MDEV-17721: Corrupted data dictionary after instant DROP COLUMN

dict_index_t::clear_instant_alter(): Correctly move all fields
corresponding to instantly dropped columns to the end of the array.
This fixes a regression that was introduced in
commit 5aaee374.

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

@@ -547,6 +547,12 @@ ALTER TABLE t1 ADD COLUMN f VARCHAR(8), ADD COLUMN dt DATETIME;
 ALTER TABLE t1 ADD COLUMN b BIT, DROP COLUMN f, ADD COLUMN t TIME FIRST;
 ALTER TABLE t1 ADD COLUMN ts2 TIMESTAMP;
 DROP TABLE t1;
+CREATE TABLE t1 (f1 INT, f2 INT, f3 INT) ENGINE=InnoDB ROW_FORMAT=REDUNDANT;
+INSERT INTO t1 VALUES (4,4,4);
+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
 (id INT PRIMARY KEY, c2 INT UNIQUE,
 c3 POINT NOT NULL DEFAULT ST_GeomFromText('POINT(3 4)'),
@@ -1040,6 +1046,12 @@ ALTER TABLE t1 ADD COLUMN f VARCHAR(8), ADD COLUMN dt DATETIME;
 ALTER TABLE t1 ADD COLUMN b BIT, DROP COLUMN f, ADD COLUMN t TIME FIRST;
 ALTER TABLE t1 ADD COLUMN ts2 TIMESTAMP;
 DROP TABLE t1;
+CREATE TABLE t1 (f1 INT, f2 INT, f3 INT) ENGINE=InnoDB ROW_FORMAT=COMPACT;
+INSERT INTO t1 VALUES (4,4,4);
+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
 (id INT PRIMARY KEY, c2 INT UNIQUE,
 c3 POINT NOT NULL DEFAULT ST_GeomFromText('POINT(3 4)'),
@@ -1533,10 +1545,16 @@ ALTER TABLE t1 ADD COLUMN f VARCHAR(8), ADD COLUMN dt DATETIME;
 ALTER TABLE t1 ADD COLUMN b BIT, DROP COLUMN f, ADD COLUMN t TIME FIRST;
 ALTER TABLE t1 ADD COLUMN ts2 TIMESTAMP;
 DROP TABLE t1;
+CREATE TABLE t1 (f1 INT, f2 INT, f3 INT) ENGINE=InnoDB ROW_FORMAT=DYNAMIC;
+INSERT INTO t1 VALUES (4,4,4);
+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;
 disconnect analyze;
 SELECT variable_value-@old_instant instants
 FROM information_schema.global_status
 WHERE variable_name = 'innodb_instant_alter_column';
 instants
-96
+102
 SET GLOBAL innodb_purge_rseg_truncate_frequency= @saved_frequency;

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

@@ -424,6 +424,13 @@ ALTER TABLE t1 ADD COLUMN b BIT, DROP COLUMN f, ADD COLUMN t TIME FIRST;
 ALTER TABLE t1 ADD COLUMN ts2 TIMESTAMP;
 DROP TABLE t1;
 
+eval CREATE TABLE t1 (f1 INT, f2 INT, f3 INT) $engine;
+INSERT INTO t1 VALUES (4,4,4);
+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;
+
 dec $format;
 }
 disconnect analyze;

storage/innobase/include/dict0mem.h

@@ -2153,9 +2153,11 @@ inline void dict_index_t::clear_instant_alter()
 	dict_field_t* end = &fields[n_fields];
 	while (end[-1].col->is_dropped()) end--;
 
-	for (dict_field_t* d = begin; d < end; d++) {
+	for (dict_field_t* d = begin; d < end; ) {
 		/* Move fields for dropped columns to the end. */
-		if (d->col->is_dropped()) {
+		if (!d->col->is_dropped()) {
+			d++;
+		} else {
 			if (d->col->is_nullable()) {
 				n_nullable--;
 			}