Re: [libmicrohttpd] HEAD and Keep-Alive Problems

[Christian Grothoff]

Hi!

The 'correct' answer is that you should not even have to treat HEAD/GET
any different in your application logic, by design MHD should just
notice it's a HEAD request and not send the body.

I just modified 'src/examples/demo.c' to support HEAD requests
(basically previously it checked for method matching 'GET' and
otherwise denied, so all I did was allow GET or HEAD).

I checked it, and it does keep the connection alive as it should,
including with both response_from_buffer and response_from_fd ---
as long as the client does request http/1.1 (or keep-alive).

I hope this helps!

Happy hacking!

Christian


On 06/24/2015 02:22 PM, Cristian KLEIN wrote:
> Hello all,
> 
> I am having some problems implementing HEAD requests in a way that keeps
> the connections alive. I am unsure how to properly implement HEAD
> requests, so as to send the client a correct "Content-Length" and not
> breaking keep-alive. I tried two options:
> 
> (1) using MHD_create_response_from_callback(), i.e., maintaining almost
> the same application code as for GET. For HEAD, microhttpd calls the
> callback with max=0, for which my callback simply
> returns MHD_CONTENT_READER_END_OF_STREAM.
> 
> The problem with this approach is that the HTTP connection is dropped
> after sending the HEAD headers. This is perfectly legal and clients can
> deal with it, but incurs a large performance overhead. I'm not sure if
> this is bad usage of microhttpd on my side or a bug in microhttpd.
> 
> (2) using MHD_create_response_from_buffer(). Since I sometimes return
> large files (1GB is not uncommon) it does not make sense to create a
> large, empty buffer to pass to MHD_create_response_from_buffer, so I
> have two options:
> 
> (2a) MHD_create_response_from_buffer(0, "", MHD_RESPMEM_PERSISTENT) ->
> the client gets an incorrect "Content-Length"
> 
> (2b) MHD_create_response_from_buffer(contentSize, "",
> MHD_RESPMEM_PERSISTENT) -> scary, looks like the largest buffer overflow
> ever. :) Valgrind does not seem to complain, so it seems that microhttpd
> does not actually access that buffer.
> 
> Tested with libmicrohttpd 0.9.33 and 0.9.42.
> 
> Can somebody point out an example HEAD implementation? Which of the
> above usages is recommended?
> 
> Thanks,
> Cristian