config: config section for instance config

This section describes general instance configuration options.

Part of #8778

NO_DOC=will be added later
NO_CHANGELOG=will be added later

src/box/lua/config/instance_config.lua

@@ -36,4 +36,62 @@ local schema = require('internal.config.utils.schema')
 --
 --   Create a parent directory for the given file before box.cfg().
 
-return schema.new('instance_config', schema.record({}))
+local CONFIG_VERSION = 'dev'
+
+-- * In the instance config: must be present.
+--
+--   TODO: This check is disabled for the early config schema
+--   version, because the schema is often changed and there is no
+--   schema evolution support yet.
+-- * In the cluster config: must be present in the global scope.
+--
+--   TODO: This check is disabled as well, see above.
+-- * In the cluster config: must not be present in the other
+--   scopes (group, replicaset, instance).
+-- * If present, must correspond to one of supported config
+--   versions, but is it validated by the config.version schema
+--   node definition.
+local function validate_config_version(data, w)
+    -- scope == nil means that it is the instance config.
+    local scope = w.schema.scope
+    if scope == nil or scope == 'global' then
+        -- Must be present.
+        if data.config == nil or data.config.version == nil then
+            -- TODO: Enable this check closer to 3.0.0 release.
+            -- w.error('config.version is mandatory')
+            return
+        end
+    else
+        -- Must not be present.
+        assert(scope == 'group' or scope == 'replicaset' or scope == 'instance')
+        if data.config ~= nil and data.config.version ~= nil then
+            w.error('config.version must not be present in the %s scope', scope)
+        end
+    end
+end
+
+return schema.new('instance_config', schema.record({
+    config = schema.record({
+        version = schema.enum({
+            CONFIG_VERSION,
+        }),
+        reload = schema.enum({
+            'auto',
+            'manual',
+        }, {
+            default = 'auto',
+        }),
+    }),
+}, {
+    -- Any configuration data should contain a version of the
+    -- config schema for which it is written.
+    --
+    -- This annotation cannot be placed right into the
+    -- config.version field, because missed fields are not
+    -- validated.
+    validate = validate_config_version,
+    -- Store the config schema version right in the outmost schema
+    -- node as an annotation. It simplifies accesses from other
+    -- code.
+    config_version = CONFIG_VERSION,
+}))

test/config-luatest/instance_config_schema_test.lua

@@ -3,6 +3,52 @@ local instance_config = require('internal.config.instance_config')
 
 local g = t.group()
 
+-- Check that all record element names can be found in the table and vice versa.
+local function validate_fields(config, record)
+    local config_fields = {}
+    if type(config) == 'table' then
+        for k in pairs(config) do
+            table.insert(config_fields, k)
+        end
+    end
+
+    local record_fields = {}
+    for k, v in pairs(record.fields) do
+        table.insert(record_fields, k)
+    end
+
+    t.assert_items_equals(config_fields, record_fields)
+end
+
 g.test_general = function()
     t.assert_equals(instance_config.name, 'instance_config')
 end
+
+g.test_config = function()
+    local iconfig = {
+        config = {
+            version = 'dev',
+            reload = 'auto',
+        },
+    }
+    instance_config:validate(iconfig)
+    validate_fields(iconfig.config, instance_config.schema.fields.config)
+
+    iconfig = {
+        config = {
+            version = '0.0.0',
+            reload = 'auto',
+        },
+    }
+    local err = '[instance_config] config.version: Got 0.0.0, but only the '..
+                'following values are allowed: dev'
+    t.assert_error_msg_equals(err, function()
+        instance_config:validate(iconfig)
+    end)
+
+    local exp = {
+        reload = 'auto',
+    }
+    local res = instance_config:apply_default({}).config
+    t.assert_equals(res, exp)
+end