MDEV-18219 Assertion index->n_core_null_bytes <= ... after DROP COLUMN

rec_init_offsets(): Relax a debug assertion that is too strict
during the execution of btr_cur_instant_init_low().

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

@@ -146,3 +146,32 @@ check table t;
 Table	Op	Msg_type	Msg_text
 test.t	check	status	OK
 drop table t;
+#
+# MDEV-18219 Assertion `index->n_core_null_bytes <= ...' failed
+# in rec_init_offsets after instant DROP COLUMN
+#
+CREATE TABLE t1 (a INT, b INT NOT NULL) ENGINE=InnoDB;
+INSERT INTO t1 VALUES
+(0,9),(2,7),(3,1),(3,4),(8,4),(3,7),(6,1),(3,8),(1,2),(4,1),(0,8),(5,3),
+(1,3),(1,6),(2,1),(8,7),(6,0),(1,9),(9,4),(0,6),(9,3),(0,9),(9,4),(2,4),
+(2,7),(7,8),(8,2),(2,5),(6,1),(4,5),(5,3),(6,8),(4,9),(5,7),(7,5),(5,1),
+(8,8),(5,7),(3,8),(0,1),(8,4),(8,3),(9,7),(4,8),(1,1),(0,4),(2,6),(8,5),
+(8,8),(8,7),(6,7),(1,7),(9,6),(3,6),(1,9),(0,3),(5,3),(2,4),(0,6),(2,0),
+(6,5),(1,6),(2,4),(9,1),(3,0),(6,4),(1,3),(0,8),(3,5),(3,1),(8,9),(9,9),
+(7,9),(4,5),(2,2),(3,8),(0,8),(7,1),(2,0),(1,5),(7,3),(4,4),(3,9),(7,2),
+(6,2),(0,4),(2,0),(1,5),(5,7),(4,5),(3,7),(6,0),(2,1),(5,0),(1,0),(2,0),
+(8,4),(5,7),(3,5),(0,5),(7,6),(5,9),(1,2),(4,2),(8,5),(8,7),(2,8),(1,8),
+(4,3),(1,6),(7,8),(3,7),(4,6),(1,1),(3,0),(1,6),(2,0),(3,4),(4,8),(3,9),
+(8,0),(4,9),(4,0),(3,9),(6,4),(7,4),(5,8),(4,7),(7,3),(5,9),(2,3),(7,3),
+(0,4),(5,9),(9,8),(4,2),(3,6),(2,6),(1,8),(7,0),(0,0),(2,3),(1,2),(3,3),
+(2,7),(6,0),(9,0),(6,9),(4,6),(9,8),(0,7),(9,1),(9,6),(4,3),(7,7),(7,7),
+(4,1),(4,7),(7,3),(2,8),(5,8),(8,9),(3,9),(7,7),(0,8),(4,9),(3,2),(5,0),
+(1,7),(0,3),(2,9),(9,7),(7,5),(6,9),(8,5),(3,6),(1,1),(2,8),(7,9),(4,9),
+(6,6),(5,9),(5,3),(9,8),(3,3),(5,6),(0,9),(3,9),(7,9),(7,3),(5,2),(1,4),
+(4,4),(8,2),(2,2),(8,3),(9,1),(4,9),(9,8),(1,8),(1,8),(9,1),(1,1),(3,0),
+(4,6),(9,3),(3,3),(5,2),(0,1),(3,4),(3,2),(1,3),(4,4),(7,0),(4,6),(7,2),
+(4,5),(8,7),(7,8),(8,1),(3,5),(0,6),(3,5),(2,1),(4,4),(3,4),(2,1),(4,1);
+INSERT INTO t1 SELECT * FROM t1;
+ALTER TABLE t1 DROP a;
+ALTER TABLE t1 ADD vb INT AS (b) VIRTUAL;
+DROP TABLE t1;

