeLookup(self->cls, del_str, NULL);
+        assert(del_attr);
+
+        CallattrFlags flags{.cls_only = false,
+                            .null_on_nonexistent = true,
+                            .argspec = ArgPassSpec(0, 0, false, false) };
+        return callattr(self, del_str, flags, NULL, NULL, NULL, NULL, NULL);
+    } catch (ExcInfo e) {
+        // Python does not support exceptions thrown inside finalizers. Instead, it just
+        // prints a warning that an exception was throw to stderr but ignores it.
+        setCAPIException(e);
+        PyErr_WriteUnraisable(self);
+        return NULL;
+    }
+}
+
 static int slot_tp_init(PyObject* self, PyObject* args, PyObject* kwds) noexcept {
     STAT_TIMER(t0, "us_timer_slot_tpinit", SLOT_AVOIDABILITY(self));
 
@@ -1467,6 +1487,7 @@ static slotdef slotdefs[]
                                                                           "see help(type(x)) for signature",
                PyWrapperFlag_KEYWORDS),
         TPSLOT("__new__", tp_new, slot_tp_new, NULL, ""),
+        TPSLOT("__del__", tp_del, slot_tp_del, NULL, ""),
         TPPSLOT("__hasnext__", tpp_hasnext, slotTppHasnext, wrapInquirypred, "hasnext"),
 
         BINSLOT("__add__", nb_add, slot_nb_add, "+"),                             // [force clang-format to line break]

src/runtime/classobj.cpp

@@ -251,52 +251,73 @@ Box* classobjStr(Box* _obj) {
     return boxStringTwine(llvm::Twine(static_cast<BoxedString*>(_mod)->s()) + "." + cls->name->s());
 }
 
-static Box* _instanceGetattribute(Box* _inst, Box* _attr, bool raise_on_missing) {
-    STAT_TIMER(t0, "us_timer_instance_getattribute", 0);
-
-    RELEASE_ASSERT(_inst->cls == instance_cls, "");
-    BoxedInstance* inst = static_cast<BoxedInstance*>(_inst);
-
-    RELEASE_ASSERT(_attr->cls == str_cls, "");
-    BoxedString* attr = static_cast<BoxedString*>(_attr);
-
-    // These are special cases in CPython as well:
-    if (attr->s()[0] == '_' && attr->s()[1] == '_') {
-        if (attr->s() == "__dict__")
-            return inst->getAttrWrapper();
 
-        if (attr->s() == "__class__")
-            return inst->inst_cls;
-    }
-
-    Box* r = inst->getattr(attr);
+// Analogous to CPython's instance_getattr2
+static Box* instanceGetattributeSimple(BoxedInstance* inst, BoxedString* attr_str) {
+    Box* r = inst->getattr(attr_str);
     if (r)
         return r;
 
-    r = classLookup(inst->inst_cls, attr);
+    r = classLookup(inst->inst_cls, attr_str);
     if (r) {
         return processDescriptor(r, inst, inst->inst_cls);
     }
-    RELEASE_ASSERT(!r, "");
+
+    return NULL;
+}
+
+static Box* instanceGetattributeWithFallback(BoxedInstance* inst, BoxedString* attr_str) {
+    Box* attr_obj = instanceGetattributeSimple(inst, attr_str);
+
+    if (attr_obj) {
+        return attr_obj;
+    }
 
     static BoxedString* getattr_str = internStringImmortal("__getattr__");
     Box* getattr = classLookup(inst->inst_cls, getattr_str);
 
     if (getattr) {
         getattr = processDescriptor(getattr, inst, inst->inst_cls);
-        return runtimeCall(getattr, ArgPassSpec(1), _attr, NULL, NULL, NULL, NULL);
+        return runtimeCall(getattr, ArgPassSpec(1), attr_str, NULL, NULL, NULL, NULL);
     }
 
