Bugfix for client reset with recovery

CHANGELOG.md

@@ -9,6 +9,7 @@
 * <How do the end-user experience this issue? what was the impact?> ([#????](https://github.com/realm/realm-core/issues/????), since v?.?.?)
 * CompensatingWriteErrorInfo reported string primary keys as boolean values instead ([PR #5938](https://github.com/realm/realm-core/pull/5938), since the introduction of CompensatingWriteErrorInfo in 12.1.0).
 * Fix a use-after-free if the last external reference to an encrypted Realm was closed between when a client reset error was received and when the download of the new Realm began. ([PR #5949](https://github.com/realm/realm-core/pull/5949), since 12.4.0).
+* Fixed an assertion failure during client reset with recovery when recovering a list operation on an embedded object that has a link column in the path prefix to the list from the top level object. ([PR #5957](https://github.com/realm/realm-core/issues/5957), since introduction of automatic recovery in v11.16.0).
 
 ### Breaking changes
 * Rename RealmConfig::automatic_handle_backlicks_in_migrations to RealmConfig::automatically_handle_backlinks_in_migrations ([PR #5897](https://github.com/realm/realm-core/pull/5897)).

src/realm/sync/noinst/client_reset_recovery.cpp

@@ -521,8 +521,7 @@ bool RecoverLocalChangesetsHandler::resolve_path(ListPath& path, Obj remote_obj,
                 REALM_UNREACHABLE();
             }
         }
-        else {
-            REALM_ASSERT(col.is_dictionary());
+        else if (col.is_dictionary()) {
             ++it;
             REALM_ASSERT(it != path.end());
             REALM_ASSERT(it->type == ListPath::Element::Type::InternKey);
@@ -538,6 +537,15 @@ bool RecoverLocalChangesetsHandler::resolve_path(ListPath& path, Obj remote_obj,
                 return false;
             }
         }
+        else { // single link to embedded object
+            // Neither embedded object sets nor Mixed(TypedLink) to embedded objects are supported.
+            REALM_ASSERT_EX(!col.is_collection(), col);
+            REALM_ASSERT_EX(col.get_type() == col_type_Link, col);
+            StringData col_name = remote_obj.get_table()->get_column_name(col);
+            remote_obj = remote_obj.get_linked_object(col);
+            local_obj = local_obj.get_linked_object(col_name);
+            ++it;
+        }
     }
     return false;
 }

test/object-store/sync/client_reset.cpp

@@ -3165,7 +3165,6 @@ TEST_CASE("client reset with embedded object", "[client reset][local][embedded o
     }
     SECTION("with shared initial state") {
         TopLevelContent initial;
-        initial.link_value = util::none;
         test_reset->setup([&](SharedRealm realm) {
             auto table = get_table(*realm, "TopLevel");
             REQUIRE(table);
@@ -3201,6 +3200,22 @@ TEST_CASE("client reset with embedded object", "[client reset][local][embedded o
             TopLevelContent expected_recovered = local;
             reset_embedded_object({local}, {remote}, expected_recovered);
         }
+        SECTION("local ArraySet to an embedded object through a deep link->linklist element which is removed by the "
+                "remote "
+                "triggers a list copy") {
+            local.link_value->array_vals[0] = 12345;
+            remote.link_value->array_vals.erase(remote.link_value->array_vals.begin());
+            TopLevelContent expected_recovered = local;
+            reset_embedded_object({local}, {remote}, expected_recovered);
+        }
+        SECTION("local ArrayErase to an embedded object through a deep link->linklist element which is removed by "
+                "the remote "
+                "triggers a list copy") {
+            local.link_value->array_vals.erase(local.link_value->array_vals.begin());
+            remote.link_value->array_vals.clear();
+            TopLevelContent expected_recovered = local;
+            reset_embedded_object({local}, {remote}, expected_recovered);
+        }
         SECTION("local modifications to an embedded object through a linklist cleared by the remote triggers a list "
                 "copy") {
             local.array_values.begin()->name = "modified";