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The following bug has been logged on the website:

Bug reference:      16006
Logged by:          Vinay Banakar
Email address:      vinay.s.banakar@gmail.com
PostgreSQL version: 9.5.19
Operating system:   Ubuntu 16.04.6 LTS
Description:        

Update queries fail on a table having any policy with a function that takes
a whole-row var as argument:

I stumbled upon this after I had enabled RLS on a table and have a policy
that basically raises every row read to log at INFO level (a quick hack for
benchmarking).

Table:
create table usertable(key VARCHAR PRIMARY KEY,field0 text,field1
text,field2 text,field3 text,field4 text,field5 text,field6 text,field7
text,field8 text,field9 text,timestamp timestamp NOT NULL DEFAULT NOW());

# This policy is to raise a row to INFO log when an operation is made on
that.
ALTER TABLE usertable enable ROW LEVEL SECURITY;
ALTER TABLE usertable force ROW LEVEL SECURITY;
create function log_record(_tbl usertable) returns boolean language plpgsql
immutable as $$ begin raise info 'log: %', $1; return true; end; $$;
CREATE POLICY loggerPolicy ON usertable USING (log_record(usertable));

Now any update queries on this table fail. 

This was working correctly on 9.5.15.

Thanks to RhodiumToad from #postgresql  for verifying the bug.