MDEV-24748 extern column check missing in btr_index_rec_validate()

In btr_index_rec_validate(), externally stored column
check is missing while matching the length of the field
with the length of the field data stored in record.
Fetch the length of the externally stored part and compare it
with the fixed field length.

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

@@ -3373,3 +3373,20 @@ c1	c2
 9	3
 DROP TABLE t1;
 DROP TABLE t2;
+#
+# MDEV-24748 Extern field check missing
+#	in btr_index_rec_validate()
+#
+CREATE TABLE t1 (pk INT, c1 char(255),
+c2 char(255), c3 char(255), c4 char(255),
+c5 char(255), c6 char(255), c7 char(255),
+c8 char(255), primary key (pk)
+) CHARACTER SET utf32 ENGINE=InnoDB;
+INSERT INTO t1 VALUES
+(1, 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'),
+(2, 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p');
+CHECK TABLE t1;
+Table	Op	Msg_type	Msg_text
+test.t1	check	status	OK
+ALTER TABLE t1 FORCE;
+DROP TABLE t1;

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

@@ -2628,3 +2628,21 @@ SELECT * FROM t2;
 
 DROP TABLE t1;
 DROP TABLE t2;
+
+--echo #
+--echo # MDEV-24748 Extern field check missing
+--echo #	in btr_index_rec_validate()
+--echo #
+CREATE TABLE t1 (pk INT, c1 char(255),
+c2 char(255), c3 char(255), c4 char(255),
+c5 char(255), c6 char(255), c7 char(255),
+c8 char(255), primary key (pk)
+) CHARACTER SET utf32 ENGINE=InnoDB;
+
+INSERT INTO t1 VALUES
+  (1, 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'),
+  (2, 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p');
+CHECK TABLE t1;
+ALTER TABLE t1 FORCE;
+# Cleanup
+DROP TABLE t1;

storage/innobase/btr/btr0btr.cc

@@ -4519,6 +4519,18 @@ btr_index_rec_validate(
 
 		rec_get_nth_field_offs(offsets, i, &len);
 
+		if (rec_offs_nth_extern(offsets, i)) {
+
+			const byte* data = rec_get_nth_field(
+				rec, offsets, i, &len);
+			len -= BTR_EXTERN_FIELD_REF_SIZE;
+			ulint extern_len = mach_read_from_4(
+				data + len + BTR_EXTERN_LEN + 4);
+			if (fixed_size == extern_len) {
+				continue;
+			}
+		}
+
 		/* Note that if fixed_size != 0, it equals the
 		length of a fixed-size column in the clustered index.
 		We should adjust it here.