MDEV-28919 Assertion `(((core_null) + 7) >> 3) == oindex.n_core_null_bytes ||...

MDEV-28919 Assertion `(((core_null) + 7) >> 3) == oindex.n_core_null_bytes || !not_redundant()' failed

- In case of discarded tablespace, InnoDB can't read the root page to
assign the n_core_null_bytes. Consecutive instant DDL fails because
of non-matching n_core_null_bytes.

mysql-test/suite/innodb/r/instant_alter_import.result

@@ -127,3 +127,29 @@ UNLOCK TABLES;
 ALTER TABLE t2 IMPORT TABLESPACE;
 ERROR HY000: Index for table 't2' is corrupt; try to repair it
 DROP TABLE t1, t2;
+#
+# MDEV-28919 Assertion `(((core_null) + 7) >> 3) ==
+# oindex.n_core_null_bytes || !not_redundant()' failed
+#
+call mtr.add_suppression(" InnoDB: Tablespace for table `test`.`t` is set as discarded");
+CREATE TABLE t (a INTEGER, b INTEGER as (a) VIRTUAL,
+c INTEGER)engine=innodb;
+ALTER TABLE t DISCARD TABLESPACE;
+ALTER TABLE t DROP COLUMN b, algorithm=instant;
+Warnings:
+Warning	1814	Tablespace has been discarded for table `t`
+ALTER TABLE t DROP COLUMN c, algorithm=instant;
+Warnings:
+Warning	1814	Tablespace has been discarded for table `t`
+CREATE TABLE t1(a INTEGER)ENGINE=InnoDB;
+INSERT INTO t1 VALUES(1);
+FLUSH TABLE t1 FOR EXPORT;
+unlock tables;
+ALTER TABLE t IMPORT tablespace;
+check table t;
+Table	Op	Msg_type	Msg_text
+test.t	check	status	OK
+select * from t;
+a
+1
+DROP TABLE t, t1;

mysql-test/suite/innodb/t/instant_alter_import.test

@@ -203,3 +203,27 @@ UNLOCK TABLES;
 ALTER TABLE t2 IMPORT TABLESPACE;
 
 DROP TABLE t1, t2;
+
+--echo #
+--echo # MDEV-28919 Assertion `(((core_null) + 7) >> 3) ==
+--echo # oindex.n_core_null_bytes || !not_redundant()' failed
+--echo #
+call mtr.add_suppression(" InnoDB: Tablespace for table `test`.`t` is set as discarded");
+CREATE TABLE t (a INTEGER, b INTEGER as (a) VIRTUAL,
+                c INTEGER)engine=innodb;
+ALTER TABLE t DISCARD TABLESPACE;
+# Table does reload
+ALTER TABLE t DROP COLUMN b, algorithm=instant;
+ALTER TABLE t DROP COLUMN c, algorithm=instant;
+
+CREATE TABLE t1(a INTEGER)ENGINE=InnoDB;
+INSERT INTO t1 VALUES(1);
+FLUSH TABLE t1 FOR EXPORT;
+--let $MYSQLD_DATADIR= `select @@datadir`
+--move_file $MYSQLD_DATADIR/test/t1.cfg $MYSQLD_DATADIR/test/t.cfg
+--copy_file $MYSQLD_DATADIR/test/t1.ibd $MYSQLD_DATADIR/test/t.ibd
+unlock tables;
+ALTER TABLE t IMPORT tablespace;
+check table t;
+select * from t;
+DROP TABLE t, t1;

storage/innobase/handler/handler0alter.cc

@@ -403,6 +403,15 @@ inline void dict_table_t::prepare_instant(const dict_table_t& old,
 				goto found_nullable;
 			}
 		}
+
+		/* In case of discarded tablespace, InnoDB can't
+		read the root page. So assign the null bytes based
+		on nullabled fields */
+		if (!oindex.table->space) {
+			oindex.n_core_null_bytes= UT_BITS_IN_BYTES(
+				unsigned(oindex.n_nullable));
+		}
+
 		/* The n_core_null_bytes only matters for
 		ROW_FORMAT=COMPACT and ROW_FORMAT=DYNAMIC tables. */
 		ut_ad(UT_BITS_IN_BYTES(core_null) == oindex.n_core_null_bytes