web_service: stop using std::future + callback style async

src/common/detached_tasks.h

@@ -0,0 +1,39 @@
+// Copyright 2018 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+#include <condition_variable>
+#include <functional>
+
+namespace Common {
+
+/**
+ * A background manager which ensures that all detached task is finished before program exits.
+ *
+ * Some tasks, telemetry submission for example, prefer executing asynchronously and don't care
+ * about the result. These tasks are suitable for std::thread::detach(). However, this is unsafe if
+ * the task is launched just before the program exits (which is a common case for telemetry), so we
+ * need to block on these tasks on program exit.
+ *
+ * To make detached task safe, a single DetachedTasks object should be placed in the main(), and
+ * call WaitForAllTasks() after all program execution but before global/static variable destruction.
+ * Any potentially unsafe detached task should be executed via DetachedTasks::AddTask.
+ */
+class DetachedTasks {
+public:
+    DetachedTasks();
+    ~DetachedTasks();
+    void WaitForAllTasks();
+
+    static void AddTask(std::function<void()> task);
+
+private:
+    static DetachedTasks* instance;
+
+    std::condition_variable cv;
+    std::mutex mutex;
+    int count = 0;
+};
+
+} // namespace Common