MDEV-23089 rpl_parallel2 fails in 10.5

Problem:- rpl_parallel2 was failing non-deterministically
Analysis:-
When FLUSH TABLES WITH READ LOCK is executed, it will allow all worker
threads to complete their ongoing transactions and then it will pause them.
At this state FTWRL will proceed to acquire global read lock. FTWRL first
blocks threads from starting new commits, then upgrades the lock to block
commit of existing transactions.
  Step1:
    FLUSH TABLES WITH READ LOCK - Blocks new commits
  Step2:
    * STOP SLAVE command enables 'force_abort=1' which unblocks workers,
      they continue to execute events.
    * T1: Waits in 'record_gtid' call to update 'gtid_slave_pos' table with
      its current GTID, but it is blocked becuase of Step1.
    * T2: Holds COMMIT lock and waits for T1 to commit.
  Step3:
    FLUSH TABLES WITH READ LOCK - Waiting to get BLOCK_COMMIT.
This results in deadlock. When STOP SLAVE command allows paused workers to
proceed, workers should skip the execution of all further events, similar
to 'conservative' parallel mode.
Solution:-
We will assign 1 to skip_event_group when we are aborted in do_ftwrl_wait.
rpl_parallel_entry->pause_sub_id is only reset when force_abort is off in
rpl_pause_after_ftwrl.

mysql-test/suite/rpl/r/rpl_parallel2.result

@@ -2,8 +2,10 @@ include/rpl_init.inc [topology=1->2]
 *** MDEV-5509: Incorrect value for Seconds_Behind_Master if parallel replication ***
 connection server_2;
 SET @old_parallel_threads=@@GLOBAL.slave_parallel_threads;
+set @old_parallel_mode= @@GLOBAL.slave_parallel_mode;
 include/stop_slave.inc
 SET GLOBAL slave_parallel_threads=5;
+set global slave_parallel_mode= optimistic;
 include/start_slave.inc
 connection server_1;
 CREATE TABLE t1 (a INT PRIMARY KEY, b INT);
@@ -157,6 +159,7 @@ UNLOCK TABLES;
 connection server_2;
 include/stop_slave.inc
 SET GLOBAL slave_parallel_threads=@old_parallel_threads;
+set global slave_parallel_mode= @old_parallel_mode;
 include/start_slave.inc
 connection server_1;
 DROP TABLE t1, t2;

mysql-test/suite/rpl/t/rpl_parallel2.test

@@ -8,8 +8,10 @@
 
 --connection server_2
 SET @old_parallel_threads=@@GLOBAL.slave_parallel_threads;
+set @old_parallel_mode= @@GLOBAL.slave_parallel_mode;
 --source include/stop_slave.inc
 SET GLOBAL slave_parallel_threads=5;
+set global slave_parallel_mode= optimistic;
 --source include/start_slave.inc
 
 --connection server_1
@@ -219,6 +221,7 @@ UNLOCK TABLES;
 --connection server_2
 --source include/stop_slave.inc
 SET GLOBAL slave_parallel_threads=@old_parallel_threads;
+set global slave_parallel_mode= @old_parallel_mode;
 --source include/start_slave.inc
 
 --connection server_1

sql/rpl_parallel.cc

@@ -396,13 +396,14 @@ do_gco_wait(rpl_group_info *rgi, group_commit_orderer *gco,
 }
 
 
-static void
+static bool
 do_ftwrl_wait(rpl_group_info *rgi,
               bool *did_enter_cond, PSI_stage_info *old_stage)
 {
   THD *thd= rgi->thd;
   rpl_parallel_entry *entry= rgi->parallel_entry;
   uint64 sub_id= rgi->gtid_sub_id;
+  bool aborted= false;
   DBUG_ENTER("do_ftwrl_wait");
 
   mysql_mutex_assert_owner(&entry->LOCK_parallel_entry);
@@ -425,7 +426,10 @@ do_ftwrl_wait(rpl_group_info *rgi,
     do
     {
       if (entry->force_abort || rgi->worker_error)
+      {
+        aborted= true;
         break;
+      }
       if (unlikely(thd->check_killed()))
       {
         slave_output_error_info(rgi, thd);
@@ -444,7 +448,7 @@ do_ftwrl_wait(rpl_group_info *rgi,
   if (sub_id > entry->largest_started_sub_id)
     entry->largest_started_sub_id= sub_id;
 
-  DBUG_VOID_RETURN;
+  DBUG_RETURN(aborted);
 }
 
 
@@ -530,6 +534,21 @@ rpl_unpause_after_ftwrl(THD *thd)
     mysql_mutex_lock(&e->LOCK_parallel_entry);
     rpt->pause_for_ftwrl = false;
     mysql_mutex_unlock(&rpt->LOCK_rpl_thread);
+    /*
+      Do not change pause_sub_id if force_abort is set.
+      force_abort is set in case of STOP SLAVE.
+
+      Reason: If pause_sub_id is not changed and force_abort_is set,
+      any parallel slave thread waiting in do_ftwrl_wait() will
+      on wakeup return from do_ftwrl_wait() with 1. This will set
+      skip_event_group to 1 in handle_rpl_parallel_thread() and the
+      parallel thread will abort at once.
+
+      If pause_sub_id is changed, the code in handle_rpl_parallel_thread()
+      would continue to execute the transaction in the queue, which would
+      cause some transactions to be lost.
+    */
+    if (!e->force_abort)
       e->pause_sub_id= (uint64)ULONGLONG_MAX;
     mysql_cond_broadcast(&e->COND_parallel_entry);
     mysql_mutex_unlock(&e->LOCK_parallel_entry);
@@ -1222,7 +1241,7 @@ handle_rpl_parallel_thread(void *arg)
           rgi->worker_error= 1;
         }
         if (likely(!skip_event_group))
-          do_ftwrl_wait(rgi, &did_enter_cond, &old_stage);
+          skip_event_group= do_ftwrl_wait(rgi, &did_enter_cond, &old_stage);
 
         /*
           Register ourself to wait for the previous commit, if we need to do