iobuf_left
n = read(fd, io->buf + io->wpos, iobuf_left(io));
n = SSL_read(ssl, io->buf + io->wpos, iobuf_left(io));
size_t iobuf_left(struct iobuf *);
n = read(fd, io->buf + io->wpos, iobuf_left(io));
n = tls_read(tls, io->buf + io->wpos, iobuf_left(io));
size_t iobuf_left(struct iobuf *);