[Bug 762349] matroskamux: generated files sometimes don't open on android

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=762349

--- Comment #2 from Matej Knopp <matej.knopp at gmail.com> ---
Not quite sure what you mean by that. By empty tag, I meant something that
gst_matroska_mux_write_simple_tag ignores. That means tags that can not be
mapped (by gst_matroska_tag_conv) or tags with value that failed to serialize.
Those are the tags that GST_MATROSKA_ID_SIMPLETAG will not be written for.
Hence the NULL check after serialization. 

So if you have bunch of tags, and not a single one of them maps or serializes,
you can not write the surrounding GST_MATROSKA_ID_TAGS and GST_MATROSKA_ID_TAG
element, because webm parser chokes on empty GST_MATROSKA_ID_TAG element.

Using gst_matroska_mux_tag_list_is_empty instead of gst_tag_list_is_empty
should take care of this (look at for example gst_matroska_mux_start).

That said, I'm not entirely sure if tags with empty (zero length) value would
also break the parser. I'll test it, but looking at the mkvparser.cpp code, I
actually think it might, because ParseElementHeader is also being used to parse
GST_MATROSKA_ID_TAGSTRING elements; If that's the case, we'll probably need to
check for serialized value both being null and empty string.

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