[LockedUnboundedQueue] use a std::shared_mutex instead of std::mutex

This allows multiple readers to peek and bail out without taking an exclusive lock.

emper/lib/adt/LockedUnboundedQueue.hpp

@@ -2,26 +2,33 @@
 // Copyright © 2020 Florian Schmaus
 #pragma once
 
-#include <mutex>
 #include <queue>
+#include <shared_mutex>
 
 namespace lib::adt {
 
 template <typename I>
 class LockedUnboundedQueue {
  private:
-	std::mutex queue_mutex;
+	std::shared_mutex queue_mutex;
 
 	std::queue<I*> queue;
 
  public:
 	void enqueue(I* item) {
-		std::lock_guard<std::mutex> lock(queue_mutex);
+		std::unique_lock lock(queue_mutex);
 		queue.push(item);
 	}
 
 	auto dequeue() -> I* {
-		std::lock_guard<std::mutex> lock(queue_mutex);
+		{
+			std::shared_lock lock(queue_mutex);
+			if (queue.empty()) {
+				return nullptr;
+			}
+		}
+
+		std::unique_lock lock(queue_mutex);
 		if (queue.empty()) {
 			return nullptr;
 		}