libfreemkv/docs/disc-mod.md

src/disc/mod.rs — internal notes

Overflow rationale for internal (non-pub) items in src/disc/mod.rs, relocated here by the comment-guard so the in-file comment can stay short. Each section is pointed to by a one-line // See docs/disc-mod.md — <topic> comment at the corresponding call site.

aacs_dir_present

THE ONE definition of "is this disc structurally AACS-encrypted". Both Disc::identify (fast path, filesystem only) and Disc::scan_with (full scan) need this, and both used to spell out the same two find_dir calls by hand. They agreed today; nothing made them agree tomorrow — adding a third AACS location, or excluding an empty placeholder directory, to one copy and not the other would silently desync the fast identify from the full scan (the same disc reported encrypted by one and clear by the other).

This is STRUCTURAL, not cryptographic: it says the tree looks like an encrypted disc, not that any sector actually is. A folder copied verbatim from a decrypted disc keeps its AACS/ and answers true here (see session::scan_dir, which corrects for exactly that).

correct_truehd_channels

Corrects a title's TrueHD audio-stream metadata by probing the first decrypted access units — channel count, real sample rate, and Atmos detection in a single major-sync read. The MPLS descriptors declare the BASE layout (often 5.1, or a container-guessed rate) even for a 7.1/Atmos TrueHD track; the truth is in the MLP major sync. reader must yield DECRYPTED sectors (the m2ts is AACS-encrypted, so this can only run at mux time, not scan). Reads a bounded window of the title's first extent.

Corrections, each individually guarded so a malformed field never writes a wrong header:

• Channels: from the presentation channel masks (as before).
• Sample rate: from the whitelisted rate nibble; left untouched on an unknown rate or no major sync.
• Atmos: when a 4th substream is detected AND the stream still carries the basic descriptor label, the label is promoted to the Atmos form; richer editorial labels (e.g. an existing "Dolby Atmos") are left intact.

Resolution::pixels

Unknown deliberately does NOT fabricate a plausible 1920x1080. This is the same trap already closed on AudioChannels::count (which used to return 6) and SampleRate::hz (48000.0): a plausible wrong answer is indistinguishable from a real one at every call site, so a caller can forget to check the variant first — the json:// sink had already walked into exactly that, reporting confident dimensions for a stream whose neighbouring resolution field said "unknown".

It also does not return (0, 0): that shape looks like a usable pair, so the MP4 sink stored it and serialised a 0x0 video track — a structurally complete file no player can render, written with no error. Before that it returned a fabricated 1920x1080, which was wrong but at least playable, so the sink had never needed a guard and the absence of one was invisible. A doc comment listing which callers are safe cannot hold this: one did not check, and a third kept its own duplicate Unknown test long after the accessor took the job over. Option forces every caller to make its OWN decision: Matroska omits the optional PixelWidth/PixelHeight elements, the metadata sinks report the dimensions as absent, and MP4 must refuse per ISO/IEC 14496-12 (width and height are mandatory in both tkhd 8.3.2 and VisualSampleEntry 12.1.3).

has_probable_video

The exposed streams come from the title's FIRST PlayItem only, so a feature whose first PlayItem is a non-video bumper reports has_video() == false and would be wrongly disqualified (audit R6). This predicate is deliberately MORE permissive than has_video — it only ever ADMITS a title the stricter gate would reject — so it cannot reopen the decoy hole: a size-inflated streamless decoy is demoted STRUCTURALLY by composite detection in Disc::rank_titles, not by this gate, while it still rejects tiny streamless menu/metadata playlists. The size fallback requires at least one clip, so a clip-less pure-metadata decoy (which cannot be a real bumper-lead feature — that has the feature clip) is still rejected.

image_crack_extents

The extents an image-time CSS crack scans, in the crate's CANONICAL order: the main feature's own extents, in natural playback order. Both halves are deferred to logic the crate already owns rather than re-derived here:

• WHICH title — scan_with has already sorted titles with Self::canonical_title_order, so the canonical main feature is the first title that actually has extents. No local pick.
• WHAT ORDER — playback order, i.e. the extent vector untouched, exactly as Self::decrypt_keys_for_title hands &title.extents to css::crack_key_outcome.

