cluster: add helpers to modify cluster config

This patch adds ability to keep and adjust cluster declarative
configuration with `cluster:modify_config()` and apply it later
via `сluster:apply_config_changes()` without passing an explicit
config.

Closes #426

Author: mandesero

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+- Added the ability to keep and adjust cluster declarative configuration
+  with `cluster:modify_config()` and apply it later via
+  `cluster:apply_config_changes()` without passing an explicit
+  configuration (gh-426).
 - Added an option to create `Cluster` objects without global hook management,
   allowing tests to keep clusters alive between test runs (gh-414).
 - Fixed a bug where URI search would terminate prematurely when multiple

luatest/cluster.lua

@@ -25,6 +25,11 @@
 -- * :sync() Sync the configuration and collect a new set of
 --   instances
 -- * :reload() Reload the configuration.
+-- * :config() Return the last applied configuration.
+-- * :modify_config() Initialize a configuration builder based on
+--   the current config and store it inside the cluster object.
+-- * :apply_config_changes() Apply the configuration built via
+--   :modify_config() by passing it to :sync().
 --
 -- The module can also be used for testing failure startup
 -- cases:
@@ -36,6 +41,7 @@
 local fun = require('fun')
 local yaml = require('yaml')
 local assertions = require('luatest.assertions')
+local cbuilder = require('luatest.cbuilder')
 local helpers = require('luatest.helpers')
 local hooks = require('luatest.hooks')
 local treegen = require('luatest.treegen')
@@ -148,6 +154,13 @@ local function instance_names_from_config(config)
     return instance_names
 end
 
+
+local function assert_no_pending_config_builder(self, method_name)
+    assert(self._config_builder == nil,
+        (':modify_config() was called; apply configuration changes with ' ..
+        ':apply_config_changes() before calling :%s'):format(method_name))
+end
+
 -- }}} Helpers
 
 -- {{{ Cluster management
@@ -226,12 +239,14 @@ end
 -- @bool[opt] opts.wait_until_running Wait until servers are running
 --   (default: wait_until_ready; used only if start_stop is set).
 function Cluster:sync(config, opts)
+    assert_no_pending_config_builder(self, 'sync()')
     assert(type(config) == 'table')
 
     local instance_names = instance_names_from_config(config)
 
     treegen.write_file(self._dir, self._config_file_rel, yaml.encode(config))
 
+    self._config = config
     local server_map = self._server_map
     self._server_map = {}
     self._servers = {}
@@ -277,10 +292,34 @@ function Cluster:sync(config, opts)
     end
 end
 
+--- Apply configuration changes built via :modify_config().
+--
+-- Uses the internal configuration builder created by :modify_config(),
+-- converts it to a config table and calls :sync() with it.
+-- After the call the stored builder is cleared.
+--
+-- @tab[opt] opts Options.
+-- @bool[opt] opts.start_stop Start/stop added/removed servers
+--   (default: false).
+-- @bool[opt] opts.wait_until_ready Wait until servers are ready
+--   (default: true; used only if start_stop is set).
+-- @bool[opt] opts.wait_until_running Wait until servers are running
+--   (default: wait_until_ready; used only if start_stop is set).
+function Cluster:apply_config_changes(opts)
+    assert(self._config_builder ~= nil,
+        ':modify_config() must be called before :apply_config_changes()')
+
+    local config = self._config_builder:config()
+    self._config_builder = nil
+
+    return self:sync(config, opts)
+end
+
 --- Reload configuration on all the instances.
 --
 -- @tab[opt] config New config.
 function Cluster:reload(config)
+    assert_no_pending_config_builder(self, 'reload()')
     assert(config == nil or type(config) == 'table')
 
     -- Rewrite the configuration file if a new config is provided.
@@ -333,12 +372,15 @@ function Cluster:new(config, server_opts, opts)
         assert(g._cluster == nil)
     end
 
