lua: add error autocompletion

Let's add an `__autocomplete` method to enable error autocompletion and
make the error object easier to use. It will suggest error object fields
(including own and inherited payload fields) and methods.

Closes #9107

NO_DOC=<not a documentable feature>

changelogs/unreleased/gh-9107-error-autocomplete.md

@@ -0,0 +1,3 @@
+## feature/lua
+
+* Added autocompletion (including payload fields) to `box.error` (gh-9107).

src/lua/error.lua

@@ -193,12 +193,36 @@ local function error_serialize(err)
     end
 end
 
-local error_methods = {
+local error_methods
+
+local function error_autocomplete(err)
+    local err_unpacked = {}
+
+    local cur_err = err
+    while cur_err ~= nil do
+        for key, val in pairs(cur_err:unpack()) do
+            if err_unpacked[key] == nil then
+                err_unpacked[key] = val
+            end
+        end
+        cur_err = cur_err.prev
+    end
+
+    for key, method in pairs(error_methods) do
+        if not key:startswith('__') then
+            err_unpacked[key] = method
+        end
+    end
+    return err_unpacked
+end
+
+error_methods = {
     ["unpack"] = error_unpack;
     ["raise"] = error_raise;
     ["match"] = error_match; -- Tarantool 1.6 backward compatibility
     ["__serialize"] = error_serialize;
     ["set_prev"] = error_set_prev;
+    ["__autocomplete"] = error_autocomplete;
 }
 
 local function error_index(err, key)

test/box-luatest/error_subsystem_improvements_test.lua

@@ -1,5 +1,6 @@
 local t = require('luatest')
 local compat = require('compat')
+local console = require('console')
 local json = require('json')
 local yaml = require('yaml')
 local tarantool = require('tarantool')
@@ -270,3 +271,44 @@ g.test_client_error_creation = function()
     local e = box.error.new(box.error.TEST_TYPE_INT, 1, 2, 3, 4, 5)
     t.assert_equals(e.field, 1)
 end
+
+--
+-- Get tab completion for @s
+-- @param s string
+-- @return tab completion
+local function tabcomplete(s)
+    return console.completion_handler(s, 0, #s)
+end
+
+-- Test `box.error` autocompletion (gh-9107).
+g.test_autocomplete = function()
+    -- tabcomplete always uses global table
+    rawset(_G, 'err', box.error.new{'cur', foo = 777,
+                                    prev = box.error.new{'prev', bar = 777}})
+
+    local r = tabcomplete('err.')
+    t.assert_items_equals(r, {
+        'err.',
+        'err.raise(',
+        'err.foo',
+        'err.base_type',
+        'err.prev',
+        'err.message',
+        'err.match(',
+        'err.set_prev(',
+        'err.trace',
+        'err.type',
+        'err.unpack(',
+        'err.code',
+        'err.bar',
+    })
+
+    r = tabcomplete('err:')
+    t.assert_items_equals(r, {
+        'err:',
+        'err:match(',
+        'err:raise(',
+        'err:set_prev(',
+        'err:unpack(',
+    })
+end