Always send an empty changeset to the first call of a Results notification callback

src/impl/background_collection.cpp

@@ -208,8 +208,15 @@ bool BackgroundCollection::deliver(SharedGroup& sg, std::exception_ptr err)
 
 void BackgroundCollection::call_callbacks()
 {
-    while (auto fn = next_callback()) {
-        fn(m_changes_to_deliver, m_error);
+    while (auto cb = next_callback()) {
+        auto fn = cb->fn;
+        if (!cb->initial_delivered && should_deliver_initial()) {
+            cb->initial_delivered = true;
+            fn({}, m_error); // note: may invalidate `cb`
+        }
+        else {
+            fn(m_changes_to_deliver, m_error);
+        }
     }
 
     if (m_error) {
@@ -220,7 +227,7 @@ void BackgroundCollection::call_callbacks()
     }
 }
 
-CollectionChangeCallback BackgroundCollection::next_callback()
+BackgroundCollection::Callback* BackgroundCollection::next_callback()
 {
     std::lock_guard<std::mutex> callback_lock(m_callback_mutex);
 
@@ -230,9 +237,7 @@ CollectionChangeCallback BackgroundCollection::next_callback()
         if (!m_error && !deliver_initial && m_changes_to_deliver.empty()) {
             continue;
         }
-
-        callback.initial_delivered = true;
-        return callback.fn;
+        return &callback;
     }
 
     m_callback_index = npos;

src/impl/background_collection.hpp

@@ -158,7 +158,7 @@ private:
     // remove_callback() updates this when needed
     size_t m_callback_index = npos;
 
-    CollectionChangeCallback next_callback();
+    Callback* next_callback();
 };
 
 } // namespace _impl

tests/results.cpp

@@ -283,6 +283,16 @@ TEST_CASE("Results") {
             r->notify();
             REQUIRE(notification_calls == 1);
         }
+
+        SECTION("the first call of a notification always passes an empty change even if it previously ran for a different callback") {
+            auto token2 = results.add_notification_callback([&](CollectionChangeIndices c, std::exception_ptr) {
+                REQUIRE(c.empty());
+            });
+
+            write([&] {
+                table->set_int(0, table->add_empty_row(), 5);
+            });
+        }
     }
 
     // Sort in descending order