MDEV-14062 Upgrade from 10.0 fails on assertion during ALTER TABLE

The fix in MDEV-14022 was incomplete. We must adjust some more code
for the fact that SYS_INDEXES records may lack the column MERGE_THRESHOLD
that was "instantly" added in MySQL 5.7 and MariaDB 10.2.2.

dict_boot(): Hard-core SYS_INDEXES.MERGE_THRESHOLD as DEFAULT NULL.

dict_index_t::instant_field_value(), rec_offs_make_valid():
Tolerate the missing SYS_INDEXES.MERGE_THRESHOLD column.

storage/innobase/dict/dict0boot.cc

@@ -440,6 +440,13 @@ dict_boot(void)
 	table->id = DICT_INDEXES_ID;
 
 	dict_table_add_system_columns(table, heap);
+	/* The column SYS_INDEXES.MERGE_THRESHOLD was "instantly"
+	added in MySQL 5.7 and MariaDB 10.2.2. Assign it DEFAULT NULL.
+	Because of file format compatibility, we must treat SYS_INDEXES
+	as a special case, relaxing some debug assertions
+	for DICT_INDEXES_ID. */
+	dict_table_get_nth_col(table, DICT_COL__SYS_INDEXES__MERGE_THRESHOLD)
+		->def_val.len = UNIV_SQL_NULL;
 	table->add_to_cache();
 	dict_sys->sys_indexes = table;
 	mem_heap_empty(heap);

storage/innobase/include/dict0boot.h

@@ -122,7 +122,7 @@ dict_is_sys_table(
 /* The ids for the basic system tables and their indexes */
 #define DICT_TABLES_ID		1
 #define DICT_COLUMNS_ID		2
-#define DICT_INDEXES_ID		3
+#define DICT_INDEXES_ID		dict_index_t::DICT_INDEXES_ID /* 3 */
 #define DICT_FIELDS_ID		4
 /* The following is a secondary index on SYS_TABLES */
 #define DICT_TABLE_IDS_ID	5

storage/innobase/include/dict0mem.h

@@ -906,6 +906,8 @@ struct dict_index_t{
 	/** magic value signalling that n_core_null_bytes was not
 	initialized yet */
 	static const unsigned NO_CORE_NULL_BYTES = 0xff;
+	/** The clustered index ID of the hard-coded SYS_INDEXES table. */
+	static const unsigned DICT_INDEXES_ID = 3;
 	unsigned	cached:1;/*!< TRUE if the index object is in the
 				dictionary cache */
 	unsigned	to_be_dropped:1;
@@ -1065,8 +1067,8 @@ struct dict_index_t{
 	@retval	NULL	if the default value is SQL NULL (len=UNIV_SQL_NULL) */
 	const byte* instant_field_value(uint n, ulint* len) const
 	{
-		DBUG_ASSERT(is_instant());
-		DBUG_ASSERT(n >= n_core_fields);
+		DBUG_ASSERT(is_instant() || id == DICT_INDEXES_ID);
+		DBUG_ASSERT(n + (id == DICT_INDEXES_ID) >= n_core_fields);
 		DBUG_ASSERT(n < n_fields);
 		return fields[n].col->instant_value(len);
 	}

storage/innobase/rem/rem0rec.cc

@@ -480,7 +480,8 @@ rec_offs_make_valid(
 	/* The infimum and supremum records carry 1 field. */
 	ut_ad(is_user_rec || n == 1);
 	ut_ad(is_user_rec || rec_offs_n_fields(offsets) == 1);
-	ut_ad(!is_user_rec || n >= index->n_core_fields
+	ut_ad(!is_user_rec
+	      || (n + (index->id == DICT_INDEXES_ID)) >= index->n_core_fields
 	      || n >= rec_offs_n_fields(offsets));
 	for (; n < rec_offs_n_fields(offsets); n++) {
 		ut_ad(leaf);