[core] Make the BackgroundScheduler a singleton

- Do not carry it over everywhere as parameter, it is a shared
  instance anyway and the lifecycle is pretty much the app lifecycle
  from the moment we instantiate a map.
- Rename to BackgroundScheduler because it is a Scheduler that will
  do tasks in the background, we don't make assumptions if it is a
  thread pool or a single thread.
- Most importantly, remove the dependency from `core` on `platform`.

27 files changed, 98 insertions, 170 deletions

diff --git a/test/actor/actor.test.cpp b/test/actor/actor.test.cpp
index 02308b7ce0..6db95a83f1 100644
--- a/test/actor/actor.test.cpp
+++ b/test/actor/actor.test.cpp
@@ -1,8 +1,8 @@
 #include <mbgl/actor/actor.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
-#include <mbgl/util/run_loop.hpp>
 
+#include <mbgl/actor/scheduler.hpp>
 #include <mbgl/test/util.hpp>
+#include <mbgl/util/run_loop.hpp>
 
 #include <chrono>
 #include <functional>
@@ -20,9 +20,8 @@ TEST(Actor, Construction) {
         };
     };
 
-    ThreadPool pool { 1 };
     bool constructed = false;
-    Actor<Test> test(pool, std::ref(constructed));
+    Actor<Test> test(Scheduler::GetBackground(), std::ref(constructed));
 
     EXPECT_TRUE(constructed);
 }
@@ -33,14 +32,13 @@ TEST(Actor, Destruction) {
         ~Test() {
             destructed = true;
         }
-        
+
         bool& destructed;
     };
 
-    ThreadPool pool { 1 };
     bool destructed = false;
     {
-        Actor<Test> test(pool, std::ref(destructed));
+        Actor<Test> test(Scheduler::GetBackground(), std::ref(destructed));
     }
 
     EXPECT_TRUE(destructed);
@@ -72,15 +70,13 @@ TEST(Actor, DestructionBlocksOnReceive) {
         }
     };
 
-    ThreadPool pool { 1 };
-
     std::promise<void> enteredPromise;
     std::future<void> enteredFuture = enteredPromise.get_future();
 
     std::promise<void> exitingPromise;
     std::future<void> exitingFuture = exitingPromise.get_future();
 
-    Actor<Test> test(pool, std::move(enteredPromise), std::move(exitingFuture));
+    Actor<Test> test(Scheduler::GetBackground(), std::move(enteredPromise), std::move(exitingFuture));
 
     test.self().invoke(&Test::wait);
     enteredFuture.wait();
@@ -157,11 +153,9 @@ TEST(Actor, DestructionAllowedInReceiveOnSameThread) {
         }
     };
 
-    ThreadPool pool { 1 };
-
     std::promise<void> callbackFiredPromise;
-
-    auto test = std::make_unique<Actor<Test>>(pool);
+    auto retainer = Scheduler::GetBackground();
+    auto test = std::make_unique<Actor<Test>>(retainer);
 
     // Callback (triggered while mutex is locked in Mailbox::receive())
     test->self().invoke(&Test::callMeBack, [&]() {
@@ -189,12 +183,10 @@ TEST(Actor, SelfDestructionDoesntCrashWaitingReceivingThreads) {
     };
 
 
-    ThreadPool pool { 2 };
-
     std::promise<void> actorClosedPromise;
 
-    auto closingActor = std::make_unique<Actor<Test>>(pool);
-    auto waitingActor = std::make_unique<Actor<Test>>(pool);
+    auto closingActor = std::make_unique<Actor<Test>>(Scheduler::GetBackground());
+    auto waitingActor = std::make_unique<Actor<Test>>(Scheduler::GetBackground());
 
     std::atomic<bool> waitingMessageProcessed {false};
 
@@ -251,11 +243,9 @@ TEST(Actor, OrderedMailbox) {
         }
     };
 
-    ThreadPool pool { 1 };
-
     std::promise<void> endedPromise;
     std::future<void> endedFuture = endedPromise.get_future();
-    Actor<Test> test(pool, std::move(endedPromise));
+    Actor<Test> test(Scheduler::GetBackground(), std::move(endedPromise));
 
     for (auto i = 1; i <= 10; ++i) {
         test.self().invoke(&Test::receive, i);
@@ -287,11 +277,9 @@ TEST(Actor, NonConcurrentMailbox) {
         }
     };
 
-    ThreadPool pool { 10 };
-
     std::promise<void> endedPromise;
     std::future<void> endedFuture = endedPromise.get_future();
-    Actor<Test> test(pool, std::move(endedPromise));
+    Actor<Test> test(Scheduler::GetBackground(), std::move(endedPromise));
 
     for (auto i = 1; i <= 10; ++i) {
         test.self().invoke(&Test::receive, i);
@@ -313,13 +301,12 @@ TEST(Actor, Ask) {
         }
     };
 
-    ThreadPool pool { 2 };
-    Actor<Test> test(pool);
+    Actor<Test> test(Scheduler::GetBackground());
 
     auto result = test.self().ask(&Test::doubleIt, 1);
 
     ASSERT_TRUE(result.valid());
-    
+
     auto status = result.wait_for(std::chrono::seconds(1));
     ASSERT_EQ(std::future_status::ready, status);
     ASSERT_EQ(2, result.get());
@@ -339,9 +326,8 @@ TEST(Actor, AskVoid) {
         }
     };
 
-    ThreadPool pool { 1 };
     bool executed = false;
-    Actor<Test> actor(pool, executed);
+    Actor<Test> actor(Scheduler::GetBackground(), executed);
 
     actor.self().ask(&Test::doIt).get();
     EXPECT_TRUE(executed);
@@ -349,31 +335,30 @@ TEST(Actor, AskVoid) {
 
