"Realm accessed from incorrect thread" when called from JUCE Message Thread

[adamski]

Realm-cpp version 2.2.0

When processing a realm::results<T> object, from the message thread in a JUCE project, I get a LogicError exception with error code WrongThread and message "Realm accessed from incorrect thread."

I'm not sure if this is a bug or intentional. If intentional, I'm not sure how to do the processing on the thread that Realm expects.

[sync-by-unito]

➤ PM Bot commented:

Jira ticket: RCPP-86

[leemaguire]

Hey @adamski, I'm not familiar with how JUCE does things under the hood. Realm requires all operations to be done on the thread it was opened on, other wise you should use a thread safe reference. Did you initially open this realm instance on the main thread?

[adamski]

Thanks @leemaguire. I now realise what's going wrong. I made some changes to log in on a separate thread, and I'm setting up the realm instance if successful also on that thread. That explains the issue. I need to ensure that the realm instance is created asynchronously once the log in status changes. (I'm starting with a local realm and re-initialising to a synced realm once logged in).

[adamski]

For anyone else that comes across this issue, I solved it using the realm Scheduler class. I would normally use JUCE's AsyncUpdater but in this case I want to keep the code I'm writing framework agnostic.

            auto scheduler = realm::util::Scheduler::make_default();
            std::thread loginThread ([this, username, scheduler]()
                                     {
            try
            {
                std::future_status status;
                // TODO: Improve security with JWT based login
                std::string credentials = "{ \"username\": \"" + username + "\" }";

                std::cout << "Logging in..." << std::endl;
                auto userFuture = app->login (realm::App::credentials::function (credentials));
                std::cout << "Waiting for response" << std::endl;

                do
                {
                    switch (status = userFuture.wait_for (50ms); status)
                    {
                        case std::future_status::deferred:
                            std::cout << "deferred\n";
                            break;
                        case std::future_status::timeout:
                            std::cout << "timeout\n";
                            break;
                        case std::future_status::ready:
                            std::cout << "ready!\n";
                            break;
                    }
                } while (status != std::future_status::ready);

                user = userFuture.get();
                std::cout << "Logged in. User ID is " << user.identifier() << '\n';

                scheduler->invoke ([this]
                                   {
                                       setupSyncedRealm();
                                       setupSubscriptions();
                                       setupObservers();
                                   });
            }
            catch (const realm::app_error& e)
            {
                std::cout << "Error logging in: " << e.message() << std::endl;
                return;
            } });

            loginThread.detach();

[adamski]

I realised the code above is not working - the lambda passed to invoke is never executed. @leemaguire do you have any insight on that? If necessary I can move the async call elsewhere using a different technique, but the scheduler seems like a nice way to do it.

[adamski]

Closing this issue as it was user error, I'll open another one about the scheduler