MDEV-31755 Replica's DML event deadlocks wit online alter table

The deadlock was caused by too strong MDL acquired by the start ALTER.

Replica's ALTER TABLE replication consists of two phases:
1. Start ALTER (SA) -- the event is emittd in the very beginning,
allowing replication start ALTER in parallel
2. Commit ALTER (CA) -- ensures that master finishes successfully

CA is normally received by wait_for_master call.
If parallel DML was run, the following sequence will take place:

|- SA
|- DML
|- CA

If CA is handled after MDL upgrade, it'll will deadlock with DML.

While MDL is shared by the start ALTER wait for its 2nd part
to allow concurrent DMLs to grab the lock.

The fix uses wait_for_master reentrancy -- no need to avoid a second call
in the end of mysql_alter_table.

Since SA and CA are marked with FL_DDL, the DML issued in-between cannot be
rescheduled before or after them. However, SA "commits" (by he call of
write_bin_log_start_alter and, subsequently,
thd->wakeup_subsequent_commits) before the copy stage begins, unlocking
the DMLs to run on this table. That is, these DMLs will be executed
concurrently with the copy stage, making Online alter effective on replicas
as well

Co-authored-by: Nikita Malyavin (nikitamalyavin@gmail.com)

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

+include/master-slave.inc
+[connection master]
+connection slave;
+include/stop_slave.inc
+set global slave_parallel_threads=3;
+set global slave_parallel_mode= optimistic;
+connection master;
+#
+# MDEV-31755 Replica's DML event deadlocks wit online alter table
+#
+# Three threads for SA,U,CA
+create table t (id int, a int, primary key (id)) engine=innodb;
+insert into t values (1,10), (2,20);
+set @@session.binlog_alter_two_phase=1;
+set debug_sync= 'alter_table_online_downgraded signal ready wait_for go';
+alter table t add c text default('qqq') after id, algorithm=copy, lock=none;
+connection master1;
+set debug_sync= 'now wait_for ready';
+update t set a = 1;
+set debug_sync= 'now signal go';
+connection master;
+include/save_master_gtid.inc
+connection slave;
+include/start_slave.inc
+include/sync_with_master_gtid.inc
+select * from t;
+id	c	a
+1	qqq	1
+2	qqq	1
+connection master;
+drop table t;
+connection slave;
+connection master;
+connection slave;
+include/stop_slave.inc
+set global slave_parallel_threads=0;
+set global slave_parallel_mode=   optimistic;
+include/start_slave.inc
+include/rpl_end.inc

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

+source include/have_debug_sync.inc;
+source include/have_innodb.inc;
+source include/master-slave.inc;
+
+
+--connection slave
+source include/stop_slave.inc;
+--let $slave_parallel_threads=`select @@global.slave_parallel_threads`
+--let $slave_parallel_mode=   `select @@global.slave_parallel_mode`
+set global slave_parallel_threads=3;
+set global slave_parallel_mode= optimistic;
+--connection master
+
+
+--echo #
+--echo # MDEV-31755 Replica's DML event deadlocks wit online alter table
+--echo #
+--echo # Three threads for SA,U,CA
+
+create table t (id int, a int, primary key (id)) engine=innodb;
+insert into t values (1,10), (2,20);
+
+set @@session.binlog_alter_two_phase=1;
+set debug_sync= 'alter_table_online_downgraded signal ready wait_for go';
+send alter table t add c text default('qqq') after id, algorithm=copy, lock=none;
+
+--connection master1
+set debug_sync= 'now wait_for ready';
+
+update t set a = 1;
+
+set debug_sync= 'now signal go';
+
+--connection master
+--reap
+--source include/save_master_gtid.inc
+
+--connection slave
+source include/start_slave.inc;
+--source include/sync_with_master_gtid.inc
+select * from t;
+
+# Cleanup
+--connection master
+drop table t;
+--sync_slave_with_master
+--connection master
+
+--echo #
+--echo # End of 11.2 tests
+--echo #
+
+--connection slave
+source include/stop_slave.inc;
+--eval set global slave_parallel_threads=$slave_parallel_threads
+--eval set global slave_parallel_mode=   $slave_parallel_mode
+source include/start_slave.inc;
+
+--source include/rpl_end.inc

sql/sql_table.cc

@@ -86,7 +86,7 @@ static int copy_data_between_tables(THD *, TABLE *,TABLE *,
                                     List<Create_field> &, bool, uint, ORDER *,
                                     ha_rows *, ha_rows *,
                                     Alter_info::enum_enable_or_disable,
-                                    Alter_table_ctx *, bool);
+                                    Alter_table_ctx *, bool, uint64);
 static int append_system_key_parts(THD *thd, HA_CREATE_INFO *create_info,
                                    Key *key);
 static int mysql_prepare_create_table(THD *, HA_CREATE_INFO *, Alter_info *,
@@ -11140,7 +11140,7 @@ do_continue:;
                                  alter_info->create_list, ignore,
                                  order_num, order, &copied, &deleted,
                                  alter_info->keys_onoff,
-                                 &alter_ctx, online))
+                                 &alter_ctx, online, start_alter_id))
       goto err_new_table_cleanup;
   }
   else
@@ -11696,7 +11696,8 @@ copy_data_between_tables(THD *thd, TABLE *from, TABLE *to,
                          uint order_num, ORDER *order,
                          ha_rows *copied, ha_rows *deleted,
                          Alter_info::enum_enable_or_disable keys_onoff,
-                         Alter_table_ctx *alter_ctx, bool online)
+                         Alter_table_ctx *alter_ctx, bool online,
+                         uint64 start_alter_id)
 {
   int error= 1;
   Copy_field *copy= NULL, *copy_end;
@@ -12104,6 +12105,14 @@ copy_data_between_tables(THD *thd, TABLE *from, TABLE *to,
     thd->lock= NULL;
 
     error= online_alter_read_from_binlog(thd, &rgi, binlog);
+    if (start_alter_id)
+    {
+      DBUG_ASSERT(thd->slave_thread);
+
+      int rpl_error= wait_for_master(thd);
+      if (rpl_error)
+        error= 1;
+    }
 
     DEBUG_SYNC(thd, "alter_table_online_before_lock");