The following test is based on the test innodb_gis.alter_spatial_index.

For MDEV-11369 Instant ADD COLUMN, we want consistent behaviour in ALTER TABLE. No matter if the ALTER TABLE was implemented by ALGORITHM=COPY, non-instant ALGORITHM=INPLACE, or instant ALGORITHM=INPLACE, the result must be the same. The result should also be consistent with INSERT and UPDATE operations. Here it is not the case:

--source include/have_innodb.inc

--source include/have_geometry.inc

CREATE TABLE tab(c1 int PRIMARY KEY) ENGINE=InnoDB;

INSERT INTO tab SET c1=1;

ALTER TABLE tab add COLUMN c2 POINT NOT NULL;

SELECT * FROM tab;

INSERT INTO tab SET c1=2;

SELECT * FROM tab;

DROP TABLE tab;

Currently, the ALTER TABLE (no matter if ALGORITHM=COPY or ALGORITHM=INPLACE is specified) will insert the invalid value c2='' (the empty string). But, the INSERT is returning an error:

mysqltest: At line 7: query 'INSERT INTO tab SET c1=2' failed: 1364: Field 'c2' doesn't have a default value

The behaviour of ALTER TABLE and INSERT are inconsistent with each other.

appended by HF:
I came upon a crash that i think should be fixed along with this

MariaDB [test]> create table pt(g geometry);

Query OK, 0 rows affected (0.50 sec)

MariaDB [test]> insert into pt values (st_geometryfromtext("point(1 2)"));

Query OK, 1 row affected (0.05 sec)

MariaDB [test]> alter table pt add column g2 geometry default("123123");

ERROR 2013 (HY000): Lost connection to MySQL server during query