EntityTree: only check for entity deletions when necessary (#8103)

* Fixes #8101

Before:

![image](https://github.com/user-attachments/assets/339211da-e82d-4646-9946-784f24cea88f)


After:

![image](https://github.com/user-attachments/assets/4a89eae9-6692-4415-a8c9-c336b6afe558)

crates/store/re_entity_db/src/entity_db.rs

@@ -5,8 +5,8 @@ use parking_lot::Mutex;
 
 use re_chunk::{Chunk, ChunkResult, RowId, TimeInt};
 use re_chunk_store::{
-    ChunkStore, ChunkStoreChunkStats, ChunkStoreConfig, ChunkStoreEvent, ChunkStoreHandle,
-    ChunkStoreSubscriber, GarbageCollectionOptions, GarbageCollectionTarget,
+    ChunkStore, ChunkStoreChunkStats, ChunkStoreConfig, ChunkStoreDiffKind, ChunkStoreEvent,
+    ChunkStoreHandle, ChunkStoreSubscriber, GarbageCollectionOptions, GarbageCollectionTarget,
 };
 use re_log_types::{
     ApplicationId, EntityPath, EntityPathHash, LogMsg, ResolvedTimeRange, ResolvedTimeRangeF,
@@ -385,7 +385,13 @@ impl EntityDb {
 
             // It is possible for writes to trigger deletions: specifically in the case of
             // overwritten static data leading to dangling chunks.
-            self.tree.on_store_deletions(&engine, &store_events);
+            let entity_paths_with_deletions = store_events
+                .iter()
+                .filter(|event| event.kind == ChunkStoreDiffKind::Deletion)
+                .map(|event| event.chunk.entity_path().clone())
+                .collect();
+            self.tree
+                .on_store_deletions(&engine, &entity_paths_with_deletions, &store_events);
 
             // We inform the stats last, since it measures e2e latency.
             self.stats.on_events(&store_events);
@@ -444,7 +450,7 @@ impl EntityDb {
         store_events
     }
 
-    fn gc(&mut self, gc_options: &GarbageCollectionOptions) -> Vec<ChunkStoreEvent> {
+    pub(crate) fn gc(&mut self, gc_options: &GarbageCollectionOptions) -> Vec<ChunkStoreEvent> {
         re_tracing::profile_function!();
 
         let mut engine = self.storage_engine.write();
@@ -544,7 +550,12 @@ impl EntityDb {
         time_histogram_per_timeline.on_events(store_events);
 
         let engine = engine.downgrade();
-        tree.on_store_deletions(&engine, store_events);
+        let entity_paths_with_deletions = store_events
+            .iter()
+            .filter(|event| event.kind == ChunkStoreDiffKind::Deletion)
+            .map(|event| event.chunk.entity_path().clone())
+            .collect();
+        tree.on_store_deletions(&engine, &entity_paths_with_deletions, store_events);
     }
 
     /// Key used for sorting recordings in the UI.

crates/store/re_entity_db/src/entity_tree.rs

@@ -1,7 +1,7 @@
 use std::collections::BTreeMap;
 
 use ahash::HashSet;
-use nohash_hasher::IntMap;
+use nohash_hasher::{IntMap, IntSet};
 
 use re_chunk::RowId;
 use re_chunk_store::{ChunkStoreDiffKind, ChunkStoreEvent, ChunkStoreSubscriber};
@@ -14,6 +14,7 @@ use re_types_core::ComponentName;
 /// A recursive, manually updated [`ChunkStoreSubscriber`] that maintains the entity hierarchy.
 ///
 /// The tree contains a list of subtrees, and so on recursively.
+#[derive(Debug)]
 pub struct EntityTree {
     /// Full path prefix to the root of this (sub)tree.
     pub path: EntityPath,
@@ -123,7 +124,12 @@ impl EntityTree {
     }
 
     /// Returns `true` if this entity has no children and no data.
-    pub fn is_empty(&self, engine: &StorageEngineReadGuard<'_>) -> bool {
+    ///
+    /// Checking for the absence of data is neither costly nor totally free: do it a few hundreds or
+    /// thousands times a frame and it will absolutely kill framerate.
+    /// Don't blindly call this on every existing entity every frame: use [`ChunkStoreEvent`]s to make
+    /// sure anything changed at all first.
+    pub fn check_is_empty(&self, engine: &StorageEngineReadGuard<'_>) -> bool {
         self.children.is_empty() && !engine.store().entity_has_data(&self.path)
     }
 
