Error during hash index scans can cause postgres halt!

recurred through deadlock.

client1:
create table test(a int);
create index id on test using hash(a);
insert into test values(1);
insert into test values(2);
set enable_seqscan=off;
begin;
update test set a=a+1 where a=1;

client2:
set enable_seqscan=off;
begin;
update test set a=a+1 where a=2;

client1:
update test set a=a+1 where a=2;

client2:
update test set a=a+1 where a=1;