core: add queue::pop_eventually()

It's an asynchronous version of blocking pop().

core/queue.hh

@@ -48,6 +48,10 @@ public:
 
     // Returns a future<> that becomes available when push() can be called.
     future<> not_full();
+
+    // Pops element now or when ther is some. Returns a future that becomes
+    // available when some element is available.
+    future<T> pop_eventually();
 };
 
 template <typename T>
@@ -97,6 +101,18 @@ T queue<T>::pop() {
     return data;
 }
 
+template <typename T>
+inline
+future<T> queue<T>::pop_eventually() {
+    if (empty()) {
+        return not_empty().then([this] {
+            return make_ready_future<T>(pop());
+        });
+    } else {
+        return make_ready_future<T>(pop());
+    }
+}
+
 template <typename T>
 template <typename Func>
 inline