Add acceptable_buffer_backends field in SubscriptionOptionsBase

Signed-off-by: CY Chen <cyc@nvidia.com>

rclcpp/include/rclcpp/generic_subscription.hpp

@@ -80,7 +80,7 @@ public:
       node_base,
       *rclcpp::get_message_typesupport_handle(topic_type, "rosidl_typesupport_cpp", *ts_lib),
       topic_name,
-      options.to_rcl_subscription_options(qos),
+      force_cpu_buffer_backend_(options).to_rcl_subscription_options(qos),
       options.event_callbacks,
       options.use_default_callbacks,
       DeliveredMessageKind::SERIALIZED_MESSAGE),
@@ -182,6 +182,17 @@ public:
 
 private:
   RCLCPP_DISABLE_COPY(GenericSubscription)
+
+  template<typename AllocatorT>
+  static rclcpp::SubscriptionOptionsWithAllocator<AllocatorT>
+  force_cpu_buffer_backend_(
+    const rclcpp::SubscriptionOptionsWithAllocator<AllocatorT> & options)
+  {
+    auto opts = options;
+    opts.acceptable_buffer_backends = "cpu";
+    return opts;
+  }
+
   AnySubscriptionCallback<rclcpp::SerializedMessage, std::allocator<void>> any_callback_;
   // The type support library should stay loaded, so it is stored in the GenericSubscription
   std::shared_ptr<rcpputils::SharedLibrary> ts_lib_;

rclcpp/include/rclcpp/subscription_options.hpp

@@ -89,6 +89,15 @@ struct SubscriptionOptionsBase
   QosOverridingOptions qos_overriding_options;
 
   ContentFilterOptions content_filter_options;
+
+  /// Acceptable buffer backend names for this subscription.
+  /**
+   * Empty string or "cpu" means CPU-only (default for backward compatibility).
+   * "any" means all installed backends are acceptable.
+   * Comma-separated for specific backends, e.g. "cuda,demo".
+   * CPU is always implicitly acceptable regardless of this value.
+   */
+  std::string acceptable_buffer_backends{"cpu"};
 };
 
 /// Structure containing optional configuration for Subscriptions.
@@ -145,6 +154,11 @@ struct SubscriptionOptionsWithAllocator : public SubscriptionOptionsBase
       }
     }
 
+    if (!acceptable_buffer_backends.empty()) {
+      result.rmw_subscription_options.acceptable_buffer_backends =
+        acceptable_buffer_backends.c_str();
+    }
+
     return result;
   }