MDEV-23801 Assertion failed in btr_pcur_store_position()

btr_lift_page_up(): If the leaf page only contains a hidden metadata
record for MDEV-11369 instant ADD COLUMN, convert the table to the
canonical format like we are supposed to do whenever the table
becomes empty.

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

@@ -201,4 +201,36 @@ SELECT * FROM t1;
 c1	c2	c3
 1	0	10
 DROP TABLE t1;
+#
+# MDEV-23801 Assertion index->table->instant... failed
+#            in btr_pcur_store_position()
+#
+SET @save_frequency = @@GLOBAL.innodb_purge_rseg_truncate_frequency;
+SET GLOBAL innodb_purge_rseg_truncate_frequency = 1;
+CREATE TABLE t (
+pk int auto_increment,
+c01 char(255) not null default repeat('a',255),
+c02 char(255) default repeat('a',255),
+c03 char(255) default repeat('a',255),
+c04 char(255) default repeat('a',255),
+c05 char(255) not null default repeat('a',255),
+c06 char(255) default repeat('a',255),
+c07 char(255) default repeat('a',255),
+c08 char(255) not null default repeat('a',255),
+c09 char(255) default repeat('a',255),
+c10 char(255) default repeat('a',255),
+c11 char(255) default repeat('a',255),
+c12 char(255) not null default repeat('a',255),
+primary key (pk)
+) ENGINE=InnoDB CHARACTER SET ucs2;
+INSERT INTO t () VALUES ();
+ALTER TABLE t ADD c INT;
+BEGIN;
+INSERT INTO t () VALUES (),();
+ROLLBACK;
+DELETE FROM t;
+InnoDB		0 transactions not purged
+SET GLOBAL innodb_purge_rseg_truncate_frequency = @save_frequency;
+CREATE TABLE tt ENGINE=InnoDB AS SELECT c FROM t;
+DROP TABLE t, tt;
 # End of 10.3 tests

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

@@ -204,4 +204,45 @@ ALTER TABLE t1 IMPORT TABLESPACE;
 SHOW CREATE TABLE t1;
 SELECT * FROM t1;
 DROP TABLE t1;
+
+
+--echo #
+--echo # MDEV-23801 Assertion index->table->instant... failed
+--echo #            in btr_pcur_store_position()
+--echo #
+
+SET @save_frequency = @@GLOBAL.innodb_purge_rseg_truncate_frequency;
+SET GLOBAL innodb_purge_rseg_truncate_frequency = 1;
+
+CREATE TABLE t (
+  pk int auto_increment,
+  c01 char(255) not null default repeat('a',255),
+  c02 char(255) default repeat('a',255),
+  c03 char(255) default repeat('a',255),
+  c04 char(255) default repeat('a',255),
+  c05 char(255) not null default repeat('a',255),
+  c06 char(255) default repeat('a',255),
+  c07 char(255) default repeat('a',255),
+  c08 char(255) not null default repeat('a',255),
+  c09 char(255) default repeat('a',255),
+  c10 char(255) default repeat('a',255),
+  c11 char(255) default repeat('a',255),
+  c12 char(255) not null default repeat('a',255),
+  primary key (pk)
+) ENGINE=InnoDB CHARACTER SET ucs2;
+
+INSERT INTO t () VALUES ();
+ALTER TABLE t ADD c INT;
+
+BEGIN;
+INSERT INTO t () VALUES (),();
+ROLLBACK;
+DELETE FROM t;
+
+--source include/wait_all_purged.inc
+SET GLOBAL innodb_purge_rseg_truncate_frequency = @save_frequency;
+
+CREATE TABLE tt ENGINE=InnoDB AS SELECT c FROM t;
+DROP TABLE t, tt;
+
 --echo # End of 10.3 tests

storage/innobase/btr/btr0btr.cc

@@ -3331,6 +3331,7 @@ btr_lift_page_up(
 
 	ut_ad(!page_has_siblings(page));
 	ut_ad(mtr_memo_contains(mtr, block, MTR_MEMO_PAGE_X_FIX));
+	ut_ad(!page_is_empty(page));
 
 	page_level = btr_page_get_level(page);
 	root_page_no = dict_index_get_page(index);
@@ -3419,6 +3420,16 @@ btr_lift_page_up(
 	if (index->is_instant()
 	    && father_block->page.id.page_no() == root_page_no) {
 		ut_ad(!father_page_zip);
+		if (page_is_leaf(page)) {
+			ut_d(const rec_t* rec
+			     = page_rec_get_next(page_get_infimum_rec(page)));
+			ut_ad(rec_is_metadata(rec, index));
+			if (page_get_n_recs(page) == 1) {
+				index->remove_instant();
+				goto copied;
+			}
+		}
+
 		byte* page_type = father_block->frame + FIL_PAGE_TYPE;
 		ut_ad(mach_read_from_2(page_type) == FIL_PAGE_INDEX);
 		mlog_write_ulint(page_type, FIL_PAGE_TYPE_INSTANT,
@@ -3427,8 +3438,6 @@ btr_lift_page_up(
 				 index->n_core_fields, mtr);
 	}
 
-	page_level++;
-
 	/* Copy the records to the father page one by one. */
 	if (0
 #ifdef UNIV_ZIP_COPY
@@ -3460,6 +3469,7 @@ btr_lift_page_up(
 		}
 	}
 
+copied:
 	if (!dict_table_is_locking_disabled(index->table)) {
 		/* Free predicate page locks on the block */
 		if (dict_index_is_spatial(index)) {
@@ -3471,6 +3481,8 @@ btr_lift_page_up(
 		lock_update_copy_and_discard(father_block, block);
 	}
 
+	page_level++;
+
 	/* Go upward to root page, decrementing levels by one. */
 	for (i = lift_father_up ? 1 : 0; i < n_blocks; i++, page_level++) {
 		page_t*		page	= buf_block_get_frame(blocks[i]);