MDEV-20154 Assertion `len <= col->len | ...` failed in row_merge_buf_add

len was containing garbage, since vctempl->mysql_col_offset was
containing old value while calling row_mysql_store_col_in_innobase_format
from innobase_get_computed_value().

It was not updated after the first ALTER TABLE call, because it's INPLACE
logic considered there's nothing to update, and exited immediately from
ha_innobase::inplace_alter_table().

However, vcol metadata needs an update, since vcols structure is changed
in mysql record.

The regression was introduced by 12614af1fe. There, refcount==1 condition
was removed, which turned out to be crucial, though racy. The idea was to
update vc_templ after each (sequencing) ALTER TABLE.

We should do the same another way, and there may be a plenty of solutions,
but the simplest one is to add a following condition:
  if vcol structure is changed, drop vc_templ; it will be recreated on next
  ha_innobase::open() call.

in prepare_inplace_alter_table. It is safe, since innodb inplace changes
require at least HA_ALTER_INPLACE_SHARED_LOCK_AFTER_PREPARE, which
guarantee MDL_EXCLUSIVE on this stage.

alter_templ_needs_rebuild() also has to track the columns not indexed, to
keep vc_templ correct.

Note that vc_templ is always kept constructed and available after
ha_innobase::open() call, even on INSERT, though no virtual columns are
evaluated during that statement
inside innodb.

In the test case suplied, it will be recreated on the second ALTER TABLE.

1 files changed, 14 insertions, 0 deletions

diff --git a/mysql-test/suite/gcol/r/innodb_virtual_index.result b/mysql-test/suite/gcol/r/innodb_virtual_index.result
index b8dd161c30e..4b7af8784eb 100644
--- a/mysql-test/suite/gcol/r/innodb_virtual_index.result
+++ b/mysql-test/suite/gcol/r/innodb_virtual_index.result
@@ -300,3 +300,17 @@ Table	Op	Msg_type	Msg_text
 test.t1	optimize	note	Table does not support optimize, doing recreate + analyze instead
 test.t1	optimize	status	OK
 DROP TABLE t1;
+#
+# MDEV-20154 Assertion `len <= col->len || ((col->mtype) == 5
+# || (col->mtype) == 14)' failed in row_merge_buf_add
+#
+CREATE TABLE t1 (
+a VARCHAR(2500),
+b VARCHAR(2499) AS (a) VIRTUAL
+) ENGINE=InnoDB;
+INSERT INTO t1 (a) VALUES ('foo');
+ALTER TABLE t1 MODIFY a VARCHAR(2600), ALGORITHM=INPLACE;
+ALTER TABLE t1 ADD KEY (b), ALGORITHM=INPLACE;
+# Cleanup
+DROP TABLE t1;
+# End of 10.2 tests