Remove pointers from Genode::Fifo interface

Replace methods of Genode::Fifo returning pointers with methods which
call lambdas with references.

Ref #3135

repos/dde_rump/include/rump/timed_semaphore.h

@@ -108,10 +108,12 @@ class Timed_semaphore : public Semaphore
 				 * Iterate through the queue and find the thread,
 				 * with the corresponding timeout.
 				 */
-				Element *first = Semaphore::_queue.dequeue();
+				Element *first = nullptr;
+				Semaphore::_queue.dequeue([&first] (Element &e) {
+					first = &e; });
 				Element *e     = first;
 
-				while (true) {
+				while (e) {
 
 					/*
 					 * Wakeup the thread.
@@ -124,8 +126,10 @@ class Timed_semaphore : public Semaphore
 					/*
 					 * Noninvolved threads are enqueued again.
 					 */
-					Semaphore::_queue.enqueue(e);
-					e = Semaphore::_queue.dequeue();
+					Semaphore::_queue.enqueue(*e);
+					e = nullptr;
+					Semaphore::_queue.dequeue([&e] (Element &next) {
+						e = &next; });
 
 					/*
 					 * Maybe, the alarm was triggered just after the corresponding
@@ -210,7 +214,7 @@ class Timed_semaphore : public Semaphore
 				 * in the wait queue.
 				 */
 				Element queue_element;
-				Semaphore::_queue.enqueue(&queue_element);
+				Semaphore::_queue.enqueue(queue_element);
 				Semaphore::_meta_lock.unlock();
 
 				/* Create the timeout */

repos/dde_rump/src/lib/rump/sync.cc

@@ -78,7 +78,7 @@ struct rumpuser_mtx
 				if (try_enter)
 					return false;
 
-				fifo.enqueue(&applicant);
+				fifo.enqueue(applicant);
 			}
 			applicant.block();
 		}
@@ -99,8 +99,8 @@ struct rumpuser_mtx
 			owner = nullptr;
 		}
 
-		if (Applicant *applicant = fifo.dequeue())
-			applicant->wake_up();
+		fifo.dequeue([] (Applicant &applicant) {
+			applicant.wake_up(); });
 	}
 };