Re: [libmicrohttpd] How to tell when incoming PUT has arrived in full

[Mike Durian]

Thanks for your response.  It does explain everything.  I just wasn't
sure if I could assume there would always be a final call with 0 bytes
of upload data.

mike

On Monday 17 August 2009 1:44:38 pm Christian Grothoff wrote:
> Dear Mike,
>
> The time when you should queue the response is exactly the second time you
> receive a call to your callback function with 0-bytes of additional upload
> data. The very first call (with 0-bytes of upload data) is there for you to
> initialize your internal data structures; this is followed by (possibly
> many) calls with upload data, but always with more than 0-bytes.  Once the
> client has uploaded all of the data, you get one last call with
> "upload_dataa_size==0" at which point you should queue the response.  Note
> that if you are using 'external select', you can choose to not queue a
> response (since your response may not yet be ready) and then the callback
> will be called each time the "MHD_run" function is invoked (until you queue
> a response).
>
> I hope this explains everything, if not, please don't hesitate to ask for
> further clarifications.
>
> Best,
>
> Christian
>
> > when all the data has arrived.
> >
> > Originally, I had planned on using the MHD_RequestCompletedCallback to
> > notify my application when all the data has arrived and would generate
> > the HTTP response in it.  However, it looks like
> > MHD_RequestCompletedCallback is called only after the HTTP response is
> > sent.
> >
> > My next attempt was to check the "Content-Length" header and use it to
> > determine when all the data has arrived.  While this works for
> > determining when all the data has been sent to MHD_AccessHandlerCallback,
> > I can't use it to tell when I should queue a response.  If I try to
> > enqueue a response as soon as all the data has been passed to
> > MHD_AccessHandlerCallback, MHD_queue_response fails.  I dug into the
> > MHD_Connection internals and logged the amount of data the has been
> > passed to MHD_AccessHandlerCallback and the connection state.  Here's
> > what I saw:
> >
> > state = 4, req_len = 0
> > state = 6, req_len = 1943
> > state = 6, req_len = 3886
> > state = 6, req_len = 5829
> > state = 6, req_len = 6694
> > state = 9, req_len = 6694
> >
> > The 6694 byte request, which is sent in a single TCP packet, is passed to
> > MHD_AccessHandlerCallback in a number of chunks.  Then
> > MHD_AccessHandlerCallback is called one final time with no new data.  If
> > I try to call MHD_queue_response as soon as all the data is available, it
> > fails because Connection is still in state MHD_CONNECTION_CONTINUE_SENT.
> > The final time MHD_AccessHandlerCallback is called the Connection is in
> > state MHD_CONNECTION_FOOTERS_RECEIVED and MHD_queue_response will
> > succeed.
> >
> > The problem is, MHD_queue_response is enforcing a pre-condition that the
> > application cannot verify.  Namely, the Connection state. I'm not sure
> > how to check this without breaking the API.
> >
> > Can you advise on how I can detect when the complete message has arrived
> > and it is safe to enqueue a response?
> >
> > Thanks,
> > mike