MDEV-20479 assertion failure in dict_table_get_nth_col() after INSTANT DROP COLUMN

get_col_list_to_be_dropped() incorrectly returned uninteresting instantly
dropped column which was missing in a new dict_index_t

get_col_list_to_be_dropped(): rename to collect_columns_from_dropped_indexes
and stop return dropped columns

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

@@ -261,3 +261,11 @@ a
 1
 3
 DROP TABLE t1;
+#
+# MDEV-20479: assertion failure in dict_table_get_nth_col() after INSTANT DROP COLUMN
+#
+CREATE TABLE t1 (a INT PRIMARY KEY) ENGINE=InnoDB;
+ALTER TABLE t1 ADD COLUMN (b INT, c INT, d INT, e INT NOT NULL DEFAULT 0);
+ALTER TABLE t1 ADD UNIQUE INDEX(e);
+ALTER TABLE t1 DROP b, DROP c, DROP d, DROP e;
+DROP TABLE t1;

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

@@ -266,3 +266,14 @@ ROLLBACK;
 
 SELECT * FROM t1;
 DROP TABLE t1;
+
+
+--echo #
+--echo # MDEV-20479: assertion failure in dict_table_get_nth_col() after INSTANT DROP COLUMN
+--echo #
+
+CREATE TABLE t1 (a INT PRIMARY KEY) ENGINE=InnoDB;
+ALTER TABLE t1 ADD COLUMN (b INT, c INT, d INT, e INT NOT NULL DEFAULT 0);
+ALTER TABLE t1 ADD UNIQUE INDEX(e);
+ALTER TABLE t1 DROP b, DROP c, DROP d, DROP e;
+DROP TABLE t1;

storage/innobase/handler/handler0alter.cc

@@ -9935,8 +9935,7 @@ commit_cache_rebuild(
 /** Set of column numbers */
 typedef std::set<ulint, std::less<ulint>, ut_allocator<ulint> >	col_set;
 
-/** Store the column number of the columns in a list belonging
-to indexes which are not being dropped.
+/** Collect (not instantly dropped) columns from dropped indexes
 @param[in]	ctx		In-place ALTER TABLE context
 @param[in, out]	drop_col_list	list which will be set, containing columns
 				which is part of index being dropped
@@ -9945,7 +9944,7 @@ to indexes which are not being dropped.
 				being dropped */
 static
 void
-get_col_list_to_be_dropped(
+collect_columns_from_dropped_indexes(
 	const ha_innobase_inplace_ctx*	ctx,
 	col_set&			drop_col_list,
 	col_set&			drop_v_col_list)
@@ -9966,6 +9965,12 @@ get_col_list_to_be_dropped(
 
 			} else {
 				ulint	col_no = dict_col_get_no(idx_col);
+				if (ctx->col_map
+				    && ctx->col_map[col_no]
+					   == ULINT_UNDEFINED) {
+					// this column was instantly dropped
+					continue;
+				}
 				drop_col_list.insert(col_no);
 			}
 		}
@@ -10287,7 +10292,7 @@ commit_cache_norebuild(
 	/* Check if the column, part of an index to be dropped is part of any
 	other index which is not being dropped. If it so, then set the ord_part
 	of the column to 0. */
-	get_col_list_to_be_dropped(ctx, drop_list, v_drop_list);
+	collect_columns_from_dropped_indexes(ctx, drop_list, v_drop_list);
 
 	for (col_it = drop_list.begin(); col_it != drop_list.end(); ++col_it) {
 		if (!check_col_exists_in_indexes(ctx->new_table,