+    self._config = table.deepcopy(config)
+    self._config_builder = nil
+
     -- Prepare a temporary directory and write a configuration
     -- file.
     local dir = opts.dir or treegen.prepare_directory({}, {})
     local config_file_rel = 'config.yaml'
     local config_file = treegen.write_file(dir, config_file_rel,
-                                           yaml.encode(config))
+                                           yaml.encode(self._config))
 
     -- Collect names of all the instances defined in the config
     -- in the alphabetical order.
@@ -383,6 +425,26 @@ function Cluster:new(config, server_opts, opts)
     return object
 end
 
+--- Return the last applied configuration.
+function Cluster:config()
+    assert_no_pending_config_builder(self, 'config()')
+
+    return table.deepcopy(self._config)
+end
+
+--- Initialize a configuration builder based on the current config.
+--
+-- The returned builder is stored inside the cluster object and later
+-- consumed by :apply_config_changes(), which turns it into a config
+-- table and passes it to :sync().
+function Cluster:modify_config()
+    assert(self._config_builder == nil,
+           ':modify_config() already called and changes were not applied')
+
+    self._config_builder = cbuilder:new(self:config())
+    return self._config_builder
+end
+
 -- }}} Replicaset management
 
 -- {{{ Replicaset that can't start

test/cluster_test.lua

@@ -306,6 +306,67 @@ g.test_reload = function()
     assert_instance_failover_mode(c, 'i-003', 'off')
 end
 
+g.test_modify_config = function()
+    t.run_only_if(utils.version_current_ge_than(3, 0, 0),
+                  [[Declarative configuration works on Tarantool 3.0.0+.
+                    See tarantool/tarantool@13149d65bc9d for details]])
+
+    local config = cbuilder:new()
+        :use_group('group-001')
+        :use_replicaset('replicaset-001')
+        :set_replicaset_option('replication.failover', 'manual')
+        :set_replicaset_option('leader', 'i-001')
+        :add_instance('i-001', {})
+        :config()
+
+    local c = cluster:new(config, server_opts)
+
+    c:start()
+
+    c:modify_config()
+        :use_group('group-001')
+        :use_replicaset('replicaset-001')
+        :add_instance('i-002', {})
+    c:apply_config_changes({start_stop = true})
+
+    assert_instance_running(c, 'i-001', 'replicaset-001')
+    assert_instance_running(c, 'i-002', 'replicaset-001')
+
+    c:modify_config()
+        :use_group('group-001')
+        :use_replicaset('replicaset-001')
+        :set_replicaset_option('leader', 'i-002')
+
+    local expected_msg =
+        ':modify_config() was called; apply configuration changes with ' ..
+        ':apply_config_changes() before calling :reload()'
+    t.assert_error_msg_contains(expected_msg, c.reload, c)
+
+    expected_msg =
+        ':modify_config() was called; apply configuration changes with ' ..
+        ':apply_config_changes() before calling :sync()'
+    t.assert_error_msg_contains(expected_msg, c.sync, c, config)
+
+    expected_msg =
+        ':modify_config() was called; apply configuration changes with ' ..
+        ':apply_config_changes() before calling :config()'
+    t.assert_error_msg_contains(expected_msg, c.config, c)
+
+    c:apply_config_changes()
+
+    local updated_config = c:config()
+    local replicaset =
+        updated_config.groups['group-001'].replicasets['replicaset-001']
+    t.assert_equals(replicaset.leader, 'i-002')
+    t.assert_not_equals(replicaset.instances['i-002'], nil)
+
+    t.assert_equals(c:size(), 2)
+
+    c:stop()
+    assert_instance_stopped(c, 'i-001')
+    assert_instance_stopped(c, 'i-002')
+end
+
 g.test_each = function()
     t.run_only_if(utils.version_current_ge_than(3, 0, 0),
                   [[Declarative configuration works on Tarantool 3.0.0+.