[vlc-devel] [PATCH] HLS: Add flag to start from the beginning of a broadcast, even when in live mode

Chris Smowton cs448 at cam.ac.uk
Thu May 31 18:46:05 CEST 2012

Damnit, that was the wrong version of the patch (I split it into the 
proper one, attached, and the housekeeping patch I just sent).

> Hi,
> Here's the last patch in my list of HLS features I need, and which 
> might be useful in general. This one is aimed at timeshifting 
> scenarios: when a programme is being broadcast live, but nontheless a 
> viewer arriving some minutes into the programme wants to start at the 
> beginning, not the present moment. This matches with VLC's HLS output 
> filter running with --keep-segments, which gradually assembles a VOD 
> feed, which looks 'live' whilst the broadcast is in progress.
> I simply add a flag, --hls-start-beginning, which forces it to start 
> at the first available segment regardless of whether the stream is 
> live or not. Whilst I'm at it, I clean up ChooseSegment, since it did 
> a bunch of pointless work for VOD streams before inevitably returning 
> zero.
> The simplest form of this patch broke the download thread, however: 
> this was because the DL thread was always greedy when running against 
> a "live" stream, with potentially huge memory consumption for this 
> kind of stream. It was probably that way because its wait condition 
> was broken: it tried to wait for the playlist to get longer if 
> appropriate, but it didn't re-check the list length, nor did the 
> list-update thread signal it when the list changed. Instead it broke 
> out of its wait loop and tried to download again whenever the playback 
> pointer advanced.
> I fix this by correcting its wait condition: it now checks the 
> playlist length under the correct lock, and uses a semaphore 
> ("hls_download_t::check_stream") guarded by the download lock to learn 
> when it should re-check the playlist length. The playlist update 
> thread simply sets this flag whenever it refreshes the list (ideally 
> this would be whenever it *changes*). In this way the download thread 
> can obey its buffer cap of 6 segments even when dealing with live 
> streams.
> Chris
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