Re: [libmicrohttpd] Check for client disconnection on suspended connection

[Robert D Kocisko]

Christian,

>  do whatever syscall you fancy to find out if the client disconnected.

Do you mean something like recv(fd, buffer, sizeof(buffer), MSG_PEEK | MSG_DONTWAIT) and check for it to return 0?  If so, once this returns 0 is it safe to assume that I can resume the connection and it will disconnect without invoking my access handler callback?

Thanks,

Bob

On Mon, Jul 29, 2019 at 12:08 PM Christian Grothoff <address@hidden> wrote:

Hi Robert,

You could grab the socket from the connection
(MHD_CONNECTION_INFO_CONNECTION_FD) and then do whatever syscall you
fancy to find out if the client disconnected.

However, note that there may not be a reliable way to do this, because
without a TCP FIN/CLOSE/RST from the client, the server may simply never
notice that the client is gone until it tries to send data to the client
and that then causes a RST.  This limitation also applies to the
resume/suspend approach you mapped, and fundamentally arises from the
design of TCP.

Manually setting socket options to demand TCP KEEPALIVE from the OS
should theoretically mitigate this, I don't know about the practical
implications though (= have not tried).

Happy hacking!

Christian

On 7/29/19 5:08 PM, Robert D Kocisko wrote:
> Hello all,
>
> Context: Single thread, external select with epoll.
>
> I have been using the suspend/resume flow control mechanism of
> libmicrohttpd to implement longpoll functionality, but now have a new
> use case where it is important to be able to check a particular
> connection while it is suspended to know whether the client has
> disconnected.  The only way I can imagine doing this is to resume the
> connection for some amount of time (how long?) and see whether the
> MHD_OPTION_NOTIFY_COMPLETED callback gets called and then suspend the
> connection again.  But I'm wondering if there's a simpler way to do this
> that doesn't involve resuming and waiting for a callback?
>
> Thanks,
> Bob