From 23523621fd44135102bd6e0ceea9a392eaebaae3 Mon Sep 17 00:00:00 2001
From: Mikhail Pozdnyakov <mikhail.pozdnyakov@mapbox.com>
Date: Mon, 2 Dec 2019 10:40:59 +0200
Subject: [core] Introduce Pass<> class and use it for Scheduler

Thus we enforce client to retain the returned `Scheduler` objects.
---
 include/mbgl/actor/scheduler.hpp | 21 +++++++++++++++++++--
 src/mbgl/actor/scheduler.cpp     |  4 ++--
 test/actor/actor.test.cpp        |  2 +-
 test/util/async_task.test.cpp    | 14 +++++++-------
 4 files changed, 29 insertions(+), 12 deletions(-)

diff --git a/include/mbgl/actor/scheduler.hpp b/include/mbgl/actor/scheduler.hpp
index 04cbc389e5..d7fec41ea6 100644
--- a/include/mbgl/actor/scheduler.hpp
+++ b/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>
diff --git a/src/mbgl/actor/scheduler.cpp b/src/mbgl/actor/scheduler.cpp
index 0e051d0273..c9e258b50a 100644
--- a/src/mbgl/actor/scheduler.cpp
+++ b/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;
 
@@ -42,7 +42,7 @@ std::shared_ptr<Scheduler> Scheduler::GetBackground() {
 }
 
 // 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;
diff --git a/test/actor/actor.test.cpp b/test/actor/actor.test.cpp
index c2a41fe787..03eb9b8301 100644
--- a/test/actor/actor.test.cpp
+++ b/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())
diff --git a/test/util/async_task.test.cpp b/test/util/async_task.test.cpp
index 8c79c51405..e2f988751e 100644
--- a/test/util/async_task.test.cpp
+++ b/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());
-}
\ No newline at end of file
+    EXPECT_EQ(shedulers.front(), std::shared_ptr<Scheduler>(Scheduler::GetSequenced()));
+}
-- 
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