MDEV-17735 Assertion failure in row_parse_int() on first ADD/DROP COLUMN

row_ins_clust_index_entry_low(): Do not attempt to read an AUTO_INCREMENT
column value from a metadata record, because it does not make any sense.
Moreover, the field offset would be off by one in case the AUTO_INCREMENT
column is not part of the PRIMARY KEY, because the MDEV-15562 metadata
record would contain an extra field at index->first_user_field().

On MariaDB Server 10.3 after MDEV-11369, we would unnecessarily read
a dummy AUTO_INCREMENT value from the metadata record, but that value
would always be written as NULL or 0, so there is no problem.

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

@@ -523,6 +523,17 @@ SELECT * FROM t1;
 i	t
 1	NULL
 DROP TABLE t1;
+CREATE TABLE t1 (a INT AUTO_INCREMENT, b INT, KEY(a)) ENGINE=InnoDB ROW_FORMAT=REDUNDANT;
+INSERT INTO t1 SET a=NULL;
+ALTER TABLE t1 DROP COLUMN b;
+ALTER TABLE t1 ADD COLUMN c INT NOT NULL DEFAULT 42;
+INSERT INTO t1 SET a=NULL;
+UPDATE t1 SET a=a+2;
+SELECT * FROM t1;
+a	c
+3	42
+4	42
+DROP TABLE t1;
 CREATE TABLE t1
 (id INT PRIMARY KEY, c2 INT UNIQUE,
 c3 POINT NOT NULL DEFAULT ST_GeomFromText('POINT(3 4)'),
@@ -992,6 +1003,17 @@ SELECT * FROM t1;
 i	t
 1	NULL
 DROP TABLE t1;
+CREATE TABLE t1 (a INT AUTO_INCREMENT, b INT, KEY(a)) ENGINE=InnoDB ROW_FORMAT=COMPACT;
+INSERT INTO t1 SET a=NULL;
+ALTER TABLE t1 DROP COLUMN b;
+ALTER TABLE t1 ADD COLUMN c INT NOT NULL DEFAULT 42;
+INSERT INTO t1 SET a=NULL;
+UPDATE t1 SET a=a+2;
+SELECT * FROM t1;
+a	c
+3	42
+4	42
+DROP TABLE t1;
 CREATE TABLE t1
 (id INT PRIMARY KEY, c2 INT UNIQUE,
 c3 POINT NOT NULL DEFAULT ST_GeomFromText('POINT(3 4)'),
@@ -1461,10 +1483,21 @@ SELECT * FROM t1;
 i	t
 1	NULL
 DROP TABLE t1;
+CREATE TABLE t1 (a INT AUTO_INCREMENT, b INT, KEY(a)) ENGINE=InnoDB ROW_FORMAT=DYNAMIC;
+INSERT INTO t1 SET a=NULL;
+ALTER TABLE t1 DROP COLUMN b;
+ALTER TABLE t1 ADD COLUMN c INT NOT NULL DEFAULT 42;
+INSERT INTO t1 SET a=NULL;
+UPDATE t1 SET a=a+2;
+SELECT * FROM t1;
+a	c
+3	42
+4	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
-78
+84
 SET GLOBAL innodb_purge_rseg_truncate_frequency= @saved_frequency;

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

@@ -397,6 +397,19 @@ ALTER TABLE t1 ADD COLUMN t TEXT;
 SELECT * FROM t1;
 DROP TABLE t1;
 
+#
+# MDEV-17735 Assertion failure in row_parse_int() on first ADD/DROP COLUMN
+# when an AUTO_INCREMENT column is not in PRIMARY KEY
+#
+eval CREATE TABLE t1 (a INT AUTO_INCREMENT, b INT, KEY(a)) $engine;
+INSERT INTO t1 SET a=NULL;
+ALTER TABLE t1 DROP COLUMN b;
+ALTER TABLE t1 ADD COLUMN c INT NOT NULL DEFAULT 42;
+INSERT INTO t1 SET a=NULL;
+UPDATE t1 SET a=a+2;
+SELECT * FROM t1;
+DROP TABLE t1;
+
 dec $format;
 }
 disconnect analyze;

storage/innobase/row/row0ins.cc

@@ -2597,6 +2597,11 @@ row_ins_clust_index_entry_low(
 	} else {
 		index->set_modified(mtr);
 
+		if (UNIV_UNLIKELY(entry->is_metadata())) {
+			ut_ad(index->is_instant());
+			ut_ad(!dict_index_is_online_ddl(index));
+			ut_ad(mode == BTR_MODIFY_TREE);
+		} else {
 			if (mode == BTR_MODIFY_LEAF
 			    && dict_index_is_online_ddl(index)) {
 				mode = BTR_MODIFY_LEAF_ALREADY_S_LATCHED;
@@ -2608,9 +2613,9 @@ row_ins_clust_index_entry_low(
 				from the index entry to PAGE_ROOT_AUTO_INC. */
 				const dfield_t* dfield = dtuple_get_nth_field(
 					entry, ai - 1);
-			auto_inc = dfield_is_null(dfield)
-				? 0
-				: row_parse_int(static_cast<const byte*>(
+				if (!dfield_is_null(dfield)) {
+					auto_inc = row_parse_int(
+						static_cast<const byte*>(
 							dfield->data),
 						dfield->len,
 						dfield->type.mtype,
@@ -2618,6 +2623,8 @@ row_ins_clust_index_entry_low(
 						& DATA_UNSIGNED);
 				}
 			}
+		}
+	}
 
 	/* Note that we use PAGE_CUR_LE as the search mode, because then
 	the function will return in both low_match and up_match of the