MDEV-37281 incorrect isolation level in update with unique using hash or without overlap

in 11.4 multi_update::prepare() is called both for single and multi update.
as it's invoked before lock_tables(), it cannot do prepare_for_insert(),
which calls external_lock() and only can be used on an already locked table

Author: vuvova

mysql-test/main/update_innodb.result

@@ -251,3 +251,21 @@ update  orders, customer, nation set orders.o_comment = "+++" where o_orderDATE
 o_custkey =  c_custkey and c_nationkey = n_nationkey and n_name='PERU';
 DROP DATABASE dbt3_s001;
 set default_storage_engine=@save_default_storage_engine;
+use test;
+#
+# MDEV-37281 incorrect isolation level in update with unique using hash or without overlap
+#
+create table t1 (id int, e varchar(100), a int, unique (e) using hash) engine=innodb;
+insert t1 values(10, '2000-01-01', 0);
+insert t1 values(20, '2000-01-02', 1);
+insert t1 values(30, '2000-01-03', 2);
+set transaction isolation level read committed;
+start transaction;
+update t1 set a=10 where a=0;
+connect con1,localhost,root;
+set transaction isolation level read committed;
+update t1 set a=20 where a=1;
+connection default;
+drop table t1;
+disconnect con1;
+# End of 11.4.tests

mysql-test/main/update_innodb.test

@@ -193,7 +193,6 @@ let $c1=
         c_nationkey = n_nationkey and
         n_name='PERU';
 
-
 explain
 update  orders, customer, nation set orders.o_comment = "+++" where o_orderDATE between '1992-01-01' and '1992-06-30' and
  o_custkey =  c_custkey and c_nationkey = n_nationkey and n_name='PERU';
@@ -208,4 +207,23 @@ update  orders, customer, nation set orders.o_comment = "+++" where o_orderDATE
 DROP DATABASE dbt3_s001;
 
 set default_storage_engine=@save_default_storage_engine;
+use test;
+
+--echo #
+--echo # MDEV-37281 incorrect isolation level in update with unique using hash or without overlap
+--echo #
+create table t1 (id int, e varchar(100), a int, unique (e) using hash) engine=innodb;
+insert t1 values(10, '2000-01-01', 0);
+insert t1 values(20, '2000-01-02', 1);
+insert t1 values(30, '2000-01-03', 2);
+set transaction isolation level read committed;
+start transaction;
+update t1 set a=10 where a=0;
+--connect con1,localhost,root
+set transaction isolation level read committed;
+update t1 set a=20 where a=1;
+--connection default
+drop table t1;
+--disconnect con1
 
+--echo # End of 11.4.tests

sql/sql_update.cc

@@ -1842,8 +1842,6 @@ int multi_update::prepare(List<Item> &not_used_values,
     {
       table->read_set= &table->def_read_set;
       bitmap_union(table->read_set, &table->tmp_set);
-      if (!(thd->lex->context_analysis_only & CONTEXT_ANALYSIS_ONLY_PREPARE))
-        table->file->prepare_for_insert(1);
     }
   }
   if (unlikely(error))
@@ -2076,6 +2074,8 @@ multi_update::initialize_tables(JOIN *join)
     ORDER     group;
     TMP_TABLE_PARAM *tmp_param;
 
+    table->file->prepare_for_insert(1);
+
     if (ignore)
       table->file->extra(HA_EXTRA_IGNORE_DUP_KEY);
     if (table == main_table)			// First table in join