Why this function exists (do not let the copy grow back): Disc::scan_image used to re-implement both halves inline — it picked the title with the largest total sector count, then re-sorted that title's extents LARGEST-CELL-FIRST before handing them to crack_key_outcome. Both had drifted from the canonical rules:

• Largest-cell-first is the 1.5.1 garbage bug. A CSS DVD's biggest cell opens with a long CLEAR run, and the crack's 50_000-sector budget is shared across the whole extent list, so starting there can exhaust the budget without ever reaching a scrambled sector — the crack reports Unencrypted and the mux emits scrambled MPEG as plaintext. Playback order reaches the scrambled feature body after only the small clear front matter that precedes it on a real disc (CSS: the title key is recovered from the scrambled data itself, so the scan must actually MEET scrambled data).
• The sector-count pick ignores canonical_title_order's capacity gate, so it selects the oversize "play-all" composite (whose declared cells double-count data shared with other playlists) instead of the real feature — i.e. a DIFFERENT title, and on a multi-VTS disc a different VTS, whose CSS title key does not descramble the feature at all.

The result: the same disc cracked as an ISO could disagree with the same disc cracked from the drive, which is precisely what the duplicate was free to do. Keep the derivation here, shared, so it cannot recur.

read_aacs_inputs_from_reader

Reads a disc's AACS key-input files from a sector source: returns (Unit_Key_RO.inf, MKB) raw bytes. Shared body for Disc::read_aacs_inputs (ISO) and Disc::read_aacs_inputs_from_drive (live drive). Prefers MKB_RO, falls back to MKB_RW, then TRIMS to the real record length. Both files are allocated to a fixed ~128 MiB and zero-padded, so reading either ships up to ~124 MiB of nothing — trim to the record stream so callers send/store a few MB, not 128 MiB.

read_aacs_version

AACS major version (crate::aacs::mkb::AACS_MAJOR_BD / crate::aacs::mkb::AACS_MAJOR_UHD) from the content certificate. Drives the Unit_Key_RO.inf parse stride (48-byte V10 vs 64-byte V20/V21), so the out-of-band key-fetch path parses enc_title_keys at the right stride (a server VUK then derives the correct unit keys).

When no content certificate is readable/parseable, defaults to UHD (V20, 64-byte stride) — the conservative choice the pre-1.2.0 fetch path hardcoded — and logs it: a wrong stride here folds a server VUK against mis-strided title keys (silent wrong unit keys), so a missing cert must not quietly pick the V10 stride for a UHD disc.

read_mkb_content

Reads the AACS MKB's real record stream — NOT its zero padding. MKB_RO.inf / MKB_RW.inf are allocated to a fixed ~128 MiB and zero-padded; the actual record stream is a few MiB. Reads a bounded prefix, finds the record-stream length via crate::aacs::mkb::mkb_content_len and returns exactly that, growing the prefix if the records run past it. This avoids reading 100+ MiB of padding on every scan AND avoids the read_file MAX_FILE_BYTES cap that (since 0.31.0) rejected the padded 128 MiB MKB outright — which made read_aacs_inputs fail and autorip report "could not read this disc's key files" without ever contacting the keyserver.

CANONICAL_TITLE_ORDER_KEYS

Why not just sort by duration descending? Branching UHDs (and some BD authoring) ship a "play-all" virtual playlist that references the same source clips multiple times for seamless alternate-angle / alternate-ending playback. Those playlists report an inflated duration_secs (often 4+ hours) and an inflated size_bytes greater than the disc's physical capacity. Example seen in the wild — The Amateur (2025) UHD, 58.5 GB BD-100 disc:

Title	Playlist	Duration	Size	Clips
1	00020.mpls	4h 13m	92.4 GB	253
2	00800.mpls	2h 02m	57.2 GB	1

Title 1's 92.4 GB cannot fit on a 58.5 GB disc unless the same clip data is referenced multiple times — proof it's a virtual composite. A duration-only sort would put it at titles[0], so freemkv -t 1, disc.titles.first(), and autorip's main-feature picker all grab the 4-hour composite instead of the 2-hour movie that actually matches TMDB.

Effect on non-branching discs: unchanged — the main movie is already the longest 1-clip title. Effect on branching UHDs: the virtual play-all playlist is pushed to the back, the actual movie surfaces at index 0.

Defined immediately beside the comparator, so a diagnostic can NAME the ordering instead of restating it. The freemkv::diag main-feature decision row used to carry its own hand-written copy of this list, and it drifted: it still advertised a fewest-clips key long after the comparator replaced clip-count with largest-physical-size, so the --log-level 3 bug-report log explained freemkv's top-level pick with a rule freemkv does not apply. Any change to the keys must change this list in the same edit.

probe_forced_subtitles_for_bdts_titles

Pulled out of scan_with as its own callable predicate: the decision is otherwise reachable only by driving the full scan pipeline (tree dispatch -> title parsing -> this loop), so a test could not pin the gate without also authoring an entire synthetic disc image. Running it against an HD-DVD/DVD (MpegPs) title's extents would demux the wrong container, since only BdTs titles carry PES-wrapped PGS the shared classifier (pgs_forced_probe) understands.

