Add postRetrieve hook to UnboundedBlockingMpscQueue

and use that hook in Actor.

This fixes a race condition in the actor loop between 'queue.get()'
and 'setState(Running)' racing against waitUntilIdle(long)'s check if

queue.size() == 0 && state == Retrieving

Because right after the get() operation is finished, the queue size is
0, but the state is not yet at Running, while the Actor actually is
not idle.

Thanks Raim! Best rubber duck ever!

emper/Actor.hpp

@@ -32,9 +32,10 @@ private:
 
 		while (state == Running) {
 			setState(Retrieving);
-			T t = queue.get();
+			T t = queue.get([this] {
+					setState(Running);
+				});
 
-			setState(Running);
 			receive(t);
 		}
 

emper/UnboundedBlockingMpscQueue.hpp

@@ -26,12 +26,13 @@ private:
 		}
 	}
 
-	void tryToGetElement() {
+	void tryToGetElement(std::function<void(void)> postRetrieve) {
 		std::lock_guard<std::mutex> lock(queueMutex);
 		if (!mpscQueue.empty()) {
 			t = mpscQueue.front();
 			mpscQueue.pop();
 			tPopped = true;
+			postRetrieve();
 		}
 	}
 
@@ -57,16 +58,16 @@ public:
 		}
 	}
 
-	T get() {
+	T get(std::function<void(void)> postRetrieve) {
 		tPopped = false;
-		tryToGetElement();
+		tryToGetElement(postRetrieve);
 
 		if (!tPopped) {
 			Context* context = Context::getCurrentContext();
-			block([this, context] {
+			block([this, context, postRetrieve] {
 					blockedContext = context;
 
-					tryToGetElement();
+					tryToGetElement(postRetrieve);
 					if (tPopped) {
 						tryToWakeupBlockedContext();
 					}
@@ -79,6 +80,7 @@ public:
 				assert(!mpscQueue.empty());
 				t = mpscQueue.front();
 				mpscQueue.pop();
+				postRetrieve();
 			}
 		}
 		return t;