[FFmpeg-devel] [PATCH 4/4] lavf/matroskaenc: don't try to end the segment when the output isn't seekable

[Rodger Combs]

I can see the reasoning here, but as "small enough" here means 32K, I don't think this would be worth doing on a per-muxer basis. Maybe if we had a more general way to tell the muxer about segment boundaries, or if segment.c provided a single AVIOContext, with a write() function that passes through to a sub-context and a seek() function that reopened (and seeked within) previous segments if necessary? (Though we'd need an option to disable seeks into previous files to avoid inter-process read/write races).

> On Mar 29, 2015, at 14:04, Michael Niedermayer <michaelni at gmx.at> wrote:
> 
> On Sun, Mar 29, 2015 at 11:54:59AM -0600, Rodger Combs wrote:
>> In the case of most calls to end_ebml_master, the target is within the current segment, but in this case, it's in the first segment or header file, so if the context was marked as non-seekable manually (as in segment.c), avio_seek will ignore that and execute it anyway. This is fine in other uses of end_ebml_master, since both the current and target position are within the current segment, but breaks when ending mkv->segment, since `master.pos` refers to an offset in the first segment. So instead of failing the seek and returning, we end up seeking to an early point in the last segment, overwriting 8 unrelated bytes, and jumping back.
> 
> maybe it would be better to add a private option to the matroska
> muxer that keeps it from messing with past "segments"
> and set this from the segment muxer
> 
> a non seekable file thats small enough could before this patch
> be written with mkv->segment properly closed but i think the patch
> would break this.
> 
> [...]
> -- 
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
> 
> If a bugfix only changes things apparently unrelated to the bug with no
> further explanation, that is a good sign that the bugfix is wrong.
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