Too long value in the Duplicate entry message with online alter and utf8

--source include/have_innodb.inc

--source include/have_debug.inc

--source include/have_debug_sync.inc

--source include/have_sequence.inc

CREATE TABLE t1(f1 INT NOT NULL, f2 CHAR(200), f3 CHAR(200))

  ENGINE=InnoDB CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci;

INSERT INTO t1 VALUES(3, "innodb", "alter log");

# InnoDB fails with DUPLICATE KEY error in commit phase

SET DEBUG_SYNC="inplace_after_index_build SIGNAL dml_start WAIT_FOR dml_commit";

send ALTER TABLE t1 ADD PRIMARY KEY(f3(10)), ADD UNIQUE KEY(f2(10));

CONNECT(con1,localhost,root,,,);

SET DEBUG_SYNC="now WAIT_FOR dml_start";

BEGIN;

INSERT INTO t1 VALUES(1, repeat('b', 100), repeat('c', 100));

INSERT INTO t1 VALUES(2, repeat('b', 100), repeat('a', 100));

COMMIT;

SET DEBUG_SYNC="now SIGNAL dml_commit";

connection default;

--error ER_DUP_ENTRY

reap;

DROP TABLE t1;

SET DEBUG_SYNC=reset;

TEST                                      RESULT   TIME (ms) or COMMENT

--------------------------------------------------------------------------

worker[01] Using MTR_BUILD_THREAD 300, with reserved ports 19000..19029

CREATE TABLE t1(f1 INT NOT NULL, f2 CHAR(200), f3 CHAR(200))

ENGINE=InnoDB CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci;

INSERT INTO t1 VALUES(3, "innodb", "alter log");

SET DEBUG_SYNC="inplace_after_index_build SIGNAL dml_start WAIT_FOR dml_commit";

ALTER TABLE t1 ADD PRIMARY KEY(f3(10)), ADD UNIQUE KEY(f2(10));

CONNECT con1,localhost,root,,,;

SET DEBUG_SYNC="now WAIT_FOR dml_start";

BEGIN;

INSERT INTO t1 VALUES(1, repeat('b', 100), repeat('c', 100));

INSERT INTO t1 VALUES(2, repeat('b', 100), repeat('a', 100));

COMMIT;

SET DEBUG_SYNC="now SIGNAL dml_commit";

connection default;

ERROR 23000: Duplicate entry 'bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb' for key 'f2'

DROP TABLE t1;

SET DEBUG_SYNC=reset;

ERROR 23000: Duplicate entry 'bbbbbbbbbb' for key 'f2'

    field_unpack(to, key_part->field, table->record[0], key_part->length,

                 MY_TEST(key_part->key_part_flag & HA_PART_KEY_SEG));