config: allow to extend the module on Tarantool CE

Part of #8862

NO_DOC=this API is not marked as public, at least now

changelogs/unreleased/config-extras.md

+## feature/config
+
+* Introduced a non-public API for extending the declarative config
+  functionality in Tarantool Community Edition (gh-8862).

src/box/lua/config/init.lua

@@ -5,11 +5,37 @@ local log = require('internal.config.utils.log')
 local tarantool = require('tarantool')
 local datetime = require('datetime')
 
-local extras = nil
-if tarantool.package == 'Tarantool Enterprise' then
-    extras = require('internal.config.extras')
+-- Tarantool Enterprise Edition has its own additions
+-- for config module.
+--
+-- Tarantool Community Edition may be extended using the same
+-- mechanism.
+local function load_extras()
+    local has_extras, extras = pcall(require, 'internal.config.extras')
+
+    -- The module is built into Tarantool EE executable. If it
+    -- can't be loaded, something is definitely going wrong.
+    if not has_extras and tarantool.package == 'Tarantool Enterprise' then
+        error('Tarantool Enterprise Edition build seems broken: built-in ' ..
+            'module internal.config.extras is not found')
+    end
+
+    -- The module may be provided by a user for Tarantool CE, but
+    -- it is optional.
+    if not has_extras then
+        return
+    end
+
+    -- Verify the contract.
+    assert(type(extras) == 'table')
+    assert(type(extras.initialize) == 'function')
+    assert(type(extras.post_apply) == 'function')
+
+    return extras
 end
 
+local extras = load_extras()
+
 -- {{{ Helpers
 
 -- Remove indent from a text.
@@ -125,8 +151,6 @@ function methods._initialize(self)
     self:_register_applier(require('internal.config.applier.fiber'))
     self:_register_applier(require('internal.config.applier.app'))
 
-    -- Tarantool Enterprise Edition has its own additions
-    -- for this module.
     if extras ~= nil then
         extras.initialize(self)
     end

test/config-luatest/basic_test.lua

@@ -453,3 +453,68 @@ for case_name, case in pairs({
         end
     end
 end
+
+-- Verify that it is possible to extend config's functionality
+-- using a non-public API.
+--
+-- The test registers an additional configuration source and
+-- verifies that values from it are taken into account.
+--
+-- It also verifies that the extension can add a post-apply hook.
+g.test_extras_on_community_edition = function(g)
+    -- Tarantool EE has its own internal.config.extras module, so
+    -- the external one will be ignored (if it is not placed into
+    -- the `override` directory).
+    t.tarantool.skip_if_enterprise()
+
+    local extras = string.dump(function()
+        local mysource = setmetatable({
+            name = 'mysource',
+            type = 'instance',
+            _values = {},
+        }, {
+            __index = {
+                sync = function(self, _config, _iconfig)
+                    self._values = {
+                        fiber = {
+                            slice = {
+                                warn = 10,
+                                err = 15,
+                            },
+                        },
+                    }
+                end,
+                get = function(self)
+                    return self._values
+                end,
+            },
+        })
+
+        local function initialize(config)
+            config:_register_source(mysource)
+        end
+
+        local function post_apply(_config)
+            rawset(_G, 'foo', 'extras.post_apply() is called')
+        end
+
+        return {
+            initialize = initialize,
+            post_apply = post_apply,
+        }
+    end)
+
+    local dir = treegen.prepare_directory(g, {}, {})
+    treegen.write_script(dir, 'internal/config/extras.lua', extras)
+
+    local verify = function()
+        local config = require('config')
+        t.assert_equals(config:get('fiber.slice'), {warn = 10, err = 15})
+        t.assert_equals(rawget(_G, 'foo'), 'extras.post_apply() is called')
+    end
+
+    helpers.success_case(g, {
+        dir = dir,
+        verify = verify,
+    })
+end