MDEV-28943 Online alter fails under LOCK TABLE with ER_ALTER_OPERATION_NOT_SUPPORTED_REASON

if ALTER TABLE ... LOCK=xxx is executed under LOCK TABLES,
ignore the LOCK clause, because ALTER should not downgrade
already taken EXCLUSIVE table lock to SHARED or NONE.

This commit preserves the existing behavior (LOCK was de facto ignored),
but makes it explicit.

mysql-test/main/alter_table_lock.result

@@ -14,4 +14,49 @@ ALTER TABLE t1 CHANGE COLUMN IF EXISTS b c INT;
 SET SESSION max_session_mem_used = @max_session_mem_used_save;
 UNLOCK TABLES;
 DROP TABLE t1;
+#
 # End of 10.5 tests
+#
+#
+# MDEV-28943 Online alter fails under LOCK TABLE with ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
+#
+create table t1 (f int) engine=innodb;
+insert t1 values (1);
+alter table t1 force, algorithm=copy, lock=none;
+alter table t1 force, algorithm=inplace, lock=none;
+alter table t1 force, algorithm=copy, lock=shared;
+alter table t1 force, algorithm=inplace, lock=shared;
+alter table t1 force, algorithm=copy, lock=exclusive;
+alter table t1 force, algorithm=inplace, lock=exclusive;
+lock table t1 write;
+connect con1, localhost, root;
+select count(*) as 'must be 0' from t1;
+connection default;
+alter table t1 force, algorithm=copy, lock=none;
+alter table t1 force, algorithm=inplace, lock=none;
+alter table t1 force, algorithm=copy, lock=shared;
+alter table t1 force, algorithm=inplace, lock=shared;
+alter table t1 force, algorithm=copy, lock=exclusive;
+alter table t1 force, algorithm=inplace, lock=exclusive;
+delete from t1;
+unlock tables;
+connection con1;
+must be 0
+0
+connection default;
+drop table t1;
+#
+# MDEV-29056 Replica SQL thread stops with 1846 error on ALTER ONLINE after LOCK WRITE
+#
+create table t1 (c varchar(1), key (c)) engine=innodb;
+insert into t1 (c) values ('g') ;
+alter table t1 add fulltext key(c), algorithm=inplace;
+alter online table t1 add column s blob not null, algorithm=inplace;
+ERROR 0A000: LOCK=NONE is not supported. Reason: Fulltext index creation requires a lock. Try LOCK=SHARED
+lock table t1 write;
+alter online table t1 add column s blob not null, algorithm=inplace;
+ERROR 0A000: LOCK=NONE is not supported. Reason: Fulltext index creation requires a lock. Try LOCK=SHARED
+drop table t1;
+#
+# End of 10.11 tests
+#

mysql-test/main/alter_table_lock.test

 --source include/not_msan.inc
+--source include/have_innodb.inc
 
 --echo #
 --echo # MDEV-23836: Assertion `! is_set() || m_can_overwrite_status' in
@@ -26,4 +27,54 @@ SET SESSION max_session_mem_used = @max_session_mem_used_save;
 UNLOCK TABLES;
 DROP TABLE t1;
 
+--echo #
 --echo # End of 10.5 tests
+--echo #
+
+--echo #
+--echo # MDEV-28943 Online alter fails under LOCK TABLE with ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
+--echo #
+
+# test that any lock=xxx works under LOCK TABLES
+# and that it is ignored, does not actually downgrade the lock
+create table t1 (f int) engine=innodb;
+insert t1 values (1);
+alter table t1 force, algorithm=copy, lock=none;
+alter table t1 force, algorithm=inplace, lock=none;
+alter table t1 force, algorithm=copy, lock=shared;
+alter table t1 force, algorithm=inplace, lock=shared;
+alter table t1 force, algorithm=copy, lock=exclusive;
+alter table t1 force, algorithm=inplace, lock=exclusive;
+lock table t1 write;
+connect con1, localhost, root;
+--send select count(*) as 'must be 0' from t1
+--connection default
+alter table t1 force, algorithm=copy, lock=none;
+alter table t1 force, algorithm=inplace, lock=none;
+alter table t1 force, algorithm=copy, lock=shared;
+alter table t1 force, algorithm=inplace, lock=shared;
+alter table t1 force, algorithm=copy, lock=exclusive;
+alter table t1 force, algorithm=inplace, lock=exclusive;
+delete from t1;
+unlock tables;
+--connection con1
+--reap
+--connection default
+drop table t1;
+
+--echo #
+--echo # MDEV-29056 Replica SQL thread stops with 1846 error on ALTER ONLINE after LOCK WRITE
+--echo #
+create table t1 (c varchar(1), key (c)) engine=innodb;
+insert into t1 (c) values ('g') ;
+alter table t1 add fulltext key(c), algorithm=inplace;
+--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
+alter online table t1 add column s blob not null, algorithm=inplace;
+lock table t1 write;
+--error ER_ALTER_OPERATION_NOT_SUPPORTED_REASON
+alter online table t1 add column s blob not null, algorithm=inplace;
+drop table t1;
+
+--echo #
+--echo # End of 10.11 tests
+--echo #

sql/sql_table.cc

@@ -10027,7 +10027,6 @@ bool mysql_alter_table(THD *thd, const LEX_CSTRING *new_db,
   if (alter_info->requested_lock == Alter_info::ALTER_TABLE_LOCK_SHARED
       || alter_info->requested_lock > Alter_info::ALTER_TABLE_LOCK_NONE
       || alter_info->flags & ALTER_DROP_SYSTEM_VERSIONING
-      || thd->locked_tables_mode == LTM_LOCK_TABLES
       || thd->lex->sql_command == SQLCOM_OPTIMIZE
       || alter_info->algorithm(thd) > Alter_info::ALTER_TABLE_ALGORITHM_COPY)
     online= false;
@@ -10904,6 +10903,9 @@ do_continue:;
       goto err_new_table_cleanup;
     }
 
+    if (thd->locked_tables_mode == LTM_LOCK_TABLES)
+      online= false;
+
     // If EXCLUSIVE lock is requested, upgrade already.
     if (alter_info->requested_lock == Alter_info::ALTER_TABLE_LOCK_EXCLUSIVE &&
         wait_while_table_is_used(thd, table, HA_EXTRA_FORCE_REOPEN))