fix(id): avoid comparing partial fixtures before head (#4820)

pkgs/id/src/world_fixture.rs

@@ -390,7 +390,7 @@ pub fn replay(
     let verdict = verdict(
         header,
         records.as_deref(),
-        final_epoch_covered(&fixture.events),
+        final_epoch_covered(header, &fixture.events),
     )?;
     Ok(Replayed {
         records,
@@ -477,7 +477,7 @@ fn replay_collecting(
     let verdict = verdict(
         header,
         records.as_deref(),
-        final_epoch_covered(&fixture.events),
+        final_epoch_covered(header, &fixture.events),
     )?;
     let final_records = match records.as_deref() {
         Some(text) => parse_records(text).context("the replayed records")?,
@@ -772,8 +772,14 @@ fn segments(events: &[FixtureEvent]) -> Result<Vec<&[FixtureEvent]>> {
 /// its install, so replaying the final segment is a complete replay of
 /// that epoch and its records can be compared with the header's.
 #[cfg(feature = "sandbox")]
-fn final_epoch_covered(events: &[FixtureEvent]) -> bool {
-    events.iter().any(is_install)
+fn final_epoch_covered(header: &FixtureHeader, events: &[FixtureEvent]) -> bool {
+    let has_install = events.iter().any(is_install);
+    let reaches_head = events
+        .iter()
+        .rev()
+        .find(|event| event.sequence() != 0)
+        .is_some_and(|event| event.sequence() == header.current_sequence);
+    has_install && reaches_head
 }
 
 /// The event a replay feeds the program, or `None` for events it never sees.
@@ -1311,9 +1317,8 @@ mod tests {
         #[test]
         fn partial_fixture_covering_the_final_epoch_is_compared() {
             let (runner, base) = checked_in_game();
-            // A partial window that still contains the last install covers
-            // the final epoch in full, so its records compare with the
-            // header's instead of reporting NotCompared.
+            // A front-truncated partial window still reaches the head and
+            // contains the last install, so its records remain comparable.
             let mut live = runner.instantiate(7).unwrap();
             live.update(2, b"3").unwrap();
             live.update(1, b"4").unwrap();
@@ -1327,19 +1332,38 @@ mod tests {
                     records: Some(records),
                     ..base.header
                 },
-                events: vec![
-                    install(1),
-                    input(2, 1, "0"),
-                    install(3),
-                    input(4, 2, "3"),
-                    input(5, 1, "4"),
-                ],
+                events: vec![install(3), input(4, 2, "3"), input(5, 1, "4")],
             };
             assert!(replay(&runner, 7, &partial, false).is_err());
             let replayed = replay(&runner, 7, &partial, true).unwrap();
             assert_eq!(replayed.verdict, Verdict::Match);
         }
 
+        #[test]
+        fn partial_fixture_ending_before_header_sequence_is_not_compared() {
+            let (runner, base) = checked_in_game();
+            let mut live = runner.instantiate(7).unwrap();
+            live.update(2, b"3").unwrap();
+            live.update(1, b"4").unwrap();
+            let records =
+                parse_records(&String::from_utf8(live.records().unwrap().unwrap()).unwrap())
+                    .unwrap();
+            let partial = Fixture {
+                header: FixtureHeader {
+                    current_sequence: 5,
+                    complete: false,
+                    records: Some(records),
+                    ..base.header
+                },
+                // The retained window contains the final install, but ends
+                // before the head sequence recorded in the header.
+                events: vec![install(3), input(4, 2, "3")],
+            };
+
+            let replayed = replay(&runner, 7, &partial, true).unwrap();
+            assert_eq!(replayed.verdict, Verdict::NotCompared("partial history"));
+        }
+
         #[test]
         fn chat_before_the_install_does_not_block_replay() {
             let (runner, base) = checked_in_game();