Disable the loaned messages inside the executor. (#2335)

* Disable the loaned messages inside the executor.

They are currently unsafe to use; see the comment in the
commit for more information.

Signed-off-by: Chris Lalancette <clalancette@gmail.com>

rclcpp/src/rclcpp/executor.cpp

@@ -669,6 +669,11 @@ Executor::execute_subscription(rclcpp::SubscriptionBase::SharedPtr subscription)
             subscription->get_topic_name(),
             [&]() {return subscription->take_type_erased(message.get(), message_info);},
             [&]() {subscription->handle_message(message, message_info);});
+          // TODO(clalancette): In the case that the user is using the MessageMemoryPool,
+          // and they take a shared_ptr reference to the message in the callback, this can
+          // inadvertently return the message to the pool when the user is still using it.
+          // This is a bug that needs to be fixed in the pool, and we should probably have
+          // a custom deleter for the message that actually does the return_message().
           subscription->return_message(message);
         }
         break;

rclcpp/src/rclcpp/subscription_base.cpp

@@ -298,7 +298,20 @@ SubscriptionBase::setup_intra_process(
 bool
 SubscriptionBase::can_loan_messages() const
 {
-  return rcl_subscription_can_loan_messages(subscription_handle_.get());
+  bool retval = rcl_subscription_can_loan_messages(subscription_handle_.get());
+  if (retval) {
+    // TODO(clalancette): The loaned message interface is currently not safe to use with
+    // shared_ptr callbacks.  If a user takes a copy of the shared_ptr, it can get freed from
+    // underneath them via rcl_return_loaned_message_from_subscription().  The correct solution is
+    // to return the loaned message in a custom deleter, but that needs to be carefully handled
+    // with locking.  Warn the user about this until we fix it.
+    RCLCPP_WARN_ONCE(
+      this->node_logger_,
+      "Loaned messages are only safe with const ref subscription callbacks. "
+      "If you are using any other kind of subscriptions, "
+      "set the ROS_DISABLE_LOANED_MESSAGES environment variable to 1 (the default).");
+  }
+  return retval;
 }
 
 rclcpp::Waitable::SharedPtr