MDEV-17721: Corrupted data dictionary after instant DROP COLUMN

dict_index_t::clear_instant_alter(): Remove a loop that became
redundant in commit ae2004c6
and caused corruption of n_nullable when dropping the last column
which was not declared NOT NULL.

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

@@ -553,6 +553,11 @@ ALTER TABLE t1 DROP f1, DROP f2, ADD f4 INT, ADD f5 INT;
 DELETE FROM t1;
 ALTER TABLE t1 DROP COLUMN f4;
 DROP TABLE t1;
+CREATE TABLE t1 (f1 INT, f2 INT, f3 INT) ENGINE=InnoDB ROW_FORMAT=REDUNDANT;
+ALTER TABLE t1 DROP f2, ADD COLUMN f4 INT;
+ALTER TABLE t1 DROP f4;
+ALTER TABLE t1 DROP f1;
+DROP TABLE t1;
 CREATE TABLE t1
 (id INT PRIMARY KEY, c2 INT UNIQUE,
 c3 POINT NOT NULL DEFAULT ST_GeomFromText('POINT(3 4)'),
@@ -1052,6 +1057,11 @@ ALTER TABLE t1 DROP f1, DROP f2, ADD f4 INT, ADD f5 INT;
 DELETE FROM t1;
 ALTER TABLE t1 DROP COLUMN f4;
 DROP TABLE t1;
+CREATE TABLE t1 (f1 INT, f2 INT, f3 INT) ENGINE=InnoDB ROW_FORMAT=COMPACT;
+ALTER TABLE t1 DROP f2, ADD COLUMN f4 INT;
+ALTER TABLE t1 DROP f4;
+ALTER TABLE t1 DROP f1;
+DROP TABLE t1;
 CREATE TABLE t1
 (id INT PRIMARY KEY, c2 INT UNIQUE,
 c3 POINT NOT NULL DEFAULT ST_GeomFromText('POINT(3 4)'),
@@ -1551,10 +1561,15 @@ ALTER TABLE t1 DROP f1, DROP f2, ADD f4 INT, ADD f5 INT;
 DELETE FROM t1;
 ALTER TABLE t1 DROP COLUMN f4;
 DROP TABLE t1;
+CREATE TABLE t1 (f1 INT, f2 INT, f3 INT) ENGINE=InnoDB ROW_FORMAT=DYNAMIC;
+ALTER TABLE t1 DROP f2, ADD COLUMN f4 INT;
+ALTER TABLE t1 DROP f4;
+ALTER TABLE t1 DROP f1;
+DROP TABLE t1;
 disconnect analyze;
 SELECT variable_value-@old_instant instants
 FROM information_schema.global_status
 WHERE variable_name = 'innodb_instant_alter_column';
 instants
-102
+111
 SET GLOBAL innodb_purge_rseg_truncate_frequency= @saved_frequency;

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

@@ -431,6 +431,12 @@ DELETE FROM t1;
 ALTER TABLE t1 DROP COLUMN f4;
 DROP TABLE t1;
 
+eval CREATE TABLE t1 (f1 INT, f2 INT, f3 INT) $engine;
+ALTER TABLE t1 DROP f2, ADD COLUMN f4 INT;
+ALTER TABLE t1 DROP f4;
+ALTER TABLE t1 DROP f1;
+DROP TABLE t1;
+
 dec $format;
 }
 disconnect analyze;

storage/innobase/include/dict0mem.h

@@ -2151,7 +2151,6 @@ inline void dict_index_t::clear_instant_alter()
 #endif
 	dict_field_t* const begin = &fields[first_user_field()];
 	dict_field_t* end = &fields[n_fields];
-	while (end[-1].col->is_dropped()) end--;
 
 	for (dict_field_t* d = begin; d < end; ) {
 		/* Move fields for dropped columns to the end. */