Window functions: multiple cursors to read filesort result

This is to keep track of a known task:

Computation of window function value involves moving multiple cursors through the result of filesort:

• frame bound #1 (Implemented in classes produced by get_frame_cursor())
• frame bound #2 (Implemented in classes produced by get_frame_cursor())
• current row ( has no explicit cursor, just calls read_record.read_record()).

right now, we are using "stub" code that only works when filesort result fits into an in-memory buffer:

bool clone_read_record(const READ_RECORD *src, READ_RECORD *dst)

{

  DBUG_ASSERT(src->table->sort.record_pointers);

  DBUG_ASSERT(src->read_record == rr_from_pointers);

  memcpy(dst, src, sizeof(READ_RECORD));

  return false;

}

This task is about being able to handle the case where filesort result didnt fit into the memory buffer and was partially dumped into a temp. file.