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;