[core] Block in Mailbox::close() until neither receive nor push are in progress

Otherwise, an ActorRef that's in the process of sending a message could attempt to access an invalid Scheduler reference:

      Thread 1                     Thread 2
--------------------------------------------------
 Scheduler::Scheduler
     Actor::Actor
                              weakMailbox.lock()
     Actor::~Actor
 Scheduler::~Scheduler
                               mailbox->push()
                             scheduler.schedule() 💣

include/mbgl/actor/mailbox.hpp

@@ -23,8 +23,10 @@ public:
 private:
     Scheduler& scheduler;
 
-    std::mutex closingMutex;
-    bool closing { false };
+    std::mutex receivingMutex;
+    std::mutex pushingMutex;
+
+    bool closed { false };
 
     std::mutex queueMutex;
     std::queue<std::unique_ptr<Message>> queue;

src/mbgl/actor/mailbox.cpp

@@ -10,8 +10,23 @@ Mailbox::Mailbox(Scheduler& scheduler_)
     : scheduler(scheduler_) {
 }
 
+void Mailbox::close() {
+    // Block until neither receive() nor push() are in progress. Two mutexes are used because receive()
+    // 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);
+    std::lock_guard<std::mutex> pushingLock(pushingMutex);
+
+    closed = true;
+}
+
 void Mailbox::push(std::unique_ptr<Message> message) {
-    assert(!closing);
+    std::lock_guard<std::mutex> pushingLock(pushingMutex);
+
+    if (closed) {
+        return;
+    }
 
     std::lock_guard<std::mutex> queueLock(queueMutex);
     bool wasEmpty = queue.empty();
@@ -21,16 +36,10 @@ void Mailbox::push(std::unique_ptr<Message> message) {
     }
 }
 
-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);
+    std::lock_guard<std::mutex> receivingLock(receivingMutex);
 
-    if (closing) {
+    if (closed) {
         return;
     }
 

test/actor/actor.test.cpp

@@ -26,7 +26,7 @@ TEST(Actor, Construction) {
     EXPECT_TRUE(constructed);
 }
 
-TEST(Actor, DestructionClosesMailbox) {
+TEST(Actor, DestructionBlocksOnReceive) {
     // Destruction blocks until the actor is not receiving.
 
     struct Test {
@@ -67,6 +67,62 @@ TEST(Actor, DestructionClosesMailbox) {
     exitingPromise.set_value();
 }
 
+TEST(Actor, DestructionBlocksOnSend) {
+    // Destruction blocks until the actor is not being sent a message.
+
+    struct TestScheduler : public Scheduler {
+        std::promise<void> promise;
+        std::future<void> future;
+        std::atomic<bool> waited;
+
+        TestScheduler(std::promise<void> promise_, std::future<void> future_)
+            : promise(std::move(promise_)),
+              future(std::move(future_)),
+              waited(false) {
+        }
+
+        ~TestScheduler() {
+            EXPECT_TRUE(waited.load());
+        }
+
+        void schedule(std::weak_ptr<Mailbox>) final {
+            promise.set_value();
+            future.wait();
+            std::this_thread::sleep_for(1ms);
+            waited = true;
+        }
+    };
+
+    struct Test {
+        Test(ActorRef<Test>) {}
+        void message() {}
+    };
+
+    std::promise<void> enteredPromise;
+    std::future<void> enteredFuture = enteredPromise.get_future();
+
+    std::promise<void> exitingPromise;
+    std::future<void> exitingFuture = exitingPromise.get_future();
+
+    auto scheduler = std::make_unique<TestScheduler>(std::move(enteredPromise), std::move(exitingFuture));
+    auto actor = std::make_unique<Actor<Test>>(*scheduler);
+
+    std::thread thread {
+        [] (ActorRef<Test> ref) {
+            ref.invoke(&Test::message);
+        },
+        actor->self()
+    };
+
+    enteredFuture.wait();
+    exitingPromise.set_value();
+
+    actor.reset();
+    scheduler.reset();
+
+    thread.join();
+}
+
 TEST(Actor, OrderedMailbox) {
     // Messages are processed in order.