[core] Use an actor model for tile worker concurrency

src/mbgl/actor/mailbox.cpp

@@ -0,0 +1,55 @@
+#include <mbgl/actor/mailbox.hpp>
+#include <mbgl/actor/message.hpp>
+#include <mbgl/actor/scheduler.hpp>
+
+#include <cassert>
+
+namespace mbgl {
+
+Mailbox::Mailbox(Scheduler& scheduler_)
+    : scheduler(scheduler_) {
+}
+
+void Mailbox::push(std::unique_ptr<Message> message) {
+    assert(!closing);
+
+    std::lock_guard<std::mutex> queueLock(queueMutex);
+    bool wasEmpty = queue.empty();
+    queue.push(std::move(message));
+    if (wasEmpty) {
+        scheduler.schedule(shared_from_this());
+    }
+}
+
+void Mailbox::close() {
+    // Block until the scheduler is guaranteed not to be executing receive().
+    std::lock_guard<std::mutex> closingLock(closingMutex);
+    closing = true;
+}
+
+void Mailbox::receive() {
+    std::lock_guard<std::mutex> closingLock(closingMutex);
+
+    if (closing) {
+        return;
+    }
+
+    std::unique_ptr<Message> message;
+    bool wasEmpty;
+
+    {
+        std::lock_guard<std::mutex> queueLock(queueMutex);
+        assert(!queue.empty());
+        message = std::move(queue.front());
+        queue.pop();
+        wasEmpty = queue.empty();
+    }
+
+    (*message)();
+
+    if (!wasEmpty) {
+        scheduler.schedule(shared_from_this());
+    }
+}
+
+} // namespace mbgl