raft: factor out the code to wakeup worker fiber

Raft has a worker fiber to perform async tasks such as WAL write,
state broadcast.

The worker was created and woken up from 2 places, leading at
least to code duplication. The patch wraps it into a new function
raft_worker_wakeup(), and uses it.

The patch is not need for anything functional, but was created
while working on #5339 and trying ideas. The patch seems to be
good refactoring making the code simpler, and therefore it is
submitted.

src/box/raft.c

@@ -178,6 +178,13 @@ raft_election_quorum(void)
 	return MIN(replication_synchro_quorum, replicaset.registered_count);
 }
 
+/**
+ * Wakeup the Raft worker fiber in order to do some async work. If the fiber
+ * does not exist yet, it is created.
+ */
+static void
+raft_worker_wakeup(void);
+
 /** Schedule broadcast of the complete Raft state to all the followers. */
 static void
 raft_schedule_broadcast(void);
@@ -670,10 +677,8 @@ raft_sm_pause_and_dump(void)
 	if (raft.is_write_in_progress)
 		return;
 	ev_timer_stop(loop(), &raft.timer);
+	raft_worker_wakeup();
 	raft.is_write_in_progress = true;
-	if (raft.worker == NULL)
-		raft.worker = fiber_new("raft_worker", raft_worker_f);
-	fiber_wakeup(raft.worker);
 }
 
 static void
@@ -988,21 +993,43 @@ raft_new_term(void)
 }
 
 static void
-raft_schedule_broadcast(void)
+raft_worker_wakeup(void)
 {
-	raft.is_broadcast_scheduled = true;
+	if (raft.worker == NULL) {
+		raft.worker = fiber_new("raft_worker", raft_worker_f);
+		if (raft.worker == NULL) {
+			/*
+			 * XXX: should be handled properly, no need to panic.
+			 * The issue though is that most of the Raft state
+			 * machine functions are not supposed to fail, and also
+			 * they usually wakeup the fiber when their work is
+			 * finished. So it is too late to fail. On the other
+			 * hand it looks not so good to create the fiber when
+			 * Raft is initialized. Because then it will occupy
+			 * memory even if Raft is not used.
+			 */
+			diag_log();
+			panic("Could't create Raft worker fiber");
+			return;
+		}
+		fiber_set_joinable(raft.worker, true);
+	}
 	/*
 	 * Don't wake the fiber if it writes something. Otherwise it would be a
-	 * spurious wakeup breaking the WAL write not adapted to this.
+	 * spurious wakeup breaking the WAL write not adapted to this. Also
+	 * don't wakeup the current fiber - it leads to undefined behaviour.
 	 */
-	if (raft.is_write_in_progress)
-		return;
-	if (raft.worker == NULL)
-		raft.worker = fiber_new("raft_worker", raft_worker_f);
-	if (raft.worker != fiber())
+	if (!raft.is_write_in_progress && fiber() != raft.worker)
 		fiber_wakeup(raft.worker);
 }
 
+static void
+raft_schedule_broadcast(void)
+{
+	raft.is_broadcast_scheduled = true;
+	raft_worker_wakeup();
+}
+
 void
 raft_init(void)
 {