Subtitle side-demuxer's 5 s analyze ceiling adds a flat ~5 s startup stall for PGS/bitmap tracks on URL sources

[rrgomes]

On a remote (URL/HTTP) source, opening the embedded-subtitle side-demuxer for a PGS/HDMV bitmap track blocks for the full analyzeCeiling (5.0 s) before it yields: such tracks never satisfy has_codec_parameters, so find_stream_info reads to the budget cap. When that open is on the startup path (host selectSubtitleTrack at load, or LoadOptions.preferredSubtitleLanguages auto-select), the 5 s lands as a flat pause before playback begins.

Root cause

Demuxer.swift, DemuxerOpenProfile.subtitleSideDemuxer:

let probeCeiling:   Int64 = 4 * 1024 * 1024   // 4 MB
let analyzeCeiling: Int64 = 5 * 1_000_000     // 5.0 s

The profile's own doc comment already describes it: PGS streams "keep has_codec_parameters false to the budget cap (the #75 pattern)," so find_stream_info runs to the full ceiling. A fast LAN hides it; a slower/remote URL blocks ~5 s. Since EmbeddedSubtitleDecoder only needs codec_id / codec_type (resolved by avformat_open_input from the container header / MPEG-TS PMT) and seeds the bitmap canvas from the source video size, the find_stream_info chase is pure cost.

Repro

A 1080p/4K HEVC file with embedded PGS (HDMV bitmap) subtitle tracks served over an HTTP/URL source (custom AVIO); select that PGS track at load. Result: ~5 s paused before playback self-starts. The same open is instant on a local file.

Suggested fix direction

Cap the subtitle side-demuxer's analyzeCeiling to sub-second (codec params come from the header/PMT before find_stream_info), or skip find_stream_info for the side-demuxer and read codec_type / codec_id directly, or make the open non-blocking with respect to the startup gate. Related: #52, #75, #76.

Host-side note: integrators can mitigate by deferring selectSubtitleTrack past the first playing state, but the 5 s budget is unnecessary regardless.

[superuser404notfound]

Fixed on main (e1cf6c2), taking the skip-find_stream_info direction.

DemuxerOpenProfile gains a skipStreamInfo flag, set by subtitleSideDemuxer. probeStreams drops both the avformat_find_stream_info pass and the cover-art reclassify (which only existed to bound that pass) when the flag is set. avformat_open_input already carries codec_id / codec_type for every subtitle track from the container header / MPEG-TS PMT, and EmbeddedSubtitleDecoder seeds the bitmap canvas from the source video size, so the chase was pure cost. As the report notes, even the #76 5 s ceiling was paid in full for the selected-PGS case (the target track never satisfies has_codec_parameters), so capping the budget would not have helped that path.

Safety net for any container that does not declare the subtitle codec in its header: the side reader checks the target stream's codec_id after open and runs a bounded resolveStreamInfo() (capped by the same subtitle-sized probe ceiling) only when it is AV_CODEC_ID_NONE. The common path is now zero find_stream_info.

Tests: added skipStreamInfo coverage to DemuxerProfileTests; full suite green (265).

Remaining: device-verify the actual startup delta on a 1080p/4K HEVC + embedded PGS file over an HTTP source. A before/after on the flat pause against a slow (429/503-ish) origin would confirm the measured win.