Corruption after instant ADD/DROP and shrinking the index tree

diff --git a/storage/innobase/btr/btr0btr.cc b/storage/innobase/btr/btr0btr.cc

--- a/storage/innobase/btr/btr0btr.cc

+++ b/storage/innobase/btr/btr0btr.cc

@@ -3453,7 +3453,8 @@ btr_lift_page_up(

 	/* btr_page_empty() is supposed to zero-initialize the field. */

 	ut_ad(!page_get_instant(father_block->frame));

-	if (page_level == 0 && index->is_instant()) {

+	if (index->is_instant()

+	    && father_block->page.id.page_no() == root_page_no) {

 		ut_ad(!father_page_zip);

 		byte* page_type = father_block->frame + FIL_PAGE_TYPE;

 		ut_ad(mach_read_from_2(page_type) == FIL_PAGE_INDEX);

diff --git a/mysql-test/suite/innodb/t/instant_alter_debug.test b/mysql-test/suite/innodb/t/instant_alter_debug.test

index e54623b9cbd..1e2dbcd98bb 100644

--- a/mysql-test/suite/innodb/t/instant_alter_debug.test

+++ b/mysql-test/suite/innodb/t/instant_alter_debug.test

@@ -267,4 +267,26 @@ SET DEBUG_SYNC = RESET;

 SELECT * FROM t1;

 DROP TABLE t1;

+CREATE TABLE t1 (a INT PRIMARY KEY) ENGINE=InnoDB;

+

+# Create a tree with 2 levels of node pointers.

+

+SET @old_limit = @@innodb_limit_optimistic_insert_debug;

+SET GLOBAL innodb_limit_optimistic_insert_debug = 2;

+INSERT INTO t1 VALUES (1),(5),(4),(3),(2);

+SET GLOBAL innodb_limit_optimistic_insert_debug = @old_limit;

+

+ALTER TABLE t1 ADD COLUMN b INT, ALGORITHM=INSTANT;

+

+SET @old_defragment = @@innodb_defragment;

+SET GLOBAL innodb_defragment = 1;

+OPTIMIZE TABLE t1;

+SET GLOBAL innodb_defragment = @old_defragment;

+

+# Exploit MDEV-17468 to force the table definition to be reloaded

+ALTER TABLE t1 ADD vb INT AS (b) VIRTUAL;

+CHECK TABLE t1;

+SELECT * FROM t1;

+DROP TABLE t1;

+

 SET GLOBAL innodb_purge_rseg_truncate_frequency = @save_frequency;

mysqld: /mariadb/10.3/storage/innobase/rem/rem0rec.cc:322: void rec_init_offsets_comp_ordinary(const rec_t *, const dict_index_t *, ulint *, ulint, const dict_col_t::def_t *, rec_leaf_format): Assertion `index->is_instant()' failed.

…

#7  0x000055ba28037d8d in rec_init_offsets_comp_ordinary (rec=0x7f4a2fb840ab "\200", index=0x7f49d4046838, offsets=<optimized out>, n_core=4, def_val=0x0, format=REC_LEAF_COLUMNS_ADDED) at /mariadb/10.3/storage/innobase/rem/rem0rec.cc:322

#8  0x000055ba2803466e in rec_init_offsets (rec=<optimized out>, index=<optimized out>, offsets=0x7f49d40a8e20, leaf=<optimized out>) at /mariadb/10.3/storage/innobase/rem/rem0rec.cc:603

#9  rec_get_offsets_func (rec=0x7f4a2fb840ab "\200", index=0x7f49d4046838, offsets=0x7f49d40a8e20, leaf=<optimized out>, n_fields=<optimized out>, file=<optimized out>, line=6706, heap=0x7f4a2f102640) at /mariadb/10.3/storage/innobase/rem/rem0rec.cc:864

#10 0x000055ba28171ae8 in btr_estimate_number_of_different_key_vals (index=0x7f49d4046838) at /mariadb/10.3/storage/innobase/btr/btr0cur.cc:6704

#11 0x000055ba28212ee8 in dict_stats_update_transient_for_index (index=0x7f49d4046838) at /mariadb/10.3/storage/innobase/dict/dict0stats.cc:890

#12 0x000055ba28213261 in dict_stats_update_transient (table=<optimized out>) at /mariadb/10.3/storage/innobase/dict/dict0stats.cc:948

#13 dict_stats_update (table=0x7f49d4045628, stats_upd_option=<optimized out>) at /mariadb/10.3/storage/innobase/dict/dict0stats.cc:3359

#14 0x000055ba27f3fae3 in ha_innobase::open (this=0x7f49d41becb0, name=0x7f49d409edf8 "./test/t1") at /mariadb/10.3/storage/innobase/handler/ha_innodb.cc:6154

#15 0x000055ba27d8180e in handler::ha_open (this=0x7f49d41becb0, table_arg=0x7f49d41d4438, name=0x7f49d409edf8 "./test/t1", mode=2, test_if_locked=18, mem_root=0x0, partitions_to_open=0x0) at /mariadb/10.3/sql/handler.cc:2760

#16 0x000055ba27c22d92 in open_table_from_share (thd=0x7f49d4000cf8, share=0x7f49d409e8c0, alias=<optimized out>, db_stat=33, prgflag=<optimized out>, ha_open_flags=0, outparam=0x7f49d41d4438, is_create_table=<optimized out>, partitions_to_open=0x0) at /mariadb/10.3/sql/table.cc:3506

#17 0x000055ba27ae0d04 in open_table (thd=<optimized out>, table_list=<optimized out>, ot_ctx=0x7f4a2f105250) at /mariadb/10.3/sql/sql_base.cc:1979

#18 0x000055ba27bfe223 in mysql_inplace_alter_table (thd=<optimized out>, table_list=0x7f49d4011de0, table=0x0, altered_table=<optimized out>, ha_alter_info=<optimized out>, inplace_supported=HA_ALTER_INPLACE_INSTANT, target_mdl_request=<optimized out>, alter_ctx=<optimized out>) at /mariadb/10.3/sql/sql_table.cc:7631

#19 mysql_alter_table (thd=<optimized out>, new_db=<optimized out>, new_name=<optimized out>, create_info=<optimized out>, table_list=0x7f49d4011de0, alter_info=<optimized out>, order_num=<optimized out>, order=<optimized out>, ignore=<optimized out>) at /mariadb/10.3/sql/sql_table.cc:9797

#20 0x000055ba27c5f320 in Sql_cmd_alter_table::execute (this=<optimized out>, thd=0x7f49d4000cf8) at /mariadb/10.3/sql/sql_alter.cc:500

#21 0x000055ba27b5068f in mysql_execute_command (thd=0x7f49d4000cf8) at /mariadb/10.3/sql/sql_parse.cc:6023

#22 0x000055ba27b4d183 in mysql_parse (thd=0x7f49d4000cf8, rawbuf=0x7f49d4011ce0 "ALTER TABLE t1 ADD vb INT AS (b) VIRTUAL", length=<optimized out>, parser_state=0x7f4a2f1076a0, is_com_multi=<optimized out>, is_next_command=<optimized out>) at /mariadb/10.3/sql/sql_parse.cc:7829