MAIN_FEATURE_ORDER_KEYS

• nav-feature: the title the disc's own HDMV navigation plays as the feature, resolved by running First-Play in the crate::bdnav VM the way a real player would. This is the authoritative, size-independent pick; it is video-gated (a mis-resolve cannot select a streamless playlist) and simply absent when the VM abstains (BD-J discs, or non-convergence), in which case the lower keys decide.
• authoring-feature: a title the disc's own menu authoring designates as the feature (has video, is not itself a composite, and is within a sane fraction of the longest title's duration) outranks everything else — a size-independent signal an obfuscation decoy cannot forge.
• standalone: a title that is a play-all / wrapper COMPOSITE — its clip set properly contains another substantial video title's whole clip set (a real UHD title's decoy playlist 00245 = [bumper][00001 feature clip][outro] case) — is demoted below the standalone title it wraps. This is the STRUCTURAL anti-decoy signal; it does not depend on stream counts, physical size, or disc capacity, so it survives where those are spoofed or unknown.
• has-video: a title with no plausible video content (see DiscTitle::has_probable_video) is demoted below every title that has some — a floor that drops streamless menu/metadata playlists.

rank_titles

This catches both the wrapper decoy (00245 ⊋ 00001) and the oversize play-all — independent of capacity, which the fits-disc gate needs and which is 0/unknown on drives that fail READ CAPACITY. The corpus title-selection gate is the backstop across the disc hoard for the known residual (a real feature with a separate "resume from the middle" branch playlist reusing most of its clips).

aligned_unit_keys_validate

Conservative: a sample that is not AACS-scrambled proves nothing, and with no scrambled sample at all there is nothing to disprove against, so it returns true (accept). It returns false when ANY scrambled sample cannot be restored to clear MPEG-TS by ANY unit key in the set. That covers two distinct failure shapes: (1) a wholly wrong key (a keydb VK that does not match this disc) — no sample decrypts; and (2) a partially applicable key set on a multi-CPS-unit disc — the resolved keys cover CPS unit 0 but not CPS unit 1. Accepting on the first sample that decrypts (the old behaviour) would commit such a set, after which CPS-unit-1 sectors pass through as raw encrypted bytes into the ISO/MKV with no error surfaced anywhere. Requiring every scrambled sample to decrypt rejects the incomplete set so the caller falls through to the next candidate. Reuses the ecosystem's single is_clean content-clarity predicate and the full (bus + AACS) unit decrypt, so it agrees with the actual mux decrypt.

ensure_decryptable

The verdict, in order:

• raw == true → Ok(()). --raw intentionally skips decryption and needs no key (the caller wants an encrypted image).
• self.css_error.is_some() → Err(Error::CssNoDiscKey). The scan saw scrambled CSS sectors but recovered no title key (self.css is None yet the content IS encrypted). Treating css.is_none() as "unencrypted" would mux scrambled MPEG as plaintext garbage. A DISC-LEVEL verdict (error::is_disc_level_no_key) — the main feature's crack failed, so every title fails the same way and the rip loop must stop rather than skip each title in turn.
• AACS-encrypted (self.aacs.is_some()) with no usable key (decrypt_keys() is None) → Err(Error::NoDiscKey { .. }), naming the disc by hash.
• CSS-encrypted (self.css.is_some()) with no usable key → Err(Error::CssKeyMissing). (The disc-wide decrypt_keys() yields Css{..} whenever css.is_some(), so this is defensive; the live multi-VTS case is gated by Self::ensure_decryptable_keys.)
• otherwise → Ok(()). A genuinely unencrypted disc has None keys legitimately, and a CSS disc whose keyless crack succeeded has a key.

decrypt_keys_for_title

For a DVD the CSS title key MUST be recovered before descrambling: a scrambled sector without a recoverable crib cannot self-crack, and CSS leaves the pack/PES header clear, so a sector left un-descrambled would mux as a structurally-valid but corrupt PES packet with no loss reported. Two ways to get it:

• Fast path — the scan already cracked a key whose LBA span (crate::css::CssState::crack_span) covers this title's VTS: reuse it. No re-read, and on a live drive no second CSS bus-auth round-trip. CSS title keys are per-VTS, so an overlapping span is the same key.
• Crack — when up-front detection missed (self.css == None) or the title lives in a different VTS: crack the key from this title's OWN extents in a SINGLE scan, in natural PLAYBACK ORDER (never largest-cell-first, which starved the crack in a big cell's clear prefix — the 1.5.1 bug). Playback order reaches the scrambled feature body after only the small clear front matter (logo / rating card) that precedes it. One scan = one CSS-locked early-bail, so a locked title is not re-hammered per cell against a live drive (hard rule #2); its 50k-sector budget is the same accepted bound the disc-wide scan uses.

