RCORE-2168: Replicate clear instruction unconditionally (#7810)

This will fix inconsistent merge behavior

CHANGELOG.md

@@ -33,6 +33,7 @@
 * Opening an FLX realm asynchronously may not wait to download all data ([#7720](https://github.com/realm/realm-core/issues/7720), since FLX sync was introduced).
 * Clearing a List of Mixed in an upgraded file would lead to an assertion failing ([#7771](https://github.com/realm/realm-core/issues/7771), since 14.0.0)
 * Fix compilation with Xcode 16 ([PR #7802](https://github.com/realm/realm-core/pull/7802))
+* You could get unexpected merge results when assigning to a list ([#7809](https://github.com/realm/realm-core/issues/7809), since v10.2.0)
 
 ### Breaking changes
 * None.

src/realm/dictionary.cpp

@@ -823,10 +823,11 @@ size_t Dictionary::find_first(Mixed value) const
 
 void Dictionary::clear()
 {
-    if (size() > 0) {
-        if (Replication* repl = get_replication()) {
-            repl->dictionary_clear(*this);
-        }
+    auto sz = size();
+    if (Replication* repl = get_replication()) {
+        repl->dictionary_clear(*this);
+    }
+    if (sz > 0) {
         CascadeState cascade_state(CascadeState::Mode::Strong);
         bool recurse = remove_backlinks(cascade_state);
 

src/realm/list.cpp

@@ -503,10 +503,11 @@ void Lst<Mixed>::remove(size_t from, size_t to)
 
 void Lst<Mixed>::clear()
 {
-    if (size() > 0) {
-        if (Replication* repl = Base::get_replication()) {
-            repl->list_clear(*this);
-        }
+    auto sz = size();
+    if (Replication* repl = Base::get_replication()) {
+        repl->list_clear(*this);
+    }
+    if (sz > 0) {
         CascadeState state;
         bool recurse = remove_backlinks(state);
 

src/realm/list.hpp

@@ -930,10 +930,11 @@ inline LnkLst Obj::get_linklist(StringData col_name) const
 template <class T>
 void Lst<T>::clear()
 {
-    if (size() > 0) {
-        if (Replication* repl = Base::get_replication()) {
-            repl->list_clear(*this);
-        }
+    auto sz = size();
+    if (Replication* repl = Base::get_replication()) {
+        repl->list_clear(*this);
+    }
+    if (sz > 0) {
         do_clear();
         bump_content_version();
     }

test/object-store/object.cpp

@@ -2011,8 +2011,8 @@ TEST_CASE("object") {
 
         r1->begin_transaction();
         r2->begin_transaction();
-        auto object1 = Object::create(c1, r1, *r1->schema().find("pk after list"), std::any(v1));
-        auto object2 = Object::create(c2, r2, *r2->schema().find("pk after list"), std::any(v2));
+        auto object1 = Object::create(c1, r1, "pk after list", std::any(v1), CreatePolicy::UpdateModified);
+        auto object2 = Object::create(c2, r2, "pk after list", std::any(v2), CreatePolicy::UpdateModified);
         r2->commit_transaction();
         r1->commit_transaction();
 

test/test_sync.cpp

@@ -6204,6 +6204,79 @@ TEST(Sync_DeleteCollectionInCollection)
     }
 }
 
+TEST(Sync_CollectionClear)
+{
+    TEST_CLIENT_DB(db_1);
+    TEST_CLIENT_DB(db_2);
+
+    TEST_DIR(dir);
+    fixtures::ClientServerFixture fixture{dir, test_context};
+    fixture.start();
+
+    Session session_1 = fixture.make_session(db_1, "/test");
+    Session session_2 = fixture.make_session(db_2, "/test");
+
+    Timestamp now{std::chrono::system_clock::now()};
+
+    write_transaction(db_1, [&](WriteTransaction& tr) {
+        auto& g = tr.get_group();
+        auto table = g.add_table_with_primary_key("class_Table", type_Int, "id");
+        auto col_list = table->add_column_list(type_Int, "ints");
+        auto col_list_mixed = table->add_column_list(type_Mixed, "any");
+        auto col_dict = table->add_column_dictionary(type_String, "features");
+
+        auto foo = table->create_object_with_primary_key(123);
+        auto list = foo.get_list<Int>(col_list);
+        list.clear();
+        list.add(1);
+        auto list_mixed = foo.get_list<Mixed>(col_list_mixed);
+        list_mixed.clear();
+        list_mixed.add("Hello");
+        auto dict = foo.get_dictionary(col_dict);
+        dict.clear();
+        dict.insert("address", "Any Road 3");
+    });
+
+    write_transaction(db_2, [&](WriteTransaction& tr) {
+        auto& g = tr.get_group();
+        auto table = g.add_table_with_primary_key("class_Table", type_Int, "id");
+        auto col_list = table->add_column_list(type_Int, "ints");
+        auto col_list_mixed = table->add_column_list(type_Mixed, "any");
+        auto col_dict = table->add_column_dictionary(type_String, "features");
+
+        auto foo = table->create_object_with_primary_key(123);
+        auto list = foo.get_list<Int>(col_list);
+        list.clear();
+        list.add(2);
+        auto list_mixed = foo.get_list<Mixed>(col_list_mixed);
+        list_mixed.clear();
+        list_mixed.add("Godbye");
+        auto dict = foo.get_dictionary(col_dict);
+        dict.clear();
+        dict.insert("city", "Andeby");
+    });
+
+    session_1.wait_for_upload_complete_or_client_stopped();
+    session_2.wait_for_upload_complete_or_client_stopped();
+    session_1.wait_for_download_complete_or_client_stopped();
+    session_2.wait_for_download_complete_or_client_stopped();
+
+    {
+        ReadTransaction read_1{db_1};
+        auto table = read_1.get_table("class_Table");
+        auto obj = table->get_object_with_primary_key(123);
+        auto list = obj.get_list<Int>("ints");
+        auto list_mixed = obj.get_list<Mixed>("any");
+        auto dict = obj.get_dictionary("features");
+        CHECK_EQUAL(list.size(), 1);
+        CHECK_EQUAL(list_mixed.size(), 1);
+        CHECK_EQUAL(dict.size(), 1);
+
+        ReadTransaction read_2{db_2};
+        CHECK(compare_groups(read_1, read_2));
+    }
+}
+
 TEST(Sync_Dictionary_Links)
 {
     TEST_CLIENT_DB(db_1);