MDEV-32523 ASAN errors or assertion failure in row_merge_buf_add

innobase_build_col_map_add(): InnoDB wrongly copies the old field
record into default value even when new field length has been changed.

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

@@ -507,3 +507,14 @@ name	pos	mtype	prtype	len
 a	0	1	524303	110
 drop table t;
 # End of 10.2 tests
+#
+# MDEV-32523  ASAN errors or assertion failure in row_merge_buf_add
+#
+CREATE TABLE t (a INT, b VARCHAR(16)) ENGINE=InnoDB;
+INSERT INTO t (a) VALUES (1),(2);
+ALTER TABLE t MODIFY b VARCHAR(16) DEFAULT '0';
+ALTER IGNORE TABLE t MODIFY b VARCHAR(12289) NOT NULL;
+Warnings:
+Warning	1265	Data truncated for column 'b' at row 1
+Warning	1265	Data truncated for column 'b' at row 2
+DROP TABLE t;

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

@@ -373,3 +373,13 @@ on sc.table_id=st.table_id where st.name='test/t' and sc.name='a';
 drop table t;
 
 --echo # End of 10.2 tests
+
+--echo #
+--echo # MDEV-32523  ASAN errors or assertion failure in row_merge_buf_add
+--echo #
+CREATE TABLE t (a INT, b VARCHAR(16)) ENGINE=InnoDB;
+INSERT INTO t (a) VALUES (1),(2);
+ALTER TABLE t MODIFY b VARCHAR(16) DEFAULT '0';
+ALTER IGNORE TABLE t MODIFY b VARCHAR(12289) NOT NULL;
+# Cleanup
+DROP TABLE t;

storage/innobase/handler/handler0alter.cc

@@ -4558,7 +4558,11 @@ static void innobase_build_col_map_add(
 		return;
 	}
 
-	const Field& from = old_field ? *old_field : *field;
+	const Field& from =
+		old_field && (old_field->field_length
+			      == field->field_length)
+		? *old_field : *field;
+
 	ulint	size	= from.pack_length();
 
 	byte*	buf	= static_cast<byte*>(mem_heap_alloc(heap, size));