[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`.

20 files changed, 128 insertions, 42 deletions

diff --git a/src/core-files.json b/src/core-files.json
index c0412dd3a8..d721935268 100644
--- a/src/core-files.json
+++ b/src/core-files.json
@@ -302,6 +302,7 @@
         "src/mbgl/util/rapidjson.cpp",
         "src/mbgl/util/stopwatch.cpp",
         "src/mbgl/util/string.cpp",
+        "src/mbgl/util/thread_pool.cpp",
         "src/mbgl/util/tile_cover.cpp",
         "src/mbgl/util/tile_cover_impl.cpp",
         "src/mbgl/util/tiny_sdf.cpp",
@@ -762,6 +763,7 @@
         "mbgl/util/std.hpp": "src/mbgl/util/std.hpp",
         "mbgl/util/stopwatch.hpp": "src/mbgl/util/stopwatch.hpp",
         "mbgl/util/thread_local.hpp": "src/mbgl/util/thread_local.hpp",
+        "mbgl/util/thread_pool.hpp": "src/mbgl/util/thread_pool.hpp",
         "mbgl/util/tile_coordinate.hpp": "src/mbgl/util/tile_coordinate.hpp",
         "mbgl/util/tile_cover.hpp": "src/mbgl/util/tile_cover.hpp",
         "mbgl/util/tile_cover_impl.hpp": "src/mbgl/util/tile_cover_impl.hpp",
diff --git a/src/mbgl/actor/scheduler.cpp b/src/mbgl/actor/scheduler.cpp
index d7cdb2737b..cb0c7728ec 100644
--- a/src/mbgl/actor/scheduler.cpp
+++ b/src/mbgl/actor/scheduler.cpp
@@ -1,8 +1,11 @@
 #include <mbgl/actor/scheduler.hpp>
 #include <mbgl/util/thread_local.hpp>
+#include <mbgl/util/thread_pool.hpp>
 
 namespace mbgl {
-    
+
+util::ThreadLocal<Scheduler> g_currentScheduler;
+
 static auto& current() {
     static util::ThreadLocal<Scheduler> scheduler;
     return scheduler;
@@ -16,4 +19,19 @@ Scheduler* Scheduler::GetCurrent() {
     return current().get();
 }
 
+// static
+std::shared_ptr<Scheduler> Scheduler::GetBackground() {
+    static std::weak_ptr<Scheduler> weak;
+    static std::mutex mtx;
+
+    std::lock_guard<std::mutex> lock(mtx);
+    std::shared_ptr<Scheduler> scheduler = weak.lock();
+
+    if (!scheduler) {
+        weak = scheduler = std::make_shared<ThreadPool>(4);
+    }
+
+    return scheduler;
+}
+
 } //namespace mbgl
diff --git a/src/mbgl/map/map.cpp b/src/mbgl/map/map.cpp
index c31bb8ca34..c35f33305c 100644
--- a/src/mbgl/map/map.cpp
+++ b/src/mbgl/map/map.cpp
@@ -17,7 +17,6 @@
 #include <mbgl/util/exception.hpp>
 #include <mbgl/util/mapbox.hpp>
 #include <mbgl/util/tile_coordinate.hpp>
-#include <mbgl/actor/scheduler.hpp>
 #include <mbgl/util/logging.hpp>
 #include <mbgl/math/log2.hpp>
 
@@ -29,10 +28,9 @@ using namespace style;
 
 Map::Map(RendererFrontend& frontend,
          MapObserver& observer,
-         Scheduler& scheduler,
          const MapOptions& mapOptions,
          const ResourceOptions& resourceOptions)
-    : impl(std::make_unique<Impl>(frontend, observer, scheduler,
+    : impl(std::make_unique<Impl>(frontend, observer,
                                   FileSource::getSharedFileSource(resourceOptions),
                                   mapOptions)) {}
 
diff --git a/src/mbgl/map/map_impl.cpp b/src/mbgl/map/map_impl.cpp
index 1ec7255822..fa15461ff2 100644
--- a/src/mbgl/map/map_impl.cpp
+++ b/src/mbgl/map/map_impl.cpp
@@ -9,18 +9,16 @@ namespace mbgl {
 
