unexpected X lock on Supremum in READ COMMITTED

CREATE TABLE `t2_text` (

  `a` int(11) NOT NULL,

  `b` int(11) DEFAULT NULL,

  `c` text DEFAULT NULL,

  PRIMARY KEY (`a`)

) ENGINE=InnoDB

create procedure foo1()

begin

     declare av int;

     declare ni int;

     declare mk int;

     declare u0,u1,d int;

     declare skip_max int;

     set av = ceil(rand()*100);

     set ni = ceil(rand()*10);

     set skip_max = 2;

     set d = av + mod(ceil(rand()*100), skip_max*ni);

     set u0 = av + mod(ceil(rand()*100), skip_max*ni);

     set u1 = av + mod(ceil(rand()*100), skip_max*ni);

     while (ni > 0) do

       set mk = mod(ceil(rand()*100),4);

       replace into t2_text values (av, av, repeat('a', mk*1024));

       set ni = ni - 1;

       set av = av + 1 + mod(ceil(rand()*100), skip_max);

     end while;  

     delete from t2_text where a = d;

     update t2_text set a=u1,b=u1 where a = u0;

end|

delimiter ;

--connection one

--send \

     select sleep(0.1);xa start '1'; call foo1(); xa end '1'; xa prepare '1'; select sleep(0.05); xa commit '1'; select sleep(0.1);

--connection two

--send \

     select sleep(0.1);xa start '2'; call foo1(); xa end '2'; xa prepare '2'; select sleep(0.05); xa commit '2'; select sleep(0.1);