Could we push more than one buffer in pad chain function?

[Nirbheek Chauhan]

On Tue, Sep 6, 2016 at 5:21 AM, Jake Zhang <zjpseudo at gmail.com> wrote:
> I am implementing a fairly complicate custom plugin and this plugin
> sometimes will hold on some input buffers by pushing them into a queue if
> some timestamp condition does not meet yet, and sometimes it might push more
> than one buffer to srcpads if certain condition is met. Do you guys see any
> issue of doing this? I just want to make sure I will not have a big issue
> with this unconventional chain function.
>
> I have went through the gstpad.c source code, and if my understanding is
> correctly the pad chain function will be called once every time each time
> there is a input buffer. And I do not see any issue with my above
> implementation.
>
> Please correct me if I am wrong. Any input is appropriated.
>

Yes, it is a very common use-case to not always push a single buffer
when chaining a buffer. All you have to do is make sure that you
return GST_FLOW_OK when everything is fine. You can either push
nothing, a single buffer, or multiple buffers, or a buffer list.

There's roughly three things you can do here, depending on what your
data looks like.

If your data is meant to be concatenated/merged, then you can merge
the contents of buffers and push them as a single buffer as needed.
One way to do this is with GstAdapter[1]. This is commonly done in,
for instance, payloaders and depayloaders.

1. https://developer.gnome.org/gstreamer-libs/stable/GstAdapter.html

If you want to keep GstBuffers as they are, you can maintain a GList
or GPtrArray of them and push multiple ones individually
(gst_pad_push) in a single chain function invocation (be careful to do
error checking for each push!). The third option is to both store and
push them as a single GstBufferList[2] (gst_pad_push_list).

2. https://developer.gnome.org/gstreamer/stable/gstreamer-GstBufferList.html

The first and third options have the least overhead, if that matters
for your use-case.

Cheers,
Nirbheek

-- 
~Nirbheek Chauhan