 Map::Impl::Impl(RendererFrontend& frontend_,
                 MapObserver& observer_,
-                Scheduler& scheduler_,
                 std::shared_ptr<FileSource> fileSource_,
                 const MapOptions& mapOptions)
         : observer(observer_),
           rendererFrontend(frontend_),
-          scheduler(scheduler_),
           transform(observer, mapOptions.constrainMode(), mapOptions.viewportMode()),
           mode(mapOptions.mapMode()),
           pixelRatio(mapOptions.pixelRatio()),
           crossSourceCollisions(mapOptions.crossSourceCollisions()),
           fileSource(std::move(fileSource_)),
-          style(std::make_unique<style::Style>(scheduler, *fileSource, pixelRatio)),
+          style(std::make_unique<style::Style>(*fileSource, pixelRatio)),
           annotationManager(*style) {
     transform.setNorthOrientation(mapOptions.northOrientation());
     style->impl->setObserver(this);
diff --git a/src/mbgl/map/map_impl.hpp b/src/mbgl/map/map_impl.hpp
index f233f78275..84b43c8343 100644
--- a/src/mbgl/map/map_impl.hpp
+++ b/src/mbgl/map/map_impl.hpp
@@ -1,7 +1,5 @@
 #pragma once
 
-#include <mbgl/actor/actor.hpp>
-#include <mbgl/actor/scheduler.hpp>
 #include <mbgl/annotation/annotation_manager.hpp>
 #include <mbgl/map/map.hpp>
 #include <mbgl/map/map_observer.hpp>
@@ -30,7 +28,7 @@ struct StillImageRequest {
 
 class Map::Impl : public style::Observer, public RendererObserver {
 public:
-    Impl(RendererFrontend&, MapObserver&, Scheduler&, std::shared_ptr<FileSource>, const MapOptions&);
+    Impl(RendererFrontend&, MapObserver&, std::shared_ptr<FileSource>, const MapOptions&);
     ~Impl() final;
 
     // StyleObserver
@@ -54,7 +52,6 @@ public:
 
     MapObserver& observer;
     RendererFrontend& rendererFrontend;
-    Scheduler& scheduler;
 
     Transform transform;
 
diff --git a/src/mbgl/renderer/renderer.cpp b/src/mbgl/renderer/renderer.cpp
index 18cb0ebc07..ba213c435d 100644
--- a/src/mbgl/renderer/renderer.cpp
+++ b/src/mbgl/renderer/renderer.cpp
@@ -9,12 +9,10 @@ namespace mbgl {
 
 Renderer::Renderer(gfx::RendererBackend& backend,
                    float pixelRatio_,
-                   Scheduler& scheduler_,
                    const optional<std::string> programCacheDir_,
                    const optional<std::string> localFontFamily_)
     : impl(std::make_unique<Impl>(backend,
                                   pixelRatio_,
-                                  scheduler_,
                                   std::move(programCacheDir_),
                                   std::move(localFontFamily_))) {
 }
diff --git a/src/mbgl/renderer/renderer_impl.cpp b/src/mbgl/renderer/renderer_impl.cpp
index c756234e8d..3db9439b5c 100644
--- a/src/mbgl/renderer/renderer_impl.cpp
+++ b/src/mbgl/renderer/renderer_impl.cpp
@@ -40,11 +40,9 @@ static RendererObserver& nullObserver() {
 
 Renderer::Impl::Impl(gfx::RendererBackend& backend_,
                      float pixelRatio_,
-                     Scheduler& scheduler_,
                      const optional<std::string> programCacheDir_,
                      const optional<std::string> localFontFamily_)
     : backend(backend_)
-    , scheduler(scheduler_)
     , observer(&nullObserver())
     , pixelRatio(pixelRatio_)
     , programCacheDir(std::move(programCacheDir_))
@@ -114,7 +112,6 @@ void Renderer::Impl::render(const UpdateParameters& updateParameters) {
         updateParameters.pixelRatio,
         updateParameters.debugOptions,
         updateParameters.transformState,
-        scheduler,
         updateParameters.fileSource,
         updateParameters.mode,
         updateParameters.annotationManager,
diff --git a/src/mbgl/renderer/renderer_impl.hpp b/src/mbgl/renderer/renderer_impl.hpp
index f0d4720ce2..270ea4d517 100644
--- a/src/mbgl/renderer/renderer_impl.hpp
+++ b/src/mbgl/renderer/renderer_impl.hpp
@@ -26,7 +26,6 @@ class UpdateParameters;
 class RenderStaticData;
 class RenderedQueryOptions;
 class SourceQueryOptions;
-class Scheduler;
 class GlyphManager;
 class ImageManager;
 class LineAtlas;
@@ -42,7 +41,6 @@ class Renderer::Impl : public GlyphManagerObserver,
 public:
     Impl(gfx::RendererBackend&,
          float pixelRatio_,
-         Scheduler&,
          const optional<std::string> programCacheDir,
          const optional<std::string> localFontFamily_);
     ~Impl() final;
@@ -99,7 +97,6 @@ private:
     friend class Renderer;
 
     gfx::RendererBackend& backend;
-    Scheduler& scheduler;
 
