Prom client doesn't like you creating two stats with the same name,
even though they have different labels and this makes perfect sense.
I feel like I just need to re-write the prom client at some point - it doesn't actually
do all that much except get in your way, apart from the actual text encoding which I
can steal.
Anyway, in the meantime, we get around this by breaking up metrics into two names,
a "foo_all" and a "foo_ENDPOINT". The foo_all lacks the detailed labels,
but is still labelled by endpoint and can be used more easily.
The foo_ENDPOINT labels have more information but require messier PromQL as you need to
match on a name regex if you want to look at more than one specific endpoint.
I had to go to some effort to get nice labelling,
which also meant none of the existing libs for this were any good,
but this works well enough.
Exposes the metrics on /metrics.
The calculations were backwards, so instead of cutting a video by, say, 2 seconds,
it would cut by -2 seconds, which was clamped to 0. So it would never actually cut,
it would always use the closest segment.
Also, once we were actually cutting, we hit an issue where ffmpeg would finish and close
its input early, because we'd reached the end of the cut video, but not all input had been written yet.
This resulted in an EPIPE error (write to closed pipe) in the input feeder. We now ignore that.
This cutter works by only cutting the first and last segments to size,
then concatting them with the other segments, so we only ever process a few seconds
of video instead of the entire video duration.
However, to make this work, care must be taken that the cut segments use the same codecs
as the other segments.
The reason it's experimental is that we are not yet confident in its ability
to cut accurately and without sync issues. We have seen some minor issues when trying to play
back the raw output files, but youtube's re-encoding has consistently smoothed out those issues
and they seem to be highly player-specific. Vigorous testing is needed.
Also note that both methods right now (cat then cut, and cut then cat) only work if all the segments
are cattable, that is they all use the same codecs, have the same resolution, etc.
If a stream were to change its encoding settings, and we were cutting over that change,
both approaches would not work. We should add checks for that scenario (which can only happen
over a stream drop), and if so fallback to a slow method using ffmpeg's concat filter,
which will work even for disparate codecs, though reconciling mismatched resolutions or frame rates
may require further work.
This is mainly just for testing until we get the database and proper cutter up,
but it might prove useful to have in the long run too.
This code will probably end up being totally rewritten,
as it uses the most naive form of cutting and reencoding,
and it has a whole bunch of http-serving specifics intertwined with the cutting logic.
This prevents clients from picking a variant that they then can't play any content for.
In general we expect the same content to be available on all variants being captured,
but if the set of captured variants changes we still want to handle that gracefully.
Mike Lang
Christopher Usher