`-readrate 1.05` paces input reading at wall clock so a channel behaves like
live TV, but it applies from the very first read. With 4s HLS segments and the
segmenter waiting for the first one, the playlist could not appear sooner than
~4/1.05 = 3.8s, so every tune-in that did not win a work-ahead slot paid a
multi-second wait.
Add `-readrate_initial_burst` (FFmpeg >= 6.1) next to `-readrate` on the normal
playback path, gated on runtime capability detection via the existing
`FFmpegKnownOption`/`HasOption` machinery, whose option list had simply been
empty. Measured on real prod media: time-to-first-playlist 5369/5344ms ->
648/649ms.
Root cause detail: the cold-start bimodality earlier rounds could not explain
was never about the media. `HlsSessionWorker` grants an unthrottled start only
while `_workAheadCount < work_ahead_limit` (prod: 1), so concurrent tune-ins
fall back to the throttled path. Confirmed on prod with three concurrent tunes:
firstGop 866ms for the slot winner vs 3845ms and 6357ms. This also falsifies the
issue's ranked #1 driver — accurate-seek decode-discard measures 30-100ms on
real media, and probe caps 20-50ms; neither can account for seconds.
Still images are excluded: their video input is paced by the realtime filter and
takes no readrate, so a burst would only run a song's separate audio input ahead
of the video. Concat/WrapSegmenter keep the unburst single-arg constructor.
fixes#350
timothy
Jason Dove