Update descriptor ops to use non-owning pointers

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

Author: nvcyc

rosidl_buffer/include/rosidl_buffer/buffer.hpp

@@ -475,10 +475,12 @@ class Buffer
     return impl_->get_backend_type();
   }
 
-  /// Get the implementation pointer (for serialization) - returns raw pointer
-  /// for read-only access
+  /// Get the implementation pointer (read-only).
   const BufferImplBase<T> * get_impl() const {return impl_.get();}
 
+  /// Get the implementation pointer (mutable, e.g. for descriptor creation).
+  BufferImplBase<T> * get_impl() {return impl_.get();}
+
   /// Throw exception if not CPU backend.
   /// @throws std::runtime_error if backend is not CPU.
   void throw_if_not_cpu_backend() const

rosidl_buffer_backend/include/rosidl_buffer_backend/buffer_backend.hpp

@@ -55,22 +55,22 @@ class BufferBackend
   virtual std::shared_ptr<void> create_empty_descriptor() const = 0;
 
   /// Create a descriptor message with endpoint awareness.
-  /// @param impl Type-erased BufferImplBase pointer.
+  /// @param impl Non-owning, type-erased BufferImplBase pointer (may be mutated).
   /// @param endpoint_info Endpoint info for the peer.
   /// @return Type-erased descriptor message, or nullptr if the backend cannot
   ///         handle this endpoint (e.g., the peer does not support this backend).
   ///         Returning nullptr signals the serialization layer to fall back to
   ///         CPU-based serialization.
   virtual std::shared_ptr<void> create_descriptor_with_endpoint(
-    const std::shared_ptr<void> & impl,
+    void * impl,
     const rmw_topic_endpoint_info_t & endpoint_info) const = 0;
 
   /// Create a BufferImpl from descriptor with endpoint awareness.
-  /// @param descriptor Type-erased descriptor message pointer.
+  /// @param descriptor Non-owning, type-erased descriptor message pointer (read-only).
   /// @param endpoint_info Endpoint info for the peer.
   /// @return Type-erased BufferImplBase pointer.
   virtual std::shared_ptr<void> from_descriptor_with_endpoint(
-    const std::shared_ptr<void> & descriptor,
+    const void * descriptor,
     const rmw_topic_endpoint_info_t & endpoint_info) const = 0;
 
   /// Hook invoked when creating a local endpoint.

rosidl_buffer_backend/include/rosidl_buffer_backend/buffer_descriptor_ops.hpp

@@ -27,15 +27,17 @@ namespace rosidl
 struct BufferDescriptorOps
 {
   /// Create descriptor with endpoint awareness.
-  /// Input pointer (`impl`) is a type-erased `BufferImplBase<T>` instance.
+  /// Input pointer (`impl`) is a non-owning, type-erased `BufferImplBase<T>` instance
+  /// that the backend may mutate.
   /// Return value is a type-erased backend descriptor message instance.
-  std::function<std::shared_ptr<void>(const std::shared_ptr<void> &,
+  std::function<std::shared_ptr<void>(void *,
     const rmw_topic_endpoint_info_t &)> create_descriptor_with_endpoint;
 
   /// Create buffer impl from descriptor with endpoint awareness.
-  /// Input pointer (`descriptor`) is a type-erased backend descriptor message.
+  /// Input pointer (`descriptor`) is a non-owning, type-erased backend descriptor
+  /// message (read-only).
   /// Return value is a type-erased `BufferImplBase<T>` instance.
-  std::function<std::shared_ptr<void>(const std::shared_ptr<void> &,
+  std::function<std::shared_ptr<void>(const void *,
     const rmw_topic_endpoint_info_t &)> from_descriptor_with_endpoint;
 };