Guard short attribute names in fast lookup

Fixes #971

Author: msimacek

graalpython/com.oracle.graal.python.test/src/tests/test_object.py

@@ -87,6 +87,13 @@ def set_dict_attr():
     assert_raises(AttributeError, set_dict_attr)
 
 
+def test_lookup_single_underscore_attr():
+    class MyClass:
+        pass
+
+    assert_raises(AttributeError, lambda: MyClass()._)
+
+
 def test_set_dict_attr():
     class MyClass(object):
         def __init__(self):

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/lib/PyObjectLookupAttr.java

@@ -335,7 +335,8 @@ static Object readAttributeQuickly(Object type, TpSlots slots, Object receiver,
                 PythonAbstractClass base = bases[0];
                 if (base instanceof PythonBuiltinClass &&
                                 ((PythonBuiltinClass) base).getType() == PythonBuiltinClassType.PythonObject) {
-                    if (!(codePointAtIndexNode.execute(stringName, 0) == '_' && codePointAtIndexNode.execute(stringName, 1) == '_')) {
+                    int length = codePointLengthNode.execute(stringName, TS_ENCODING);
+                    if (!(length >= 2 && codePointAtIndexNode.execute(stringName, 0) == '_' && codePointAtIndexNode.execute(stringName, 1) == '_')) {
                         // not a special name, so this attribute cannot be inherited, and can
                         // only be on the type or the object. If it's on the type, return to
                         // the generic code.