can_loan_messages for subscription

Signed-off-by: Karsten Knese <karsten@openrobotics.org>

rcl/include/rcl/subscription.h

@@ -471,6 +471,15 @@ RCL_WARN_UNUSED
 const rmw_qos_profile_t *
 rcl_subscription_get_actual_qos(const rcl_subscription_t * subscription);
 
+/// Check if subscription instance can loan messages.
+/**
+ * Depending on the middleware and the message type, this will return true if the middleware
+ * can allocate a ROS message instance.
+ */
+RCL_PUBLIC
+bool
+rcl_subscription_can_loan_messages(const rcl_subscription_t * subscription);
+
 #ifdef __cplusplus
 }
 #endif

rcl/src/rcl/publisher.c

@@ -250,7 +250,7 @@ rcl_allocate_loaned_message(
   size_t message_size)
 {
   if (!rcl_publisher_is_valid(publisher)) {
-    return RCL_RET_PUBLISHER_INVALID;  // error already set
+    return NULL;  // error already set
   }
   return rmw_allocate_loaned_message(publisher->impl->rmw_handle, type_support, message_size);
 }

rcl/src/rcl/subscription.c

@@ -379,6 +379,15 @@ rcl_subscription_get_actual_qos(const rcl_subscription_t * subscription)
   return &subscription->impl->actual_qos;
 }
 
+bool
+rcl_subscription_can_loan_messages(const rcl_subscription_t * subscription)
+{
+  if (!rcl_subscription_is_valid(subscription)) {
+    return false;  // error message already set
+  }
+  return subscription->impl->rmw_handle->can_loan_messages;
+}
+
 #ifdef __cplusplus
 }
 #endif