Re: [libmicrohttpd] unable to respond before reading entire incoming payload

[Ken Zangelin]

Hi, thanksfor the fast reply.


On 12/12/16 20:39, Evgeny Grin wrote:
> Hi,
>
> > I am provoking a request for 'Payload too large' for our MHD server, by
> > using curl sending a short request (~25 bytes) but adding a Content-Len
> > header of a lot more (2 megabytes).
> To clarify: you are not providing some large request body, you are just
> added Content-Length header?

That's it, I lie to my server (simulating a bug in a client).
I send a request with a 25-byte payload but the Content-Length header 
says it has 2 megabytes.

> > In our case the maximum payload we accept is a megabyte.
> >
> > So, in the mhd callback routines we detect the  Content-Len header (and
> > probably read the 25 bytes of payload as well, haven't checked) and when
> > we see that the Content-Len header value is beyond our allowed max size,
> > we want to return a response with a payload telling the user about the
> > error.
> >
> > We've tried to set the upload_data_size pointer to point to ZERO,
> > awaiting the last call to the callback, but it never comes.
> >
> > Probably due to that MHD still believes there is more data to read.
> >
> > Using MHD_OPTION_CONNECTION_TIMEOUT, at least the connection resets
> > instead of hangs, but ... we'd like to send that response ...
> >
> > Is there any way to accomplish what we want?
> If request has header that indicated 2megs size of request body, MHD
> will read whole request before sending response. So if request has only
> header indicating that body is 2MB, but body is absent or smaller - MHD
> will wait for for whole 2MB (indefinitely or until timeout).
Yes, this is what happens. I've set the timeout to 5 secs and it times out.

> If you request is real 2MB size and you don't want to process it, you
> can read whole 2MB, but ignore it and send response with error.
> Alternatively - you can abort connection, but if client is web-browser,
> it will repeat sending request.
Sure, if that was the case ...

Now, the problem I have is that I am not able to send a reply to the 
faulty client.
I detect the error and I try to reply but MHD is in a state that doesn't 
accept writes, still "reading" the incoming package.

So, my question is:

Is there anyway I can respond to this 'lying client'?
[ perhaps forcing MHD to stop reading on the socket, just send a reply 
and closing it ]

Thanks!

/KZ