[gst-devel] oggdemux push seek support

[Jay L. T. Cornwall]

Hi,

I am interested in putting together a patch to enable seeking in the 
oggdemux element when configured in push mode. My use case is for .ogg 
files streamed over the DAAP protocol, which takes place over HTTP. The 
Rhythmbox audio player implements this with a GStreamer chain beginning 
with souphttpsrc (in push mode).

The Ogg format is inherently difficult to seek because the stream 
headers (intentionally, by virtue of its streaming-oriented design) do 
not provide enough information to do this and its codecs are typically 
VBR. In pull mode, oggdemux performs a bisection search of the stream to 
identify chains (Ogg files concatenated together) and to compute the 
total stream length, among other things. This process is too seek 
intensive for push mode and so the demuxer currently has no seek support 
in push mode.

Going on a FIXME suggestion in gstoggdemux.c I wondered if it might be 
possible to add support for a less accurate seek via GST_FORMAT_PERCENT. 
A single seek request could be issued to a location, in bytes, 
proportional to the file length. libogg would then resync the stream and 
playback could continue from the approximated position.

A DAAP server typically scans media files for stream length and presents 
the total time to the client. Patching Rhythmbox to issue 
GST_FORMAT_PERCENT seek requests when GST_FORMAT_TIME is unavailable was 
quite easy and makes sense in the UI.

The main challenge I see is:

* Chained files could drop you into a part of the stream without having 
read the page headers first. This is bad and, worse, even undetectable 
if the same Ogg page serial numbers are used by different chains.

This seems like a showstopper for such an idea and I can't see a way 
around it. VLC does a similar thing and simply crashes if you seek too 
far into a chained file streamed over HTTP. Since chained files are an 
intentional design feature of the Ogg format I am loathe to consider 
them outliers and implement this anyway.

What do you think?

-- 
Jay L. T. Cornwall
http://www.jcornwall.me.uk/