Implement no-op on IPC subscription

Author: Mauro Passerino

rclcpp/include/rclcpp/experimental/buffers/ring_buffer_implementation.hpp

@@ -91,7 +91,12 @@ class RingBufferImplementation : public BufferImplementationBase<BufferT>
 
     if (!has_data_()) {
       RCLCPP_ERROR(rclcpp::get_logger("rclcpp"), "Calling dequeue on empty intra-process buffer");
-      throw std::runtime_error("Calling dequeue on empty intra-process buffer");
+      // This situation can happen on the EventsExecutor if we have more events in the queue
+      // than messages in the history cache (set by the qos_policies.depth of the subscription)
+      // For example if we set depth=1 and we get 2 messages really fast (so no time for processing),
+      // we could end up with 2 events in the queue but only 1 msg is actually stored on the buffer.
+      // In this case we return an empty buffer.
+      return BufferT();
     }
 
     auto request = std::move(ring_buffer_[read_index_]);

rclcpp/include/rclcpp/experimental/subscription_intra_process.hpp

@@ -129,8 +129,14 @@ class SubscriptionIntraProcess : public SubscriptionIntraProcessBase
 
     if (any_callback_.use_take_shared_method()) {
       shared_msg = buffer_->consume_shared();
+      if(!shared_msg) {
+        return nullptr;
+      }
     } else {
       unique_msg = buffer_->consume_unique();
+      if(!unique_msg) {
+        return nullptr;
+      }
     }
     return std::static_pointer_cast<void>(
       std::make_shared<std::pair<ConstMessageSharedPtr, MessageUniquePtr>>(
@@ -185,7 +191,8 @@ class SubscriptionIntraProcess : public SubscriptionIntraProcessBase
   execute_impl(std::shared_ptr<void> & data)
   {
     if (!data) {
-      throw std::runtime_error("'data' is empty");
+      RCLCPP_ERROR(rclcpp::get_logger("rclcpp"), "Subscription intra-process: 'data' is empty");
+      return;
     }
 
     rmw_message_info_t msg_info;