[core] Avoid blocking in Thread<Object> constructor (#12151)

* Introduce AspiringActor, EstablishedActor

This pair of objects represents the two-phase (parent-thread /
child-thread) construction that's needed to support constructing
Thread<Object> without blocking until the child thread is up and
running.

An `AspiringActor<O>` is responsible for:
 - ownership of the actor's `Mailbox`
 - allocating the memory for (but *not* constructing) the target object `O`

Using these two pieces--the mailbox and a stable address for `O`--an
`AspiringActor<O>` can accept messages for the target object, or provide
`ActorRef<O>`s that do so, before the object has actually been
constructed by the corresponding `EstablishedActor<O>`.  (Such messages
are queued in the mailbox until after the object is constructed.)

This allows for an `AspiringActor<O>` to be created and safely used by a
thread other than the one on which the target object will (eventually)
live.

An `EstablishedActor<O>` is responsible for managing the lifetime of the
target object `O` and the open/closed state of the parent's `mailbox`.

The `O` object's lifetime is contained by that of its owning
`EstablishedActor<O>`: the `EstablishedActor` constructor executes the
`O` constructor via "placement new", constructing it at the address
provided by the parent `AspiringActor`, and the `~EstablishedActor`
destructor similarly executes the `~O` destructor (after closing the
mailbox). `EstablishedActor` should therefore live entirely on the
thread intended to own `O`.

* Remove Actor#{invoke,ask}

1 files changed, 11 insertions, 9 deletions

diff --git a/platform/default/mbgl/map/map_snapshotter.cpp b/platform/default/mbgl/map/map_snapshotter.cpp
index a909e3fe9b..149ef22e7a 100644
--- a/platform/default/mbgl/map/map_snapshotter.cpp
+++ b/platform/default/mbgl/map/map_snapshotter.cpp
@@ -13,8 +13,8 @@ namespace mbgl {
 
 class MapSnapshotter::Impl {
 public:
-    Impl(FileSource&,
-         Scheduler&,
+    Impl(FileSource*,
+         std::shared_ptr<Scheduler>,
          const std::pair<bool, std::string> style,
          const Size&,
          const float pixelRatio,
@@ -40,20 +40,22 @@ public:
     void snapshot(ActorRef<MapSnapshotter::Callback>);
 
 private:
+    std::shared_ptr<Scheduler> scheduler;
     HeadlessFrontend frontend;
     Map map;
 };
 
-MapSnapshotter::Impl::Impl(FileSource& fileSource,
-           Scheduler& scheduler,
+MapSnapshotter::Impl::Impl(FileSource* fileSource,
+           std::shared_ptr<Scheduler> scheduler_,
            const std::pair<bool, std::string> style,
            const Size& size,
            const float pixelRatio,
            const optional<CameraOptions> cameraOptions,
            const optional<LatLngBounds> region,
            const optional<std::string> programCacheDir)
-    : frontend(size, pixelRatio, fileSource, scheduler, programCacheDir)
-    , map(frontend, MapObserver::nullObserver(), size, pixelRatio, fileSource, scheduler, MapMode::Static) {
+    : scheduler(std::move(scheduler_))
+    , frontend(size, pixelRatio, *fileSource, *scheduler, programCacheDir)
+    , map(frontend, MapObserver::nullObserver(), size, pixelRatio, *fileSource, *scheduler, MapMode::Static) {
 
     if (style.first) {
         map.getStyle().loadJSON(style.second);
@@ -149,15 +151,15 @@ LatLngBounds MapSnapshotter::Impl::getRegion() const {
     return map.latLngBoundsForCamera(getCameraOptions());
 }
 
-MapSnapshotter::MapSnapshotter(FileSource& fileSource,
-                               Scheduler& scheduler,
+MapSnapshotter::MapSnapshotter(FileSource* fileSource,
+                               std::shared_ptr<Scheduler> scheduler,
                                const std::pair<bool, std::string> style,
                                const Size& size,
                                const float pixelRatio,
                                const optional<CameraOptions> cameraOptions,
                                const optional<LatLngBounds> region,
                                const optional<std::string> programCacheDir)
-   : impl(std::make_unique<util::Thread<MapSnapshotter::Impl>>("Map Snapshotter", fileSource, scheduler, style, size, pixelRatio, cameraOptions, region, programCacheDir)) {
+   : impl(std::make_unique<util::Thread<MapSnapshotter::Impl>>("Map Snapshotter", fileSource, std::move(scheduler), style, size, pixelRatio, cameraOptions, region, programCacheDir)) {
 }
 
 MapSnapshotter::~MapSnapshotter() = default;