     RendererObserver* observer;
 
diff --git a/src/mbgl/renderer/tile_parameters.hpp b/src/mbgl/renderer/tile_parameters.hpp
index 665c7490d2..cbe8dfb3ba 100644
--- a/src/mbgl/renderer/tile_parameters.hpp
+++ b/src/mbgl/renderer/tile_parameters.hpp
@@ -5,7 +5,6 @@
 namespace mbgl {
 
 class TransformState;
-class Scheduler;
 class FileSource;
 class AnnotationManager;
 class ImageManager;
@@ -16,7 +15,6 @@ public:
     const float pixelRatio;
     const MapDebugOptions debugOptions;
     const TransformState& transformState;
-    Scheduler& workerScheduler;
     FileSource& fileSource;
     const MapMode mode;
     AnnotationManager& annotationManager;
diff --git a/src/mbgl/sprite/sprite_loader.cpp b/src/mbgl/sprite/sprite_loader.cpp
index ce8fbe5894..bfb0c570d6 100644
--- a/src/mbgl/sprite/sprite_loader.cpp
+++ b/src/mbgl/sprite/sprite_loader.cpp
@@ -20,9 +20,9 @@ namespace mbgl {
 static SpriteLoaderObserver nullObserver;
 
 struct SpriteLoader::Loader {
-    Loader(Scheduler& scheduler, SpriteLoader& imageManager)
+    Loader(SpriteLoader& imageManager)
         : mailbox(std::make_shared<Mailbox>(*Scheduler::GetCurrent())),
-          worker(scheduler, ActorRef<SpriteLoader>(imageManager, mailbox)) {
+          worker(Scheduler::GetBackground(), ActorRef<SpriteLoader>(imageManager, mailbox)) {
     }
 
     std::shared_ptr<const std::string> image;
@@ -40,14 +40,14 @@ SpriteLoader::SpriteLoader(float pixelRatio_)
 
 SpriteLoader::~SpriteLoader() = default;
 
-void SpriteLoader::load(const std::string& url, Scheduler& scheduler, FileSource& fileSource) {
+void SpriteLoader::load(const std::string& url, FileSource& fileSource) {
     if (url.empty()) {
         // Treat a non-existent sprite as a successfully loaded empty sprite.
         observer->onSpriteLoaded({});
         return;
     }
 
-    loader = std::make_unique<Loader>(scheduler, *this);
+    loader = std::make_unique<Loader>(*this);
 
     loader->jsonRequest = fileSource.request(Resource::spriteJSON(url, pixelRatio), [this](Response res) {
         if (res.error) {
diff --git a/src/mbgl/sprite/sprite_loader.hpp b/src/mbgl/sprite/sprite_loader.hpp
index 0daf46be9c..0b7d37fa14 100644
--- a/src/mbgl/sprite/sprite_loader.hpp
+++ b/src/mbgl/sprite/sprite_loader.hpp
@@ -12,7 +12,6 @@
 
 namespace mbgl {
 
-class Scheduler;
 class FileSource;
 class SpriteLoaderObserver;
 
@@ -21,7 +20,7 @@ public:
     SpriteLoader(float pixelRatio);
     ~SpriteLoader();
 
-    void load(const std::string& url, Scheduler&, FileSource&);
+    void load(const std::string& url, FileSource&);
 
     void setObserver(SpriteLoaderObserver*);
 
diff --git a/src/mbgl/style/sources/custom_geometry_source.cpp b/src/mbgl/style/sources/custom_geometry_source.cpp
index 7420be9320..6e9d8d65fb 100644
--- a/src/mbgl/style/sources/custom_geometry_source.cpp
+++ b/src/mbgl/style/sources/custom_geometry_source.cpp
@@ -4,7 +4,6 @@
 #include <mbgl/actor/actor.hpp>
 #include <mbgl/actor/scheduler.hpp>
 #include <mbgl/tile/tile_id.hpp>
-#include <mbgl/util/shared_thread_pool.hpp>
 #include <tuple>
 #include <map>
 
@@ -14,8 +13,7 @@ namespace style {
 CustomGeometrySource::CustomGeometrySource(std::string id,
                                        const CustomGeometrySource::Options options)
     : Source(makeMutable<CustomGeometrySource::Impl>(std::move(id), options)),
-    threadPool(sharedThreadPool()),
-    loader(std::make_unique<Actor<CustomTileLoader>>(*threadPool, options.fetchTileFunction, options.cancelTileFunction)) {
+    loader(std::make_unique<Actor<CustomTileLoader>>(Scheduler::GetBackground(), options.fetchTileFunction, options.cancelTileFunction)) {
 }
 
