py: Allow subclass of native object to delegate to the native buffer_p.

Addresses issue #1109.

py/objtype.c

@@ -666,6 +666,25 @@ STATIC mp_obj_t instance_getiter(mp_obj_t self_in) {
     return mp_call_function_n_kw(meth, 0, 0, NULL);
 }
 
+STATIC mp_int_t instance_get_buffer(mp_obj_t self_in, mp_buffer_info_t *bufinfo, mp_uint_t flags) {
+    mp_obj_instance_t *self = self_in;
+    mp_obj_t member[2] = {MP_OBJ_NULL};
+    struct class_lookup_data lookup = {
+        .obj = self,
+        .attr = MP_QSTR_, // don't actually look for a method
+        .meth_offset = offsetof(mp_obj_type_t, buffer_p.get_buffer),
+        .dest = member,
+        .is_type = false,
+    };
+    mp_obj_class_lookup(&lookup, self->base.type);
+    if (member[0] == MP_OBJ_SENTINEL) {
+        mp_obj_type_t *type = mp_obj_get_type(self->subobj[0]);
+        return type->buffer_p.get_buffer(self->subobj[0], bufinfo, flags);
+    } else {
+        return 1; // object does not support buffer protocol
+    }
+}
+
 /******************************************************************************/
 // type object
 //  - the struct is mp_obj_type_t and is defined in obj.h so const types can be made
@@ -807,13 +826,16 @@ mp_obj_t mp_obj_new_type(qstr name, mp_obj_t bases_tuple, mp_obj_t locals_dict)
     o->name = name;
     o->print = instance_print;
     o->make_new = instance_make_new;
+    o->call = mp_obj_instance_call;
     o->unary_op = instance_unary_op;
     o->binary_op = instance_binary_op;
     o->load_attr = mp_obj_instance_load_attr;
     o->store_attr = mp_obj_instance_store_attr;
     o->subscr = instance_subscr;
-    o->call = mp_obj_instance_call;
     o->getiter = instance_getiter;
+    //o->iternext = ; not implemented
+    o->buffer_p.get_buffer = instance_get_buffer;
+    //o->stream_p = ; not implemented
     o->bases_tuple = bases_tuple;
     o->locals_dict = locals_dict;
 

tests/basics/subclass_native_buffer.py

@@ -0,0 +1,16 @@
+# test when we subclass a type with the buffer protocol
+
+class my_bytes(bytes):
+    pass
+
+b1 = my_bytes([0, 1])
+b2 = my_bytes([2, 3])
+b3 = bytes([4, 5])
+
+# addition will use the buffer protocol on the RHS
+print(b1 + b2)
+print(b1 + b3)
+print(b3 + b1)
+
+# bytearray construction will use the buffer protocol
+print(bytearray(b1))