 TEST(Actor, NoSelfActorRef) {
     // Not all actors need a reference to self
-    
+
     // Trivially constructable
     struct Trivial {};
-    
-    ThreadPool pool { 2 };
-    Actor<Trivial> trivial(pool);
-    
-    
+
+    Actor<Trivial> trivial(Scheduler::GetBackground());
+
+
     // With arguments
     struct WithArguments {
         std::promise<void> promise;
-        
+
         WithArguments(std::promise<void> promise_)
         : promise(std::move(promise_)) {
         }
-        
+
         void receive() {
             promise.set_value();
         }
     };
-    
+
     std::promise<void> promise;
     auto future = promise.get_future();
-    Actor<WithArguments> withArguments(pool, std::move(promise));
-    
+    Actor<WithArguments> withArguments(Scheduler::GetBackground(), std::move(promise));
+
     withArguments.self().invoke(&WithArguments::receive);
     future.wait();
 }
@@ -386,32 +371,32 @@ TEST(Actor, TwoPhaseConstruction) {
     struct Test {
         Test(ActorRef<Test>, std::shared_ptr<bool> destroyed_)
             : destroyed(std::move(destroyed_)) {};
-        
+
         ~Test() {
             *destroyed = true;
         }
-        
+
         void callMe(std::promise<void> p) {
             p.set_value();
         }
-        
+
         void stop() {
             util::RunLoop::Get()->stop();
         }
-        
+
         std::shared_ptr<bool> destroyed;
     };
 
     AspiringActor<Test> parent;
-    
+
     auto destroyed = std::make_shared<bool>(false);
-    
+
     std::promise<void> queueExecuted;
     auto queueExecutedFuture = queueExecuted.get_future();
-    
+
     parent.self().invoke(&Test::callMe, std::move(queueExecuted));
     parent.self().invoke(&Test::stop);
-    
+
     auto thread = std::thread([
         capturedArgs = std::make_tuple(destroyed),
         &parent
@@ -420,11 +405,11 @@ TEST(Actor, TwoPhaseConstruction) {
         EstablishedActor<Test> test(loop, parent, capturedArgs);
         loop.run();
     });
-    
+
     // should not hang
     queueExecutedFuture.get();
     thread.join();
-    
+
     EXPECT_TRUE(*destroyed);
 }
 
diff --git a/test/actor/actor_ref.test.cpp b/test/actor/actor_ref.test.cpp
index 20aa1c35c1..221a220ed9 100644
--- a/test/actor/actor_ref.test.cpp
+++ b/test/actor/actor_ref.test.cpp
@@ -1,6 +1,6 @@
 #include <mbgl/actor/actor.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
 
+#include <mbgl/actor/scheduler.hpp>
 #include <mbgl/test/util.hpp>
 
 #include <future>
@@ -27,11 +27,10 @@ TEST(ActorRef, CanOutliveActor) {
         }
     };
 
-    ThreadPool pool { 1 };
     bool died = false;
 
     ActorRef<Test> test = [&] () {
-        return Actor<Test>(pool, std::ref(died)).self();
+        return Actor<Test>(Scheduler::GetBackground(), std::ref(died)).self();
     }();
 
     EXPECT_TRUE(died);
@@ -54,8 +53,7 @@ TEST(ActorRef, Ask) {
         }
     };
 
-    ThreadPool pool { 1 };
-    Actor<Test> actor(pool);
+    Actor<Test> actor(Scheduler::GetBackground());
     ActorRef<Test> ref = actor.self();
 
     EXPECT_EQ(20, ref.ask(&Test::gimme).get());
@@ -76,9 +74,8 @@ TEST(ActorRef, AskVoid) {
         }
     };
     
-    ThreadPool pool { 1 };
     bool executed = false;
-    Actor<Test> actor(pool, executed);
+    Actor<Test> actor(Scheduler::GetBackground(), executed);
     ActorRef<Test> ref = actor.self();
     
     ref.ask(&Test::doIt).get();
@@ -104,8 +101,7 @@ TEST(ActorRef, AskOnDestroyedActor) {
     };
     bool died = false;
 
-    ThreadPool pool { 1 };
-    auto actor = std::make_unique<Actor<Test>>(pool, died);
+    auto actor = std::make_unique<Actor<Test>>(Scheduler::GetBackground(), died);
     ActorRef<Test> ref = actor->self();
 
     actor.reset();
diff --git a/test/api/annotations.test.cpp b/test/api/annotations.test.cpp
index 11920d1624..2d76a3d154 100644
--- a/test/api/annotations.test.cpp
+++ b/test/api/annotations.test.cpp
@@ -2,7 +2,6 @@
 #include <mbgl/test/stub_file_source.hpp>
 #include <mbgl/test/map_adapter.hpp>
 
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/annotation/annotation.hpp>
 #include <mbgl/style/style.hpp>
 #include <mbgl/style/image.hpp>
@@ -28,10 +27,9 @@ std::unique_ptr<style::Image> namedMarker(const std::string& name) {
 class AnnotationTest {
 public:
     util::RunLoop loop;
-    ThreadPool threadPool { 4 };
-    HeadlessFrontend frontend { 1, threadPool };
+    HeadlessFrontend frontend { 1 };
 
