MDEV-24512 Assertion failed in rec_is_metadata() in btr_discard_only_page_on_level()

btr_discard_only_page_on_level(): Attempt to read the MDEV-15562 metadata
record from the leaf page, not the root page. In the root, the leftmost
(in this case, the only) node pointer would look like a metadata record.

This corruption bug was introduced in
commit 0e5a4ac2 (MDEV-15562).
The scenario is rare: a column was dropped instantly or the order of
columns was changed instantly, and then the table became empty in such
a way that in the last step, the root page had one child page.
Normally, a non-leaf B-tree page would always contain at least 2 children.

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

@@ -327,7 +327,6 @@ FROM information_schema.global_status
 WHERE variable_name = 'innodb_instant_alter_column';
 instants
 22
-SET GLOBAL innodb_purge_rseg_truncate_frequency = @save_frequency;
 #
 # MDEV-21045 AddressSanitizer: use-after-poison in mem_heap_dup / row_log_table_get_pk_col
 #
@@ -370,3 +369,18 @@ b
 SET DEBUG_SYNC='RESET';
 disconnect con2;
 DROP TABLE t1;
+#
+# MDEV-24512 Assertion failed in rec_is_metadata()
+#            in btr_discard_only_page_on_level()
+#
+SET @save_limit= @@GLOBAL.innodb_limit_optimistic_insert_debug;
+SET GLOBAL innodb_limit_optimistic_insert_debug=2;
+CREATE TABLE t1 (c CHAR(1) UNIQUE) ENGINE=InnoDB;
+ALTER TABLE t1 ADD c2 INT NOT NULL DEFAULT 0 FIRST;
+INSERT INTO t1 (c) VALUES ('x'),('d'),('r'),('f'),('y'),('u'),('m'),('d');
+ERROR 23000: Duplicate entry 'd' for key 'c'
+SET GLOBAL innodb_limit_optimistic_insert_debug=@save_limit;
+SELECT * FROM t1;
+c2	c
+DROP TABLE t1;
+SET GLOBAL innodb_purge_rseg_truncate_frequency = @save_frequency;

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

@@ -361,8 +361,6 @@ SELECT variable_value-@old_instant instants
 FROM information_schema.global_status
 WHERE variable_name = 'innodb_instant_alter_column';
 
-SET GLOBAL innodb_purge_rseg_truncate_frequency = @save_frequency;
-
 --echo #
 --echo # MDEV-21045 AddressSanitizer: use-after-poison in mem_heap_dup / row_log_table_get_pk_col
 --echo #
@@ -414,3 +412,21 @@ SELECT * FROM t1;
 SET DEBUG_SYNC='RESET';
 --disconnect con2
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-24512 Assertion failed in rec_is_metadata()
+--echo #            in btr_discard_only_page_on_level()
+--echo #
+
+SET @save_limit= @@GLOBAL.innodb_limit_optimistic_insert_debug;
+SET GLOBAL innodb_limit_optimistic_insert_debug=2;
+CREATE TABLE t1 (c CHAR(1) UNIQUE) ENGINE=InnoDB;
+
+ALTER TABLE t1 ADD c2 INT NOT NULL DEFAULT 0 FIRST;
+--error ER_DUP_ENTRY
+INSERT INTO t1 (c) VALUES ('x'),('d'),('r'),('f'),('y'),('u'),('m'),('d');
+SET GLOBAL innodb_limit_optimistic_insert_debug=@save_limit;
+SELECT * FROM t1;
+DROP TABLE t1;
+
+SET GLOBAL innodb_purge_rseg_truncate_frequency = @save_frequency;

storage/innobase/btr/btr0btr.cc

@@ -4094,12 +4094,13 @@ btr_discard_only_page_on_level(
 	mtr_t*		mtr)	/*!< in: mtr */
 {
 	ulint		page_level = 0;
-	trx_id_t	max_trx_id;
 
 	ut_ad(!index->is_dummy);
 
 	/* Save the PAGE_MAX_TRX_ID from the leaf page. */
-	max_trx_id = page_get_max_trx_id(buf_block_get_frame(block));
+	const trx_id_t max_trx_id = page_get_max_trx_id(block->frame);
+	const rec_t* r = page_rec_get_next(page_get_infimum_rec(block->frame));
+	ut_ad(rec_is_metadata(r, *index) == index->is_instant());
 
 	while (block->page.id.page_no() != dict_index_get_page(index)) {
 		btr_cur_t	cursor;
@@ -4154,9 +4155,6 @@ btr_discard_only_page_on_level(
 	const rec_t* rec = NULL;
 	rec_offs* offsets = NULL;
 	if (index->table->instant) {
-		const rec_t* r = page_rec_get_next(page_get_infimum_rec(
-							   block->frame));
-		ut_ad(rec_is_metadata(r, *index) == index->is_instant());
 		if (rec_is_alter_metadata(r, *index)) {
 			heap = mem_heap_create(srv_page_size);
 			offsets = rec_get_offsets(r, index, NULL, true,