disc: detect forced PGS subtitles from stream content for info

Give `info` the same forced-subtitle verdict the muxer derives during a
rip, so the two agree. A shared classifier (mux::codec::pgs::ForcedTracker)
folds a PGS track's display sets — forced iff every one carries the
forced_on_flag — and is used by BOTH the MKV writer and a new scan-time
probe that reads the title's PGS streams (reusing the TS demuxer and PGS
parser). The probe only overrides a track it actually observed content
for, so an undecrypted/unread stream keeps its vendor-derived flag. Gated
behind ScanOptions::probe_forced_subtitles (off for the rip path, which
detects forced while muxing without a second read).

src/disc/mod.rs

@@ -16,6 +16,7 @@ mod extract;
 mod hddvd;
 pub mod mapfile;
 mod patch;
+pub(crate) mod pgs_forced_probe;
 pub mod read_error;
 mod section_recover;
 mod sweep;
@@ -1491,6 +1492,13 @@ pub struct ScanOptions {
     /// 50_000 sectors on a live DVD) poll it and bail out cleanly so a
     /// scan-phase watchdog or operator Stop is never stuck behind a hang.
     pub halt: Option<crate::halt::Halt>,
+    /// Read the PGS subtitle streams during the scan to detect forced-narrative
+    /// tracks from their content (the `forced_on_flag`), matching what the mux
+    /// derives during a rip. OFF by default — it reads the clip's PGS content,
+    /// which is slow, so only callers that want authoritative forced flags in the
+    /// scanned title (e.g. `freemkv info`) opt in. The rip path leaves it off:
+    /// the muxer detects forced during muxing without a second read.
+    pub probe_forced_subtitles: bool,
 }
 
 /// Quick disc identification — name, format, capacity. No title/stream parsing.
@@ -1983,6 +1991,18 @@ impl Disc {
         // 4. Metadata + labels
         let meta_title = Self::read_meta_title(reader, &udf_fs);
         crate::labels::apply(reader, &udf_fs, &mut titles);
+
+        // Optional content-based forced-subtitle detection. `info` opts in so its
+        // forced flags match what the muxer derives during a rip (both use the
+        // one shared PGS classifier); the rip path leaves it off — the muxer
+        // detects forced while muxing, without a second read of the clip.
+        if _opts.probe_forced_subtitles {
+            for title in &mut titles {
+                if title.content_format == ContentFormat::BdTs {
+                    pgs_forced_probe::probe_and_set_forced(reader, title);
+                }
+            }
+        }
         crate::labels::fill_defaults(&mut titles);
 