-    MapAdapter map { frontend, MapObserver::nullObserver(), std::make_shared<StubFileSource>(), threadPool,
+    MapAdapter map { frontend, MapObserver::nullObserver(), std::make_shared<StubFileSource>(),
                   MapOptions().withMapMode(MapMode::Static).withSize(frontend.getSize())};
 
     void checkRendering(const char * name) {
diff --git a/test/api/api_misuse.test.cpp b/test/api/api_misuse.test.cpp
index b65bdf9918..54d8eb5912 100644
--- a/test/api/api_misuse.test.cpp
+++ b/test/api/api_misuse.test.cpp
@@ -6,7 +6,6 @@
 #include <mbgl/map/map_options.hpp>
 #include <mbgl/gfx/backend_scope.hpp>
 #include <mbgl/gl/headless_frontend.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/util/exception.hpp>
 #include <mbgl/util/run_loop.hpp>
 
@@ -21,11 +20,10 @@ TEST(API, RenderWithoutCallback) {
 
     util::RunLoop loop;
 
-    ThreadPool threadPool(4);
-    HeadlessFrontend frontend { 1, threadPool };
+    HeadlessFrontend frontend { 1 };
 
     auto map = std::make_unique<MapAdapter>(frontend, MapObserver::nullObserver(),
-                                            std::make_shared<StubFileSource>(), threadPool,
+                                            std::make_shared<StubFileSource>(),
                                             MapOptions().withMapMode(MapMode::Static).withSize(frontend.getSize()));
     map->renderStill(nullptr);
 
diff --git a/test/api/custom_geometry_source.test.cpp b/test/api/custom_geometry_source.test.cpp
index f796e3086b..df65dc2703 100644
--- a/test/api/custom_geometry_source.test.cpp
+++ b/test/api/custom_geometry_source.test.cpp
@@ -2,7 +2,6 @@
 
 #include <mbgl/map/map.hpp>
 #include <mbgl/map/map_options.hpp>
-#include <mbgl/util/shared_thread_pool.hpp>
 #include <mbgl/gl/headless_frontend.hpp>
 #include <mbgl/storage/resource_options.hpp>
 #include <mbgl/style/style.hpp>
@@ -20,9 +19,8 @@ using namespace mbgl::style;
 TEST(CustomGeometrySource, Grid) {
     util::RunLoop loop;
 
-    auto threadPool = sharedThreadPool();
-    HeadlessFrontend frontend { 1, *threadPool };
-    Map map(frontend, MapObserver::nullObserver(), *threadPool,
+    HeadlessFrontend frontend { 1 };
+    Map map(frontend, MapObserver::nullObserver(),
             MapOptions().withMapMode(MapMode::Static).withSize(frontend.getSize()),
             ResourceOptions().withCachePath(":memory:").withAssetPath("test/fixtures/api/assets"));
     map.getStyle().loadJSON(util::read_file("test/fixtures/api/water.json"));
diff --git a/test/api/custom_layer.test.cpp b/test/api/custom_layer.test.cpp
index 4a0a34bafc..e9b9aee9bc 100644
--- a/test/api/custom_layer.test.cpp
+++ b/test/api/custom_layer.test.cpp
@@ -3,7 +3,6 @@
 #include <mbgl/platform/gl_functions.hpp>
 #include <mbgl/map/map.hpp>
 #include <mbgl/map/map_options.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/gl/defines.hpp>
 #include <mbgl/gl/headless_frontend.hpp>
 #include <mbgl/storage/resource_options.hpp>
@@ -90,9 +89,8 @@ public:
 TEST(CustomLayer, Basic) {
     util::RunLoop loop;
 
-    ThreadPool threadPool(4);
-    HeadlessFrontend frontend { 1, threadPool };
-    Map map(frontend, MapObserver::nullObserver(), threadPool,
+    HeadlessFrontend frontend { 1 };
+    Map map(frontend, MapObserver::nullObserver(),
             MapOptions().withMapMode(MapMode::Static).withSize(frontend.getSize()),
             ResourceOptions().withCachePath(":memory:").withAssetPath("test/fixtures/api/assets"));
     map.getStyle().loadJSON(util::read_file("test/fixtures/api/water.json"));
diff --git a/test/api/query.test.cpp b/test/api/query.test.cpp
index ddf5df6aab..b80d3b7d8a 100644
--- a/test/api/query.test.cpp
+++ b/test/api/query.test.cpp
@@ -1,7 +1,6 @@
 #include <mbgl/test/map_adapter.hpp>
 
 #include <mbgl/map/map_options.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/test/stub_file_source.hpp>
 #include <mbgl/test/util.hpp>
 #include <mbgl/util/image.hpp>
@@ -35,9 +34,8 @@ public:
 
     util::RunLoop loop;
     std::shared_ptr<StubFileSource> fileSource = std::make_shared<StubFileSource>();
-    ThreadPool threadPool { 4 };
-    HeadlessFrontend frontend { 1, threadPool };
-    MapAdapter map { frontend, MapObserver::nullObserver(), fileSource, threadPool,
+    HeadlessFrontend frontend { 1 };
+    MapAdapter map { frontend, MapObserver::nullObserver(), fileSource,
                   MapOptions().withMapMode(MapMode::Static).withSize(frontend.getSize())};
 };
 
diff --git a/test/api/recycle_map.cpp b/test/api/recycle_map.cpp
index 18cc9c00aa..55e19e3cd9 100644
--- a/test/api/recycle_map.cpp
+++ b/test/api/recycle_map.cpp
@@ -9,7 +9,6 @@
 #include <mbgl/style/sources/geojson_source.hpp>
 #include <mbgl/style/image.hpp>
 #include <mbgl/style/style.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/util/exception.hpp>
 #include <mbgl/util/geometry.hpp>
 #include <mbgl/util/geojson.hpp>
@@ -23,11 +22,9 @@ using namespace mbgl::style;
 TEST(API, RecycleMapUpdateImages) {
     util::RunLoop loop;
 
