settings: implement KeyValueStore (#1659)

* settings: implement KeyValueStore and dependencies

* settings: update KeyValueStore for recent refactoring

* settings: address feedback

libraries/libstratosphere/source/settings/impl/settings_static_object.hpp

@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) Atmosphère-NX
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#include <stratosphere.hpp>
+
+namespace ams::settings::impl {
+
+    template<typename T, typename Tag>
+    class StaticObject final {
+        NON_COPYABLE(StaticObject);
+        NON_MOVEABLE(StaticObject);
+        private:
+        StaticObject();
+        public:
+            static T &Get() {
+                /* Declare static instance variables. */
+                static constinit util::TypedStorage<T> s_storage = {};
+                static constinit bool s_initialized = false;
+                static constinit os::SdkMutex s_mutex;
+
+                /* If we haven't already done so, construct the instance. */
+                if (AMS_UNLIKELY(!s_initialized)) {
+                    std::scoped_lock lk(s_mutex);
+
+                    /* Check that we didn't concurrently construct the instance. */
+                    if (AMS_LIKELY(!s_initialized)) {
+                        /* Construct the instance. */
+                        util::ConstructAt(s_storage);
+
+                        /* Note that we constructed. */
+                        s_initialized = true;
+                    }
+                }
+
+                /* Return the constructed instance. */
+                return util::GetReference(s_storage);
+            }
+    };
+
+}