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

@@ -154,3 +154,34 @@ alter table t modify c int without system versioning, algorithm=instant;
 set @@system_versioning_alter_history = error;
 check table t;
 drop table t;
+
+--echo #
+--echo # MDEV-18219 Assertion `index->n_core_null_bytes <= ...' failed
+--echo # in rec_init_offsets after instant DROP COLUMN
+--echo #
+CREATE TABLE t1 (a INT, b INT NOT NULL) ENGINE=InnoDB;
+INSERT INTO t1 VALUES
+(0,9),(2,7),(3,1),(3,4),(8,4),(3,7),(6,1),(3,8),(1,2),(4,1),(0,8),(5,3),
+(1,3),(1,6),(2,1),(8,7),(6,0),(1,9),(9,4),(0,6),(9,3),(0,9),(9,4),(2,4),
+(2,7),(7,8),(8,2),(2,5),(6,1),(4,5),(5,3),(6,8),(4,9),(5,7),(7,5),(5,1),
+(8,8),(5,7),(3,8),(0,1),(8,4),(8,3),(9,7),(4,8),(1,1),(0,4),(2,6),(8,5),
+(8,8),(8,7),(6,7),(1,7),(9,6),(3,6),(1,9),(0,3),(5,3),(2,4),(0,6),(2,0),
+(6,5),(1,6),(2,4),(9,1),(3,0),(6,4),(1,3),(0,8),(3,5),(3,1),(8,9),(9,9),
+(7,9),(4,5),(2,2),(3,8),(0,8),(7,1),(2,0),(1,5),(7,3),(4,4),(3,9),(7,2),
+(6,2),(0,4),(2,0),(1,5),(5,7),(4,5),(3,7),(6,0),(2,1),(5,0),(1,0),(2,0),
+(8,4),(5,7),(3,5),(0,5),(7,6),(5,9),(1,2),(4,2),(8,5),(8,7),(2,8),(1,8),
+(4,3),(1,6),(7,8),(3,7),(4,6),(1,1),(3,0),(1,6),(2,0),(3,4),(4,8),(3,9),
+(8,0),(4,9),(4,0),(3,9),(6,4),(7,4),(5,8),(4,7),(7,3),(5,9),(2,3),(7,3),
+(0,4),(5,9),(9,8),(4,2),(3,6),(2,6),(1,8),(7,0),(0,0),(2,3),(1,2),(3,3),
+(2,7),(6,0),(9,0),(6,9),(4,6),(9,8),(0,7),(9,1),(9,6),(4,3),(7,7),(7,7),
+(4,1),(4,7),(7,3),(2,8),(5,8),(8,9),(3,9),(7,7),(0,8),(4,9),(3,2),(5,0),
+(1,7),(0,3),(2,9),(9,7),(7,5),(6,9),(8,5),(3,6),(1,1),(2,8),(7,9),(4,9),
+(6,6),(5,9),(5,3),(9,8),(3,3),(5,6),(0,9),(3,9),(7,9),(7,3),(5,2),(1,4),
+(4,4),(8,2),(2,2),(8,3),(9,1),(4,9),(9,8),(1,8),(1,8),(9,1),(1,1),(3,0),
+(4,6),(9,3),(3,3),(5,2),(0,1),(3,4),(3,2),(1,3),(4,4),(7,0),(4,6),(7,2),
+(4,5),(8,7),(7,8),(8,1),(3,5),(0,6),(3,5),(2,1),(4,4),(3,4),(2,1),(4,1);
+INSERT INTO t1 SELECT * FROM t1;
+ALTER TABLE t1 DROP a;
+# Exploit MDEV-17468 to force the table definition to be reloaded
+ALTER TABLE t1 ADD vb INT AS (b) VIRTUAL;
+DROP TABLE t1;

storage/innobase/rem/rem0rec.cc

@@ -612,7 +612,13 @@ rec_init_offsets(
 	ulint	i	= 0;
 	ulint	offs;
 
-	ut_ad(index->n_core_null_bytes <= UT_BITS_IN_BYTES(index->n_nullable));
+	/* This assertion was relaxed for the btr_cur_open_at_index_side()
+	call in btr_cur_instant_init_low(). We cannot invoke
+	index->is_instant(), because the same assertion would fail there
+	until btr_cur_instant_init_low() has invoked
+	dict_table_t::deserialise_columns(). */
+	ut_ad(index->n_core_null_bytes <= UT_BITS_IN_BYTES(index->n_nullable)
+	      || (!leaf && index->n_core_fields != index->n_fields));
 	ut_d(offsets[2] = ulint(rec));
 	ut_d(offsets[3] = ulint(index));