@@ -160,6 +166,7 @@ impl EntityTree {
     pub fn on_store_deletions(
         &mut self,
         engine: &StorageEngineReadGuard<'_>,
+        entity_paths_with_deletions: &IntSet<EntityPath>,
         events: &[ChunkStoreEvent],
     ) {
         re_tracing::profile_function!();
@@ -171,9 +178,22 @@ impl EntityTree {
 
         self.children.retain(|_, entity| {
             // this is placed first, because we'll only know if the child entity is empty after telling it to clear itself.
-            entity.on_store_deletions(engine, events);
+            entity.on_store_deletions(engine, entity_paths_with_deletions, events);
+
+            let has_children = || !entity.children.is_empty();
+            // Checking for lack of data is not free, so make sure there is any reason to believe
+            // that any relevant data has changed first.
+            let has_recursive_deletion_events = || {
+                entity_paths_with_deletions
+                    .iter()
+                    .any(|removed_entity_path| removed_entity_path.starts_with(&entity.path))
+            };
+            let has_data = || engine.store().entity_has_data(&entity.path);
+
+            let should_be_removed =
+                !has_children() && (has_recursive_deletion_events() && !has_data());
 
-            !entity.is_empty(engine)
+            !should_be_removed
         });
     }
 
@@ -237,3 +257,84 @@ impl EntityTree {
         visit(self, &mut predicate)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use std::sync::Arc;
+
+    use re_chunk::{Chunk, RowId};
+    use re_log_types::{example_components::MyPoint, EntityPath, StoreId, TimePoint, Timeline};
+
+    use crate::EntityDb;
+
+    #[test]
+    fn deleting_descendants() -> anyhow::Result<()> {
+        re_log::setup_logging();
+
+        let mut db = EntityDb::new(StoreId::random(re_log_types::StoreKind::Recording));
+
+        let timeline_frame = Timeline::new_sequence("frame");
+
+        let entity_path_parent: EntityPath = "parent".into();
+        let entity_path_child: EntityPath = "parent/child1".into();
+        let entity_path_grandchild: EntityPath = "parent/child1/grandchild".into();
+
+        assert!(db.tree().check_is_empty(&db.storage_engine()));
+
+        {
+            let row_id = RowId::new();
+            let timepoint = TimePoint::from_iter([(timeline_frame, 10)]);
+            let point = MyPoint::new(1.0, 2.0);
+            let chunk = Chunk::builder(entity_path_grandchild.clone())
+                .with_component_batches(row_id, timepoint, [&[point] as _])
+                .build()?;
+
+            db.add_chunk(&Arc::new(chunk))?;
+        }
+
+        {
+            let parent = db
+                .tree()
+                .find_first_child_recursive(|entity_path| *entity_path == entity_path_parent)
+                .unwrap();
+            let child = db
+                .tree()
+                .find_first_child_recursive(|entity_path| *entity_path == entity_path_child)
+                .unwrap();
+            let grandchild = db
+                .tree()
+                .find_first_child_recursive(|entity_path| *entity_path == entity_path_grandchild)
+                .unwrap();
+
+            assert_eq!(1, parent.children.len());
+            assert_eq!(1, child.children.len());
+            assert_eq!(0, grandchild.children.len());
+
+            assert!(!db.tree().check_is_empty(&db.storage_engine()));
+            assert!(!parent.check_is_empty(&db.storage_engine()));
+            assert!(!child.check_is_empty(&db.storage_engine()));
+            assert!(!grandchild.check_is_empty(&db.storage_engine()));
+        }
+
+        let _store_events = db.gc(&re_chunk_store::GarbageCollectionOptions::gc_everything());
+
+        {
+            let parent = db
+                .tree()
+                .find_first_child_recursive(|entity_path| *entity_path == entity_path_parent);
+            let child = db
+                .tree()
+                .find_first_child_recursive(|entity_path| *entity_path == entity_path_child);
+            let grandchild = db
+                .tree()
+                .find_first_child_recursive(|entity_path| *entity_path == entity_path_grandchild);
+
+            assert!(db.tree().check_is_empty(&db.storage_engine()));
+            assert!(parent.is_none());
+            assert!(child.is_none());
+            assert!(grandchild.is_none());
+        }
+
+        Ok(())
+    }
+}