Outcomes (batch_sectors sizes the crack's batched reads):

• (Css{title_key}, false) — descramble with the recovered/reused key.
• (None, true) — a genuinely-clear title needs no key; the gate passes it.
• (None, false) — scrambled but no key recoverable → hard failure via Self::ensure_title_decryptable, never a silent garbage mux.

inject_unit_keys

Injects pre-resolved AACS unit keys into a scanned disc — the deferred-mux / resume path. The keys come from the mapfile's # freemkv-uk: header (persisted at sweep time when the disc was keyed), so the mux decrypts directly with NO key-service round-trip. Populates self.aacs.unit_keys so Self::decrypt_keys returns them and marks the source ExternalUk.

If the scan built no AACS state (self.aacs == None) — which happens when the keydb was absent at scan time (scan_aacs_no_keydb -> aacs_error = KeydbLoad) — this synthesizes a minimal ExternalUk state for an encrypted AACS disc. A Unit Key is the FINAL per-title decryption key; the keydb is only needed to derive it, and that derivation already happened at sweep (the UK is in the mapfile). So a UK alone is sufficient to decrypt the on-disk ISO — AACS 2.0 bus decryption was applied by the drive at read time, so read_data_key is unused for file-backed mux. Without this, a keyed disc swept without a keydb would recover its UK yet still report E8005 (no usable decrypt_keys) at remux. No-op for an unencrypted or CSS (DVD) disc.

scan_with_passes_the_halt_flag_to_the_bluray_enumerator

Every BD and DVD cancellation test in this crate calls scan_bluray_titles / scan_dvd_titles DIRECTLY, so the three-line wiring in scan_with that connects the operator's Stop to them was covered by nothing at all: replacing opts.halt.as_ref() with None on either branch left the whole suite green while a Stop during a BD or DVD scan silently did nothing. Only the HD-DVD branch was pinned.

The fixture is deliberately the smallest disc that takes the BD branch — a bare /BDMV with no PLAYLIST — because the point under test is the ARGUMENT, not the enumerator's own (separately tested) halt polling. Nothing between scan_with's entry and the enumerator reads the flag (the disc is unencrypted, so the AACS path is skipped), so a green here can only mean the flag arrived. Mutation: Self::scan_bluray_titles(reader, &udf_fs, None)? fails here.

halted_reads_do_not_report_the_hddvd_scan_as_successful

A Stop on a LIVE DRIVE never touches ScanOptions::halt: Drive has its own flag and checked_exec fails every SCSI command with Error::Halted once it is set. The HD-DVD enumerator must not swallow that into a successful scan. Measured before this was fixed: the scan returned Ok with both titles present and ZERO streams on each — a cancelled scan wearing the shape of a disc whose clips carry no video or audio. Downstream that is a title list to cache, display and rip from.

scan_with_capacity_bytes tests

scan_with's capacity_bytes = capacity as u64 * 2048 feeds canonical_title_order's "bigger than the whole disc = play-all composite" threshold.

..._not_addition: chosen so capacity * 2048 clears both titles (neither is oversize, so the bigger one — OTHER — ranks first), but capacity + 2048 lands BETWEEN the two sizes: MAIN stays non-oversize while OTHER flips to oversize and gets demoted, changing titles[0]. A * -> + mutation is caught by this ranking flip, not merely a wrong numeric threshold value.

..._not_division: same mechanism, tuned to catch a * -> / mutation instead: capacity * 2048 clears both titles, but capacity / 2048 collapses to a threshold far below MAIN's declared size while still leaving MAIN under it, so only OTHER flips to oversize.

read_mkb_content_grows_prefix_past_16mib_when_records_run_longer

This fixture's record stream is deliberately built so the first 16 records land EXACTLY on the 16 MiB boundary (so the 16 MiB prefix alone parses as a clean, complete record stream — the case a naive "n < buf.len() means done" check would wrongly accept) and 4 more MiB of records follow, terminated by the explicit 00 000000 end marker. Only a caller that actually grows past 16 MiB recovers the true 20 MiB content length.

