MDEV-27218 mtr test galera.MDEV-20793 has sporadic false positive failures

Fixes to make the galera.MDEV-20793 test deterministic.
Specifically, after --send COMMIT, there is now a sync point to catch a known state of the COMMIT execution

mysql-test/suite/galera/r/MDEV-20793.result

@@ -9,7 +9,6 @@ connection node_1;
 SET SESSION wsrep_retry_autocommit = 0;
 START TRANSACTION;
 UPDATE t1 SET f2 = 1;
-SET SESSION debug_sync = "wsrep_before_replay SIGNAL reached WAIT_FOR continue";
 connection node_1_ctrl;
 SET GLOBAL wsrep_provider_options = 'dbug=d,apply_monitor_slave_enter_sync';
 connection node_2;
@@ -24,9 +23,15 @@ UPDATE t1 SET f2 = 2 WHERE f1 = 5;
 connection node_1_ctrl;
 SET SESSION wsrep_on = 0;
 SET SESSION wsrep_on = 1;
+SET GLOBAL wsrep_provider_options = 'dbug=';
+SET GLOBAL wsrep_provider_options = 'dbug=d,after_replicate_sync';
 connection node_1;
+SET SESSION debug_sync = "wsrep_before_replay SIGNAL reached WAIT_FOR continue";
 COMMIT;
 connection node_1_ctrl;
+SET SESSION wsrep_on = 0;
+SET SESSION wsrep_on = 1;
+SET GLOBAL wsrep_provider_options = 'signal=after_replicate_sync';
 SET GLOBAL wsrep_provider_options = 'signal=apply_monitor_slave_enter_sync';
 SET SESSION debug_sync = "now WAIT_FOR reached";
 SET GLOBAL wsrep_provider_options = 'signal=local_monitor_slave_enter_sync';

mysql-test/suite/galera/t/MDEV-20793.test

@@ -30,9 +30,9 @@ SET GLOBAL wsrep_slave_threads = 2;
 SET SESSION wsrep_retry_autocommit = 0;
 START TRANSACTION;
 UPDATE t1 SET f2 = 1;
-SET SESSION debug_sync = "wsrep_before_replay SIGNAL reached WAIT_FOR continue";
 
 --connection node_1_ctrl
+# set sync point for incoming applier
 --let $galera_sync_point = apply_monitor_slave_enter_sync
 --source include/galera_set_sync_point.inc
 
@@ -42,8 +42,11 @@ SET SESSION debug_sync = "wsrep_before_replay SIGNAL reached WAIT_FOR continue";
 INSERT INTO t1 VALUES (2, 2);
 
 --connection node_1_ctrl
+# wait to see the INSERT from node_2 reaching applier sync point
 --source include/galera_wait_sync_point.inc
 --source include/galera_clear_sync_point.inc
+
+# set sync point to catch other write set applying from node_2
 --let $galera_sync_point = local_monitor_slave_enter_sync
 --source include/galera_set_sync_point.inc
 
@@ -52,17 +55,38 @@ INSERT INTO t1 VALUES (2, 2);
 UPDATE t1 SET f2 = 2 WHERE f1 = 5;
 
 --connection node_1_ctrl
+# wait until both appliers are stopped in sync points
 --let $galera_sync_point = apply_monitor_slave_enter_sync local_monitor_slave_enter_sync
 --source include/galera_wait_sync_point.inc
+--source include/galera_clear_sync_point.inc
+
+# set sync point for catching node_1 transaction just before committing
+--let $galera_sync_point = after_replicate_sync
+--source include/galera_set_sync_point.inc
 
 --connection node_1
+# set sync point, which will stop execution after COMMIT has been BF aborted
+# and send the COMMIT, it should stop in commit_monitor_master_enter_sync point
+SET SESSION debug_sync = "wsrep_before_replay SIGNAL reached WAIT_FOR continue";
 --send COMMIT
 
 --connection node_1_ctrl
+# wait until both appliers and local COMMIT are idle in their sync points
+--let $galera_sync_point = after_replicate_sync apply_monitor_slave_enter_sync local_monitor_slave_enter_sync
+--source include/galera_wait_sync_point.inc
+
+# release local COMMIT processing, it will continue and pause first before certification,
+--let $galera_sync_point = after_replicate_sync
+--source include/galera_signal_sync_point.inc
+
+# release first applier (INSERT), it should BF abort the local COMMIT processing
 --let $galera_sync_point = apply_monitor_slave_enter_sync
 --source include/galera_signal_sync_point.inc
+
+# wait to see that COMMIT was BF aborted and has reached replaying state
 SET SESSION debug_sync = "now WAIT_FOR reached";
 
+# release the latter applier, with real lock conflict
 --let $galera_sync_point = local_monitor_slave_enter_sync
 --source include/galera_signal_sync_point.inc
 --source include/galera_clear_sync_point.inc