[core] Use an actor model for tile worker concurrency

test/actor/actor_ref.cpp

@@ -0,0 +1,42 @@
+#include <mbgl/actor/actor.hpp>
+#include <mbgl/actor/thread_pool.hpp>
+
+#include <mbgl/test/util.hpp>
+
+#include <chrono>
+#include <functional>
+#include <future>
+
+using namespace mbgl;
+using namespace std::chrono_literals;
+
+TEST(ActorRef, CanOutliveActor) {
+    // An ActorRef can outlive its actor. Doing does not extend the actor's lifetime.
+    // Sending a message to an ActorRef whose actor has died is a no-op.
+
+    struct Test {
+        bool& died;
+
+        Test(ActorRef<Test>, bool& died_)
+            : died(died_) {
+        }
+
+        ~Test() {
+            died = true;
+        }
+
+        void receive() {
+            FAIL();
+        }
+    };
+
+    ThreadPool pool { 1 };
+    bool died = false;
+
+    ActorRef<Test> test = [&] () {
+        return Actor<Test>(pool, std::ref(died)).self();
+    }();
+
+    EXPECT_TRUE(died);
+    test.invoke(&Test::receive);
+}