[daemon] ensure all outstanding futures are done before exit

The daemon object hosts to QFuture and QFutureWatcher objects,
which may be still active while the daemon is being destructed.

This patch ensures that the destructor call waits for all outstanding
futures, and all the raised signals during the lifetime of the futures
are delivered to the recipients.

Signed-off-by: Mustafa Kemal Gilor <[email protected]>

Author: xmkg

src/daemon/daemon.cpp

@@ -1465,6 +1465,25 @@ mp::Daemon::~Daemon()
     mp::top_catch_all(category, [this] {
         MP_SETTINGS.unregister_handler(instance_mod_handler);
         MP_SETTINGS.unregister_handler(snapshot_mod_handler);
+
+        /**
+         * Wait until all futures are finished, so there will
+         * be no outstanding requests left behind. Otherwise, the
+         * futures might be interrupted halfway and the actions
+         * associated with the futures are not going to be
+         * executed.
+         */
+        for (auto& [_, watcher] : async_future_watchers)
+        {
+            watcher->waitForFinished();
+        }
+
+        // waitForFinished() ensures that the futures are finished gracefully
+        // but there's a chance that the signals which are queued during their
+        // execution haven't got executed yet. So, process all the remaining events
+        // in the event loop immediately to ensure that all recipients are notified
+        // before the daemon object destructs.
+        QCoreApplication::processEvents(QEventLoop::AllEvents);
     });
 }