MDEV-11525 Assertion `cp + len <= buff + buff_size' failed in JOIN_CACHE::write_record_data

Workaround for join_cache + index on vcols + keyread bug.
Initialize the record to avoid caching garbage in non-read fields.

A proper fix (do not cache non-read fields at all) is done in 10.2
in commits 5d7607f340f..8d99166c697

mysql-test/suite/vcol/r/vcol_select_myisam.result

@@ -295,3 +295,112 @@ Note	1003	select `test`.`t1`.`b` AS `b`,`test`.`t2`.`a` AS `a` from `test`.`t1`
 SELECT * FROM t1 NATURAL JOIN t2;
 b	a
 DROP TABLE t1,t2;
+create table t1 (
+pk integer auto_increment,
+bi integer not null,
+vi integer generated always as (bi) persistent,
+bc varchar(1) not null,
+vc varchar(2) generated always as (concat(bc, bc)) persistent,
+primary key (pk),
+key (vi, vc));
+insert t1 (bi, bc) values (0, 'x'), (0, 'n'), (1, 'w'), (7, 's'), (0, 'a'), (4, 'd'), (1, 'w'), (1, 'j'), (1, 'm'), (4, 'k'), (7, 't'), (4, 'k'), (2, 'e'), (0, 'i'), (1, 't'), (6, 'z'), (3, 'c'), (6, 'i'), (8, 'v');
+create table t2 (
+pk integer auto_increment,
+bi integer not null,
+vi integer generated always as (bi) persistent,
+bc varchar(257) not null,
+vc varchar(2) generated always as (concat(bc, bc)) persistent,
+primary key (pk),
+key (vi, vc));
+insert t2 (bi, bc) values (1, 'c'), (8, 'm'), (9, 'd'), (6, 'y'), (1, 't'), (6, 'd'), (2, 's'), (4, 'r'), (8, 'm'), (4, 'b'), (4, 'x'), (7, 'g'), (4, 'p'), (1, 'q'), (9, 'w'), (4, 'd'), (8, 'e'), (4, 'b'), (8, 'y');
+explain # should be using join buffer
+select t2.vi from (t2 as t3 right join (t2 left join t1 on (t1.bi = t2.vi)) on (t1.vc = t2.vc));
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t2	index	NULL	vi	10	NULL	19	Using index
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	19	Using where; Using join buffer (flat, BNL join)
+1	SIMPLE	t3	index	NULL	PRIMARY	4	NULL	19	Using where; Using index; Using join buffer (incremental, BNL join)
+select t2.vi from (t2 as t3 right join (t2 left join t1 on (t1.bi = t2.vi)) on (t1.vc = t2.vc));
+vi
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+1
+2
+4
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+4
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+4
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+4
+6
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+7
+8
+8
+8
+8
+9
+9
+drop table t2,t1;

mysql-test/suite/vcol/t/vcol_select_myisam.test

@@ -68,3 +68,35 @@ SELECT * FROM t1 NATURAL JOIN t2;
 SELECT * FROM t1 NATURAL JOIN t2;
 
 DROP TABLE t1,t2;
+
+#
+# MDEV-11525 Assertion `cp + len <= buff + buff_size' failed in JOIN_CACHE::write_record_data
+#
+
+create table t1 (
+  pk integer auto_increment,
+  bi integer not null,
+  vi integer generated always as (bi) persistent,
+  bc varchar(1) not null,
+  vc varchar(2) generated always as (concat(bc, bc)) persistent,
+  primary key (pk),
+  key (vi, vc));
+insert t1 (bi, bc) values (0, 'x'), (0, 'n'), (1, 'w'), (7, 's'), (0, 'a'), (4, 'd'), (1, 'w'), (1, 'j'), (1, 'm'), (4, 'k'), (7, 't'), (4, 'k'), (2, 'e'), (0, 'i'), (1, 't'), (6, 'z'), (3, 'c'), (6, 'i'), (8, 'v');
+create table t2 (
+  pk integer auto_increment,
+  bi integer not null,
+  vi integer generated always as (bi) persistent,
+  bc varchar(257) not null,
+  vc varchar(2) generated always as (concat(bc, bc)) persistent,
+  primary key (pk),
+  key (vi, vc));
+insert t2 (bi, bc) values (1, 'c'), (8, 'm'), (9, 'd'), (6, 'y'), (1, 't'), (6, 'd'), (2, 's'), (4, 'r'), (8, 'm'), (4, 'b'), (4, 'x'), (7, 'g'), (4, 'p'), (1, 'q'), (9, 'w'), (4, 'd'), (8, 'e'), (4, 'b'), (8, 'y');
+explain # should be using join buffer
+select t2.vi from (t2 as t3 right join (t2 left join t1 on (t1.bi = t2.vi)) on (t1.vc = t2.vc));
+--sorted_result
+select t2.vi from (t2 as t3 right join (t2 left join t1 on (t1.bi = t2.vi)) on (t1.vc = t2.vc));
+drop table t2,t1;
+
+#
+# End of 5.5 tests
+#

sql/sql_join_cache.cc

@@ -589,6 +589,11 @@ void JOIN_CACHE::create_remaining_fields()
   {
     MY_BITMAP *rem_field_set;
     TABLE *table= tab->table;
+#if MYSQL_VERSION_ID < 100204
+    empty_record(table);
+#else
+#error remove
+#endif
 
     if (all_read_fields)
       rem_field_set= table->read_set;