FOR EXECUTE STATEMENT cursors are unstable

[dyemanov]

We have announced 'cursor stability' for most types of queries in v3, but this improvement does not affect FOR EXECUTE STATEMENT cursors. Consider the following example:

create table t (id int, val int);
insert into t values (1, 1);
insert into t values (2, 2);
insert into t values (3, 3);
commit;

set term ^;

execute block returns (val int)
as begin
  for
    select val from t into :val
  do begin
    delete from t where id = 3;
  end
  
  suspend;
end^

execute block returns (val int)
as begin
  for
    execute statement 'select val from t' into :val
  do begin
    delete from t where id = 3;
  end

  suspend;
end^

set term ;^

rollback;

Here we have two kinds of loop iterating through all records in table T and returning the value of the last record. Inside the loop there's a DELETE statement that affects the record set. Case with a native FOR loop returns 3 but case with FOR EXECUTE STATEMENT loop returns 2, i.e. its cursor does not return the deleted record. This behaviour is contradictory to what is expected from a stable cursor.