MDEV-21148: Assertion index->n_core_fields + n_add >= index->n_fields

Revert part of commit 6cedb671
because it turns out to be theoretically impossible to parse a
ROW_FORMAT=COMPACT or ROW_FORMAT=DYNAMIC metadata record where
the variable-length fields in the PRIMARY KEY have been written
as nonempty strings.

mysql-test/suite/innodb/r/instant_alter,4k.rdiff

@@ -318,8 +318,8 @@
  FROM information_schema.global_status
  WHERE variable_name = 'innodb_instant_alter_column';
  instants
--187
-+189
+-193
++195
  SET GLOBAL innodb_purge_rseg_truncate_frequency= @saved_frequency;
  #
  # MDEV-18266: Changing an index comment unnecessarily rebuilds index

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

@@ -912,6 +912,19 @@ SELECT * FROM t1;
 b	a	vb
 NULL	barf	NULL
 DROP TABLE t1;
+CREATE TABLE t1 (a INT, b TEXT, PRIMARY KEY(b(9))) ENGINE=InnoDB ROW_FORMAT=REDUNDANT;
+ALTER TABLE t1 ADD COLUMN c TEXT FIRST;
+ALTER TABLE t1 ADD COLUMN d TEXT GENERATED ALWAYS AS (SUBSTR(b,1,499)) FIRST;
+DROP TABLE t1;
+CREATE TABLE t1(a CHAR(5), b INT, c CHAR(1), d CHAR(1), PRIMARY KEY(a,b))
+DEFAULT CHARACTER SET utf8 ENGINE=InnoDB ROW_FORMAT=REDUNDANT;
+INSERT INTO t1 SET a='fubar',b=42;
+ALTER TABLE t1 DROP c, DROP d, ALGORITHM=INSTANT;
+ALTER TABLE t1 ADD vb INT AS (b);
+SELECT * FROM t1;
+a	b	vb
+fubar	42	42
+DROP TABLE t1;
 CREATE TABLE t1
 (id INT PRIMARY KEY, c2 INT UNIQUE,
 c3 POINT NOT NULL DEFAULT ST_GeomFromText('POINT(3 4)'),
@@ -1769,6 +1782,19 @@ SELECT * FROM t1;
 b	a	vb
 NULL	barf	NULL
 DROP TABLE t1;
+CREATE TABLE t1 (a INT, b TEXT, PRIMARY KEY(b(9))) ENGINE=InnoDB ROW_FORMAT=COMPACT;
+ALTER TABLE t1 ADD COLUMN c TEXT FIRST;
+ALTER TABLE t1 ADD COLUMN d TEXT GENERATED ALWAYS AS (SUBSTR(b,1,499)) FIRST;
+DROP TABLE t1;
+CREATE TABLE t1(a CHAR(5), b INT, c CHAR(1), d CHAR(1), PRIMARY KEY(a,b))
+DEFAULT CHARACTER SET utf8 ENGINE=InnoDB ROW_FORMAT=COMPACT;
+INSERT INTO t1 SET a='fubar',b=42;
+ALTER TABLE t1 DROP c, DROP d, ALGORITHM=INSTANT;
+ALTER TABLE t1 ADD vb INT AS (b);
+SELECT * FROM t1;
+a	b	vb
+fubar	42	42
+DROP TABLE t1;
 CREATE TABLE t1
 (id INT PRIMARY KEY, c2 INT UNIQUE,
 c3 POINT NOT NULL DEFAULT ST_GeomFromText('POINT(3 4)'),
@@ -2626,12 +2652,25 @@ SELECT * FROM t1;
 b	a	vb
 NULL	barf	NULL
 DROP TABLE t1;
+CREATE TABLE t1 (a INT, b TEXT, PRIMARY KEY(b(9))) ENGINE=InnoDB ROW_FORMAT=DYNAMIC;
+ALTER TABLE t1 ADD COLUMN c TEXT FIRST;
+ALTER TABLE t1 ADD COLUMN d TEXT GENERATED ALWAYS AS (SUBSTR(b,1,499)) FIRST;
+DROP TABLE t1;
+CREATE TABLE t1(a CHAR(5), b INT, c CHAR(1), d CHAR(1), PRIMARY KEY(a,b))
+DEFAULT CHARACTER SET utf8 ENGINE=InnoDB ROW_FORMAT=DYNAMIC;
+INSERT INTO t1 SET a='fubar',b=42;
+ALTER TABLE t1 DROP c, DROP d, ALGORITHM=INSTANT;
+ALTER TABLE t1 ADD vb INT AS (b);
+SELECT * FROM t1;
+a	b	vb
+fubar	42	42
+DROP TABLE t1;
 disconnect analyze;
 SELECT variable_value-@old_instant instants
 FROM information_schema.global_status
 WHERE variable_name = 'innodb_instant_alter_column';
 instants
-187
+193
 SET GLOBAL innodb_purge_rseg_truncate_frequency= @saved_frequency;
 #
 # MDEV-18266: Changing an index comment unnecessarily rebuilds index

