config: fix names not always applying after config reload

We need to apply instance/replicaset_name as soon as the instance
becomes RW, so currently we try to do so at every box.status broadcast.
Even though broadcast happens pretty often, it's not enough:

The bug is reproduced in config-luatest/set_names_reload, which checks
the following situation:

    1. Cluster is recovered from the xlogs without names set.
    2. User forgets to set UUID for one replica, starts the cluster.
    3. Replica, UUID of which have not been set, fails to start.
    4. User notices that and updates config, reloading it on the
       instances, which succeeded to start, starting failed one.
    5. Master must apply name for a failed replica.

The test worked all right in the majority of runs, because box.status
broadcast happens often: e.g. it's broadcasted, when master's applier
synced with replica. However, under heavy load on CPU, the test failed
sometimes, when master fails to subscribe on replica and broadcast
doesn't happen.

Let's try to set names not only, when box.status is broadcasted, but
immediately after reload, as at this time new names, which must be
set, might appear. Let's also change test so that, it doesn't rely on
broadcast anymore.

Closes tarantool/tarantool-qa#328

NO_DOC=bugfix

changelogs/unreleased/config-set-names-on-reload.md

+## bugfix/config
+
+* Increased stability and speed of automatic names applying after config reload.

src/box/lua/config/applier/box_cfg.lua

@@ -90,8 +90,7 @@ local names_state = {
 -- from box.info) and which have not be alerted already.
 local function names_alert_missing(config, missing_names)
     local msg = 'box_cfg.apply: name %s for %s uuid is missing from the ' ..
-                'snapshot. It will be automatically set when master switches' ..
-                ' to Read Write mode'
+                'snapshot. It will be automatically set when possible.'
     local replicaset_name = config._configdata._replicaset_name
     if missing_names[replicaset_name] ~= nil and
        config._alerts[replicaset_name] == nil then
@@ -158,13 +157,15 @@ local function missing_names_is_empty(missing_names, replicaset_name)
            table.equals(missing_names._peers, {})
 end
 
-local function names_on_rw_transit()
+local function names_try_set_missing()
     local configdata = names_state.config._configdata
     local replicaset_name = configdata._replicaset_name
     local missing_names = configdata:missing_names()
 
-    if missing_names_is_empty(missing_names, replicaset_name) then
-        -- Somebody have done work for us, nothing to update.
+    if box.info.ro or missing_names_is_empty(missing_names,
+                                             replicaset_name) then
+        -- Somebody have done work for us, nothing to update or we're not
+        -- a rw, which is possible if the function was invoked after reload.
         return
     end
 
@@ -186,10 +187,10 @@ end
 
 local function names_rw_watcher()
     return box.watch('box.status', function(_, status)
-        -- It's ok, if names_on_rw_transit will be triggered
+        -- It's ok, if names_try_set_missing will be triggered
         -- several times. It's NoOp after first execution.
         if status.is_ro == false then
-            schedule_task(names_on_rw_transit)
+            schedule_task(names_try_set_missing)
         end
     end)
 end
@@ -270,6 +271,10 @@ local function names_apply(config, missing_names, schema_version)
     -- Even if everything is configured we try to make alerts one
     -- more time, as new instances without names may be found.
     names_alert_missing(config, missing_names)
+    -- Don't wait for box.status to change, we may be already rw, set names
+    -- on reload, if it's possible and needed.
+    names_try_set_missing()
+
     if names_state.is_configured then
         -- All triggers are already configured, nothing to do, but wait.
         return

test/config-luatest/set_names_reload_test.lua

@@ -170,19 +170,25 @@ g.test_names_are_set_after_reload = function(g)
         instance_uuid = g.uuids['instance-002']
     }
 
+    -- Reload config on instance-001 and wait for the name to be assigned
     local cfg = yaml.encode(g.config)
     opts.config_file = treegen.write_script(g.dir, 'cfg.yaml', cfg)
-    g.instance_2 = server:new(fun.chain(opts, {alias = 'instance-002'}):tomap())
+    g.instance_1:exec(function(uuid)
+        require('config'):reload()
+        t.helpers.retrying({timeout = 20}, function()
+            t.assert_equals(box.space._cluster.index.uuid:select(uuid)[1][3],
+                            'instance-002')
+        end)
+    end, {g.uuids['instance-002']})
+
+    g.instance_1:grep_log(msg:format('instance-002', g.uuids['instance-002']),
+                          1024, {filename = g.dir .. log_postfix})
 
+    -- Start instance-002
+    g.instance_2 = server:new(fun.chain(opts, {alias = 'instance-002'}):tomap())
     g.instance_2:start({wait_until_ready = false})
-    g.instance_1:exec(function()
-        require('config'):reload()
-    end)
 
     g.instance_2:wait_until_ready()
     g.instance_2:wait_for_vclock_of(g.instance_1)
     g.instance_2:exec(check_names, {rs_name, 'instance-002', g.uuids})
-
-    g.instance_1:grep_log(msg:format('instance-002', g.uuids['instance-002']),
-                          1024, {filename = g.dir .. log_postfix})
 end