To use: .setsockopt(SOL_SOCKET, 20, lambda sock: print(sock)). There's a
single underlying callback slot. For normal sockets, it serves as data
received callback, for listening sockets - connection arrived callback.
When lwIP creates a incoming connection socket of a listen socket, it
sets its recv callback to one which discards incoming data. We set
proper callback only in accept() call, when we allocate Python-level
socket where we can queue incoming data. So, in lwIP accept callback
be sure to set recv callback to one which tells lwIP to not discard
incoming data.
Per POSIX http://pubs.opengroup.org/onlinepubs/9699919799/functions/send.html :
"If space is not available at the sending socket to hold the message to be
transmitted, and the socket file descriptor does not have O_NONBLOCK set,
send() shall block until space is available. If space is not available at the
sending socket to hold the message to be transmitted, and the socket file
descriptor does have O_NONBLOCK set, send() shall fail [with EAGAIN]."
The code is based on Damien George's implementation for esp8266 port,
avoids use of global variables and associated re-entrancy issues, and
fixes returning stale data in some cases.
It can happen that a socket gets closed while the pbuf is not completely
drained by the application. It can also happen that a new pbuf comes in
via the recv callback, and then a "peer closed" event comes via the same
callback (pbuf=NULL) before the previous event has been handled. In both
cases the socket is closed but there is remaining data. This patch makes
sure such data is passed to the application.
Paul Sokolovsky
Damien George