the callback returns 0 to indicate that the buffer switch should not
occur yet, i.e. until the consumer has had a chance to read the
current set of ready sub-buffers. For the relay_buf_full() function
the callback returns 0 to indicate that the buffer switch should not
occur yet, i.e. until the consumer has had a chance to read the
current set of ready sub-buffers. For the relay_buf_full() function