main_feature_order_demotes_wrapper_composite_decoy

The decoy 00245.mpls is a WRAPPER COMPOSITE — [00339 2s bumper][00001 the whole feature clip][00336 outro] — so its first-PlayItem STN reports v=0, and it is the LONGEST and LARGEST title (8350s / 78.4 GB vs the feature's 8038s / 76.5 GB). Muxing its zero frames produced E6008 forever. It must be demoted STRUCTURALLY (its clip set properly contains the feature's), not by the fragile first-PlayItem stream count — this is what makes the fix robust to the video-bearing-composite variant (see main_feature_order_demotes_video_composite).

key_source_failure_is_not_reported_as_a_missing_disc_key

Drives the real chain end to end — KeySource -> resolve_and_apply_traced -> Disc::aacs_error -> ensure_decryptable — twice over the SAME disc, and asserts the two operator-visible verdicts differ:

• source returns Err(KeyServiceUnavailable) (the HTTP-502 outage) → E7028 "the key service could not answer; retry later",
• source returns Ok(vec![]) (the service answered, no entry) → E7022 "no key source has a decryption key for this disc".

The credential and rate-limit verdicts are asserted alongside, since each is a different operator action (fix the token / back off).

css_disc_wide_no_key_is_disc_level_while_per_title_stays_skippable

The two CSS no-key conditions are NOT the same verdict and must classify oppositely through the public predicates:

• disc-wide — css_error is set: the MAIN feature's crack failed, so every title of this disc fails identically. Must be crate::error::is_disc_level_no_key (the rip loop fail-fasts) and must NOT be crate::error::is_skippable_title_stub. While both conditions shared E_CSS_KEY_MISSING, an uncrackable CSS disc iterated all N titles logging "title skipped" and exited 0 — a total failure reported as success.
• per-title — one title's own re-crack failed on a multi-VTS disc (title_is_clear == false, no key): skipping it and finishing the rest is correct policy, so it must STAY skippable and must NOT be disc-level.

Pinned in both directions so a future change cannot silently flip either.

correct_truehd_channels_full_correction_and_atmos_promotion

Kills the was_basic ==, the channels/rate !=/&& guards' "already correct" branch, the !matches! per-stream skip, the is_atmos == Some(true) branch, and the whole-function no-op mutant.

byte_offset_in_title_accumulates_across_extents

Kills: lba >= start && lba < end flipped to || (the first extent's disjunction would trivially match almost any lba and return the wrong, too-early offset); cumulative += flipped to *= (cumulative is seeded at 0, so *= freezes it at 0 forever); and sector_count * SECTOR_BYTES_U64 flipped to + or / (wrong per-extent byte length folded into cumulative).

chapter_at_offset_concrete_arithmetic

Kills: the / (time-fraction) flipped to % or *; the * (duration scale) flipped to +; the <= chapter-scan comparison flipped to >; the final chapter_idx + 1 flipped to - or *; and every whole-function fixed-tuple replacement (none produce (2, 60.0)).

locate_ranges_positive_bps_duration_and_at_risk

Kills bps > 0.0 flipped to ==/< (both would take the else 0.0 branch here, wrongly reporting 0); (*size as f64) / bps flipped to % or *; and the trailing * MILLIS_PER_SEC flipped to + or /.

decrypt_keys_for_title_css_span_reuse_is_half_open

A CSS title key is per-VTS: reusing the scan's cracked key for a title that lives OUTSIDE the cracked span descrambles that title with the wrong key, and the mux emits garbage at exit 0. crack_span is documented as the half-open LBA span [start, end), so overlap is extent.start < span.end && span.start < extent.end — both comparisons STRICT. A title that merely ABUTS the span (ends exactly where it begins, or begins exactly where it ends) shares no sector with it and must NOT reuse the key.

The reader serves only clear (all-zero) sectors, so a title that falls through to its own crack is reported unencrypted — distinguishable from the reused-key answer both in the key and in the is-clear flag.

merge_content_key_ranges

Merges per-title AACS key ranges into the sorted, disjoint set the whole-disc map needs (crate::decrypt::AacsKeyMap::entry_for requires disjoint ranges). Titles that share a clip resolve the SAME physical span (same LBAs -> same CPS unit -> same key). When a later range overlaps a kept one that carries the SAME key index and phase, the two are UNIONED (end extended to the max) — this covers both the exact-duplicate (shared clip) case and any partial overlap without ever dropping coverage, so no encrypted LBA is left in no range (which would pass through as ciphertext). A real disc never produces two DIFFERENT keys for one LBA; if that malformed case ever appeared, the later range is dropped to keep the set disjoint rather than extend one key over another key's LBAs.