-    ThreadPool threadPool(4);
-
-    HeadlessFrontend frontend { 1, threadPool };
+    HeadlessFrontend frontend { 1 };
     auto map = std::make_unique<MapAdapter>(frontend, MapObserver::nullObserver(),
-                                         std::make_shared<StubFileSource>(), threadPool,
+                                         std::make_shared<StubFileSource>(),
                                          MapOptions().withMapMode(MapMode::Static).withSize(frontend.getSize()));
 
     EXPECT_TRUE(map);
diff --git a/test/gl/context.test.cpp b/test/gl/context.test.cpp
index b8fe895c44..756f2c58d4 100644
--- a/test/gl/context.test.cpp
+++ b/test/gl/context.test.cpp
@@ -4,7 +4,6 @@
 #include <mbgl/gl/context.hpp>
 #include <mbgl/map/map.hpp>
 #include <mbgl/map/map_options.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/gfx/backend_scope.hpp>
 #include <mbgl/gl/defines.hpp>
 #include <mbgl/gl/headless_frontend.hpp>
@@ -88,11 +87,9 @@ struct Buffer {
 TEST(GLContextMode, Shared) {
     util::RunLoop loop;
 
-    ThreadPool threadPool(4);
+    HeadlessFrontend frontend { 1, {}, gfx::ContextMode::Shared };
 
-    HeadlessFrontend frontend { 1, threadPool, {}, gfx::ContextMode::Shared };
-
-    Map map(frontend, MapObserver::nullObserver(), threadPool,
+    Map map(frontend, MapObserver::nullObserver(),
             MapOptions().withMapMode(MapMode::Static).withSize(frontend.getSize()),
             ResourceOptions().withCachePath(":memory:").withAssetPath("test/fixtures/api/assets"));
     map.getStyle().loadJSON(util::read_file("test/fixtures/api/water.json"));
diff --git a/test/map/map.test.cpp b/test/map/map.test.cpp
index 3af2acdb8b..ee86ce7f77 100644
--- a/test/map/map.test.cpp
+++ b/test/map/map.test.cpp
@@ -9,7 +9,6 @@
 #include <mbgl/gfx/backend_scope.hpp>
 #include <mbgl/gl/context.hpp>
 #include <mbgl/gl/headless_frontend.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/storage/resource_options.hpp>
 #include <mbgl/storage/network_status.hpp>
 #include <mbgl/storage/default_file_source.hpp>
@@ -34,15 +33,14 @@ class MapTest {
 public:
     util::RunLoop runLoop;
     std::shared_ptr<FileSource> fileSource;
-    ThreadPool threadPool { 4 };
     StubMapObserver observer;
     HeadlessFrontend frontend;
     MapAdapter map;
 
     MapTest(float pixelRatio = 1, MapMode mode = MapMode::Static)
         : fileSource(std::make_shared<FileSource>())
-        , frontend(pixelRatio, threadPool)
-        , map(frontend, observer, fileSource, threadPool,
+        , frontend(pixelRatio)
+        , map(frontend, observer, fileSource,
               MapOptions().withMapMode(mode).withSize(frontend.getSize()).withPixelRatio(pixelRatio)) {}
 
     template <typename T = FileSource>
@@ -50,8 +48,8 @@ public:
             float pixelRatio = 1, MapMode mode = MapMode::Static,
             typename std::enable_if<std::is_same<T, DefaultFileSource>::value>::type* = nullptr)
             : fileSource(std::make_shared<T>(cachePath, assetPath))
-            , frontend(pixelRatio, threadPool)
-            , map(frontend, observer, fileSource, threadPool,
+            , frontend(pixelRatio)
+            , map(frontend, observer, fileSource,
                   MapOptions().withMapMode(mode).withSize(frontend.getSize()).withPixelRatio(pixelRatio)) {}
 };
 
@@ -744,7 +742,6 @@ TEST(Map, DontLoadUnneededTiles) {
 
 TEST(Map, TEST_DISABLED_ON_CI(ContinuousRendering)) {
     util::RunLoop runLoop;
-    ThreadPool threadPool { 4 };
 
     using namespace std::chrono_literals;
 
@@ -756,7 +753,7 @@ TEST(Map, TEST_DISABLED_ON_CI(ContinuousRendering)) {
 
     util::Timer timer;
 
-    HeadlessFrontend frontend(1, threadPool);
+    HeadlessFrontend frontend(1);
 
     StubMapObserver observer;
     observer.didFinishRenderingFrameCallback = [&] (MapObserver::RenderMode) {
@@ -768,7 +765,7 @@ TEST(Map, TEST_DISABLED_ON_CI(ContinuousRendering)) {
         });
     };
 
-    Map map(frontend, observer, threadPool,
+    Map map(frontend, observer,
             MapOptions().withMapMode(MapMode::Continuous).withSize(frontend.getSize()),
             ResourceOptions().withCachePath(":memory:").withAssetPath("test/fixtures/api/assets"));
     map.getStyle().loadJSON(util::read_file("test/fixtures/api/water.json"));
diff --git a/test/map/prefetch.test.cpp b/test/map/prefetch.test.cpp
index 3232c99a22..071c918427 100644
--- a/test/map/prefetch.test.cpp
+++ b/test/map/prefetch.test.cpp
@@ -6,7 +6,6 @@
 #include <mbgl/map/map_options.hpp>
 #include <mbgl/gl/headless_frontend.hpp>
 #include <mbgl/style/style.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/util/image.hpp>
 #include <mbgl/util/io.hpp>
 #include <mbgl/util/run_loop.hpp>
@@ -22,7 +21,6 @@ using namespace std::chrono_literals;
 
