[YSQL] Unique constraint violations don't populate constraint_name in stacked diagnostics

[pkj415]

Jira Link: DB-3097

Description

The "constraint" field is empty in the below test case:

NOTICE:  function f(text) does not exist, skipping


drop table if exists t cascade;
NOTICE:  table "t" does not exist, skipping


create table t(k serial primary key, v varchar(5) not null);
create unique index t_v_unq on t(v);


insert into t(v) values ('dog'), ('cat'), ('frog');


create function f(new_v in text)
  returns table(z text)
  security definer
  language plpgsql
as $body$
declare
  sqlstate_    text not null := '';
  constraint_  text not null := '';
  msg          text not null := '';
begin
  z := 'Inserting "'||new_v||'"'; return next;
  begin
    insert into t(v) values (new_v);
    z := 'No exception'; return next;
  exception
    when others then
      get stacked diagnostics
        sqlstate_ = returned_sqlstate,
        constraint_ = constraint_name,
        msg = message_text;
      if sqlstate_ = '23505' then
        z := 'unique_violation'; return next;
        z := 'constraint: '||constraint_; return next;
        z := 'message:    '||msg; return next;
      else
        z := 'others'; return next;
        z := 'sqlstate:   '||sqlstate_; return next;
        z := 'message:    '||msg; return next;
    end if;
  end;
end;
$body$;

select z from f('dog');
                                  z                                   
----------------------------------------------------------------------
 Inserting "dog"
 unique_violation
 constraint: 
 message:    duplicate key value violates unique constraint "t_v_unq"
(4 rows)```