25.07.2016, 22:13, Anssi Hannula kirjoitti: > Hi all, > > Commit d0fc5de3a643fe7f974ed14e410c2ac2f4147d7e [1] merged in > commit 81306fd4bdeb5c17d4db771e4fec684773b5790f "hls: eliminate ffurl_* > usage" from libav, which changed the hls demuxer to use AVIOContext > instead of URLContext for its HTTP requests. > > HLS demuxer uses the "offset" option for the http demuxer, requesting > the initial file offset for the I/O (http URLProtocol uses the "Range:" > HTTP header to try to accommodate that). > > However, the code in libavformat/aviobuf.c seems to be doing its own > accounting for the current file offset (AVIOContext.pos), with the > assumption that the initial offset is always zero. > > HLS demuxer does an explicit seek after open_url to account for cases > where the "offset" was not effective (due to the URL being a local file > or the HTTP server not obeying it), which should be a no-op in case the > file offset is already at that position. > > However, since aviobuf.c code thinks the starting offset is 0, this > doesn't work properly. > > E.g. this command, which worked fine in FFmpeg 3.0, now fails after > approx. 10 seconds of playback: > ffplay > https://devimages.apple.com.edgekey.net/streaming/examples/bipbop_16x9/bipbop_16x9_variant.m3u8 > > > Now, what should be done to fix this regression? > > I guess there are at least these options: > > 1. Make aviobuf.c call seek() to check the offset after open: > > --- a/libavformat/aviobuf.c > +++ b/libavformat/aviobuf.c > @@ -913,6 +913,14 @@ int ffio_fdopen(AVIOContext **s, URLContext *h) > (*s)->read_pause = io_read_pause; > (*s)->read_seek = io_read_seek; > } > + > + /* get initial buffer position (may be non-zero with e.g. offset > option for http) */ > + if ((*s)->seek) { > + int64_t pos = (*s)->seek((*s)->opaque, 0, SEEK_CUR); > + if (pos >= 0) > + (*s)->pos = pos; > + } > + > (*s)->av_class = &ff_avio_class; > return 0; > fail: > > I guess that has a high chance of further regressions (*if* it is even > correct), though, as I guess there may be seek() handlers that don't > have SEEK_CUR = 0 as a no-op. > > 2. Revert the commit. I'm not familiar enough (or I've just forgotten > that stuff) with avio/ffurl to know how big the benefit of using avio_ > instead of ffurl_ is, though... > > 3. Drop the seek call from HLS demuxer in case protocol is http*, and > assume the HTTP server obeyed the ranged request.
After thinking about it a bit, I think I'll go with this 3rd one to fix the regression, unless a better idea surfaces. It is quite trivial to implement, and server obeying ranged requests is mandatory anyway in HLS specs when byte-ranged segments are used - the failure mode in the non-spec-compliant will just be a bit different. > > Any other ideas / opinions for the course of action? > > > [1] https://github.com/FFmpeg/FFmpeg/commit/d0fc5de3a643fe7f974ed14 > -- Anssi Hannula _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
