MDEV-33512 Corrupted table after IMPORT TABLESPACE and restart

In commit d74d9596 (MDEV-18543)
there was an error that would cause the hidden metadata record
to be deleted, and therefore cause the table to appear corrupted
when it is reloaded into the data dictionary cache.

PageConverter::update_records(): Do not delete the metadata record,
but do validate it.

RecIterator::open(): Make the API more similar to 10.6, to simplify
merges.

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

@@ -61,6 +61,7 @@ alter table t1 discard tablespace;
 flush tables t2 for export;
 unlock tables;
 alter table t1 import tablespace;
+# restart
 select * from t1;
 z
 42

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

@@ -79,6 +79,7 @@ flush tables t2 for export;
 unlock tables;
 
 alter table t1 import tablespace;
+--source include/restart_mysqld.inc
 select * from t1;
 
 drop table t2;

storage/innobase/row/row0import.cc

@@ -252,13 +252,13 @@ class RecIterator {
 	}
 
 	/** Position the cursor on the first user record. */
-	void	open(buf_block_t* block) UNIV_NOTHROW
+	rec_t* open(buf_block_t* block, const dict_index_t* index) noexcept
+		MY_ATTRIBUTE((warn_unused_result))
 	{
+		m_cur.index = const_cast<dict_index_t*>(index);
 		page_cur_set_before_first(block, &m_cur);
-
-		if (!end()) {
 		next();
-		}
+		return page_cur_get_rec(&m_cur);
 	}
 
 	/** Move to the next record. */
@@ -1848,12 +1848,39 @@ PageConverter::update_records(
 	bool	clust_index = m_index->m_srv_index == m_cluster_index;
 
 	/* This will also position the cursor on the first user record. */
+	rec_t* rec = m_rec_iter.open(block, m_index->m_srv_index);
+
+	if (!rec) {
+		return DB_CORRUPTION;
+	}
+
+	ulint deleted;
+
+	if (!page_has_prev(block->frame)
+	    && m_index->m_srv_index->is_instant()) {
+		/* Expect to find the hidden metadata record */
+		if (page_rec_is_supremum(rec)) {
+			return DB_CORRUPTION;
+		}
 
-	m_rec_iter.open(block);
+		const ulint info_bits = rec_get_info_bits(rec, comp);
+
+		if (!(info_bits & REC_INFO_MIN_REC_FLAG)) {
+			return DB_CORRUPTION;
+		}
+
+		if (!(info_bits & REC_INFO_DELETED_FLAG)
+		    != !m_index->m_srv_index->table->instant) {
+			return DB_CORRUPTION;
+		}
+
+		deleted = 0;
+		goto first;
+	}
 
 	while (!m_rec_iter.end()) {
-		rec_t*	rec = m_rec_iter.current();
-		ibool	deleted = rec_get_deleted_flag(rec, comp);
+		rec = m_rec_iter.current();
+		deleted = rec_get_deleted_flag(rec, comp);
 
 		/* For the clustered index we have to adjust the BLOB
 		reference and the system fields irrespective of the
@@ -1861,6 +1888,7 @@ PageConverter::update_records(
 		cluster records is required for purge to work later. */
 
 		if (deleted || clust_index) {
+first:
 			m_offsets = rec_get_offsets(
 				rec, m_index->m_srv_index, m_offsets,
 				m_index->m_srv_index->n_core_fields,
@@ -3158,7 +3186,7 @@ static size_t get_buf_size()
       ;
 }
 
-/* find, parse instant metadata, performing variaous checks,
+/* find, parse instant metadata, performing various checks,
 and apply it to dict_table_t
 @return DB_SUCCESS or some error */
 static dberr_t handle_instant_metadata(dict_table_t *table,