MDEV-19030: Assertion failed in rec_init_offsets() after DROP COLUMN

This basically is a duplicate of MDEV-18219, proving that the
assertion was not relaxed correctly.

dict_index_t::in_instant_init: A debug flag that will only be set in
btr_cur_instant_init_low() in order to suppress the assertion failure
in rec_init_offsets() for that code path only.

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

@@ -175,3 +175,21 @@ INSERT INTO t1 SELECT * FROM t1;
 ALTER TABLE t1 DROP a;
 ALTER TABLE t1 ADD vb INT AS (b) VIRTUAL;
 DROP TABLE t1;
+#
+# MDEV-19030 Assertion index->n_core_null_bytes <= ... failed
+# in rec_init_offsets after instant DROP COLUMN
+#
+CREATE TABLE t1 (a INT, b INT NOT NULL DEFAULT 0) ENGINE=InnoDB;
+INSERT INTO t1 () VALUES (),(),(),();
+INSERT INTO t1 SELECT * FROM t1;
+INSERT INTO t1 SELECT * FROM t1;
+INSERT INTO t1 SELECT * FROM t1;
+INSERT INTO t1 SELECT * FROM t1;
+INSERT INTO t1 SELECT * FROM t1;
+ALTER TABLE t1 FORCE;
+INSERT INTO t1 SELECT * FROM t1;
+ALTER TABLE t1 DROP a, ADD a SMALLINT NOT NULL;
+INSERT INTO t1 SELECT * FROM t1;
+INSERT INTO t1 SELECT * FROM t1;
+ALTER TABLE t1 ADD vb INT AS (b) VIRTUAL;
+DROP TABLE t1;

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

@@ -185,3 +185,23 @@ 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;
+
+--echo #
+--echo # MDEV-19030 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 DEFAULT 0) ENGINE=InnoDB;
+INSERT INTO t1 () VALUES (),(),(),();
+INSERT INTO t1 SELECT * FROM t1;
+INSERT INTO t1 SELECT * FROM t1;
+INSERT INTO t1 SELECT * FROM t1;
+INSERT INTO t1 SELECT * FROM t1;
+INSERT INTO t1 SELECT * FROM t1;
+ALTER TABLE t1 FORCE;
+INSERT INTO t1 SELECT * FROM t1;
+ALTER TABLE t1 DROP a, ADD a SMALLINT NOT NULL;
+INSERT INTO t1 SELECT * FROM t1;
+INSERT INTO t1 SELECT * FROM t1;
+# 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/btr/btr0cur.cc

@@ -423,8 +423,12 @@ static dberr_t btr_cur_instant_init_low(dict_index_t* index, mtr_t* mtr)
 	}
 
 	btr_cur_t cur;
+	/* Relax the assertion in rec_init_offsets(). */
+	ut_ad(!index->in_instant_init);
+	ut_d(index->in_instant_init = true);
 	dberr_t err = btr_cur_open_at_index_side(true, index, BTR_SEARCH_LEAF,
 						 &cur, 0, mtr);
+	ut_d(index->in_instant_init = false);
 	if (err != DB_SUCCESS) {
 		index->table->corrupted = true;
 		return err;

storage/innobase/dict/dict0load.cc

@@ -426,6 +426,8 @@ dict_process_sys_indexes_rec(
 	const char*	err_msg;
 	byte*		buf;
 
+	ut_d(index->is_dummy = true);
+	ut_d(index->in_instant_init = false);
 	buf = static_cast<byte*>(mem_heap_alloc(heap, 8));
 
 	/* Parse the record, and get "dict_index_t" struct filled */

storage/innobase/include/dict0mem.h

@@ -964,6 +964,8 @@ struct dict_index_t {
 #ifdef UNIV_DEBUG
 	/** whether this is a dummy index object */
 	bool		is_dummy;
+	/** whether btr_cur_instant_init() is in progress */
+	bool		in_instant_init;
 	uint32_t	magic_n;/*!< magic number */
 /** Value of dict_index_t::magic_n */
 # define DICT_INDEX_MAGIC_N	76789786

storage/innobase/rem/rem0rec.cc

@@ -618,7 +618,7 @@ rec_init_offsets(
 	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));
+	      || index->in_instant_init);
 	ut_d(offsets[2] = ulint(rec));
 	ut_d(offsets[3] = ulint(index));