Fix a bug in metadata record creation

dict_index_t::instant_metadata(): For instantly dropped NOT NULL
variable-length columns, store an empty string for the default value.

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

@@ -495,6 +495,13 @@ INSERT INTO t1 SET a=NULL;
 ROLLBACK;
 DELETE FROM t1;
 DROP TABLE t1;
+CREATE TABLE t1 (a INT PRIMARY KEY, t VARCHAR(33101) NOT NULL) ENGINE=InnoDB ROW_FORMAT=REDUNDANT;
+INSERT INTO t1 VALUES(347,'');
+ALTER TABLE t1 DROP COLUMN t, ALGORITHM=INSTANT;
+SELECT * FROM t1;
+a
+347
+DROP TABLE t1;
 CREATE TABLE t1
 (id INT PRIMARY KEY, c2 INT UNIQUE,
 c3 POINT NOT NULL DEFAULT ST_GeomFromText('POINT(3 4)'),
@@ -936,6 +943,13 @@ INSERT INTO t1 SET a=NULL;
 ROLLBACK;
 DELETE FROM t1;
 DROP TABLE t1;
+CREATE TABLE t1 (a INT PRIMARY KEY, t VARCHAR(33101) NOT NULL) ENGINE=InnoDB ROW_FORMAT=COMPACT;
+INSERT INTO t1 VALUES(347,'');
+ALTER TABLE t1 DROP COLUMN t, ALGORITHM=INSTANT;
+SELECT * FROM t1;
+a
+347
+DROP TABLE t1;
 CREATE TABLE t1
 (id INT PRIMARY KEY, c2 INT UNIQUE,
 c3 POINT NOT NULL DEFAULT ST_GeomFromText('POINT(3 4)'),
@@ -1377,10 +1391,17 @@ INSERT INTO t1 SET a=NULL;
 ROLLBACK;
 DELETE FROM t1;
 DROP TABLE t1;
+CREATE TABLE t1 (a INT PRIMARY KEY, t VARCHAR(33101) NOT NULL) ENGINE=InnoDB ROW_FORMAT=DYNAMIC;
+INSERT INTO t1 VALUES(347,'');
+ALTER TABLE t1 DROP COLUMN t, ALGORITHM=INSTANT;
+SELECT * FROM t1;
+a
+347
+DROP TABLE t1;
 disconnect analyze;
 SELECT variable_value-@old_instant instants
 FROM information_schema.global_status
 WHERE variable_name = 'innodb_instant_alter_column';
 instants
-63
+66
 SET GLOBAL innodb_purge_rseg_truncate_frequency= @saved_frequency;

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

@@ -371,6 +371,12 @@ BEGIN;INSERT INTO t1 SET a=NULL;ROLLBACK;
 DELETE FROM t1;
 DROP TABLE t1;
 
+eval CREATE TABLE t1 (a INT PRIMARY KEY, t VARCHAR(33101) NOT NULL) $engine;
+INSERT INTO t1 VALUES(347,'');
+ALTER TABLE t1 DROP COLUMN t, ALGORITHM=INSTANT;
+SELECT * FROM t1;
+DROP TABLE t1;
+
 dec $format;
 }
 disconnect analyze;

storage/innobase/handler/handler0alter.cc

@@ -4774,7 +4774,7 @@ dict_index_t::instant_metadata(const dtuple_t& row, mem_heap_t* heap) const
 				dfield_set_null(dfield);
 			} else {
 				dfield_set_data(dfield, field_ref_zero,
-						field->col->len);
+						field->fixed_len);
 			}
 			continue;
 		}