 CustomGeometrySource::~CustomGeometrySource() = default;
diff --git a/src/mbgl/style/style.cpp b/src/mbgl/style/style.cpp
index bd8631fc52..783c850097 100644
--- a/src/mbgl/style/style.cpp
+++ b/src/mbgl/style/style.cpp
@@ -8,8 +8,8 @@
 namespace mbgl {
 namespace style {
 
-Style::Style(Scheduler& scheduler, FileSource& fileSource, float pixelRatio)
-    : impl(std::make_unique<Impl>(scheduler, fileSource, pixelRatio)) {
+Style::Style(FileSource& fileSource, float pixelRatio)
+    : impl(std::make_unique<Impl>(fileSource, pixelRatio)) {
 }
 
 Style::~Style() = default;
diff --git a/src/mbgl/style/style_impl.cpp b/src/mbgl/style/style_impl.cpp
index d527b8440b..10fee73cdd 100644
--- a/src/mbgl/style/style_impl.cpp
+++ b/src/mbgl/style/style_impl.cpp
@@ -27,9 +27,8 @@ namespace style {
 
 static Observer nullObserver;
 
-Style::Impl::Impl(Scheduler& scheduler_, FileSource& fileSource_, float pixelRatio)
-    : scheduler(scheduler_),
-      fileSource(fileSource_),
+Style::Impl::Impl(FileSource& fileSource_, float pixelRatio)
+    : fileSource(fileSource_),
       spriteLoader(std::make_unique<SpriteLoader>(pixelRatio)),
       light(std::make_unique<Light>()),
       observer(&nullObserver) {
@@ -111,7 +110,7 @@ void Style::Impl::parse(const std::string& json_) {
     setLight(std::make_unique<Light>(parser.light));
 
     spriteLoaded = false;
-    spriteLoader->load(parser.spriteURL, scheduler, fileSource);
+    spriteLoader->load(parser.spriteURL, fileSource);
     glyphURL = parser.glyphURL;
 
     loaded = true;
diff --git a/src/mbgl/style/style_impl.hpp b/src/mbgl/style/style_impl.hpp
index 3dc222bfad..4c56f6785b 100644
--- a/src/mbgl/style/style_impl.hpp
+++ b/src/mbgl/style/style_impl.hpp
@@ -25,7 +25,6 @@
 
 namespace mbgl {
 
-class Scheduler;
 class FileSource;
 class AsyncRequest;
 class SpriteLoader;
@@ -38,7 +37,7 @@ class Style::Impl : public SpriteLoaderObserver,
                     public LightObserver,
                     public util::noncopyable {
 public:
-    Impl(Scheduler&, FileSource&, float pixelRatio);
+    Impl(FileSource&, float pixelRatio);
     ~Impl() override;
 
     void loadJSON(const std::string&);
@@ -98,7 +97,6 @@ public:
 private:
     void parse(const std::string&);
 
-    Scheduler& scheduler;
     FileSource& fileSource;
 
     std::string url;
diff --git a/src/mbgl/tile/geometry_tile.cpp b/src/mbgl/tile/geometry_tile.cpp
index 029c761e53..d110b2f826 100644
--- a/src/mbgl/tile/geometry_tile.cpp
+++ b/src/mbgl/tile/geometry_tile.cpp
@@ -1,4 +1,5 @@
 #include <mbgl/tile/geometry_tile.hpp>
+
 #include <mbgl/tile/geometry_tile_worker.hpp>
 #include <mbgl/tile/geometry_tile_data.hpp>
 #include <mbgl/tile/tile_observer.hpp>
@@ -45,7 +46,7 @@ GeometryTile::GeometryTile(const OverscaledTileID& id_,
       ImageRequestor(parameters.imageManager),
       sourceID(std::move(sourceID_)),
       mailbox(std::make_shared<Mailbox>(*Scheduler::GetCurrent())),
-      worker(parameters.workerScheduler,
+      worker(Scheduler::GetBackground(),
              ActorRef<GeometryTile>(*this, mailbox),
              id_,
              sourceID,
diff --git a/src/mbgl/tile/raster_dem_tile.cpp b/src/mbgl/tile/raster_dem_tile.cpp
index 224c5c9062..8b4e0113c7 100644
--- a/src/mbgl/tile/raster_dem_tile.cpp
+++ b/src/mbgl/tile/raster_dem_tile.cpp
@@ -1,4 +1,5 @@
 #include <mbgl/tile/raster_dem_tile.hpp>
+
 #include <mbgl/tile/raster_dem_tile_worker.hpp>
 #include <mbgl/tile/tile_observer.hpp>
 #include <mbgl/tile/tile_loader_impl.hpp>
@@ -17,7 +18,7 @@ RasterDEMTile::RasterDEMTile(const OverscaledTileID& id_,
     : Tile(Kind::RasterDEM, id_),
       loader(*this, id_, parameters, tileset),
       mailbox(std::make_shared<Mailbox>(*Scheduler::GetCurrent())),
-      worker(parameters.workerScheduler,
+      worker(Scheduler::GetBackground(),
              ActorRef<RasterDEMTile>(*this, mailbox)) {
 
