Don't copy contiguous bytes on reception

This uses the slices iterator API of zenoh-cpp to avoid unecessarily
copying bytes into a vecotr, if and only if the bytes is made up of
exactly one slice.

rmw_zenoh_cpp/src/detail/payload.hpp

@@ -0,0 +1,88 @@
+// Copyright 2024 Open Source Robotics Foundation, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef DETAIL__PAYLOAD_HPP_
+#define DETAIL__PAYLOAD_HPP_
+
+#include <zenoh.h>
+
+#include <variant>
+#include <utility>
+#include <vector>
+
+namespace rmw_zenoh_cpp
+{
+///=============================================================================
+class Payload
+{
+public:
+  explicit Payload(const zenoh::Bytes & bytes)
+  {
+    auto slices = bytes.slice_iter();
+    auto slice = slices.next();
+    if (!slice.has_value()) {
+      bytes_ = nullptr;
+    } else {
+      if (!slices.next().has_value()) {
+        bytes_ = Contiguous {slice.value(), bytes.clone()};
+      } else {
+        bytes_ = bytes.as_vector();
+      }
+    }
+  }
+
+  ~Payload() = default;
+
+  const uint8_t * data()
+  {
+    if (std::holds_alternative<Empty>(bytes_)) {
+      return nullptr;
+    } else if (std::holds_alternative<NonContiguous>(bytes_)) {
+      return std::get<NonContiguous>(bytes_).data();
+    } else {
+      return std::get<Contiguous>(bytes_).slice.data;
+    }
+  }
+
+  size_t size()
+  {
+    if (std::holds_alternative<Empty>(bytes_)) {
+      return 0;
+    } else if (std::holds_alternative<NonContiguous>(bytes_)) {
+      return std::get<NonContiguous>(bytes_).size();
+    } else {
+      return std::get<Contiguous>(bytes_).slice.len;
+    }
+  }
+
+  bool empty()
+  {
+    return std::holds_alternative<Empty>(bytes_);
+  }
+
+private:
+  struct Contiguous
+  {
+    zenoh::Slice slice;
+    zenoh::Bytes bytes;
+  };
+  using NonContiguous = std::vector<uint8_t>;
+  using Empty = std::nullptr_t;
+  // Is `std::vector<uint8_t>` in case of a non-contiguous payload
+  // and `zenoh::Slice` plus a `zenoh::Bytes` otherwise.
+  std::variant<NonContiguous, Contiguous, Empty> bytes_;
+};
+}  // namespace rmw_zenoh_cpp
+
+#endif  // DETAIL__PAYLOAD_HPP_

rmw_zenoh_cpp/src/detail/rmw_subscription_data.cpp

@@ -44,10 +44,10 @@ namespace rmw_zenoh_cpp
 {
 ///=============================================================================
 SubscriptionData::Message::Message(
-  std::vector<uint8_t> && p,
+  const zenoh::Bytes & p,
   uint64_t recv_ts,
   AttachmentData && attachment_)
-: payload(std::move(p)), recv_timestamp(recv_ts), attachment(std::move(attachment_))
+: payload(p), recv_timestamp(recv_ts), attachment(std::move(attachment_))
 {
 }
 
@@ -225,7 +225,7 @@ bool SubscriptionData::init()
 
         sub_data->add_new_message(
           std::make_unique<SubscriptionData::Message>(
-            sample.get_payload().as_vector(),
+            sample.get_payload(),
             std::chrono::system_clock::now().time_since_epoch().count(),
             std::move(attachment_data)),
           std::string(sample.get_keyexpr().as_string_view()));

rmw_zenoh_cpp/src/detail/rmw_subscription_data.hpp

@@ -36,6 +36,7 @@
 #include "attachment_helpers.hpp"
 #include "type_support_common.hpp"
 #include "zenoh_utils.hpp"
+#include "payload.hpp"
 
 #include "rcutils/allocator.h"
 
@@ -51,13 +52,13 @@ class SubscriptionData final : public std::enable_shared_from_this<SubscriptionD
   struct Message
   {
     explicit Message(
-      std::vector<uint8_t> && p,
+      const zenoh::Bytes & bytes,
       uint64_t recv_ts,
       AttachmentData && attachment);
 
     ~Message();
 
-    std::vector<uint8_t> payload;
+    Payload payload;
     uint64_t recv_timestamp;
     AttachmentData attachment;
   };