 TEST(Map, PrefetchTiles) {
     util::RunLoop runLoop;
-    ThreadPool threadPool(4);
     std::shared_ptr<StubFileSource> fileSource = std::make_shared<StubFileSource>();
 
     util::Timer emergencyShutoff;
@@ -36,8 +34,8 @@ TEST(Map, PrefetchTiles) {
         runLoop.stop();
     };
 
-    HeadlessFrontend frontend { { 512, 512 }, 1, threadPool };
-    MapAdapter map(frontend, observer, fileSource, threadPool,
+    HeadlessFrontend frontend { { 512, 512 }, 1 };
+    MapAdapter map(frontend, observer, fileSource,
                 MapOptions().withMapMode(MapMode::Continuous).withSize(frontend.getSize()));
 
     std::vector<int> tiles;
diff --git a/test/renderer/image_manager.test.cpp b/test/renderer/image_manager.test.cpp
index 7f0d670846..87a24e3bd3 100644
--- a/test/renderer/image_manager.test.cpp
+++ b/test/renderer/image_manager.test.cpp
@@ -9,7 +9,6 @@
 #include <mbgl/util/io.hpp>
 #include <mbgl/util/image.hpp>
 #include <mbgl/util/run_loop.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/util/string.hpp>
 
 #include <utility>
diff --git a/test/sprite/sprite_loader.test.cpp b/test/sprite/sprite_loader.test.cpp
index 3691572265..122e711f51 100644
--- a/test/sprite/sprite_loader.test.cpp
+++ b/test/sprite/sprite_loader.test.cpp
@@ -8,7 +8,6 @@
 #include <mbgl/util/io.hpp>
 #include <mbgl/util/image.hpp>
 #include <mbgl/util/run_loop.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/util/string.hpp>
 
 #include <utility>
@@ -37,7 +36,6 @@ public:
     util::RunLoop loop;
     StubFileSource fileSource;
     StubSpriteLoaderObserver observer;
-    ThreadPool threadPool { 1 };
     SpriteLoader spriteLoader{ 1 };
 
     void run() {
@@ -45,7 +43,7 @@ public:
         Log::setObserver(std::make_unique<Log::NullObserver>());
 
         spriteLoader.setObserver(&observer);
-        spriteLoader.load("test/fixtures/resources/sprite", threadPool, fileSource);
+        spriteLoader.load("test/fixtures/resources/sprite", fileSource);
 
         loop.run();
     }
diff --git a/test/src/mbgl/test/map_adapter.hpp b/test/src/mbgl/test/map_adapter.hpp
index 5bdab1d164..d4af579ae1 100644
--- a/test/src/mbgl/test/map_adapter.hpp
+++ b/test/src/mbgl/test/map_adapter.hpp
@@ -13,9 +13,8 @@ public:
     explicit MapAdapter(RendererFrontend& frontend,
                         MapObserver& observer,
                         std::shared_ptr<FileSource> fileSource,
-                        Scheduler& scheduler,
                         const MapOptions& options)
-        : Map(std::make_unique<Map::Impl>(frontend, observer, scheduler, std::move(fileSource), options)) {}
+        : Map(std::make_unique<Map::Impl>(frontend, observer, std::move(fileSource), options)) {}
 };
 
 } // namespace mbgl
diff --git a/test/src/mbgl/test/test.cpp b/test/src/mbgl/test/test.cpp
index 4c3dad1727..1c77aae2a1 100644
--- a/test/src/mbgl/test/test.cpp
+++ b/test/src/mbgl/test/test.cpp
@@ -1,3 +1,4 @@
+#include <mbgl/actor/scheduler.hpp>
 #include <mbgl/test.hpp>
 #include <mbgl/test/util.hpp>
 
diff --git a/test/style/source.test.cpp b/test/style/source.test.cpp
index f0ff1f81b4..289e41c75f 100644
--- a/test/style/source.test.cpp
+++ b/test/style/source.test.cpp
@@ -31,7 +31,6 @@
 #include <mbgl/util/image.hpp>
 
 #include <mbgl/util/tileset.hpp>
-#include <mbgl/util/shared_thread_pool.hpp>
 #include <mbgl/util/logging.hpp>
 #include <mbgl/util/optional.hpp>
 #include <mbgl/util/range.hpp>
@@ -55,8 +54,7 @@ public:
     StubRenderSourceObserver renderSourceObserver;
     Transform transform;
     TransformState transformState;
-    ThreadPool threadPool { 1 };
-    Style style { loop, fileSource, 1 };
+    Style style { fileSource, 1 };
     AnnotationManager annotationManager { style };
     ImageManager imageManager;
     GlyphManager glyphManager { fileSource };
@@ -65,7 +63,6 @@ public:
         1.0,
         MapDebugOptions(),
         transformState,
-        threadPool,
         fileSource,
         MapMode::Continuous,
         annotationManager,
@@ -763,7 +760,6 @@ TEST(Source, ImageSourceImageUpdate) {
 
 TEST(Source, CustomGeometrySourceSetTileData) {
     SourceTest test;
-    std::shared_ptr<ThreadPool> threadPool = sharedThreadPool();
     CustomGeometrySource source("source", CustomGeometrySource::Options());
     source.loadDescription(test.fileSource);
 
diff --git a/test/style/style.test.cpp b/test/style/style.test.cpp
index 7f57651331..72f74d3b01 100644
--- a/test/style/style.test.cpp
+++ b/test/style/style.test.cpp
@@ -9,7 +9,6 @@
 #include <mbgl/style/layers/line_layer.hpp>
 #include <mbgl/util/io.hpp>
 #include <mbgl/util/run_loop.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
 
