Re: [libmicrohttpd] Clarification on receiving on one thread, replying on another

[Tom Cornell]

On Mon, Oct 24, 2016 at 11:16:05PM +0200, Christian Grothoff wrote:
> On 10/24/2016 10:13 PM, Tom Cornell wrote:
> > 
> > Generating a response requires forwarding another request off to 
> > a different web service, and then possibly waiting a while for that
> > service to respond to us, so we can respond to our client. I'm looking
> > at different ways to avoid tying up an MHD thread while waiting for
> > that response. External select is an obvious choice, but I was 
> > thinking I could get away with using internal select as well. Looks
> > like a "no". The work could be done on another thread and signaled
> > back to the MHD thread, but the latter would still be stuck waiting
> > for that signal, since it can't return from the access handler
> > until it has queued the response.
> 
> The latter is incorrect. You can totally return from the access handler
> without queueing a response. Simply return "MHD_YES".  MHD will call you
> again "immediately" in this case (next iteration of event loop). To
> avoid busy waiting, suspend the connection.  Then resume it whenever you
> have the response ready, wait to be called again, then give MHD the
> now-ready response.

Hmmm. Okay. I was already returning from the access handler without 
queuing a response (since that was happening on another thread), 
but I am not seeing this subsequent extra call to the access handler. 
Is that because I am suspending the connection before returning?
(I also don't see it after resuming the connection, but since at that
point I've queued a response in a different thread, presumably I'm 
well into the world of Undefined Behavior at that point.)

I will rework the code along the lines you suggest. Thanks for the 
quick reply!