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

[Jan Heylen <heyleke@xxxxxxxxx>]

On Sun, Mar 20, 2016 at 2:35 PM, Azat Khuzhin <a3at.mail@xxxxxxxxx> wrote:
> 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()

Ah, indeed, did think about that one in the scope of using the
evbuffer callbacks, but the problem remains the same at that moment:
once you have the bufferevent_get_output, one should not alter the
callbacks for the output buffer of bev. So I'm not sure what that
would bring me in this case?

>
>> 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.
OK, understood :-)

>
> Cheers,
> Azat.
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