 #include <memory>
 
@@ -19,9 +18,8 @@ using namespace mbgl::style;
 TEST(Style, Properties) {
     util::RunLoop loop;
 
-    ThreadPool threadPool{ 1 };
     StubFileSource fileSource;
-    Style::Impl style { threadPool, fileSource, 1.0 };
+    Style::Impl style { fileSource, 1.0 };
 
     style.loadJSON(R"STYLE({"name": "Test"})STYLE");
     ASSERT_EQ("Test", style.getName());
@@ -62,9 +60,8 @@ TEST(Style, Properties) {
 TEST(Style, DuplicateSource) {
     util::RunLoop loop;
 
-    ThreadPool threadPool{ 1 };
     StubFileSource fileSource;
-    Style::Impl style { threadPool, fileSource, 1.0 };
+    Style::Impl style { fileSource, 1.0 };
 
     style.loadJSON(util::read_file("test/fixtures/resources/style-unused-sources.json"));
 
@@ -84,9 +81,8 @@ TEST(Style, RemoveSourceInUse) {
     auto log = new FixtureLogObserver();
     Log::setObserver(std::unique_ptr<Log::Observer>(log));
 
-    ThreadPool threadPool{ 1 };
     StubFileSource fileSource;
-    Style::Impl style { threadPool, fileSource, 1.0 };
+    Style::Impl style { fileSource, 1.0 };
 
     style.loadJSON(util::read_file("test/fixtures/resources/style-unused-sources.json"));
 
diff --git a/test/style/style_layer.test.cpp b/test/style/style_layer.test.cpp
index 7598d888e9..d6a926c631 100644
--- a/test/style/style_layer.test.cpp
+++ b/test/style/style_layer.test.cpp
@@ -21,7 +21,6 @@
 #include <mbgl/test/stub_file_source.hpp>
 #include <mbgl/util/color.hpp>
 #include <mbgl/util/run_loop.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/util/io.hpp>
 
 #include <memory>
@@ -283,9 +282,8 @@ TEST(Layer, DuplicateLayer) {
     util::RunLoop loop;
 
     // Setup style
-    ThreadPool threadPool{ 1 };
     StubFileSource fileSource;
-    Style::Impl style { threadPool, fileSource, 1.0 };
+    Style::Impl style { fileSource, 1.0 };
     style.loadJSON(util::read_file("test/fixtures/resources/style-unused-sources.json"));
 
     // Add initial layer
diff --git a/test/text/local_glyph_rasterizer.test.cpp b/test/text/local_glyph_rasterizer.test.cpp
index f21ac0b883..4c0719e16c 100644
--- a/test/text/local_glyph_rasterizer.test.cpp
+++ b/test/text/local_glyph_rasterizer.test.cpp
@@ -8,7 +8,6 @@
 #include <mbgl/util/color.hpp>
 #include <mbgl/renderer/renderer.hpp>
 #include <mbgl/gl/headless_frontend.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/style/style.hpp>
 
 /*
@@ -34,15 +33,14 @@ namespace {
 class LocalGlyphRasterizerTest {
 public:
     LocalGlyphRasterizerTest(const optional<std::string> fontFamily)
-        : frontend(1, threadPool, optional<std::string>(), gfx::ContextMode::Unique, fontFamily)
+        : frontend(1, optional<std::string>(), gfx::ContextMode::Unique, fontFamily)
     {
     }
 
     util::RunLoop loop;
     std::shared_ptr<StubFileSource> fileSource = std::make_shared<StubFileSource>();
-    ThreadPool threadPool { 4 };
     HeadlessFrontend frontend;
-    MapAdapter map { frontend, MapObserver::nullObserver(), fileSource, threadPool,
+    MapAdapter map { frontend, MapObserver::nullObserver(), fileSource,
                   MapOptions().withMapMode(MapMode::Static).withSize(frontend.getSize())};
 
     void checkRendering(const char * name) {
diff --git a/test/tile/custom_geometry_tile.test.cpp b/test/tile/custom_geometry_tile.test.cpp
index 7a627bad88..50bc7d0e8f 100644
--- a/test/tile/custom_geometry_tile.test.cpp
+++ b/test/tile/custom_geometry_tile.test.cpp
@@ -5,7 +5,6 @@
 #include <mbgl/tile/custom_geometry_tile.hpp>
 #include <mbgl/style/custom_tile_loader.hpp>
 
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/util/run_loop.hpp>
 #include <mbgl/map/transform.hpp>
 #include <mbgl/renderer/tile_parameters.hpp>
@@ -26,8 +25,7 @@ public:
     FakeFileSource fileSource;
     TransformState transformState;
     util::RunLoop loop;
-    ThreadPool threadPool { 1 };
-    style::Style style { loop, fileSource, 1 };
+    style::Style style { fileSource, 1 };
     AnnotationManager annotationManager { style };
     ImageManager imageManager;
     GlyphManager glyphManager { fileSource };
@@ -36,7 +34,6 @@ public:
         1.0,
         MapDebugOptions(),
         transformState,
-        threadPool,
         fileSource,
         MapMode::Continuous,
         annotationManager,
diff --git a/test/tile/geojson_tile.test.cpp b/test/tile/geojson_tile.test.cpp
index 772590efb2..4f4435182f 100644
--- a/test/tile/geojson_tile.test.cpp
+++ b/test/tile/geojson_tile.test.cpp
@@ -4,7 +4,6 @@
 #include <mbgl/tile/geojson_tile.hpp>
 #include <mbgl/tile/tile_loader_impl.hpp>
 
