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

From: Evgeny Grin
Subject: Re: [libmicrohttpd] unable to respond before reading entire incoming payload
Date: Mon, 12 Dec 2016 22:39:53 +0300
User-agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.5.1


> 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?

> 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).

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.

Best Wishes,
Evgeny Grin

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