Re: About a websockets write callback

From: Daniel F via curl-library <curl-library_at_lists.haxx.se>
Date: Fri, 17 Jun 2022 19:22:21 +0200

W dniu 2022-06-17 13:39, Daniel Stenberg via curl-library napisał(a):
> Hello,
>
> Last year when we brainstormed different takes on a WebSockets idea,
> we landed on a separate ws callback for writing data, similar to the
> regular CURLOPT_WRITEFUNCTION. [*]
>
> A separate callback, because it needs to provide some additional data
> that the plain callback doesn't receive.
>
> Now when I've started working on implementing websockets, I'm having
> second thoughts on that design but I can't decide on what's a better
> way. So I'm here to see if someone else can fix this for me! =)
>
> I would like to provide websockets data with the CURLOPT_WRITEFUNCTION
> callback as well. But how would libcurl provide the additional
> necessary metadata?
>
> My initial thought was to provide a special websocket function that
> only can be used from within the callback that would provide the extra
> information (type of data, opcode, and if the FIN bit is set or not
> etc). Like maybe
> 'curl_ws_metadata()'.
>
> This is what makes me hesitate on this idea:
>
> This new function would need to get the easy handle as an argument, so
> that it can find and return the correct information, but the
> CURLOPT_WRITEFUNCTION does not pass in the easy handle! So in order to
> use this function within the callback, it would require that the
> callback on its own design gets the easy handle passed into it via the
> CURLOPT_WRITEDATA option in some manner.
>
> Would that be a sensible requirement or would be asking too much of an
> application?
>
> Or can we come up with a better way?
>
> [*] = https://github.com/curl/curl/wiki/WebSockets

Hi,
For me this new proposal is fine. In my apps I always pass pointer to
some structure or class to CURLOPT_WRITEDATA. and this structure/class
contains curl handle in addition to other data.

I can guess that apps written by others either pass pointer to
struct/class to CURLOPT_WRITEDATA, pass index from some array to
CURLOPT_WRITEDATA, or use single curl handle in whole app, which is
stored globally or in other well-known place. I do not see any special
issue in any of these approaches.

Make sure to properly document it with example how to use this. Full
info should be in docs for curl_ws_metadata function, plus some new note
in existing docs for CURLOPT_WRITEFUNCTION and CURLOPT_WRITEDATA that
WebSockets have extra requirement.

-- 
Regards,
Daniel
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Received on 2022-06-17