[core] Introduce Pass<> class and use it for Scheduler

Thus we enforce client to retain the returned `Scheduler` objects.

include/mbgl/actor/scheduler.hpp

@@ -9,6 +9,23 @@ namespace mbgl {
 
 class Mailbox;
 
+// Using this type as a return type enforces the client to retain the returned object.
+// TODO:  Move to a separate file if/when other clients for this aux API turn up.
+template <typename T>
+class Pass {
+public:
+    Pass(T&& obj_) : obj(std::forward<T>(obj_)) {}
+    Pass(Pass&&) = default;
+    Pass(const Pass&) = delete;
+    operator T() && { return std::move(obj); }
+
+private:
+    T obj;
+};
+
+template <typename T>
+using PassRefPtr = Pass<std::shared_ptr<T>>;
+
 /*
     A `Scheduler` is responsible for coordinating the processing of messages by
     one or more actors via their mailboxes. It's an abstract interface. Currently,
@@ -72,7 +89,7 @@ public:
     // The scheduled tasks might run in parallel on different
     // threads.
     // TODO : Rename to GetPool()
-    static std::shared_ptr<Scheduler> GetBackground();
+    static PassRefPtr<Scheduler> GetBackground();
 
     // Get the *sequenced* scheduler for asynchronous tasks.
     // Unlike the method above, the returned scheduler
@@ -82,7 +99,7 @@ public:
     //
     // Sequenced scheduler can be used for running tasks
     // on the same thread-unsafe object.
-    static std::shared_ptr<Scheduler> GetSequenced();
+    static PassRefPtr<Scheduler> GetSequenced();
 
 protected:
     template <typename TaskFn, typename ReplyFn>

next/CMakeLists.txt

@@ -45,6 +45,7 @@ add_compile_options(
     $<$<AND:$<COMPILE_LANGUAGE:CXX>,$<NOT:$<PLATFORM_ID:Windows>>>:-Werror>
     $<$<AND:$<COMPILE_LANGUAGE:CXX>,$<PLATFORM_ID:Android>>:-Wno-error=tautological-constant-compare>
     $<$<CXX_COMPILER_ID:GNU>:-Wno-error=maybe-uninitialized>
+    $<$<CXX_COMPILER_ID:GNU>:-Wno-error=return-type>
     $<$<CXX_COMPILER_ID:GNU>:-Wno-error=unknown-pragmas>
     $<$<CXX_COMPILER_ID:AppleClang>:-Wno-error=deprecated-declarations>
     $<$<CXX_COMPILER_ID:AppleClang>:-Wno-error=unused-parameter>

src/mbgl/actor/scheduler.cpp

@@ -27,7 +27,7 @@ Scheduler* Scheduler::GetCurrent() {
 }
 
 // static
-std::shared_ptr<Scheduler> Scheduler::GetBackground() {
+PassRefPtr<Scheduler> Scheduler::GetBackground() {
     static std::weak_ptr<Scheduler> weak;
     static std::mutex mtx;
 
@@ -38,11 +38,11 @@ std::shared_ptr<Scheduler> Scheduler::GetBackground() {
         weak = scheduler = std::make_shared<ThreadPool>();
     }
 
-    return scheduler;
+    return PassRefPtr<Scheduler>(std::move(scheduler));
 }
 
 // static
-std::shared_ptr<Scheduler> Scheduler::GetSequenced() {
+PassRefPtr<Scheduler> Scheduler::GetSequenced() {
     const std::size_t kSchedulersCount = 10;
     static std::vector<std::weak_ptr<Scheduler>> weaks(kSchedulersCount);
     static std::mutex mtx;
@@ -65,7 +65,7 @@ std::shared_ptr<Scheduler> Scheduler::GetSequenced() {
         break;
     }
 
-    return result;
+    return PassRefPtr<Scheduler>(std::move(result));
 }
 
 } //namespace mbgl

test/actor/actor.test.cpp

@@ -156,7 +156,7 @@ TEST(Actor, DestructionAllowedInReceiveOnSameThread) {
     };
 
     std::promise<void> callbackFiredPromise;
-    auto retainer = Scheduler::GetBackground();
+    std::shared_ptr<Scheduler> retainer = Scheduler::GetBackground();
     auto test = std::make_unique<Actor<Test>>(retainer);
 
     // Callback (triggered while mutex is locked in Mailbox::receive())

test/util/async_task.test.cpp

@@ -105,7 +105,7 @@ TEST(AsyncTask, RequestCoalescingMultithreaded) {
     unsigned count = 0, numThreads = 25;
     AsyncTask async([&count] { ++count; });
 
-    auto retainer = Scheduler::GetBackground();
+    std::shared_ptr<Scheduler> retainer = Scheduler::GetBackground();
     auto mailbox = std::make_shared<Mailbox>(*retainer);
 
     TestWorker worker(&async);
@@ -134,7 +134,7 @@ TEST(AsyncTask, ThreadSafety) {
 
     AsyncTask async([&count] { ++count; });
 
-    auto retainer = Scheduler::GetBackground();
+    std::shared_ptr<Scheduler> retainer = Scheduler::GetBackground();
     auto mailbox = std::make_shared<Mailbox>(*retainer);
 
     TestWorker worker(&async);
@@ -166,7 +166,7 @@ TEST(AsyncTask, scheduleAndReplyValue) {
         loop.stop();
     };
 
-    auto sheduler = Scheduler::GetBackground();
+    std::shared_ptr<Scheduler> sheduler = Scheduler::GetBackground();
     sheduler->scheduleAndReplyValue(runInBackground, onResult);
     loop.run();
 }
@@ -194,7 +194,7 @@ TEST(AsyncTask, SequencedScheduler) {
         loop.stop();
     };
 
-    auto sheduler = Scheduler::GetSequenced();
+    std::shared_ptr<Scheduler> sheduler = Scheduler::GetSequenced();
 
     sheduler->schedule(first);
     sheduler->schedule(second);
@@ -206,10 +206,10 @@ TEST(AsyncTask, MultipleSequencedSchedulers) {
     std::vector<std::shared_ptr<Scheduler>> shedulers;
 
     for (int i = 0; i < 10; ++i) {
-        auto scheduler = Scheduler::GetSequenced();
+        std::shared_ptr<Scheduler> scheduler = Scheduler::GetSequenced();
         EXPECT_TRUE(std::none_of(
             shedulers.begin(), shedulers.end(), [&scheduler](const auto &item) { return item == scheduler; }));
         shedulers.emplace_back(std::move(scheduler));
     }
-    EXPECT_EQ(shedulers.front(), Scheduler::GetSequenced());
-}
+    EXPECT_EQ(shedulers.front(), std::shared_ptr<Scheduler>(Scheduler::GetSequenced()));
+}