Audit round: an ADTS frame that cannot hold its own CRC, and three drifts

An ADTS header that declares a CRC follows must be at least nine bytes —
seven of header plus the two the CRC occupies — because the declared frame
length counts them. The structural gate compared against a flat seven and
never read the bit that says whether a CRC is there at all, so a frame
whose own header describes something impossible was accepted and handed to
the muxer as decodable.

The pipeline's spawn doc named Sweep, and the thread Sweep would have
created, as callers to look for. Neither has been in this crate since the
recovery passes moved out. The same paragraph already records fixing this
once, for a different departed caller — it simply drifted again a sentence
later, so it now says to name callers that live here or name none.

Whether a disc is structurally AACS-encrypted was spelled out by hand in
both the fast identify and the full scan. They agreed today; nothing made
them agree tomorrow, and disagreeing would mean the same disc reported
encrypted by one path and clear by the other. There is one definition now,
and the comment that pointed at it by line number points at its name.

The AC-3 parser built a fresh buffer on every packet — of the order of a
hundred thousand times per title — to work around a borrow it cannot
avoid. The copy stays; the allocation does not. The buffer is now lent out
and handed back, and a test pins that, because reverting it would be
invisible in behaviour.

Left alone deliberately: send/send_with_halt and finish/finish_with_halt
look like one action under two names, and are not. After the consumer
fails, one must still accept items and the other must refuse them; that
difference is what stops a producer reading an entire disc for a write
that died on its first frame. Collapsing them was tried here and the
existing test caught it. Both now say so where the choice is made.

src/mux/codec/ac3.rs

@@ -67,6 +67,13 @@ pub struct Ac3Parser {
     /// began in an earlier packet takes THAT packet's source offset, not the
     /// one that happened to complete it.
     acc: super::pesbuf::PesBuf,
+    /// Reused working copy of `acc`, kept across calls so the per-PES scan does
+    /// not allocate. `parse` runs once per PES packet on an audio track — of
+    /// the order of 10^5 times per title — and previously built a fresh `Vec`
+    /// each time. The copy itself is unavoidable without restructuring the
+    /// borrow relationship (the scanner needs `&mut self.tally` while it reads
+    /// these bytes); the ALLOCATION is not.
+    scratch: Vec<u8>,
     /// PTS (ns) to stamp on the frame that begins the carry-over `buf` — i.e.
     /// the running per-frame PTS at the point the partial tail was retained.
     /// Used by `flush()` to time the final buffered frame at EOS.
@@ -95,6 +102,7 @@ impl Default for Ac3Parser {
 impl Ac3Parser {
     pub fn new() -> Self {
         Self {
+            scratch: Vec::new(),
             acc: super::pesbuf::PesBuf::with_capacity(4096),
             flush_pts_ns: 0,
             tally: super::dropgate::DropTally::new("ac3"),
@@ -500,7 +508,12 @@ impl CodecParser for Ac3Parser {
         // Copy the working bytes out so the scanner can borrow `self.tally`;
         // the buffer keeps its marks, so the unconsumed tail stays attributed
         // to the packet that carried it.
-        let buf = self.acc.as_slice().to_vec();
+        // Take the scratch OUT of `self` so the scanner can borrow `self.tally`
+        // while reading it; it is put back at the end of the call, keeping its
+        // capacity for the next PES. There is no early return after this point.
+        let mut buf = std::mem::take(&mut self.scratch);
+        buf.clear();
+        buf.extend_from_slice(self.acc.as_slice());
         let marks = self.acc.marks_snapshot();
         let data = &buf;
         let (frames, keep_from, frame_pts_ns) =
@@ -541,6 +554,9 @@ impl CodecParser for Ac3Parser {
             self.flush_pts_ns = frame_pts_ns;
         }
 
+        // Hand the working buffer back so the next PES reuses its capacity.
+        // `data` borrowed it; that borrow ends here, at its last use.
+        self.scratch = buf;
         frames
     }
 
@@ -807,6 +823,53 @@ mod tests {
         frame[n - 1] = (c & 0xFF) as u8;
     }
 
+    /// The per-PES working buffer must be REUSED, not reallocated.
+    ///
+    /// `parse` runs once per PES packet — of the order of 10^5 times on a
+    /// feature's audio track — and used to build a fresh `Vec` every call. The
+    /// copy itself cannot go without restructuring the borrow relationship, but
+    /// the allocation can: the buffer is taken out of `self`, filled, and put
+    /// back with its capacity intact.
+    ///
+    /// Asserting on capacity is deliberately white-box, because that is exactly
+    /// what regresses if someone reverts to `to_vec()` — the behaviour would be
+    /// identical and no other test would notice.
+    #[test]
+    fn the_working_buffer_is_reused_across_packets_not_reallocated() {
+        let mut parser = Ac3Parser::new();
+        let frame = make_ac3_frame(0, 0);
+
+        parser.parse(&PesPacket {
+            source: None,
+            pid: 0x1100,
+            pts: Some(90_000),
+            dts: None,
+            data: frame.clone(),
+            discontinuity: false,
+        });
+        let cap_after_first = parser.scratch.capacity();
+        assert!(
+            cap_after_first > 0,
+            "the buffer must be handed back to the parser, not dropped"
+        );
+
+        for _ in 0..8 {
+            parser.parse(&PesPacket {
+                source: None,
+                pid: 0x1100,
+                pts: None,
+                dts: None,
+                data: frame.clone(),
+                discontinuity: false,
+            });
+        }
+        assert!(
+            parser.scratch.capacity() >= cap_after_first,
+            "capacity must persist across calls; a fresh Vec each time would \
+             show it dropping back to the last packet's size"
+        );
+    }
+
     #[test]
     fn parse_empty_pes() {
         let mut parser = Ac3Parser::new();