Instead of rekeying, delete a quick mode if we have a fresher instance

If both peers initiate quick mode rekeying simultaneously, we end up
with duplicate SAs for a configuration. This can't be avoided, nor do
the standards provide an appropriate solution. Instead of closing one
SA immediately, we keep both. But once rekeying triggers, we don't
refresh the SA with the shorter soft lifetime, but delete it.

src/libcharon/sa/ikev1/task_manager_v1.c

@@ -1366,6 +1366,34 @@ METHOD(task_manager_t, queue_child, void,
 	queue_task(this, &task->task);
 }
 
+/**
+ * Check if a CHILD_SA is redundant and we should delete instead of rekey
+ */
+static bool is_redundant(private_task_manager_t *this, child_sa_t *child_sa)
+{
+	enumerator_t *enumerator;
+	child_sa_t *current;
+	bool redundant = FALSE;
+
+	enumerator = this->ike_sa->create_child_sa_enumerator(this->ike_sa);
+	while (enumerator->enumerate(enumerator, &current))
+	{
+		if (current->get_state(current) == CHILD_INSTALLED &&
+			streq(current->get_name(current), child_sa->get_name(child_sa)) &&
+			current->get_lifetime(current, FALSE) >
+				child_sa->get_lifetime(child_sa, FALSE))
+		{
+			DBG1(DBG_IKE, "deleting redundant CHILD_SA %s{%d}",
+				 child_sa->get_name(child_sa), child_sa->get_reqid(child_sa));
+			redundant = TRUE;
+			break;
+		}
+	}
+	enumerator->destroy(enumerator);
+
+	return redundant;
+}
+
 METHOD(task_manager_t, queue_child_rekey, void,
 	private_task_manager_t *this, protocol_id_t protocol, u_int32_t spi)
 {
@@ -1380,13 +1408,21 @@ METHOD(task_manager_t, queue_child_rekey, void,
 	}
 	if (child_sa && child_sa->get_state(child_sa) == CHILD_INSTALLED)
 	{
-		child_sa->set_state(child_sa, CHILD_REKEYING);
-		cfg = child_sa->get_config(child_sa);
-		task = quick_mode_create(this->ike_sa, cfg->get_ref(cfg), NULL, NULL);
-		task->use_reqid(task, child_sa->get_reqid(child_sa));
-		task->rekey(task, child_sa->get_spi(child_sa, TRUE));
+		if (is_redundant(this, child_sa))
+		{
+			queue_task(this, (task_t*)quick_delete_create(this->ike_sa,
+												protocol, spi, FALSE, FALSE));
+		}
+		else
+		{
+			child_sa->set_state(child_sa, CHILD_REKEYING);
+			cfg = child_sa->get_config(child_sa);
+			task = quick_mode_create(this->ike_sa, cfg->get_ref(cfg), NULL, NULL);
+			task->use_reqid(task, child_sa->get_reqid(child_sa));
+			task->rekey(task, child_sa->get_spi(child_sa, TRUE));
 
-		queue_task(this, &task->task);
+			queue_task(this, &task->task);
+		}
 	}
 }