[libmicrohttpd] Developing async responses

[Marcos Pindado Sebastian]

Hi, thanks for this fantastic library.
We have used in the past microhttpd with internal select and thread per 
connection successfully.
This time however, we are interested in developing a way to produce 
asynchronous responses, because we need to use another asynchronous
Framework. Besides, we do not want the thread per connection this time to avoid 
spawning many threads.

We have implemented several tests based in the archive posts for async 
responses using the EXTERNAL SELECT mode, in two ways:

- The first way is to queue a ContentReaderCallback in the accessHandler 
function, which returns 0 while response has not arrived.
- The second way, is not queuing any response in the accessHandler function and 
return MHD_YES, queuing the response when it arrives.
We use an eventFd in the external loop to notify the main select of the arrival 
of the asynchronous responses.

The first solution Works great but has a problem. Apparently, there is no way 
to change the HTTP responseCode after the response is queued. So, if i was to 
return an specific http error code from the async framework I could not do it 
(without hacking the library or including the internal types).

The second solution has the main disadvantage that does busy waiting. The 
workaround for this is suspending the connection on the accessHandler and 
resume it when the response arrives just before queuing the response.

Do you find any problem in using approach 2? (Appart from not being notified 
when a client disconnects)
Is there a way of changing statusResponse in approach 1?

Thanks in advance

Marcos

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