[Bug 747414] New: basesrc: Enable pushing frames from OS thread

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=747414

            Bug ID: 747414
           Summary: basesrc: Enable pushing frames from OS thread
    Classification: Platform
           Product: GStreamer
           Version: git master
                OS: All
            Status: NEW
          Severity: normal
          Priority: Normal
         Component: gst-plugins-base
          Assignee: gstreamer-bugs at lists.freedesktop.org
          Reporter: ilya.konstantinov at gmail.com
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

Current GstBaseSrc in push-mode calls into element code whenever it expects
element to yield data. It's desired to have a mode of operation where
GstBaseSrc is called-into when data is available (i.e. inversion of control in
relation to our current mode).

There are OS frameworks that deliver data, as it becomes available, through a
callback called on an OS thread. It would be desirable, therefore, to push data
from this OS thread, rather than having to deliver it (e.g. via GAsyncQueue)
back to the GstTask thread.

The rationale -- "solving" this with a GAsyncQueue has two problems:
 - with a small queue, latency, perf will suffer due to blocking on kernel lock 
 - with a big queue, latency can grow too large
Both problems can cause frame-drops -- the first at the source (due to buffer
overrun), the later at the sink (due to too-late frames)

Both problems can be avoided by simply not doing it in the first place.

Also, each element that's currently in this situation ends up rolling its own
solution -- GAsyncQueue etc. -- complicating the code.

ELEMENTS THAT COULD BENEFIT

Basically, a lot of Apple elements:

* osxaudiosrc
* avfvideosrc
* vtenc_h264
* vtdec

ANSWERS TO POSSIBLE CONCERNS

Such OS frameworks typically implement their own buffering, and their callbacks
shouldn't be treated as strictly as - say - interrupts in the kernel. In other
words, one shouldn't rush to copy the data aside from the OS buffer and finish
the operation in another thread. In fact, usually zero-copy is possible.

Elements don't expect each other to be on the same thread (e.g. due to GstQueue
elements), so switching elements mid-pipeline is not expected to upset anybody.

Since things expect a GstTask to associate with, the original GstTask can be
used. The original task's open and close (or lock / unlock?) will ensure that
the OS framework callback is enabled/disabled.

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