Created attachment 259301 [details]
Program which listens for x-tags-changed events on playbin, then compares sticky tag events and "tag" property

I'm attaching a program for testing this.

Here's some example output:

$ gst-git ./flakey-tags.py https://trac.webkit.org/export/158942/trunk/LayoutTests/media/content/counting-subtitled-srt.mkv
text pad 0 got tags:
input-selector pad tags: taglist, language-code=(string)fr;
sticky event tags: taglist;

text pad 1 got tags:
input-selector pad tags: taglist, language-code=(string)fr, container-format=(string)Matroska;
sticky event tags: taglist, language-code=(string)fr;

text pad 0 got tags:
input-selector pad tags: taglist, language-code=(string)en;
sticky event tags: taglist;

text pad 1 got tags:
input-selector pad tags: taglist, language-code=(string)en, container-format=(string)Matroska;
sticky event tags: taglist, language-code=(string)en;

EOS

Notice how pad 0's input-selector pad "tag" property shows the language correctly, but the sticky events never show anything.

With pad 0 there's still a problem, since the container-format isn't showing up in the sticky events.

I'm guessing what's happening is that the tag sticky events are getting cleared or replaced, which doesn't matter to the input-selector pad since it just merges tags as it sees them and never clears it.

This causes problems because there's basically no reliable way to intercept tags with a pad probe, which I'd like to do here: https://bugs.webkit.org/show_bug.cgi?id=124020

Some interesting files to run this program on:

Text and video: https://trac.webkit.org/export/158942/trunk/LayoutTests/media/content/counting-subtitled-srt.mkv
Audio and video: https://trac.webkit.org/export/158943/trunk/LayoutTests/media/content/two-audio-and-video-tracks.mkv