[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() 💣

1 files changed, 4 insertions, 2 deletions

diff --git a/include/mbgl/actor/mailbox.hpp b/include/mbgl/actor/mailbox.hpp
index cff0de243a..1327e8e6fe 100644
--- a/include/mbgl/actor/mailbox.hpp
+++ b/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;