config: allow to call config:get() from app script

It is convenient to access configuration using `config:get()` from the
application script (`app.file` or `app.module`).

However, before this commit, it was not possible, because the
configuration was not considered as applied before the application
script is loaded.

Now, the script loading is moved into the post-apply phase.

The resulting sequence of steps on startup/reload is the following.

* collect configuration information (from all the sources)
* <if the previous step is failed, set `check_errors` status and break>
* apply the configuration (call all the appliers)
* <if the previous step is failed, set `check_errors` status and break>
* <set the new successful status: `ready` or `check_warnings`>
* call post-apply hooks (including application script loading)
* <if the previous step is failed, set `check_errors` status and break>
* <set the new successful status: `ready` or `check_warnings`>

I would like to briefly comment the changes in the tests.

* `app_test.lua`: added the check for the new behavior (call
  config:get() from the app script)
* `appliers_test.lua`: fixed the applier call (`.apply` ->
  `.post_apply`)
* `config_test.lua`: fixed status observed in the app script
  (`*_in_progress` -> `ready`)

Part of #8862

NO_DOC=reflected in https://github.com/tarantool/doc/issues/3544

changelogs/unreleased/config-get-configuration-from-app.md

+## feature/config
+
+* It is now possible to access configuration from the application script using
+  the `config:get()` method (gh-8862).

src/box/lua/config/applier/app.lua

 local log = require('internal.config.utils.log')
 
-local function apply(config)
+local function apply(_config)
+    log.verbose('app.apply: do nothing')
+end
+
+local function post_apply(config)
     local configdata = config._configdata
     local file = configdata:get('app.file', {use_default = true})
     local module = configdata:get('app.module', {use_default = true})
     if file ~= nil then
         assert(module == nil)
         local fn = assert(loadfile(file))
-        log.verbose('app.apply: loading '..file)
+        log.verbose('app.post_apply: loading '..file)
         fn(file)
     elseif module ~= nil then
-        log.verbose('app.apply: loading '..module)
+        log.verbose('app.post_apply: loading '..module)
         require(module)
     end
 end
@@ -18,4 +22,5 @@ end
 return {
     name = 'app',
     apply = apply,
+    post_apply = post_apply,
 }

src/box/lua/config/init.lua

@@ -279,10 +279,6 @@ function methods._apply_on_startup(self, opts)
         end
 
         self._configdata_applied = self._configdata
-
-        if extras ~= nil then
-            extras.post_apply(self)
-        end
     else
         assert(false)
     end
@@ -294,12 +290,35 @@ function methods._apply(self)
     end
 
     self._configdata_applied = self._configdata
+end
+
+function methods._post_apply(self)
+    for _, applier in ipairs(self._appliers) do
+        if applier.post_apply ~= nil then
+            applier.post_apply(self)
+        end
+    end
 
     if extras ~= nil then
         extras.post_apply(self)
     end
 end
 
+-- Set proper status depending on received alerts.
+function methods._set_status_based_on_alerts(self)
+    local status = 'ready'
+    for _, alert in pairs(self._alerts) do
+        assert(alert.type == 'error' or alert.type == 'warn')
+        if alert.type == 'error' then
+            status = 'check_errors'
+            break
+        end
+        status = 'check_warnings'
+    end
+    self._status = status
+    broadcast(self)
+end
+
 function methods._startup(self, instance_name, config_file)
     assert(self._status == 'uninitialized')
     self._status = 'startup_in_progress'
@@ -331,9 +350,10 @@ function methods._startup(self, instance_name, config_file)
     else
         self:_apply_on_startup({phase = 2})
     end
+    self:_set_status_based_on_alerts()
 
-    self._status = 'ready'
-    broadcast(self)
+    self:_post_apply()
+    self:_set_status_based_on_alerts()
 end
 
 function methods.get(self, path)
@@ -360,27 +380,21 @@ function methods._reload_noexc(self, opts)
 
     self._alerts = {}
     self._metadata = {}
-    local ok, err = pcall(self._collect, self, opts)
-    if ok then
-        ok, err = pcall(self._apply, self)
-    end
+
+    local ok, err = pcall(function(opts)
+        self:_collect(opts)
+        self:_apply()
+        self:_set_status_based_on_alerts()
+        self:_post_apply()
+    end, opts)
+
     assert(not ok or err == nil)
     if not ok then
         self:_alert({type = 'error', message = err})
     end
 
-    -- Set proper status depending on received alerts.
-    local status = 'ready'
-    for _, alert in pairs(self._alerts) do
-        assert(alert.type == 'error' or alert.type == 'warn')
-        if alert.type == 'error' then
-            status = 'check_errors'
-            break
-        end
-        status = 'check_warnings'
-    end
-    self._status = status
-    broadcast(self)
+    self:_set_status_based_on_alerts()
+
     return ok, err
 end
 

test/config-luatest/app_test.lua

@@ -5,13 +5,14 @@ local g = helpers.group()
 
 -- Start a script that is pointed by app.file or app.module.
 --
--- TODO: Verify that the script has access to config:get() values.
+-- Verify that the script has access to config:get() values.
 g.test_startup_success = function(g)
     local script = [[
-        -- TODO: Verify that the script has access to config:get()
-        -- values.
-        -- local config = require('config')
-        -- assert(config:get('app.cfg.foo') == 42)
+        local config = require('config')
+        assert(config:get('app.cfg.foo') == 42)
+        local info = config:info()
+        assert(info.status == 'ready')
+        assert(#info.alerts == 0)
 
         _G.foo = 42
     ]]

test/config-luatest/appliers_test.lua

@@ -52,7 +52,7 @@ local appliers_script = [[
     local fiber = require('internal.config.applier.fiber')
     fiber.apply(config)
     local app = require('internal.config.applier.app')
-    local ok, err = pcall(app.apply, config)
+    local ok, err = pcall(app.post_apply, config)
     %s
     os.exit(0)
 ]]

test/config-luatest/config_test.lua

@@ -237,8 +237,7 @@ g.test_config_broadcast = function()
         local fiber = require('fiber')
         local status = ''
         box.watch('config.info', function(_, v) status = v.status end)
-        while status ~= 'startup_in_progress' and
-              status ~= 'reload_in_progress' do
+        while status ~= 'ready' do
             fiber.sleep(0.1)
         end
         print(status)
@@ -249,7 +248,7 @@ g.test_config_broadcast = function()
     local args = {'main.lua'}
     local res = justrun.tarantool(dir, {}, args, opts)
     t.assert_equals(res.exit_code, 0)
-    local exp = {'startup_in_progress', 'ready', 'reload_in_progress', 'ready'}
+    local exp = {'ready', 'ready', 'ready', 'ready'}
     t.assert_equals(res.stdout, table.concat(exp, "\n"))
 end