nxt_sendbuf_completion() has been renamed to nxt_port_buf_completion()
and moved to src/nxt_port_socket.c. nxt_sendbuf_completion0() has been
renamed to nxt_sendbuf_completion().
- Each sendmsg() transmits no more than port->max_size payload data.
- Longer buffers are fragmented and send using multiple sendmsg() calls.
- On receive side, buffers are connected in chain.
- Number of handler calls is the same as number of nxt_port_socket_write()
calls.
- nxt_buf_make_plain() function introduced to make single plain buffer from
the chain.
