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

[Azat Khuzhin <a3at.mail@xxxxxxxxx>]

On Sun, Mar 20, 2016 at 04:11:18PM +0100, Jan Heylen wrote:
> On Sun, Mar 20, 2016 at 3:55 PM, Azat Khuzhin <a3at.mail@xxxxxxxxx> wrote:
> > On Sun, Mar 20, 2016 at 03:11:31PM +0100, Jan Heylen wrote:
> >> On Sun, Mar 20, 2016 at 3:01 PM, Azat Khuzhin <a3at.mail@xxxxxxxxx> wrote:

> > Why do you need this special case evbuffer_write(), can you just use
> > bufferevent_write_buffer() always?
> In most cases, that would be overkill, as long as the fd/socket is not
> overloaded, the data should go out immediately (so the data can be on
> the stack of the function that was trying to send it, and no
> 'memcpy'/malloc/data housekeeping has to happen at all)

There is no memcpy/malloc since bufferevent_write_buffer() is
evbuffer_add_buffer() and it only needs to copy *chains* (structures
that contains pointer to your refs), of course maybe a one/two malloc's
for chain itself, but does it critical for you?

> > Since cleanup_cb must be called only when reference didn't exist,
> > example:
> >  struct evbuffer *buf = evbuffer_new();
> >  evbuffer_add_reference(buf, data, len, cleanup, buf);
> >  bufferevent_write_buffer(evbuf, buf);
> >  evbuffer_free(buf);
> > And during this calls "cleanup" will not be called because
> > bufferevent_write_buffer() will take the chain ownership (part of
> > evbuffer that contains you reference), so when you call evbuffer_free()
> > there is no chain in evbuffer so "cleanup" will not be called yet.
> Indeed, I know, the reason I want to use cleanup, suits my needs.

You can also re-create buffer with cleanup callback if evbuffer_write()
return an error, in this case you don't need special case in cleanup
callback.

> > "cleanup" will be called only when it will be drained from
> > bufferevent_get_output() and it will happen only it will be written to
> > the client, or I'm missing something and it doesn't work like this?
> No, that is indeed what I was planning to do, but I thought I was
> missing something, as these cleanup_cbs
> didn't come with deferred thingy. At the moment I'm going to do this
> without deferring. But I'll probably have to add that later.
> 
> Btw, is there a reason why the deferral infrastructure is 'internal'?
> Wouldn't it be useful as an extra interface of libevent? Especially
> the deferral implementation of 2.1.x, where some issues with
> priorities where solved I understood from the documentation. Or is
> there a reason not to use it as a more 'generic' callback deferral
> infrastructure?

Once you export something you can't change it, or you have to support
two versions (to avoid breaking binary compatibility), and besides
nobody ask about this before (AFAIR).

But we have finalizers (event_finalize()) and some parts (as you already
mentioned) of library support them via flags (bufferevent/evbuffer).

Regards,
Azat.
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