MDEV-29010 Table cannot be loaded after instant ALTER

Reason:
======
- InnoDB fails to load the instant alter table metadata from
clustered index while loading the table definition.
The reason is that InnoDB metadata blob has the column length
exceeds maximum fixed length column size.

Fix:
===
- InnoDB should treat the long fixed length column as variable
length fields that needs external storage while initializing
the field map for instant alter operation

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

@@ -521,3 +521,12 @@ COUNT(*)
 1
 DROP TABLE t1;
 # End of 10.4 tests
+#
+#  MDEV-29010  Table cannot be loaded after instant ALTER
+#
+CREATE TABLE t1 (a CHAR(255), b INT,
+c INT as (b) VIRTUAL)ENGINE=InnoDB CHARACTER SET utf32;
+ALTER TABLE t1 DROP COLUMN a;
+ALTER TABLE t1 DROP COLUMN c;
+DROP TABLE t1;
+# End of 10.5 tests

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

@@ -553,3 +553,14 @@ SELECT COUNT(*) FROM t1;
 DROP TABLE t1;
 
 --echo # End of 10.4 tests
+
+--echo #
+--echo #  MDEV-29010  Table cannot be loaded after instant ALTER
+--echo #
+CREATE TABLE t1 (a CHAR(255), b INT,
+		 c INT as (b) VIRTUAL)ENGINE=InnoDB CHARACTER SET utf32;
+ALTER TABLE t1 DROP COLUMN a;
+ALTER TABLE t1 DROP COLUMN c;
+DROP TABLE t1;
+
+--echo # End of 10.5 tests

storage/innobase/handler/handler0alter.cc

@@ -181,6 +181,13 @@ inline void dict_table_t::init_instant(const dict_table_t& table)
 
 		auto fixed_len = dict_col_get_fixed_size(
 			f.col, not_redundant());
+
+		/* Long fixed length can be treated as variable
+		length fields that needs external storage */
+		if (fixed_len > DICT_MAX_FIXED_COL_LEN) {
+			fixed_len = 0;
+		}
+
 		field_map_it->set_dropped();
 		if (!f.col->is_nullable()) {
 			field_map_it->set_not_null();