Small processing_queue improvements (#4159)

* Add batch type alias

* Add `joinable` function

* Use universal reference

nano/lib/processing_queue.hpp

@@ -6,6 +6,7 @@
 #include <nano/lib/threading.hpp>
 #include <nano/lib/utility.hpp>
 
+#include <algorithm>
 #include <condition_variable>
 #include <deque>
 #include <functional>
@@ -23,6 +24,7 @@ class processing_queue final
 {
 public:
 	using value_t = T;
+	using batch_t = std::deque<value_t>;
 
 	/**
 	 * @param thread_role Spawned processing threads will use this name
@@ -71,15 +73,23 @@ class processing_queue final
 		threads.clear ();
 	}
 
+	bool joinable () const
+	{
+		return std::any_of (threads.cbegin (), threads.cend (), [] (auto const & thread) {
+			return thread.joinable ();
+		});
+	}
+
 	/**
 	 * Queues item for batch processing
 	 */
-	void add (T && item)
+	template <class Item>
+	void add (Item && item)
 	{
 		nano::unique_lock<nano::mutex> lock{ mutex };
 		if (queue.size () < max_queue_size)
 		{
-			queue.emplace_back (std::forward<T> (item));
+			queue.push_back (std::forward<T> (item));
 			lock.unlock ();
 			condition.notify_one ();
 			stats.inc (stat_type, nano::stat::detail::queue);
@@ -136,7 +146,7 @@ class processing_queue final
 			for (int n = 0; n < max_batch_size; ++n)
 			{
 				debug_assert (!queue.empty ());
-				queue_l.emplace_back (std::move (queue.front ()));
+				queue_l.push_back (std::move (queue.front ()));
 				queue.pop_front ();
 			}
 			return queue_l;
@@ -160,7 +170,7 @@ class processing_queue final
 	}
 
 public:
-	std::function<void (std::deque<value_t> &)> process_batch{ [] (auto &) { debug_assert (false, "processing queue callback empty"); } };
+	std::function<void (batch_t &)> process_batch{ [] (auto &) { debug_assert (false, "processing queue callback empty"); } };
 
 private:
 	nano::stats & stats;