Concurrent DML causes data error in views with tuple dupulicates

[yugo-n]

Reported by huyajun in https://www.postgresql.org/message-id/tencent_FCAF11BCA5003FD16BDDFDDA5D6A19587809%40qq.com

Setup:
Create table t( a int);
Insert into t select 1 from generate_series(1,3);
create incremental materialized view s as select count(*) from t;

S1: begin;delete from t where ctid in (select ctid from t limit 1);
S2: begin;delete from t where ctid in (select ctid from t limit 1 offset 1);
S1: commit;
S2: commit;

[yugo-n]

It was resolved by always taking exclusive lock when a view is maintained for now. However, I would like to look for another way to reduce the lock. Anyway, we need some code cleaning for updating the patch.