Re: [libmicrohttpd] More useful MHD_quiesce_daemon in threaded mode

[Christian Grothoff]

On 10/27/2014 05:13 PM, Milan Straka wrote:
> Hi all,
> 
> I am from Charles University in Prague and I am using libmicrohttpd as
> HTTP server in several RESTful services provided by our department
> (linguistics, for example http://lindat.mff.cuni.cz/services/morphodita/).
> I use MHD_USE_THREAD_PER_CONNECTION | MHD_USE_POLL and need to continue
> using some threaded mode (the services response takes quite some time to
> compute).

Sure, makes sense.

> I am now facing an issue how to upgrade the running service seamlessly.
> I need to stop listening on the socket, wait reasonable time for the
> requests being served to finish, and stop.
> 
> Nevertheless, as stated in the documentation, the MHD_select_thread(s)
> still has the listening socket after MHD_quiesce_daemon and can accept
> one incoming connection at any time.

I don't quite see the documentation saying that.  The documentation says:

@deftypefun int MHD_quiesce_daemon (struct MHD_Daemon *daemon)
@cindex quiesce
Stop accepting connections from the listening socket.  Allows clients
to continue processing, but stops accepting new connections.

So after MHD_quiesce_daemon, MHD will no longer accept connections on
that socket. You're not supposed to 'close()' on it in multi-threaded
mode because the socket _may_ be still in a 'select()' set and if you
close() it we get some semi-defined behavior, but that does not mean
that MHD might still call 'accept()' on the socket.

It is also safe to pass the listen socket to another process or another
MHD instance to start accepting again.

> I believe it would be better to stop the MHD_select_thread(s) in the
> MHD_quesce_daemon. That would allow to return the listening FD which
> would no longer be referenced anywhere in the daemon, making sure that
> no more connections are accepted and allowing user to manipulate the FD
> as they see fit. This behaviour is actually the one I was expecting when
> discovering MHD_quiesce_daemon.

Well, the issue is that then the active connections would break. So
instead, we allow your process to pass on listening activity AND still
cleanly finish existing requests.

> I think the patch would be simple -- you could just kill the threads
> in the MHD_quiesce_daemon. Or, if you do not like that, you could
> make the MHD_select_thread(s) exit not only if daemon->shutdown, but
> also if daemon->socket_fd == MHD_INVALID_SOCKET, and use wpipe at the
> end of MHD_quiesce_daemon to wake them up and pthread_join them.
> (Opened wpipe would be needed, but there are already some (not really
> needed) tests for opened wpipe in MHD_USE_SELECT_INTERNALLY at the
> beginning of MHD_quiesce_daemon.)

The semantics you propose you can already get by calling 'dup()' on the
listen FD (which you can get from MHD_get_daemon_info()) and then
calling MHD_daemon_stop().  But that brutally closes ongoing
connections, which may not be ideal.

Happy hacking!

Christian

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