INSERT SELECT ON DUPLICATE KEY rejects NULL before a BEFORE UPDATE trigger can replace it

CREATE DATABASE trigger_test;

USE trigger_test;

SET SESSION sql_mode='STRICT_ALL_TABLES';

CREATE TABLE t1 (id INT PRIMARY KEY, b INT NOT NULL) ENGINE=InnoDB;

CREATE TRIGGER bu BEFORE UPDATE ON t1 FOR EACH ROW SET NEW.b=COALESCE(NEW.b,99);

INSERT INTO t1 VALUES (1,5);

INSERT INTO t1 (id,b) VALUES (1,2)

  ON DUPLICATE KEY UPDATE b=NULL;

SELECT 'control', b, b=99 AS oracle FROM t1 WHERE id=1;

UPDATE t1 SET b=5 WHERE id=1;

DELIMITER //

CREATE PROCEDURE run_case()

BEGIN

  DECLARE CONTINUE HANDLER FOR SQLEXCEPTION

    GET DIAGNOSTICS CONDITION 1 @errno=MYSQL_ERRNO, @message=MESSAGE_TEXT;

  SET @errno=0, @message='';

  INSERT INTO t1 (id,b)

    SELECT 1,2 UNION ALL SELECT 2,3

    ON DUPLICATE KEY UPDATE b=NULL;

END//

DELIMITER ;

CALL run_case();

SELECT 'candidate_error', @errno, @message;

SELECT 'candidate_rows', id,b FROM t1 ORDER BY id;

SELECT 'candidate_oracle',

       @errno=0

       AND COUNT(*)=2

       AND SUM(id=1 AND b=99)=1

       AND SUM(id=2 AND b=3)=1

FROM t1;

DROP DATABASE trigger_test;

SELECT 'cleanup', COUNT(*)=0

FROM information_schema.schemata

WHERE schema_name='trigger_test';

control             99    1

candidate_error     1048  Column 'b' cannot be null

candidate_rows      1     5

candidate_oracle    0

cleanup             1