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/util/run_loop.hpp>
 #include <mbgl/map/transform.hpp>
 #include <mbgl/renderer/tile_parameters.hpp>
@@ -25,8 +24,7 @@ public:
     FakeFileSource fileSource;
     TransformState transformState;
     util::RunLoop loop;
-    ThreadPool threadPool { 1 };
-    style::Style style { loop, fileSource, 1 };
+    style::Style style { fileSource, 1 };
     AnnotationManager annotationManager { style };
     ImageManager imageManager;
     GlyphManager glyphManager { fileSource };
@@ -36,7 +34,6 @@ public:
         1.0,
         MapDebugOptions(),
         transformState,
-        threadPool,
         fileSource,
         MapMode::Continuous,
         annotationManager,
diff --git a/test/tile/raster_dem_tile.test.cpp b/test/tile/raster_dem_tile.test.cpp
index 5a6f5a8c9a..e2e7d44b9b 100644
--- a/test/tile/raster_dem_tile.test.cpp
+++ b/test/tile/raster_dem_tile.test.cpp
@@ -4,7 +4,6 @@
 #include <mbgl/tile/tile_loader_impl.hpp>
 
 #include <mbgl/style/style.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/util/run_loop.hpp>
 #include <mbgl/map/transform.hpp>
 #include <mbgl/annotation/annotation_manager.hpp>
@@ -20,8 +19,7 @@ public:
     FakeFileSource fileSource;
     TransformState transformState;
     util::RunLoop loop;
-    ThreadPool threadPool { 1 };
-    style::Style style { loop, fileSource, 1 };
+    style::Style style { fileSource, 1 };
     AnnotationManager annotationManager { style };
     ImageManager imageManager;
     GlyphManager glyphManager { fileSource };
@@ -31,7 +29,6 @@ public:
         1.0,
         MapDebugOptions(),
         transformState,
-        threadPool,
         fileSource,
         MapMode::Continuous,
         annotationManager,
diff --git a/test/tile/raster_tile.test.cpp b/test/tile/raster_tile.test.cpp
index 8b2b3dee61..2e35a94025 100644
--- a/test/tile/raster_tile.test.cpp
+++ b/test/tile/raster_tile.test.cpp
@@ -4,7 +4,6 @@
 #include <mbgl/tile/tile_loader_impl.hpp>
 
 #include <mbgl/style/style.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/util/run_loop.hpp>
 #include <mbgl/map/transform.hpp>
 #include <mbgl/annotation/annotation_manager.hpp>
@@ -20,8 +19,7 @@ public:
     FakeFileSource fileSource;
     TransformState transformState;
     util::RunLoop loop;
-    ThreadPool threadPool { 1 };
-    style::Style style { loop, fileSource, 1 };
+    style::Style style { fileSource, 1 };
     AnnotationManager annotationManager { style };
     ImageManager imageManager;
     GlyphManager glyphManager { fileSource };
@@ -31,7 +29,6 @@ public:
         1.0,
         MapDebugOptions(),
         transformState,
-        threadPool,
         fileSource,
         MapMode::Continuous,
         annotationManager,
diff --git a/test/tile/vector_tile.test.cpp b/test/tile/vector_tile.test.cpp
index ed3eda7863..c610d8c1a7 100644
--- a/test/tile/vector_tile.test.cpp
+++ b/test/tile/vector_tile.test.cpp
@@ -4,7 +4,6 @@
 #include <mbgl/tile/vector_tile_data.hpp>
 #include <mbgl/tile/tile_loader_impl.hpp>
 
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/util/run_loop.hpp>
 #include <mbgl/map/transform.hpp>
 #include <mbgl/style/style.hpp>
@@ -26,8 +25,7 @@ public:
     FakeFileSource fileSource;
     TransformState transformState;
     util::RunLoop loop;
-    ThreadPool threadPool { 1 };
-    style::Style style { loop, fileSource, 1 };
+    style::Style style { fileSource, 1 };
     AnnotationManager annotationManager { style };
     ImageManager imageManager;
     GlyphManager glyphManager { fileSource };
@@ -37,7 +35,6 @@ public:
         1.0,
         MapDebugOptions(),
         transformState,
-        threadPool,
         fileSource,
         MapMode::Continuous,
         annotationManager,
diff --git a/test/util/async_task.test.cpp b/test/util/async_task.test.cpp
index f3025e8952..682926a17d 100644
--- a/test/util/async_task.test.cpp
+++ b/test/util/async_task.test.cpp
@@ -1,14 +1,15 @@
 #include <mbgl/util/async_task.hpp>
-#include <mbgl/util/run_loop.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
-#include <mbgl/actor/actor_ref.hpp>
 
+#include <mbgl/actor/actor_ref.hpp>
+#include <mbgl/actor/scheduler.hpp>
 #include <mbgl/test/util.hpp>
+#include <mbgl/util/run_loop.hpp>
 
 #include <atomic>
 #include <future>
 #include <vector>
 
+using namespace mbgl;
 using namespace mbgl::util;
 
 namespace {
@@ -104,11 +105,11 @@ TEST(AsyncTask, RequestCoalescingMultithreaded) {
     unsigned count = 0, numThreads = 25;
     AsyncTask async([&count] { ++count; });
 
