Re: [Libevent-users] deferred callbacks and evbuffer cleanup_cb

[Azat Khuzhin <a3at.mail@xxxxxxxxx>]

On Sun, Mar 20, 2016 at 10:58:34AM +0100, Jan Heylen wrote:
> Hi,
> 
> I'm trying to use evbuffers, cleanup_cb, and the deferred callbacks.
> 
> some background:
> * using (tcp) sockets
> * sending data over the socket: in first place try to write
> (evbuffer_write), if you get 'wouldblock' postpone the write by using
> bufferevent_write_buffer. Main reason is we don't need to keep the
> user's data in most cases and the dat is send immediatly.
> * after sending data, a 'send cb' should be called to the user who
> wanted to send something.
> 
> Now, when looking at the best way to get the cleanup happening (and a
> mechanism to let the user know his data is send), I was looking at
> both evbuffer callbacks and the evbuffer cleanup_cb.
> I'm thinking on using the cleanup_cb, as it is only called when the
> chain, the actual data, gets freed, so I can be sure the data is out.
> With evbuffer callbacks, the original evbuffer gets drained upon call
> to bufferevent_write_buffer already, and not when the data itself is
> really 'out', so that means, I have to make a distinction.

Hi Jan,

bufferevent_write() just move data from user-passed to bufferevent
internal, you can get internal output buffer with
bufferevent_get_output()

> However, If I look at the code, these cleanup_cbs are not and cannot
> be 'deferred'. So in the first place, when using evbuffer_write, the
> cleanup_cb will happen on the stack, and so also my 'data is send'
> callback, would happen on stack (and so cb will happen before the
> sending data function returns).
> 
> Questions:
> * thinking about the 2 different paths on getting data out,
> (evbuffer_write and postponed via bufferevent_write_buffer), what
> would be the best mechanism to get the 'all data is out' cb mechanism
> working for them both?
> * Is it a design choice to not defer the cleanup_cb, or would it make
> sense to add it?

I suppose that the answer is: nobody need it until now.

Cheers,
Azat.
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