     encoding = tileset.encoding;
diff --git a/src/mbgl/tile/raster_tile.cpp b/src/mbgl/tile/raster_tile.cpp
index ff02301021..9da11b3595 100644
--- a/src/mbgl/tile/raster_tile.cpp
+++ b/src/mbgl/tile/raster_tile.cpp
@@ -1,4 +1,5 @@
 #include <mbgl/tile/raster_tile.hpp>
+
 #include <mbgl/tile/raster_tile_worker.hpp>
 #include <mbgl/tile/tile_observer.hpp>
 #include <mbgl/tile/tile_loader_impl.hpp>
@@ -17,7 +18,7 @@ RasterTile::RasterTile(const OverscaledTileID& id_,
     : Tile(Kind::Raster, id_),
       loader(*this, id_, parameters, tileset),
       mailbox(std::make_shared<Mailbox>(*Scheduler::GetCurrent())),
-      worker(parameters.workerScheduler,
+      worker(Scheduler::GetBackground(),
              ActorRef<RasterTile>(*this, mailbox)) {
 }
 
diff --git a/src/mbgl/util/thread_pool.cpp b/src/mbgl/util/thread_pool.cpp
new file mode 100644
index 0000000000..79c2f75012
--- /dev/null
+++ b/src/mbgl/util/thread_pool.cpp
@@ -0,0 +1,58 @@
+#include <mbgl/util/thread_pool.hpp>
+
+#include <mbgl/util/platform.hpp>
+#include <mbgl/util/string.hpp>
+
+namespace mbgl {
+
+ThreadPool::ThreadPool(std::size_t count) {
+    threads.reserve(count);
+
+    for (std::size_t i = 0; i < count; ++i) {
+        threads.emplace_back([this, i]() {
+            platform::setCurrentThreadName(std::string{ "Worker " } + util::toString(i + 1));
+
+            while (true) {
+                std::unique_lock<std::mutex> lock(mutex);
+
+                cv.wait(lock, [this] {
+                    return !queue.empty() || terminate;
+                });
+
+                if (terminate) {
+                    return;
+                }
+
+                auto mailbox = queue.front();
+                queue.pop();
+                lock.unlock();
+
+                Mailbox::maybeReceive(mailbox);
+            }
+        });
+    }
+}
+
+ThreadPool::~ThreadPool() {
+    {
+        std::lock_guard<std::mutex> lock(mutex);
+        terminate = true;
+    }
+
+    cv.notify_all();
+
+    for (auto& thread : threads) {
+        thread.join();
+    }
+}
+
+void ThreadPool::schedule(std::weak_ptr<Mailbox> mailbox) {
+    {
+        std::lock_guard<std::mutex> lock(mutex);
+        queue.push(mailbox);
+    }
+
+    cv.notify_one();
+}
+
+} // namespace mbgl
diff --git a/src/mbgl/util/thread_pool.hpp b/src/mbgl/util/thread_pool.hpp
new file mode 100644
index 0000000000..509fd06061
--- /dev/null
+++ b/src/mbgl/util/thread_pool.hpp
@@ -0,0 +1,28 @@
+#pragma once
+
+#include <mbgl/actor/mailbox.hpp>
+#include <mbgl/actor/scheduler.hpp>
+
+#include <condition_variable>
+#include <mutex>
+#include <queue>
+#include <thread>
+
+namespace mbgl {
+
+class ThreadPool final : public Scheduler {
+public:
+    explicit ThreadPool(std::size_t count);
+    ~ThreadPool() override;
+
+    void schedule(std::weak_ptr<Mailbox>) override;
+
+private:
+    std::vector<std::thread> threads;
+    std::queue<std::weak_ptr<Mailbox>> queue;
+    std::mutex mutex;
+    std::condition_variable cv;
+    bool terminate{ false };
+};
+
+} // namespace mbgl