MDEV-24443 : galera.lp1376747-4 MTR fails: Result length mismatch

Use debug_sync to force FTWRL to pause in correct state.

mysql-test/suite/galera/r/lp1376747-4.result

@@ -3,26 +3,23 @@ CREATE TABLE t1 (id INT PRIMARY KEY) ENGINE=InnoDB;
 INSERT INTO t1 VALUES (1);
 connection node_2;
 SET session wsrep_sync_wait=0;
-SET session wsrep_causal_reads=OFF;
-Warnings:
-Warning	1287	'@@wsrep_causal_reads' is deprecated and will be removed in a future release. Please use '@@wsrep_sync_wait=1' instead
-FLUSH TABLE WITH READ LOCK;
+FLUSH TABLES WITH READ LOCK;
 connection node_1;
 ALTER TABLE t1 ADD COLUMN f2 INTEGER;
 INSERT INTO t1 VALUES (2,3);
 connection node_2a;
 SET session wsrep_sync_wait=0;
-SET session wsrep_causal_reads=OFF;
-Warnings:
-Warning	1287	'@@wsrep_causal_reads' is deprecated and will be removed in a future release. Please use '@@wsrep_sync_wait=1' instead
 SHOW CREATE TABLE t1;
 Table	Create Table
 t1	CREATE TABLE `t1` (
   `id` int(11) NOT NULL,
   PRIMARY KEY (`id`)
 ) ENGINE=InnoDB DEFAULT CHARSET=latin1
+SET debug_sync='flush_tables_with_read_lock_after_acquire_locks SIGNAL parked2 WAIT_FOR go2';
 FLUSH TABLES t1 WITH READ LOCK;;
 connection node_2;
+SET debug_sync='now WAIT_FOR parked2';
+SET debug_sync='now SIGNAL go2';
 UNLOCK TABLES;
 SHOW CREATE TABLE t1;
 Table	Create Table
@@ -30,8 +27,10 @@ t1	CREATE TABLE `t1` (
   `id` int(11) NOT NULL,
   PRIMARY KEY (`id`)
 ) ENGINE=InnoDB DEFAULT CHARSET=latin1
+set debug_sync= 'RESET';
 connection node_2a;
 UNLOCK TABLES;
+SET SESSION wsrep_sync_wait = DEFAULT;
 SHOW CREATE TABLE t1;
 Table	Create Table
 t1	CREATE TABLE `t1` (

mysql-test/suite/galera/t/lp1376747-4.test

@@ -5,7 +5,8 @@
 # after provider is unpaused
 #
 --source include/galera_cluster.inc
---source include/have_innodb.inc
+--source include/have_debug.inc
+--source include/have_debug_sync.inc
 
 --let $galera_connection_name = node_2a
 --let $galera_server_number = 2
@@ -17,8 +18,7 @@ INSERT INTO t1 VALUES (1);
 
 --connection node_2
 SET session wsrep_sync_wait=0;
-SET session wsrep_causal_reads=OFF;
-FLUSH TABLE WITH READ LOCK;
+FLUSH TABLES WITH READ LOCK;
 
 --connection node_1
 ALTER TABLE t1 ADD COLUMN f2 INTEGER;
@@ -26,25 +26,33 @@ INSERT INTO t1 VALUES (2,3);
 
 --connection node_2a
 SET session wsrep_sync_wait=0;
-SET session wsrep_causal_reads=OFF;
-
 SHOW CREATE TABLE t1;
+SET debug_sync='flush_tables_with_read_lock_after_acquire_locks SIGNAL parked2 WAIT_FOR go2';
 --send FLUSH TABLES t1 WITH READ LOCK;
 
 --connection node_2
---let $wait_condition = SELECT COUNT(*) = 1 FROM INFORMATION_SCHEMA.PROCESSLIST WHERE USER = 'system user' AND STATE LIKE 'committed%';
---source include/wait_condition.inc
-
+SET debug_sync='now WAIT_FOR parked2';
+
+# let the flush table wait in pause state before we unlock
+# table otherwise there is window where-in flush table is
+# yet to wait in pause and unlock allows alter table to proceed.
+# this is because send is asynchronous.
+--let $wait_condition = SELECT COUNT(*) = 1 FROM INFORMATION_SCHEMA.PROCESSLIST WHERE Info LIKE 'FLUSH TABLES t1 WITH READ LOCK';
+--let $wait_condition_on_error_output = SHOW PROCESSLIST
+--source include/wait_condition_with_debug.inc
+
+SET debug_sync='now SIGNAL go2';
+# this will release existing lock but will not resume
+# the cluster as there is new FTRL that is still pausing it.
 UNLOCK TABLES;
 SHOW CREATE TABLE t1;
+set debug_sync= 'RESET';
 
 --connection node_2a
 --reap
 UNLOCK TABLES;
 
---let $wait_condition = SELECT COUNT(*) = 2 FROM t1;
---source include/wait_condition.inc
-
+SET SESSION wsrep_sync_wait = DEFAULT;
 SHOW CREATE TABLE t1;
 SELECT * from t1;