         // 5. Format (AACS MKB generation → BD/UHD/FMTS; tree → HD-DVD/DVD) and

src/disc/pgs_forced_probe.rs

@@ -0,0 +1,196 @@
+//! Content-based forced-subtitle detection for Blu-ray/UHD PGS tracks.
+//!
+//! `freemkv info` and the muxer must agree on which subtitle tracks are forced.
+//! The muxer derives it from the PGS `forced_on_flag` while muxing a rip; this
+//! module gives `info` the SAME verdict up front by reading the title's PGS
+//! streams and feeding them through the one shared classifier
+//! ([`crate::mux::codec::pgs::ForcedTracker`]) — so the two never diverge.
+//!
+//! Cost: a track is only confirmed forced once EVERY display set is seen to be
+//! forced, so a disc that has a forced track is read through — the
+//! accuracy-over-speed tradeoff `info` opts into. Full tracks early-exit as soon
+//! as they show a single non-forced subtitle, and a whole run stops early once
+//! every track has settled.
+//!
+//! Encrypted content: the probe reuses whatever [`SectorSource`] the scan holds.
+//! With a decrypting source it sees real PGS; without keys it reads ciphertext
+//! and observes no display sets, in which case it leaves each track's existing
+//! (vendor-label-derived) forced flag untouched rather than asserting anything.
+
+use crate::disc::{Codec, DiscTitle, Stream};
+use crate::mux::codec::CodecParser;
+use crate::mux::codec::pgs::{ForcedTracker, PgsParser};
+use crate::mux::ts::TsDemuxer;
+use crate::sector::SectorSource;
+use std::collections::HashMap;
+
+const SECTOR_BYTES: usize = 2048;
+/// Read the clip in 2 MiB chunks.
+const CHUNK_SECTORS: u16 = 1024;
+
+/// Read the title's PGS streams and set `SubtitleStream::forced` from their
+/// content. Best-effort: any read error ends the probe with whatever verdicts
+/// have accumulated. Only PGS tracks are touched (DVD VobSub forced comes from
+/// the IFO/vendor path).
+pub(crate) fn probe_and_set_forced<S: SectorSource + ?Sized>(
+    reader: &mut S,
+    title: &mut DiscTitle,
+) {
+    let pg_pids: Vec<u16> = title
+        .streams
+        .iter()
+        .filter_map(|s| match s {
+            Stream::Subtitle(sub) if sub.codec == Codec::Pgs => Some(sub.pid),
+            _ => None,
+        })
+        .collect();
+    if pg_pids.is_empty() {
+        return;
+    }
+
+    let mut demux = TsDemuxer::new(&pg_pids);
+    let mut parsers: HashMap<u16, PgsParser> =
+        pg_pids.iter().map(|&p| (p, PgsParser::new())).collect();
+    let mut trackers: HashMap<u16, ForcedTracker> =
+        pg_pids.iter().map(|&p| (p, ForcedTracker::new())).collect();
+
+    let extents = title.extents.clone();
+    let mut buf = vec![0u8; CHUNK_SECTORS as usize * SECTOR_BYTES];
+    'outer: for ext in &extents {
+        let mut lba = ext.start_lba;
+        let mut remaining = ext.sector_count;
+        while remaining > 0 {
+            let count = remaining.min(CHUNK_SECTORS as u32) as u16;
+            let want = count as usize * SECTOR_BYTES;
+            let n = match reader.read_sectors(lba, count, &mut buf[..want], false) {
+                Ok(n) => n,
+                Err(_) => break 'outer, // best-effort — stop, keep what we have
+            };
+            if n == 0 {
+                break 'outer;
+            }
+            for pes in demux.feed(&buf[..n]) {
+                if let (Some(parser), Some(tracker)) =
+                    (parsers.get_mut(&pes.pid), trackers.get_mut(&pes.pid))
+                {
+                    for frame in parser.parse(&pes) {
+                        tracker.observe(&frame.data);
+                    }
+                }
+            }
+            // Every track has already shown a non-forced set → nothing left to
+            // learn; stop reading the (huge) clip.
+            if trackers.values().all(ForcedTracker::settled_not_forced) {
+                break 'outer;
+            }
+            lba += count as u32;
+            remaining -= count as u32;
+        }
+    }
+
+    // Drain any buffered final display set.
+    for (pid, parser) in parsers.iter_mut() {
+        if let Some(tracker) = trackers.get_mut(pid) {
+            for frame in parser.flush() {
+                tracker.observe(&frame.data);
+            }
+        }
+    }
+
+    // Apply verdicts. Only override a track we actually saw content for — an
+    // undecrypted/unread track keeps its vendor-derived flag.
+    for s in &mut title.streams {
+        if let Stream::Subtitle(sub) = s {
+            if sub.codec == Codec::Pgs {
+                if let Some(t) = trackers.get(&sub.pid) {
+                    if t.observed() {
+                        sub.forced = t.is_forced();
+                    }
+                }
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::disc::{ContentFormat, Extent, LabelQualifier, SubtitleStream};
+
+    /// A reader that yields all-zeros (an encrypted / unreadable clip) for a
+    /// bounded span, then EOF.
+    struct ZeroReader {
+        served: u32,
+        cap: u32,
+    }
+    impl SectorSource for ZeroReader {
+        fn read_sectors(
+            &mut self,
+            _lba: u32,
+            count: u16,
+            buf: &mut [u8],
+            _recovery: bool,
+        ) -> crate::error::Result<usize> {
+            if self.served >= self.cap {
+                return Ok(0);
+            }
+            self.served += count as u32;
+            buf.fill(0);
+            Ok(buf.len())
+        }
+        fn capacity_sectors(&self) -> u32 {
+            self.cap
+        }
+    }
+
+    fn pgs_title(pid: u16, vendor_forced: bool) -> DiscTitle {
+        DiscTitle {
+            playlist: String::new(),
+            playlist_id: 0,
+            duration_secs: 0.0,
+            size_bytes: 0,
+            clips: vec![],
+            streams: vec![Stream::Subtitle(SubtitleStream {
+                pid,
+                codec: Codec::Pgs,
+                language: "eng".into(),
+                forced: vendor_forced,
+                qualifier: LabelQualifier::None,
+                codec_data: None,
+            })],
+            chapters: vec![],
+            extents: vec![Extent {
+                start_lba: 0,
+                sector_count: 4,
+            }],
+            content_format: ContentFormat::BdTs,
+            codec_privates: vec![None],
+        }
+    }
+
+    #[test]
+    fn no_observed_content_preserves_vendor_forced() {
+        // An unreadable/encrypted clip yields no PGS display sets — the probe must
+        // leave the existing vendor-derived forced flag untouched, never assert
+        // "not forced" from having seen nothing.
+        let mut reader = ZeroReader { served: 0, cap: 4 };
+        let mut title = pgs_title(0x1200, true);
+        probe_and_set_forced(&mut reader, &mut title);
+        let Stream::Subtitle(s) = &title.streams[0] else {
+            panic!()
+        };
+        assert!(s.forced, "no content observed → vendor forced preserved");
+    }
+
+    #[test]
+    fn no_pgs_streams_is_noop() {
+        // A title with no PGS subtitle streams is a no-op (the reader is never
+        // touched — a DVD/VobSub or audio-only title).
+        let mut reader = ZeroReader { served: 0, cap: 0 };
+        let mut title = pgs_title(0x1200, false);
+        // Swap the PGS sub for an audio stream so there are no PGS PIDs.
+        title.streams.clear();
+        probe_and_set_forced(&mut reader, &mut title);
+        assert_eq!(reader.served, 0, "no PGS PIDs → no reads");
+    }
+}