-    mbgl::ThreadPool threads(numThreads);
-    auto mailbox = std::make_shared<mbgl::Mailbox>(threads);
+    auto retainer = Scheduler::GetBackground();
+    auto mailbox = std::make_shared<Mailbox>(*retainer);
 
     TestWorker worker(&async);
-    mbgl::ActorRef<TestWorker> workerRef(worker, mailbox);
+    ActorRef<TestWorker> workerRef(worker, mailbox);
 
     for (unsigned i = 0; i < numThreads; ++i) {
         workerRef.invoke(&TestWorker::run);
@@ -133,11 +134,11 @@ TEST(AsyncTask, ThreadSafety) {
 
     AsyncTask async([&count] { ++count; });
 
-    mbgl::ThreadPool threads(numThreads);
-    auto mailbox = std::make_shared<mbgl::Mailbox>(threads);
+    auto retainer = Scheduler::GetBackground();
+    auto mailbox = std::make_shared<Mailbox>(*retainer);
 
     TestWorker worker(&async);
-    mbgl::ActorRef<TestWorker> workerRef(worker, mailbox);
+    ActorRef<TestWorker> workerRef(worker, mailbox);
 
     for (unsigned i = 0; i < numThreads; ++i) {
         // The callback runs on the worker, thus the atomic type.
diff --git a/test/util/memory.test.cpp b/test/util/memory.test.cpp
index e3ad976432..8602e6ac5b 100644
--- a/test/util/memory.test.cpp
+++ b/test/util/memory.test.cpp
@@ -5,7 +5,6 @@
 
 #include <mbgl/map/map_options.hpp>
 #include <mbgl/gl/headless_frontend.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/util/io.hpp>
 #include <mbgl/util/run_loop.hpp>
 #include <mbgl/style/style.hpp>
@@ -37,7 +36,6 @@ public:
 
     util::RunLoop runLoop;
     std::shared_ptr<StubFileSource> fileSource = std::make_shared<StubFileSource>();
-    ThreadPool threadPool { 4 };
 
 private:
     Response response(const std::string& path) {
@@ -72,8 +70,8 @@ TEST(Memory, Vector) {
     MemoryTest test;
     float ratio { 2 };
 
-    HeadlessFrontend frontend { { 256, 256 }, ratio, test.threadPool };
-    MapAdapter map(frontend, MapObserver::nullObserver(), test.fileSource, test.threadPool,
+    HeadlessFrontend frontend { { 256, 256 }, ratio };
+    MapAdapter map(frontend, MapObserver::nullObserver(), test.fileSource,
                    MapOptions().withMapMode(MapMode::Static).withSize(frontend.getSize()).withPixelRatio(ratio));
     map.jumpTo(CameraOptions().withZoom(16));
     map.getStyle().loadURL("mapbox://streets");
@@ -85,8 +83,8 @@ TEST(Memory, Raster) {
     MemoryTest test;
     float ratio { 2 };
 
-    HeadlessFrontend frontend { { 256, 256 }, ratio, test.threadPool };
-    MapAdapter map(frontend, MapObserver::nullObserver(), test.fileSource, test.threadPool,
+    HeadlessFrontend frontend { { 256, 256 }, ratio };
+    MapAdapter map(frontend, MapObserver::nullObserver(), test.fileSource,
                    MapOptions().withMapMode(MapMode::Static).withSize(frontend.getSize()).withPixelRatio(ratio));
     map.getStyle().loadURL("mapbox://satellite");
 
@@ -123,8 +121,8 @@ TEST(Memory, Footprint) {
     class FrontendAndMap {
     public:
         FrontendAndMap(MemoryTest& test_, const char* style)
-            : frontend(Size{ 256, 256 }, 2, test_.threadPool)
-            , map(frontend, MapObserver::nullObserver(), test_.fileSource, test_.threadPool,
+            : frontend(Size{ 256, 256 }, 2)
+            , map(frontend, MapObserver::nullObserver(), test_.fileSource,
                   MapOptions().withMapMode(MapMode::Static).withSize(frontend.getSize()).withPixelRatio(2)) {
             map.jumpTo(CameraOptions().withZoom(16));
             map.getStyle().loadURL(style);
diff --git a/test/util/thread.test.cpp b/test/util/thread.test.cpp
index 2bcb9d8959..49735585c6 100644
--- a/test/util/thread.test.cpp
+++ b/test/util/thread.test.cpp
@@ -1,8 +1,9 @@
+#include <mbgl/util/thread.hpp>
+
 #include <mbgl/actor/actor_ref.hpp>
+#include <mbgl/actor/scheduler.hpp>
 #include <mbgl/test/util.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/util/run_loop.hpp>
-#include <mbgl/util/thread.hpp>
 #include <mbgl/util/timer.hpp>
 
 #include <atomic>
@@ -139,8 +140,7 @@ TEST(Thread, Concurrency) {
     unsigned numMessages = 100000;
     std::atomic_uint completed(numMessages);
 
-    ThreadPool threadPool(10);
-    Actor<TestWorker> poolWorker(threadPool);
+    Actor<TestWorker> poolWorker(Scheduler::GetBackground());
     auto poolWorkerRef = poolWorker.self();
 
     Thread<TestWorker> threadedObject("Test");
@@ -165,8 +165,7 @@ TEST(Thread, Concurrency) {
 TEST(Thread, ThreadPoolMessaging) {
     auto loop = std::make_shared<RunLoop>();
 
-    ThreadPool threadPool(1);
-    Actor<TestWorker> poolWorker(threadPool);
+    Actor<TestWorker> poolWorker(Scheduler::GetBackground());
     auto poolWorkerRef = poolWorker.self();
 
     Thread<TestWorker> threadedObject("Test");