MDEV-37345 sequences and prelocking

if a bunch of tables are prelocked, when a table is actually
needed in the routine, open_tables picks one table out of the
prelocked list with a smallest "distance". Distance is simply
a difference between the actual table lock and the requested
table lock. Say, if the prelocked set contains both t1 write-locked
and t1 read-locked, than an UPDATE will prefer write-locked t1
and SELECT will prefer read-locked. if there's only write-locked
table in the set, both UPDATE and SELECT will use it.

this doesn't distingush between UPDATE and INSERT, but INSERT
marks tables with tables->for_insert_data=1, which causes
prelocking to invoke add_internal_tables() and prepare sequences
for execution.

in this bug there were two prelocked t1's, one for INSERT (with
for_insert_data=1) and one for UPDATE.
INSERT picks the second (they both are write-locked, so the distance is
the same), its sequence is not prepared and crashes.

Let's add for_insert_data as the lowest bit into the distance.

mysql-test/suite/sql_sequence/other.result

@@ -394,4 +394,20 @@ create table t (id bigint default(nextval(s))) engine=myisam;
 insert delayed into t () values();
 drop table t;
 drop sequence s;
+#
+# MDEV-37345 Item_func_nextval::val_int() crash on INSERT...SELECT with subqueries
+#
+create sequence s;
+create table t1 (a int, b int default(nextval(s)));
+insert into t1 () values ();
+create table t2 (c int);
+create procedure p() update t1 set a = 0;
+create trigger tr after insert on t2 for each row
+begin
+insert into t1 () values ();
+call p();
+end $
+insert into t2 values ();
+drop table t1, t2, s;
+drop procedure p;
 # End of 10.11 tests

mysql-test/suite/sql_sequence/other.test

@@ -428,4 +428,24 @@ insert delayed into t () values();
 drop table t;
 drop sequence s;
 
+--echo #
+--echo # MDEV-37345 Item_func_nextval::val_int() crash on INSERT...SELECT with subqueries
+--echo #
+# sequence and prelocking.
+create sequence s;
+create table t1 (a int, b int default(nextval(s)));
+insert into t1 () values ();
+create table t2 (c int);
+create procedure p() update t1 set a = 0;
+--delimiter $
+create trigger tr after insert on t2 for each row
+begin
+  insert into t1 () values ();
+  call p();
+end $
+--delimiter ;
+insert into t2 values ();
+drop table t1, t2, s;
+drop procedure p;
+
 --echo # End of 10.11 tests

sql/sql_base.cc

@@ -2010,7 +2010,18 @@ bool open_table(THD *thd, TABLE_LIST *table_list, Open_table_context *ot_ctx)
              table->query_id == 0))
         {
           int distance= ((int) table->reginfo.lock_type -
-                         (int) table_list->lock_type);
+                         (int) table_list->lock_type) * 2;
+          TABLE_LIST *tl= thd->locked_tables_mode == LTM_PRELOCKED
+                      ? table->pos_in_table_list : table->pos_in_locked_tables;
+          /*
+            note, that merge table children are automatically added to
+            prelocking set in ha_myisammrg::add_children_list(), but their
+            TABLE_LIST's are on the execution arena, so tl will be invalid
+            on the second execution. Let's just skip them below.
+          */
+          if (table_list->parent_l || !tl ||
+              table_list->for_insert_data != tl->for_insert_data)
+            distance|= 1;
 
           /*
             Find a table that either has the exact lock type requested,