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Re: [libmicrohttpd] minor issue with epoll+external select+no_timeout

From: Christian Grothoff
Subject: Re: [libmicrohttpd] minor issue with epoll+external select+no_timeout
Date: Wed, 18 Jun 2014 21:12:46 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Icedove/24.5.0


Interesting workaround, but obviously this is not how it should be.

However, I am not sure I can see how this happens.  In daemon.c:3537
MHD_start_daemon_va() makes a call to 'setup_epoll_to_listen()' and in
that function in line daemon.c:3253 the listen FD is added to the epoll set.

For the combination of flags you state, I do not quite see how the
respective paths would not be taken (without triggering a jump to
'free_and_fail' which means MHD_start_daemon() would return NULL).

So to further understand, analyze and hopefully improve the situation,
could you say which version of MHD are you using, and --- especially if
you are using a release >= 0.9.35 --- could you please try to use gdb to
see how MHD runs through the MHD_start_daemon()-routine without
triggering the epoll call in line 3253?

Happy hacking!


On 06/18/2014 11:40 AM, Marcos Pindado Sebastian wrote:
> Hi,
> I want to share this little issue I had while implementing an external select 
> webserver.
> Case: daemon configured with MHD_USE_EPOLL_LINUX_ONLY plus EXTERNAL_SELECT.
> The typical example for the external select is:
> MHD_start_daemon (epoll)
> while (1)
>  {
>    MHD_get_fdset (...)
>    MHD_get_timeout(...)
>    select(fds, timeout)
>    MHD_run
> }
> Well, the example worked as far as I noticed that timeout had value 0 (not 
> null), meaning not waiting at all. When I changed it to NULL then the example 
> stopped working, because the program got blocked in the select statement not 
> being notified of incoming connections.
> The problem is, MHD_start_daemon, creates the epollFD before the 
> listen-socket is set up. So, get_fdset returns a well epoll-fd without any 
> listen socket attached just after start_daemon returns.
> The solution for this is very simple, call MHD_run one time after the daemon 
> is started and before the select-loop:
> MHD_start_daemon (epoll)
> MHD_run // => The listen socket is attached to the epollFD 
> while (1)
>  {select/run}
> Thanks for your work!
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