box/lua: don't mask error object methods by payload fields

With commit eb2c6a4 ("box/lua: allow to set custom error payload
fields") it is possible to add a field named "raise", but it will
override the `error_object:raise()' method. Forbid it. The user can
still access masked payload fields through the `unpack()' method.

NO_DOC=unreleased
NO_CHANGELOG=unreleased

Author: Gumix

src/lua/error.lua

@@ -174,6 +174,10 @@ local error_methods = {
 }
 
 local function error_index(err, key)
+    local method = error_methods[key]
+    if method ~= nil then
+        return method
+    end
     local getter = error_fields[key]
     if getter ~= nil then
         return getter(err)
@@ -182,7 +186,6 @@ local function error_index(err, key)
     if f ~= nil then
         return mp_decode(f._data)
     end
-    return error_methods[key]
 end
 
 local function error_concat(lhs, rhs)

test/box-luatest/error_subsystem_improvements_test.lua

@@ -106,3 +106,24 @@ g.test_error_new_message_arg = function()
     t.assert_equals(box.error.new{message = msg, reason = 'other'}.message, msg)
     t.assert_equals(box.error.new{msg, reason = 'other'}.message, msg)
 end
+
+-- Test that error's methods are not masked by the payload fields.
+g.test_methods_masking = function()
+    local e = box.error.new({
+        unpack = 'hack',
+        raise = 'hack',
+        match = 'hack',
+        set_prev = 'hack',
+        __serialize = 'hack',
+    })
+    t.assert_type(e.unpack, 'function')
+    t.assert_type(e.raise, 'function')
+    t.assert_type(e.match, 'function')
+    t.assert_type(e.set_prev, 'function')
+    t.assert_type(e.__serialize, 'function')
+    t.assert_equals(e:unpack().unpack, 'hack')
+    t.assert_equals(e:unpack().raise, 'hack')
+    t.assert_equals(e:unpack().match, 'hack')
+    t.assert_equals(e:unpack().set_prev, 'hack')
+    t.assert_equals(e:unpack().__serialize, 'hack')
+end