MDEV-11369: Implement accurate checks for the metadata record

Because changes of the FIL_PAGE_TYPE or PAGE_INSTANT in the root
page are not undo-logged, it is possible that the fields suggest
that instant ADD COLUMN is in effect, even though no metadata
record exists. If the fields are set, proceed to fetch the
metadata record. If the metadata record does not exist, return
success if !index->is_instant().

Also, check that the "infimum" and "supremum" records carry the
strings in the root page. In a later format that supports instant
DROP COLUMN, we will have to store more information in the root
page, so that index->n_core_null_bytes can be determined accurately.

storage/innobase/btr/btr0cur.cc

@@ -412,10 +412,15 @@ btr_cur_instant_init_low(dict_index_t* index, mtr_t* mtr)
 
 	ut_ad(index->n_core_null_bytes != dict_index_t::NO_CORE_NULL_BYTES);
 
-	if (!index->is_instant()) {
-		if (fil_page_get_type(root) == FIL_PAGE_INDEX) {
-			return DB_SUCCESS;
-		}
+	if (fil_page_get_type(root) == FIL_PAGE_INDEX) {
+		ut_ad(!index->is_instant());
+		return DB_SUCCESS;
+	}
+
+	/* In a later format, these fields in a FIL_PAGE_TYPE_INSTANT
+	root page could be repurposed for something else. */
+	if (memcmp(page_get_infimum_rec(root), "infimum", 8)
+	    || memcmp(page_get_supremum_rec(root), "supremum", 8)) {
 incompatible:
 		ib::error() << "Table " << index->table->name
 			<< " contains unrecognizable instant ALTER metadata";
@@ -442,6 +447,17 @@ btr_cur_instant_init_low(dict_index_t* index, mtr_t* mtr)
 
 	if (page_rec_is_supremum(rec)
 	    || !(info_bits & REC_INFO_MIN_REC_FLAG)) {
+		if (!index->is_instant()) {
+			/* The FIL_PAGE_TYPE_INSTANT and PAGE_INSTANT may be
+			assigned even if instant ADD COLUMN was not
+			committed. Changes to these page header fields are not
+			undo-logged, but changes to the hidden metadata record
+			are. If the server is killed and restarted, the page
+			header fields could remain set even though no metadata
+			record is present. */
+			return DB_SUCCESS;
+		}
+
 		ib::error() << "Table " << index->table->name
 			    << " is missing instant ALTER metadata";
 		index->table->corrupted = true;