MDEV-19916 Corruption after instant ADD/DROP and shrinking the tree

btr_lift_page_up(): Correct the incorrect condition.

page_validate(): Validate the page type.

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

@@ -236,4 +236,31 @@ a	b	c	d
 1	2	NULL	1
 2	3	4	1
 DROP TABLE t1;
+#
+# MDEV-19916 Corruption after instant ADD/DROP and shrinking the tree
+#
+CREATE TABLE t1 (a INT PRIMARY KEY) ENGINE=InnoDB;
+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;
+Table	Op	Msg_type	Msg_text
+test.t1	optimize	status	OK
+SET GLOBAL innodb_defragment = @old_defragment;
+ALTER TABLE t1 ADD vb INT AS (b) VIRTUAL;
+CHECK TABLE t1;
+Table	Op	Msg_type	Msg_text
+test.t1	check	status	OK
+SELECT * FROM t1;
+a	b	vb
+1	NULL	NULL
+2	NULL	NULL
+3	NULL	NULL
+4	NULL	NULL
+5	NULL	NULL
+DROP TABLE t1;
 SET GLOBAL innodb_purge_rseg_truncate_frequency = @save_frequency;

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

@@ -267,4 +267,29 @@ SET DEBUG_SYNC = RESET;
 SELECT * FROM t1;
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-19916 Corruption after instant ADD/DROP and shrinking the tree
+--echo #
+CREATE TABLE t1 (a INT PRIMARY KEY) ENGINE=InnoDB;
+
+# Create an index tree with 2 levels of node pointer pages.
+
+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;

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);