fix(scanner): reject trailing cycle state bytes

crates/scanner/src/scanner/cycle_state.rs

@@ -86,7 +86,11 @@ pub(super) fn decode_scanner_cycle_state(buf: &[u8]) -> Result<(CurrentCycle, u6
         (0, &buf[8..])
     };
 
-    let cycle_info = rmp_serde::from_slice::<CurrentCycle>(payload)?;
+    let mut deserializer = rmp_serde::Deserializer::new(std::io::Cursor::new(payload));
+    let cycle_info = CurrentCycle::deserialize(&mut deserializer)?;
+    if deserializer.position() != u64::try_from(payload.len()).unwrap_or(u64::MAX) {
+        return Err(ScannerCycleStateError::InvalidData("scanner cycle state has trailing bytes"));
+    }
     if cycle_info.next != persisted_next {
         return Err(ScannerCycleStateError::InvalidData("scanner cycle counter disagrees with encoded state"));
     }

crates/scanner/src/scanner/tests.rs

@@ -963,6 +963,10 @@ fn scanner_cycle_state_decodes_legacy_and_fenced_formats() {
     let (fenced_cycle, fenced_epoch) = decode_scanner_cycle_state(&fenced).expect("fenced cycle state should decode");
     assert_eq!(fenced_cycle.next, 13);
     assert_eq!(fenced_epoch, 7);
+
+    let mut trailing = fenced;
+    trailing.push(0);
+    assert!(decode_scanner_cycle_state(&trailing).is_err());
 }
 
 #[test]