Recycle serialization buffers on transmission

Adds a LIFO buffer pool in the context to reuse buffers allocated
on serialization. The aim is not (only) to avoid the overhead of
dynamic allocation but rather to enhance the cache locality of
serialization buffers.

rmw_zenoh_cpp/src/detail/buffer_pool.hpp

@@ -0,0 +1,74 @@
+// 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__BUFFER_POOL_HPP_
+#define DETAIL__BUFFER_POOL_HPP_
+
+#include <mutex>
+#include <cassert>
+#include <vector>
+
+#include "rcutils/allocator.h"
+
+// FIXME(fuzzypixelz): indeed, we leak all allocated buffers ;)
+class BufferPool
+{
+public:
+  struct Buffer
+  {
+    uint8_t *data;
+    size_t size;
+  };
+
+  BufferPool() = default;
+
+  Buffer allocate(rcutils_allocator_t *allocator, size_t size)
+  {
+    std::lock_guard<std::mutex> guard(mutex_);
+
+    if (buffers_.empty()) {
+      uint8_t *data = static_cast<uint8_t *>(allocator->allocate(size, allocator->state));
+      if (data == nullptr) {
+        return {};
+      }
+      return Buffer {data, size};
+    } else {
+      Buffer buffer = buffers_.back();
+      buffers_.pop_back();
+      if (buffer.size < size) {
+        uint8_t *data = static_cast<uint8_t *>(allocator->reallocate(
+            buffer.data, size, allocator->state));
+        if (data == nullptr) {
+          return {};
+        }
+        buffer.data = data;
+        buffer.size = size;
+      }
+      return buffer;
+    }
+  }
+
+  void
+  deallocate(Buffer buffer)
+  {
+    std::lock_guard<std::mutex> guard(mutex_);
+    buffers_.push_back(buffer);
+  }
+
+private:
+  std::vector<Buffer> buffers_;
+  std::mutex mutex_;
+};
+
+#endif  // DETAIL__BUFFER_POOL_HPP_

rmw_zenoh_cpp/src/detail/rmw_context_impl_s.hpp

@@ -24,6 +24,7 @@
 
 #include "graph_cache.hpp"
 #include "rmw_node_data.hpp"
+#include "buffer_pool.hpp"
 
 #include "rmw/ret_types.h"
 #include "rmw/types.h"
@@ -92,6 +93,9 @@ struct rmw_context_impl_s final
   // Forward declaration
   class Data;
 
+  // Pool of serialization buffers.
+  BufferPool serialization_buffer_pool;
+
 private:
   std::shared_ptr<Data> data_{nullptr};
 };