-    if (!raise_on_missing)
     return NULL;
+}
 
-    raiseExcHelper(AttributeError, "%s instance has no attribute '%s'", inst->inst_cls->name->data(), attr->data());
+static Box* _instanceGetattribute(Box* _inst, BoxedString* attr_str, bool raise_on_missing) {
+    RELEASE_ASSERT(_inst->cls == instance_cls, "");
+    BoxedInstance* inst = static_cast<BoxedInstance*>(_inst);
+
+    // These are special cases in CPython as well:
+    if (attr_str->s()[0] == '_' && attr_str->s()[1] == '_') {
+        if (attr_str->s() == "__dict__")
+            return inst->getAttrWrapper();
+
+        if (attr_str->s() == "__class__")
+            return inst->inst_cls;
+    }
+
+    Box* attr = instanceGetattributeWithFallback(inst, attr_str);
+    if (attr) {
+        return attr;
+    } else if (!raise_on_missing) {
+        return NULL;
+    } else {
+        raiseExcHelper(AttributeError, "%s instance has no attribute '%s'", inst->inst_cls->name->data(),
+                       attr_str->data());
+    }
 }
 
 Box* instanceGetattribute(Box* _inst, Box* _attr) {
-    return _instanceGetattribute(_inst, _attr, true);
+    STAT_TIMER(t0, "us_timer_instance_getattribute", 0);
+
+    RELEASE_ASSERT(_attr->cls == str_cls, "");
+    BoxedString* attr = static_cast<BoxedString*>(_attr);
+    return _instanceGetattribute(_inst, attr, true);
 }
 
+
+// Analogous to CPython's instance_getattr
 static Box* instance_getattro(Box* cls, Box* attr) noexcept {
     try {
         return instanceGetattribute(cls, attr);
@@ -716,16 +737,9 @@ static Box* instanceNext(BoxedInstance* inst) {
 
 static PyObject* instance_index(PyObject* self) noexcept {
     PyObject* func, *res;
-    /*
-    static PyObject* indexstr = NULL;
 
-    if (indexstr == NULL) {
-        indexstr = PyString_InternFromString("__index__");
-        if (indexstr == NULL)
-            return NULL;
-    }
-    */
-    if ((func = instance_getattro(self, boxString("__index__"))) == NULL) {
+    static BoxedString* index_str = internStringImmortal("__index__");
+    if ((func = instance_getattro(self, index_str)) == NULL) {
         if (!PyErr_ExceptionMatches(PyExc_AttributeError))
             return NULL;
         PyErr_Clear();
@@ -737,6 +751,18 @@ static PyObject* instance_index(PyObject* self) noexcept {
     return res;
 }
 
+static void instance_dealloc(Box* _inst) {
+    RELEASE_ASSERT(_inst->cls == instance_cls, "");
+    BoxedInstance* inst = static_cast<BoxedInstance*>(_inst);
+
+    // Note that trying to call __del__ as a finalizer does not fallback to
+    // __getattr__ unlike other attributes (like __index__). This is CPython's behavior.
+    static BoxedString* del_str = internStringImmortal("__del__");
+    Box* func = instanceGetattributeSimple(inst, del_str);
+    if (func)
+        runtimeCall(func, ArgPassSpec(0), NULL, NULL, NULL, NULL, NULL);
+}
+
 static Box* _instanceBinary(Box* _inst, Box* other, BoxedString* attr) {
     RELEASE_ASSERT(_inst->cls == instance_cls, "");
     BoxedInstance* inst = static_cast<BoxedInstance*>(_inst);
@@ -893,5 +919,7 @@ void setupClassobj() {
     instance_cls->tp_getattro = instance_getattro;
     instance_cls->tp_setattro = instance_setattro;
     instance_cls->tp_as_number->nb_index = instance_index;
+    instance_cls->tp_dealloc = instance_dealloc;
+    instance_cls->has_safe_tp_dealloc = false;
 }
 }