Re: [libmicrohttpd] race condition on 0.9.73

[José Bollo]

On Thu, 23 Sep 2021 19:57:23 +0300
Evgeny Grin <k2k@yandex.ru> wrote:

> On 23.09.2021 18:43, José Bollo wrote:
> >> For sure you can run MHD with "external poll" mode, but you must
> >> ensure that only single thread is calling MHD_run() at any given
> >> moment of time.  
> > 
> > Okay, that is a good hint, thanks. I'm going to first track
> > concurrent use of MHD_run. FYI, I tried the latest git and got the
> > same issue. 
> >> If you want to call several copies of MHD_run(), make sure that you
> >> have individual copies of MHD for each thread (start MHD by
> >> MHD_start_daemon() for every thread).  
> > 
> > I don't want that, because it implies differents port I suppose.  
> 
> Even if MHD is doing internal accept(), you still can use 
> MHD_OPTION_LISTENING_ADDRESS_REUSE parameter to bind to the same
> port. Or provide your own socket.
> However as I can see below, it's not applicable for your case as you
> are not using MHD to listen for new connections.
> 
> 
> >> Alternatively, you can use MHD with its own threading mode. MHD can
> >> run treads per connection or use thread pool.
> >>
> >> Can you give more details how you use MHD? Which flags are used?  
> > 
> >     flags = MHD_USE_EPOLL
> >                     | MHD_ALLOW_UPGRADE
> >                     | MHD_USE_TCP_FASTOPEN
> >                     | MHD_USE_NO_LISTEN_SOCKET
> >                     | MHD_ALLOW_SUSPEND_RESUME;  
> 
> Do you use suspend/resume of connections?

yes for websocket upgrade but it is not relevant in the case.
 
> >   
> >> Why do you use external threads instead of MHD internal treads?  
> > 
> > The program has a job scheduler that manages threads on its own. It
> > also serves other protocols that HTTP(S).  
> 
> It is still hard to see where is the problem. Actually responses are 
> thread-safe and shouldn't create any such kind of problems.
> 
> Could you share you access handler callback code?
> Where do you generate responses?
> Where do you queue responses?

I think that the clue about re-entering MHD_run is the good one. I'm
going to follow that path tomorrow and will let you know. The code is
public but you know, it is a pain to read someone else code and this
part is poorly commented.

Best regards
José