MDEV-5788: Incorrect free of rgi->deferred_events in parallel replication

When an rpl_group_info object was returned from the free list, the
rgi->deferred_events_collecting and rgi->deferred_events was not correctly
re-inited. Additionally, the rgi->deferred_events was incorrectly freed in
free_rgi(), which causes unnecessary malloc/free (or crash when re-init is not
done).

Thanks to user nanyi607rao, who reported this bug on maria-developers@.

mysql-test/suite/rpl/r/rpl_parallel.result

@@ -706,6 +706,26 @@ SET GLOBAL binlog_format=@old_format;
 SET GLOBAL slave_parallel_threads=0;
 SET GLOBAL slave_parallel_threads=10;
 include/start_slave.inc
+*** MDEV-5788 Incorrect free of rgi->deferred_events in parallel replication  ***
+include/stop_slave.inc
+SET GLOBAL replicate_ignore_table="test.t3";
+SET GLOBAL slave_parallel_threads=2;
+include/start_slave.inc
+INSERT INTO t3 VALUES (100, rand());
+INSERT INTO t3 VALUES (101, rand());
+INSERT INTO t3 VALUES (102, rand());
+INSERT INTO t3 VALUES (103, rand());
+INSERT INTO t3 VALUES (104, rand());
+INSERT INTO t3 VALUES (105, rand());
+include/stop_slave.inc
+SET GLOBAL replicate_ignore_table="";
+include/start_slave.inc
+INSERT INTO t3 VALUES (106, rand());
+INSERT INTO t3 VALUES (107, rand());
+SELECT * FROM t3 WHERE a >= 100 ORDER BY a;
+a	b
+106	#
+107	#
 include/stop_slave.inc
 SET GLOBAL slave_parallel_threads=@old_parallel_threads;
 include/start_slave.inc

mysql-test/suite/rpl/t/rpl_parallel.test

@@ -1069,6 +1069,47 @@ SET GLOBAL slave_parallel_threads=0;
 SET GLOBAL slave_parallel_threads=10;
 --source include/start_slave.inc
 
+--echo *** MDEV-5788 Incorrect free of rgi->deferred_events in parallel replication  ***
+
+--connection server_2
+# Use just two worker threads, so we are sure to get the rpl_group_info added
+# to the free list, which is what triggered the bug.
+--source include/stop_slave.inc
+SET GLOBAL replicate_ignore_table="test.t3";
+SET GLOBAL slave_parallel_threads=2;
+--source include/start_slave.inc
+
+--connection server_1
+INSERT INTO t3 VALUES (100, rand());
+INSERT INTO t3 VALUES (101, rand());
+
+--save_master_pos
+
+--connection server_2
+--sync_with_master
+
+--connection server_1
+INSERT INTO t3 VALUES (102, rand());
+INSERT INTO t3 VALUES (103, rand());
+INSERT INTO t3 VALUES (104, rand());
+INSERT INTO t3 VALUES (105, rand());
+
+--connection server_2
+--sync_with_master
+--source include/stop_slave.inc
+SET GLOBAL replicate_ignore_table="";
+--source include/start_slave.inc
+
+--connection server_1
+INSERT INTO t3 VALUES (106, rand());
+INSERT INTO t3 VALUES (107, rand());
+--save_master_pos
+
+--connection server_2
+--sync_with_master
+--replace_column 2 #
+SELECT * FROM t3 WHERE a >= 100 ORDER BY a;
+
 
 --connection server_2
 --source include/stop_slave.inc

sql/rpl_parallel.cc

@@ -789,9 +789,10 @@ rpl_parallel_thread::get_rgi(Relay_log_info *rli, Gtid_log_event *gtid_ev,
       return NULL;
     }
     rgi->is_parallel_exec = true;
-    if ((rgi->deferred_events_collecting= rli->mi->rpl_filter->is_on()))
-      rgi->deferred_events= new Deferred_log_events(rli);
   }
+  if ((rgi->deferred_events_collecting= rli->mi->rpl_filter->is_on()) &&
+      !rgi->deferred_events)
+    rgi->deferred_events= new Deferred_log_events(rli);
   if (event_group_new_gtid(rgi, gtid_ev))
   {
     free_rgi(rgi);
@@ -810,11 +811,6 @@ rpl_parallel_thread::free_rgi(rpl_group_info *rgi)
   mysql_mutex_assert_owner(&LOCK_rpl_thread);
   DBUG_ASSERT(rgi->commit_orderer.waitee == NULL);
   rgi->free_annotate_event();
-  if (rgi->deferred_events)
-  {
-    delete rgi->deferred_events;
-    rgi->deferred_events= NULL;
-  }
   rgi->next= rgi_free_list;
   rgi_free_list= rgi;
 }