In MDEV-31086, SET FOREIGN_KEY_CHECKS=0 cannot bypass checks that
make column types of foreign keys incompatible. An unfortunate
consequence is that adding an AUTO_INCREMENT is considered
incompatible in Field_{num,decimal}::is_equal and for the purpose
of FK checks this isn't relevant.

innodb.foreign_key - pragmaticly left wait_until_count_sessions.inc at
end of test to match the second line of test.

Reporter: horrockss@github - https://github.com/MariaDB/mariadb-docker/issues/528
Co-Author: Marko Mäkelä <marko.makela@mariadb.com>
Reviewer: Nikita Malyavin

For the future reader this was attempted:

Removing AUTO_INCREMENT checks from Field_{num,decimal}::is_equals
failed in the following locations (noted for future fixing):
* MyISAM and Aria (not InnoDB) don't adjust AUTO_INCREMENT next number
  correctly, hence added a test to main.auto_increment to catch
  the next person that attempts this fix.
* InnoDB must perform an ALGORITHM=COPY to populate NULL values of
  an original table (MDEV-19190 mtr test period.copy), this requires
  ALTER_STORED_COLUMN_TYPE to be set in fill_alter_inplace_info
  which doesn't get hit because field->is_equal is true.
* InnoDB must not perform the change inplace (below patch)
* innodb.innodb-alter-timestamp main.partition_innodb test would
  also need futher investigation.

InnoDB ha_innobase::check_if_supported_inplace_alter to support the
removal of Field_{num,decimal}::is_equal AUTO_INCREMENT checks would need the following change

diff --git a/storage/innobase/handler/handler0alter.cc b/storage/innobase/handler/handler0alter.cc
index a5ccb1957f3..9d778e2d39a 100644
--- a/storage/innobase/handler/handler0alter.cc
+++ b/storage/innobase/handler/handler0alter.cc
@@ -2455,10 +2455,15 @@ ha_innobase::check_if_supported_inplace_alter(
                        /* An AUTO_INCREMENT attribute can only
                        be added to an existing column by ALGORITHM=COPY,
                        but we can remove the attribute. */
-                       ut_ad((MTYP_TYPENR((*af)->unireg_check)
-                              != Field::NEXT_NUMBER)
-                             || (MTYP_TYPENR(f->unireg_check)
-                                 == Field::NEXT_NUMBER));
+                       if ((MTYP_TYPENR((*af)->unireg_check)
+                              == Field::NEXT_NUMBER)
+                             && (MTYP_TYPENR(f->unireg_check)
+                                 != Field::NEXT_NUMBER))
+                       {
+                               ha_alter_info->unsupported_reason = my_get_err_msg(
+                                       ER_ALTER_OPERATION_NOT_SUPPORTED_REASON_AUTOINC);
+                               DBUG_RETURN(HA_ALTER_INPLACE_NOT_SUPPORTED);
+                       }

With this change the main.auto_increment test for bug #14573, under
innodb, will pass without the 2 --error ER_DUP_ENTRY entries.

The function header comment was updated to reflect the MDEV-31086
changes.