box/lua: implement inheritance of error payload fields

Suppose an error has a cause with some payload fields, for example:
  local e1 = box.error.new{'e1', foo = 'bar'} -- cause
  local e2 = box.error.new{'e2', prev = e1}   -- effect

Now it is possible to access cause payload fields via e2 directly:
  e2.foo -- 'bar'

While looking for a payload field with a given name, we always stop at
the topmost (closest to the effect) field. If there's a field with the
same name deeper in the stack it is masked.

Closes tarantool#9106

@TarantoolBot document
Title: Document inheritance of error payload fields
Product: Tarantool
Since: 3.1
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_error/error_object/

[Link to the design document](https://www.notion.so/tarantool/Error-subsystem-improvements-90faa0a4714b4143abaf8bed2c10b2fc?pvs=4#c080fe2ac28b46c8b0eda7234a8852ce)

Author: Gumix

changelogs/unreleased/gh-9106-error-payload-inheritance.md

@@ -0,0 +1,4 @@
+## feature/lua
+
+* Now it is possible to access a payload field of an error's cause directly
+  from the error (gh-9106).

src/lua/error.lua

@@ -182,9 +182,14 @@ local function error_index(err, key)
     if getter ~= nil then
         return getter(err)
     end
-    local f = ffi.C.error_find_field(err, key)
-    if f ~= nil then
-        return mp_decode(f._data)
+    -- Look for a payload field, starting from the topmost error in the stack.
+    local cur_err = err
+    while cur_err ~= nil do
+        local f = ffi.C.error_find_field(cur_err, key)
+        if f ~= nil then
+            return mp_decode(f._data)
+        end
+        cur_err = cur_err.prev
     end
 end
 

test/box-luatest/error_subsystem_improvements_test.lua

@@ -127,3 +127,14 @@ g.test_methods_masking = function()
     t.assert_equals(e:unpack().set_prev, 'hack')
     t.assert_equals(e:unpack().__serialize, 'hack')
 end
+
+-- Test that payload fields of a cause are inherited by the effect (gh-9106).
+g.test_payload_inheritance = function()
+    local e1 = box.error.new({foo = 11, bar = 22})
+    local e2 = box.error.new({bar = 33, baz = 44, prev = e1})
+    local e3 = box.error.new({baz = 55, prev = e2})
+
+    t.assert_equals(e3.foo, 11) -- Inherited from e1.
+    t.assert_equals(e3.bar, 33) -- Inherited from e2.
+    t.assert_equals(e3.baz, 55) -- Own payload field.
+end