MDEV-4506: Parallel replication: intermediate commit.

Fix typo in worker thread free list management.
Simple parallel INSERT from worker threads runs now.

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

@@ -20,7 +20,24 @@ eval CHANGE MASTER TO master_host = '127.0.0.1', master_port = $MASTER_MYPORT,
      master_user='root', master_use_gtid=current_pos;
 
 --connection s1
+SET gtid_domain_id=0;
 INSERT INTO t1 VALUES (1);
+SET gtid_domain_id=1;
+INSERT INTO t1 VALUES (2);
+SET gtid_domain_id=2;
+INSERT INTO t1 VALUES (3);
+SET gtid_domain_id=0;
+INSERT INTO t1 VALUES (4);
+SET gtid_domain_id=1;
+INSERT INTO t1 VALUES (5);
+SET gtid_domain_id=2;
+INSERT INTO t1 VALUES (6);
+SET gtid_domain_id=0;
+INSERT INTO t1 VALUES (7);
+SET gtid_domain_id=1;
+INSERT INTO t1 VALUES (8);
+SET gtid_domain_id=2;
+INSERT INTO t1 VALUES (9);
 
 --connection s2
 query_vertical SHOW SLAVE STATUS;

sql/rpl_parallel.cc

@@ -132,9 +132,9 @@ handle_rpl_parallel_thread(void *arg)
         mysql_mutex_lock(&rpt->pool->LOCK_rpl_thread_pool);
         list= rpt->pool->free_list;
         rpt->next= list;
-        rpt->pool->free_list= list;
+        rpt->pool->free_list= rpt;
         if (!list)
-          mysql_cond_signal(&rpt->pool->COND_rpl_thread_pool);
+          mysql_cond_broadcast(&rpt->pool->COND_rpl_thread_pool);
         mysql_mutex_unlock(&rpt->pool->LOCK_rpl_thread_pool);
         rpt->free= true;
       }