[core] allow self closing mailbox/actor

include/mbgl/actor/mailbox.hpp

@@ -23,7 +23,7 @@ public:
 private:
     Scheduler& scheduler;
 
-    std::mutex receivingMutex;
+    std::recursive_mutex receivingMutex;
     std::mutex pushingMutex;
 
     bool closed { false };

src/mbgl/actor/mailbox.cpp

@@ -15,7 +15,8 @@ void Mailbox::close() {
     // must not block send(). Of the two, the receiving mutex must be acquired first, because that is
     // the order that an actor will obtain them when it self-sends a message, and consistent lock
     // acquisition order prevents deadlocks.
-    std::lock_guard<std::mutex> receivingLock(receivingMutex);
+    // The receiving mutex is recursive to allow a mailbox (and thus the actor) to close itself.
+    std::lock_guard<std::recursive_mutex> receivingLock(receivingMutex);
     std::lock_guard<std::mutex> pushingLock(pushingMutex);
 
     closed = true;
@@ -37,7 +38,7 @@ void Mailbox::push(std::unique_ptr<Message> message) {
 }
 
 void Mailbox::receive() {
-    std::lock_guard<std::mutex> receivingLock(receivingMutex);
+    std::lock_guard<std::recursive_mutex> receivingLock(receivingMutex);
 
     if (closed) {
         return;

test/actor/actor.test.cpp

@@ -6,6 +6,7 @@
 #include <chrono>
 #include <functional>
 #include <future>
+#include <memory>
 
 using namespace mbgl;
 using namespace std::chrono_literals;
@@ -123,6 +124,37 @@ TEST(Actor, DestructionBlocksOnSend) {
     thread.join();
 }
 
+TEST(Actor, DestructionAllowedInReceiveOnSameThread) {
+    // Destruction doesn't block if occurring on the same
+    // thread as receive(). This prevents deadlocks and
+    // allows for self-closing actors
+
+    struct Test {
+
+        Test(ActorRef<Test>){};
+
+        void callMeBack(std::function<void ()> callback) {
+            callback();
+        }
+    };
+
+    ThreadPool pool { 1 };
+
+    std::promise<void> callbackFiredPromise;
+
+    auto test = std::make_unique<Actor<Test>>(pool);
+
+    // Callback (triggered while mutex is locked in Mailbox::receive())
+    test->invoke(&Test::callMeBack, [&]() {
+        // Destroy the Actor/Mailbox in the same thread
+        test.reset();
+        callbackFiredPromise.set_value();
+    });
+
+    auto status = callbackFiredPromise.get_future().wait_for(std::chrono::seconds(1));
+    ASSERT_EQ(std::future_status::ready, status);
+}
+
 TEST(Actor, OrderedMailbox) {
     // Messages are processed in order.