MDEV-18507 can't update temporary table when joined with table with triggers on read-only

triggers are opened and tables used in triggers are prelocked in
open_tables(). But multi-update can detect what tables will actually
be updated only later, after all main tables are opened.

Meaning, if a table is used in multi-update, but is not actually updated,
its on-update treggers will be opened and tables will be prelocked,
even if it's unnecessary. This can cause more tables to be
write-locked than needed, causing read_only errors, privilege errors
and lock waits.

Fix: don't open/prelock triggers unless table->updating is true.
In multi-update after setting table->updating=true, do a second
open_tables() for newly added tables, if any.

1 files changed, 21 insertions, 0 deletions

diff --git a/mysql-test/r/multi_update.result b/mysql-test/r/multi_update.result
index 09df98c741e..e04637a3a54 100644
--- a/mysql-test/r/multi_update.result
+++ b/mysql-test/r/multi_update.result
@@ -1017,4 +1017,25 @@ execute stmt1;
 deallocate prepare stmt1;
 drop view v3,v2,v1;
 drop table t1,t2,t3;
+create table t1 (id int not null, v1 varchar(10) not null);
+insert into t1 values (1,1),(2,2);
+create table t2 (log varchar(10) not null);
+create trigger t1_after_update after update on t1
+for each row insert into t2 values ('triggered');
+create user foo;
+grant select, insert, update, delete, create, drop, reload, index, alter, show databases, create temporary tables, lock tables, execute, create view, show view, create routine, alter routine, trigger on *.* to 'foo'@'%';
+set global read_only=1;
+create temporary table temp_t1 (id int not null, update_me varchar(10));
+insert into temp_t1 values (1,1),(2,2),(3,3);
+update temp_t1 left join t1 on temp_t1.id = t1.id set temp_t1.update_me = 'hello';
+set global read_only = 0;
+create table t3 (id int not null);
+insert t3 values (2);
+update t1 left join t3 on t1.id = t3.id set t1.v1 = 'hello';
+select * from t2;
+log
+triggered
+triggered
+drop table t1,t2, t3;
+drop user foo;
 end of 5.5 tests