Store less data for notification tracking on collections in Mixed

We only need the very next path element, not the entire remaining path.

Author: tgoyne

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ### Enhancements
 * Add support to synchronize collections embedded in Mixed properties and other collections (except sets) ([PR #7353](https://github.com/realm/realm-core/pull/7353)).
+* Improve performance of change notifications on nested collections somewhat ([PR #7402](https://github.com/realm/realm-core/pull/7402)).
 
 ### Fixed
 * Fixed conflict resolution bug which may result in an crash when the AddInteger instruction on Mixed properties is merged against updates to a non-integer type ([PR #7353](https://github.com/realm/realm-core/pull/7353)).

src/realm/collection_parent.cpp

@@ -66,14 +66,7 @@ bool StablePath::is_prefix_of(const StablePath& other) const noexcept
 {
     if (size() > other.size())
         return false;
-
-    auto it = other.begin();
-    for (auto& p : *this) {
-        if (!(p == *it))
-            return false;
-        ++it;
-    }
-    return true;
+    return std::equal(begin(), end(), other.begin());
 }
 
 /***************************** CollectionParent ******************************/

src/realm/object-store/collection_notifications.hpp

@@ -106,7 +106,7 @@ struct CollectionChangeSet {
     // Per-column version of `modifications`
     std::unordered_map<int64_t, IndexSet> columns;
 
-    std::set<StablePath> paths;
+    std::set<StableIndex> paths;
 
     bool empty() const noexcept
     {

src/realm/object-store/impl/list_notifier.cpp

@@ -62,9 +62,11 @@ void ListNotifier::attach(CollectionBase const& src)
     if (auto obj = tr.get_table(src.get_table()->get_key())->try_get_object(src.get_owner_key())) {
         auto path = src.get_stable_path();
         m_list = std::static_pointer_cast<CollectionBase>(obj.get_collection_by_stable_path(path));
+        m_collection_parent = dynamic_cast<CollectionParent*>(m_list.get());
     }
     else {
         m_list = nullptr;
+        m_collection_parent = nullptr;
     }
 }
 
@@ -73,14 +75,9 @@ bool ListNotifier::do_add_required_change_info(TransactionChangeInfo& info)
     if (!m_list || !m_list->is_attached())
         return false; // origin row was deleted after the notification was added
 
-    // We need to have the collections with the shortest paths first
     StablePath this_path = m_list->get_stable_path();
-    auto it = std::lower_bound(info.collections.begin(), info.collections.end(), this_path.size(),
-                               [](const CollectionChangeInfo& info, size_t sz) {
-                                   return info.path.size() < sz;
-                               });
-    info.collections.insert(
-        it, {m_list->get_table()->get_key(), m_list->get_owner_key(), std::move(this_path), &m_change});
+    info.collections.push_back(
+        {m_list->get_table()->get_key(), m_list->get_owner_key(), std::move(this_path), &m_change});
 
     m_info = &info;
 
@@ -142,14 +139,14 @@ void ListNotifier::run()
         }
     }
 
+    // Modifications to nested values in Mixed are recorded in replication as
+    // StableIndex and we have to look up the actual index afterwards
     if (m_change.paths.size()) {
-        if (auto coll = dynamic_cast<CollectionParent*>(m_list.get())) {
-            for (auto& p : m_change.paths) {
-                // Report changes in substructure as modifications on this list
-                auto ndx = coll->find_index(p[0]);
-                if (ndx != realm::not_found)
-                    m_change.modifications.add(ndx); // OK to insert same index again
-            }
+        REALM_ASSERT(m_collection_parent);
+        REALM_ASSERT(m_type == PropertyType::Mixed);
+        for (auto& p : m_change.paths) {
+            if (auto ndx = m_collection_parent->find_index(p); ndx != realm::not_found)
+                m_change.modifications.add(ndx);
         }
     }
 }

src/realm/object-store/impl/list_notifier.hpp

@@ -34,6 +34,7 @@ class ListNotifier : public CollectionNotifier {
 private:
     PropertyType m_type;
     CollectionBasePtr m_list;
+    CollectionParent* m_collection_parent = nullptr;
 
     // The last-seen size of the collection so that when the parent of the collection
     // is deleted we can report each row as being deleted

src/realm/object-store/impl/results_notifier.cpp

@@ -393,13 +393,13 @@ void ListResultsNotifier::run()
         std::iota(m_run_indices->begin(), m_run_indices->end(), 0);
     }
 
+    // Modifications to nested values in Mixed are recorded in replication as
+    // StableIndex and we have to look up the actual index afterwards
     if (m_change.paths.size()) {
         if (auto coll = dynamic_cast<CollectionParent*>(m_list.get())) {
             for (auto& p : m_change.paths) {
-                // Report changes in substructure as modifications on this list
-                auto ndx = coll->find_index(p[0]);
-                if (ndx != realm::not_found)
-                    m_change.modifications.add(ndx); // OK to insert same index again
+                if (auto ndx = coll->find_index(p); ndx != realm::not_found)
+                    m_change.modifications.add(ndx);
             }
         }
     }

src/realm/object-store/impl/transact_log_handler.cpp

@@ -368,21 +368,19 @@ class TransactLogObserver : public TransactLogValidationMixin {
     {
         modify_object(col, obj);
         auto table = current_table();
+        m_active_collection = nullptr;
         for (auto& c : m_info.collections) {
-
             if (c.table_key == table && c.obj_key == obj && c.path.is_prefix_of(path)) {
                 if (c.path.size() != path.size()) {
-                    StablePath sub_path;
-                    sub_path.insert(sub_path.begin(), path.begin() + c.path.size(), path.end());
-                    c.changes->paths.insert(std::move(sub_path));
+                    c.changes->paths.insert(path[c.path.size()]);
                 }
-                else {
+                // If there are multiple exact matches for this collection we
+                // use the first and then propagate the data to the others later
+                else if (!m_active_collection) {
                     m_active_collection = c.changes;
-                    return true;
                 }
             }
         }
-        m_active_collection = nullptr;
         return true;
     }