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

[Christian Grothoff]

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

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