MDEV-18218 Assertion `0' failed in btr_page_reorganize_low upon DROP COLUMN

dict_table_t::init_instant(): Correctly initialize the length of
variable-length instantly dropped columns.

row_ins_index_entry_set_vals(): For variable-length instantly dropped
columns, write 0 bytes of data. For dropped fixed-length NOT NULL
columns, write the fixed length of NUL bytes as data.

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

@@ -708,6 +708,13 @@ INSERT INTO t1 SET a=1;
 ALTER TABLE t1 DROP c;
 ALTER TABLE t1 DROP b, ADD v INT AS (a);
 DROP TABLE t1;
+CREATE TABLE t1 (pk INT PRIMARY KEY, i INT, b BLOB NOT NULL) ENGINE=InnoDB ROW_FORMAT=REDUNDANT;
+INSERT INTO t1 VALUES (1,10,REPEAT('foobar',2000));
+ALTER TABLE t1 DROP COLUMN b;
+INSERT INTO t1 VALUES (2,20);
+ALTER TABLE t1 ADD COLUMN vpk INT AS (pk);
+ALTER TABLE t1 DROP COLUMN i;
+DROP TABLE t1;
 CREATE TABLE t1
 (id INT PRIMARY KEY, c2 INT UNIQUE,
 c3 POINT NOT NULL DEFAULT ST_GeomFromText('POINT(3 4)'),
@@ -1362,6 +1369,13 @@ INSERT INTO t1 SET a=1;
 ALTER TABLE t1 DROP c;
 ALTER TABLE t1 DROP b, ADD v INT AS (a);
 DROP TABLE t1;
+CREATE TABLE t1 (pk INT PRIMARY KEY, i INT, b BLOB NOT NULL) ENGINE=InnoDB ROW_FORMAT=COMPACT;
+INSERT INTO t1 VALUES (1,10,REPEAT('foobar',2000));
+ALTER TABLE t1 DROP COLUMN b;
+INSERT INTO t1 VALUES (2,20);
+ALTER TABLE t1 ADD COLUMN vpk INT AS (pk);
+ALTER TABLE t1 DROP COLUMN i;
+DROP TABLE t1;
 CREATE TABLE t1
 (id INT PRIMARY KEY, c2 INT UNIQUE,
 c3 POINT NOT NULL DEFAULT ST_GeomFromText('POINT(3 4)'),
@@ -2016,10 +2030,17 @@ INSERT INTO t1 SET a=1;
 ALTER TABLE t1 DROP c;
 ALTER TABLE t1 DROP b, ADD v INT AS (a);
 DROP TABLE t1;
+CREATE TABLE t1 (pk INT PRIMARY KEY, i INT, b BLOB NOT NULL) ENGINE=InnoDB ROW_FORMAT=DYNAMIC;
+INSERT INTO t1 VALUES (1,10,REPEAT('foobar',2000));
+ALTER TABLE t1 DROP COLUMN b;
+INSERT INTO t1 VALUES (2,20);
+ALTER TABLE t1 ADD COLUMN vpk INT AS (pk);
+ALTER TABLE t1 DROP COLUMN i;
+DROP TABLE t1;
 disconnect analyze;
 SELECT variable_value-@old_instant instants
 FROM information_schema.global_status
 WHERE variable_name = 'innodb_instant_alter_column';
 instants
-149
+155
 SET GLOBAL innodb_purge_rseg_truncate_frequency= @saved_frequency;

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

@@ -599,6 +599,17 @@ ALTER TABLE t1 DROP c;
 ALTER TABLE t1 DROP b, ADD v INT AS (a);
 DROP TABLE t1;
 
+# MDEV-18218 Assertion `0' failed in btr_page_reorganize_low upon DROP COLUMN
+eval CREATE TABLE t1 (pk INT PRIMARY KEY, i INT, b BLOB NOT NULL) $engine;
+INSERT INTO t1 VALUES (1,10,REPEAT('foobar',2000));
+ALTER TABLE t1 DROP COLUMN b;
+INSERT INTO t1 VALUES (2,20);
+# this evicts and reloads the table definition until MDEV-17468 is fixed
+ALTER TABLE t1 ADD COLUMN vpk INT AS (pk);
+# this would load wrong metadata from the previous DROP COLUMN b, causing a crash
+ALTER TABLE t1 DROP COLUMN i;
+DROP TABLE t1;
+
 dec $format;
 }
 disconnect analyze;

storage/innobase/handler/handler0alter.cc

@@ -199,7 +199,7 @@ inline void dict_table_t::init_instant(const dict_table_t& table)
 		}
 		field_map_it->set_ind(fixed_len
 				      ? uint16_t(fixed_len + 1)
-				      : f.col->len > 255);
+				      : DATA_BIG_COL(f.col));
 		field_map_it++;
 		ut_ad(f.col >= table.instant->dropped);
 		ut_ad(f.col < table.instant->dropped

storage/innobase/row/row0ins.cc

@@ -3462,8 +3462,9 @@ row_ins_index_entry_set_vals(
 				field->type.prtype = DATA_BINARY_TYPE;
 			} else {
 				ut_ad(col->len <= sizeof field_ref_zero);
+				ut_ad(ind_field->fixed_len <= col->len);
 				dfield_set_data(field, field_ref_zero,
-						col->len);
+						ind_field->fixed_len);
 				field->type.prtype = DATA_NOT_NULL;
 			}