wal: fix failing assertion in box_wait_limbo_acked

Before this patch there was an execution sequence in which the
assertion in box_wait_limbo_acked would fail. The assertion is that
the lsn of the last entry in limbo is always positive after wal_sync.
Fix it.

Closes tarantool#9235

NO_DOC=bugfix

changelogs/unreleased/gh-9235-assertion_in_box_wait_limbo_acked.md

@@ -0,0 +1,6 @@
+## bugfix/core
+
+* Fixed a bug when the assertion in `box_wait_limbo_acked` would fail.
+  The assertion is that the lsn of the last entry in limbo is always
+  positive after `wal_sync` (gh-9235). In the release build, the behavior
+  does not change except that the statement is now true.

src/box/box.cc

@@ -2792,6 +2792,7 @@ box_wait_limbo_acked(double timeout)
 	if (last_entry->lsn < 0) {
 		int64_t tid = last_entry->txn->id;
 
+		journal_queue_flush();
 		if (wal_sync(NULL) != 0)
 			return -1;
 

test/replication-luatest/gh_9235_assertion_in_box_wait_limbo_acked_test.lua

@@ -0,0 +1,104 @@
+local t = require('luatest')
+local cluster = require('luatest.replica_set')
+local server = require('luatest.server')
+
+local g = t.group('assertion-in-box-wait-limbo-acked')
+--
+-- gh-9235:
+-- Assertion in box_wait_limbo_acked.
+--
+local wait_timeout = 10
+
+local function wait_pair_sync(server1, server2)
+    -- Without retrying it fails sometimes when vclocks are empty and both
+    -- instances are in 'connect' state instead of 'follow'.
+    t.helpers.retrying({timeout = wait_timeout}, function()
+        server1:wait_for_vclock_of(server2)
+        server2:wait_for_vclock_of(server1)
+        server1:assert_follows_upstream(server2:get_instance_id())
+        server2:assert_follows_upstream(server1:get_instance_id())
+    end)
+end
+
+local function server_wait_wal_is_blocked(server)
+    server:exec(function(wait_timeout)
+        t.helpers.retrying({timeout = wait_timeout}, function()
+            t.assert(box.error.injection.get('ERRINJ_WAL_DELAY'))
+        end)
+    end, {wait_timeout})
+end
+
+local function server_wait_synchro_queue_len_is_equal(server, expected)
+    server:exec(function(expected, wait_timeout)
+        t.helpers.retrying({timeout = wait_timeout}, function(expected)
+            t.assert_equals(box.info.synchro.queue.len, expected)
+        end, expected)
+    end, {expected, wait_timeout})
+end
+
+g.before_each(function(cg)
+    t.tarantool.skip_if_not_debug()
+
+    cg.cluster = cluster:new({})
+    local box_cfg = {
+        replication = {
+            server.build_listen_uri('master', cg.cluster.id),
+            server.build_listen_uri('replica', cg.cluster.id),
+        },
+        election_mode = 'candidate',
+        replication_timeout = 0.1,
+        election_fencing_mode='off',
+        replication_synchro_quorum = 2,
+        replication_synchro_timeout = 100000,
+    }
+    cg.master = cg.cluster:build_and_add_server({
+        alias = 'master',
+        box_cfg = box_cfg
+    })
+    box_cfg.election_mode = 'off'
+    cg.replica = cg.cluster:build_and_add_server({
+        alias = 'replica',
+        box_cfg = box_cfg
+    })
+    cg.cluster:start()
+    cg.master:wait_until_election_leader_found()
+    cg.replica:wait_until_election_leader_found()
+    cg.master:exec(function()
+        box.schema.space.create('test', {is_sync = true})
+        box.space.test:create_index('pk')
+    end)
+    wait_pair_sync(cg.replica, cg.master)
+end)
+
+g.after_each(function(cg)
+    cg.cluster:drop()
+end)
+
+g.test_assert_last_entry_lsn_is_positive = function(cg)
+    local f = cg.replica:exec(function()
+       box.error.injection.set('ERRINJ_WAL_DELAY_COUNTDOWN', 0)
+       local f = require('fiber').create(function() box.ctl.promote() end)
+       box.cfg{wal_queue_max_size=1}
+       f:set_joinable(true)
+       return f:id()
+    end)
+    server_wait_wal_is_blocked(cg.replica)
+    cg.master:exec(function()
+        require('fiber').create(function() box.space.test:insert{1} end)
+        require('fiber').create(function() box.space.test:insert{2} end)
+    end)
+    server_wait_synchro_queue_len_is_equal(cg.replica, 1)
+    cg.replica:exec(function()
+        box.error.injection.set('ERRINJ_WAL_DELAY_COUNTDOWN', 0)
+        box.error.injection.set('ERRINJ_WAL_DELAY', false)
+    end)
+    server_wait_wal_is_blocked(cg.replica)
+    cg.replica:exec(function(f)
+        box.error.injection.set('ERRINJ_WAL_DELAY', false)
+        require('fiber').find(f):join()
+    end, {f})
+    cg.master:exec(function()
+        box.cfg{replication_synchro_quorum=1}
+    end)
+    server_wait_synchro_queue_len_is_equal(cg.replica, 0)
+end