fix(scanner): retain raw enumeration quantum

Co-Authored-By: heihutu <heihutu@gmail.com>

Co-Authored-By: zhi22915 <qiuzgang@gmail.com>

crates/scanner/src/scanner_folder.rs

@@ -846,6 +846,16 @@ impl RawEnumerationProgress {
             }
         })
     }
+
+    fn has_checkpointable_page_index(&self) -> bool {
+        self.page_index().is_some()
+    }
+
+    fn checkpointable_entry_count(&self) -> usize {
+        self.page_index()
+            .and_then(|index| index.indexed_entries().ok())
+            .map_or(0, |entries| entries.len())
+    }
 }
 
 fn update_raw_enumeration_digest(digest: &mut Sha256, label: &[u8], value: &[u8]) {
@@ -1165,20 +1175,33 @@ impl FolderScanner {
     }
 
     fn finish_raw_enumeration_parent(&mut self, parent: &str) {
+        let scan_root = self.old_cache.info.name.as_str();
         self.raw_enumeration_progress.retain(|progress| {
-            progress.parent != parent
-                && !progress
-                    .parent
-                    .strip_prefix(parent)
-                    .is_some_and(|suffix| suffix.starts_with(SLASH_SEPARATOR))
+            if progress.parent == parent {
+                return parent == scan_root && progress.has_checkpointable_page_index();
+            }
+
+            !progress
+                .parent
+                .strip_prefix(parent)
+                .is_some_and(|suffix| suffix.starts_with(SLASH_SEPARATOR))
         });
     }
 
     fn take_raw_enumeration_resume_state(&mut self) -> (Option<DataUsageRawEnumerationCursor>, Option<RawEnumerationPageIndex>) {
-        match self.raw_enumeration_progress.drain(..).next() {
-            Some(progress) => (progress.cursor(), progress.page_index()),
-            None => (None, None),
+        if self.raw_enumeration_progress.is_empty() {
+            return (None, None);
         }
+        let progress_index = self
+            .raw_enumeration_progress
+            .iter()
+            .enumerate()
+            .max_by_key(|(index, progress)| (progress.checkpointable_entry_count(), std::cmp::Reverse(*index)))
+            .map(|(index, _)| index)
+            .unwrap_or(0);
+        let progress = self.raw_enumeration_progress.swap_remove(progress_index);
+        self.raw_enumeration_progress.clear();
+        (progress.cursor(), progress.page_index())
     }
 
     fn carry_forward_old_children(&mut self, parent_hash: &DataUsageHash, entry: &mut DataUsageEntry) {

crates/scanner/src/scanner_folder/tests.rs

@@ -3512,6 +3512,80 @@ fn raw_enumeration_progress_checkpoint_commits_budgeted_page_for_oracle() {
     );
 }
 
+#[tokio::test]
+async fn raw_enumeration_root_page_survives_child_partial_boundary() {
+    let (mut scanner, temp_dir) = build_test_scanner().await;
+    let _guard = TestGuard {
+        temp_dir: Some(temp_dir),
+    };
+    scanner.old_cache.info.name = "bucket".to_string();
+
+    let mut root_progress = RawEnumerationProgress::new("bucket", None);
+    root_progress.record_entry("object-0000");
+    root_progress.record_entry("object-0001");
+    scanner.raw_enumeration_progress.push(root_progress);
+    scanner.finish_raw_enumeration_parent("bucket");
+
+    assert_eq!(scanner.raw_enumeration_progress.len(), 1);
+    let root_index = scanner.raw_enumeration_progress[0]
+        .page_index()
+        .expect("completed scan root should retain its raw-page oracle");
+    assert_eq!(
+        root_index
+            .committed_entries()
+            .expect("retained root raw-page oracle should validate"),
+        vec!["object-0000".to_string(), "object-0001".to_string()]
+    );
+
+    let mut child_progress = RawEnumerationProgress::new("bucket/object-0000", None);
+    child_progress.record_entry("xl.meta");
+    scanner.raw_enumeration_progress.push(child_progress);
+    scanner.finish_raw_enumeration_parent("bucket/object-0000");
+
+    assert_eq!(
+        scanner
+            .raw_enumeration_progress
+            .iter()
+            .map(|progress| progress.parent.as_str())
+            .collect::<Vec<_>>(),
+        vec!["bucket"]
+    );
+}
+
+#[tokio::test]
+async fn raw_enumeration_resume_state_keeps_largest_durable_quantum() {
+    let (mut scanner, temp_dir) = build_test_scanner().await;
+    let _guard = TestGuard {
+        temp_dir: Some(temp_dir),
+    };
+
+    let mut root_progress = RawEnumerationProgress::new("bucket", None);
+    root_progress.record_entry("object-0000");
+    scanner.raw_enumeration_progress.push(root_progress);
+
+    let mut child_progress = RawEnumerationProgress::new("bucket/object-0000", None);
+    child_progress.record_entry("part-0000");
+    child_progress.record_entry("part-0001");
+    child_progress.record_entry("part-0002");
+    scanner.raw_enumeration_progress.push(child_progress);
+
+    let (cursor, page_index) = scanner.take_raw_enumeration_resume_state();
+    assert_eq!(
+        cursor.as_ref().expect("largest raw quantum should include a cursor").parent,
+        "bucket/object-0000"
+    );
+    assert_eq!(
+        page_index
+            .as_ref()
+            .expect("largest raw quantum should include a page index")
+            .indexed_entries()
+            .expect("selected page index should validate")
+            .len(),
+        3
+    );
+    assert!(scanner.raw_enumeration_progress.is_empty());
+}
+
 #[test]
 fn raw_enumeration_progress_retains_resume_index_until_unordered_entries_reappear() {
     let mut index = RawEnumerationPageIndex::new("bucket", 2).expect("raw page index should initialize");