MDEV-17721: Corrupted data dictionary after instant DROP COLUMN

dict_index_t::clear_instant_alter(): Correctly sort the index fields
to correspond to the columns of the table.

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

@@ -542,6 +542,11 @@ SELECT * FROM t1;
 b	i	v
 NULL	1	1
 DROP TABLE t1;
+CREATE TABLE t1 (ts TIMESTAMP) ENGINE=InnoDB ROW_FORMAT=REDUNDANT;
+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
 (id INT PRIMARY KEY, c2 INT UNIQUE,
 c3 POINT NOT NULL DEFAULT ST_GeomFromText('POINT(3 4)'),
@@ -1030,6 +1035,11 @@ SELECT * FROM t1;
 b	i	v
 NULL	1	1
 DROP TABLE t1;
+CREATE TABLE t1 (ts TIMESTAMP) ENGINE=InnoDB ROW_FORMAT=COMPACT;
+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
 (id INT PRIMARY KEY, c2 INT UNIQUE,
 c3 POINT NOT NULL DEFAULT ST_GeomFromText('POINT(3 4)'),
@@ -1518,10 +1528,15 @@ SELECT * FROM t1;
 b	i	v
 NULL	1	1
 DROP TABLE t1;
+CREATE TABLE t1 (ts TIMESTAMP) ENGINE=InnoDB ROW_FORMAT=DYNAMIC;
+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;
 disconnect analyze;
 SELECT variable_value-@old_instant instants
 FROM information_schema.global_status
 WHERE variable_name = 'innodb_instant_alter_column';
 instants
-87
+96
 SET GLOBAL innodb_purge_rseg_truncate_frequency= @saved_frequency;

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

@@ -417,6 +417,13 @@ ALTER TABLE t1 ADD COLUMN v INT AS (i) VIRTUAL;
 SELECT * FROM t1;
 DROP TABLE t1;
 
+# MDEV-17721 Corrupted data dictionary after instant DROP COLUMN
+eval CREATE TABLE t1 (ts TIMESTAMP) $engine;
+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;
+
 dec $format;
 }
 disconnect analyze;

storage/innobase/include/dict0mem.h

@@ -2149,39 +2149,26 @@ inline void dict_index_t::clear_instant_alter()
 		DBUG_ASSERT(!fields[i].col->is_nullable());
 	}
 #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 = &fields[first_user_field()]; d < end; d++) {
+	for (dict_field_t* d = begin; d < end; d++) {
 		/* Move fields for dropped columns to the end. */
-		while (d->col->is_dropped()) {
+		if (d->col->is_dropped()) {
 			if (d->col->is_nullable()) {
 				n_nullable--;
 			}
 
 			std::swap(*d, *--end);
-
-			if (d == end) {
-				goto done;
-			}
-		}
-
-		/* Ensure that the surviving fields are sorted by
-		ascending order of columns. */
-		const unsigned c = d->col->ind;
-
-		for (dict_field_t* s = d + 1; s < end; s++) {
-			if (s->col->ind < c) {
-				std::swap(*d, *s);
-				break;
-			}
 		}
 	}
 
-done:
 	DBUG_ASSERT(&fields[n_fields - table->n_dropped()] == end);
-
 	n_core_fields = n_fields = n_def = end - fields;
 	n_core_null_bytes = UT_BITS_IN_BYTES(n_nullable);
+	std::sort(begin, end, [](const dict_field_t& a, const dict_field_t& b)
+		  { return a.col->ind < b.col->ind; });
 	table->instant = NULL;
 }