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

@@ -799,6 +799,21 @@ ALTER TABLE t1 ADD vb INT AS (b);
 SELECT * FROM t1;
 DROP TABLE t1;
 
+# MDEV-21148 Assertion failure index->n_core_fields + n_add >= index->n_fields
+eval CREATE TABLE t1 (a INT, b TEXT, PRIMARY KEY(b(9))) $engine;
+ALTER TABLE t1 ADD COLUMN c TEXT FIRST;
+ALTER TABLE t1 ADD COLUMN d TEXT GENERATED ALWAYS AS (SUBSTR(b,1,499)) FIRST;
+DROP TABLE t1;
+
+eval CREATE TABLE t1(a CHAR(5), b INT, c CHAR(1), d CHAR(1), PRIMARY KEY(a,b))
+DEFAULT CHARACTER SET utf8 $engine;
+INSERT INTO t1 SET a='fubar',b=42;
+ALTER TABLE t1 DROP c, DROP d, ALGORITHM=INSTANT;
+# this evicts and reloads the table definition until MDEV-17468 is fixed
+ALTER TABLE t1 ADD vb INT AS (b);
+SELECT * FROM t1;
+DROP TABLE t1;
+
 dec $format;
 let $redundant_4k= 0;
 }

storage/innobase/btr/btr0cur.cc

@@ -500,28 +500,32 @@ static dberr_t btr_cur_instant_init_low(dict_index_t* index, mtr_t* mtr)
 		we must duplicate some logic here. */
 		if (trx_id_offset) {
 		} else if (index->table->not_redundant()) {
-			/* PRIMARY KEY columns can never be NULL.
-			We can skip the null flag bitmap. */
-			const byte* lens = rec - (REC_N_NEW_EXTRA_BYTES + 1)
-				- index->n_core_null_bytes;
-			unsigned n_add = rec_get_n_add_field(lens);
-			ut_ad(index->n_core_fields + n_add >= index->n_fields);
-			lens -= n_add;
+			/* The PRIMARY KEY contains variable-length columns.
+			For the metadata record, variable-length columns are
+			always written with zero length. The DB_TRX_ID will
+			start right after any fixed-length columns. */
+
+			/* OK, before MDEV-21088 was fixed, for
+			variable-length encoded PRIMARY KEY column of
+			type CHAR, we wrote more than zero bytes. In
+			order to allow affected tables to be accessed,
+			it would be nice to determine the actual
+			length of each PRIMARY KEY column. However, to
+			be able to do that, we should determine the
+			size of the null-bit bitmap in the metadata
+			record. And we cannot know that before reading
+			the metadata BLOB, whose starting point we are
+			trying to find here. (Although the PRIMARY KEY
+			columns cannot be NULL, we would have to know
+			where the lengths of variable-length PRIMARY KEY
+			columns start.)
+
+			So, unfortunately we cannot help users who
+			were affected by MDEV-21088 on a ROW_FORMAT=COMPACT
+			or ROW_FORMAT=DYNAMIC table. */
 
 			for (uint i = index->n_uniq; i--; ) {
-				const dict_field_t& f = index->fields[i];
-				unsigned len = f.fixed_len;
-				if (!len) {
-					len = *lens--;
-					if ((len & 0x80)
-					    && DATA_BIG_COL(f.col)) {
-						/* 1exxxxxxx xxxxxxxx */
-						len &= 0x3f;
-						len <<= 8;
-						len |= *lens--;
-					}
-				}
-				trx_id_offset += len;
+				trx_id_offset += index->fields[i].fixed_len;
 			}
 		} else if (rec_get_1byte_offs_flag(rec)) {
 			trx_id_offset = rec_1_get_field_end_info(