RCORE-2004 Make Realm::call_completion_callbacks() resilient to trasaction being deleted (#7437)

Author: jedelbo

CHANGELOG.md

@@ -8,6 +8,7 @@
 * Fix an assertion failure "m_lock_info && m_lock_info->m_file.get_path() == m_filename" that appears to be related to opening a Realm while the file is in the process of being closed on another thread ([Swift #8507](https://github.com/realm/realm-swift/issues/8507)).
 * Fixed diverging history due to a bug in the replication code when setting default null values (embedded objects included) ([#7536](https://github.com/realm/realm-core/issues/7536)).
 * Version 19.39.33523 of MSVC would crash when compiling for arm64 in release mode ([PR #7533](https://github.com/realm/realm-core/pull/7533)).
+* Null pointer exception may be triggered when logging out and async commits callbacks not executed ([#7434](https://github.com/realm/realm-core/issues/7434), since v13.26.0)
 
 ### Breaking changes
 * Updated default base URL to be `https://services.cloud.mongodb.com` to support the new domains (was `https://realm.mongodb.com`). ([PR #7534](https://github.com/realm/realm-core/pull/7534))

src/realm/object-store/shared_realm.cpp

@@ -765,7 +765,7 @@ void Realm::run_writes_on_proper_thread()
 
 void Realm::call_completion_callbacks()
 {
-    if (m_is_running_async_commit_completions) {
+    if (m_is_running_async_commit_completions || m_async_commit_q.empty()) {
         return;
     }
 

test/object-store/realm.cpp

@@ -2440,6 +2440,68 @@ TEST_CASE("SharedRealm: async writes") {
 
     _impl::RealmCoordinator::clear_all_caches();
 }
+
+TEST_CASE("Call run_async_completions after realm has been closed") {
+    // This requires a special scheduler as we have to call Realm::close
+    // just after DB::AsyncCommitHelper has made a callback to the function
+    // that asks the scheduler to invoke run_async_completions()
+
+    struct ManualScheduler : util::Scheduler {
+        std::mutex mutex;
+        std::condition_variable cv;
+        std::vector<util::UniqueFunction<void()>> callbacks;
+
+        void invoke(util::UniqueFunction<void()>&& cb) override
+        {
+            {
+                std::lock_guard lock(mutex);
+                callbacks.push_back(std::move(cb));
+            }
+            cv.notify_all();
+        }
+
+        bool is_on_thread() const noexcept override
+        {
+            return true;
+        }
+        bool is_same_as(const Scheduler*) const noexcept override
+        {
+            return false;
+        }
+        bool can_invoke() const noexcept override
+        {
+            return true;
+        }
+    };
+
+    auto scheduler = std::make_shared<ManualScheduler>();
+
+    TestFile config;
+    config.schema_version = 0;
+    config.schema = Schema{{"object", {{"value", PropertyType::Int}}}};
+    config.scheduler = scheduler;
+    config.automatic_change_notifications = false;
+
+    auto realm = Realm::get_shared_realm(config);
+
+    realm->begin_transaction();
+    realm->async_commit_transaction([](std::exception_ptr) {});
+
+    std::vector<util::UniqueFunction<void()>> callbacks;
+    {
+        std::unique_lock lock(scheduler->mutex);
+        // Wait for scheduler to be invoked
+        scheduler->cv.wait(lock, [&] {
+            return !scheduler->callbacks.empty();
+        });
+        callbacks.swap(scheduler->callbacks);
+    }
+    realm->close();
+    // Call whatever functions that was added to scheduler.
+    for (auto& cb : callbacks)
+        cb();
+}
+
 // Our libuv scheduler currently does not support background threads, so we can
 // only run this on apple platforms
 #if REALM_PLATFORM_APPLE