prevent lost stop() calls

Currently the write to 'state' from stop() is overridden in actorLoop()
if it occurs while the Actor is executing the while loop body in actorLoop()
because the while loop body unconditionally writes 'state'.

To prevent those lost 'state' updates actorLoop() now uses compare_exchange
to switch 'state'.

emper/Actor.hpp

@@ -23,6 +23,10 @@ private:
 
 	UnboundedBlockingMpscQueue<T> queue;
 
+	bool switchState(State oldState, State newState) {
+		return state.compare_exchange_strong(oldState, newState, std::memory_order_acq_rel);
+	}
+
 	void setState(State newState) {
 		state.store(newState, std::memory_order_release);
 	}
@@ -30,16 +34,19 @@ private:
 	void actorLoop() {
 		setState(Running);
 
-		while (state == Running) {
-			setState(Retrieving);
+		while (state.load(std::memory_order_acquire) == Running) {
+			if (!switchState(Running, Retrieving)) {
+				// Someone else must have set our state to Stopped
+				break;
+			}
+
 			T t = queue.get([this] {
-					setState(Running);
+					// Prevent lost stop()
+					switchState(Retrieving, Running);
 				});
 
 			receive(t);
 		}
-
-		setState(Stopped);
 	}
 
 protected: