From 0557f4a9fd8249d7d2604fe6093a76966bf7877d Mon Sep 17 00:00:00 2001
From: Dmitry Igrishin <dmitigr@gmail.com>
Date: Wed, 16 Mar 2022 19:37:21 +0300
Subject: [PATCH] Optimize Copier by reducing memory allocations

---
 src/pgfe/copier.cpp            | 14 ++++++++------
 src/pgfe/copier.hpp            | 14 +++++++++-----
 test/pgfe/pgfe-unit-copier.cpp |  2 +-
 3 files changed, 18 insertions(+), 12 deletions(-)

diff --git a/src/pgfe/copier.cpp b/src/pgfe/copier.cpp
index b4da528..a53630b 100644
--- a/src/pgfe/copier.cpp
+++ b/src/pgfe/copier.cpp
@@ -109,21 +109,23 @@ DMITIGR_PGFE_INLINE bool Copier::end(const std::string& error_message) const
   DMITIGR_ASSERT(false);
 }
 
-DMITIGR_PGFE_INLINE std::unique_ptr<Data> Copier::receive(const bool wait) const
+DMITIGR_PGFE_INLINE Data_view Copier::receive(const bool wait) const
 {
   check_receive();
 
+  buffer_ = decltype(buffer_){nullptr, &dummy_free};
   char* buffer{};
   const int size{PQgetCopyData(connection().conn(), &buffer, !wait)};
-  DMITIGR_ASSERT(!buffer || size > 0);
+  if (buffer)
+    buffer_ = decltype(buffer_){buffer, &::PQfreemem};
+  DMITIGR_ASSERT(!buffer_ || size > 0);
 
   if (size == -1)
-    return nullptr;
+    return Data_view{};
   else if (size == 0)
-    return Data::make(std::string_view{"", 0}, data_format(0));
+    return Data_view{"", 0, data_format(0)};
   else if (size > 0)
-    return Data::make(std::unique_ptr<char, void(*)(void*)>{buffer, &::PQfreemem},
-      static_cast<std::size_t>(size), data_format(0));
+    return Data_view{buffer_.get(), static_cast<std::size_t>(size), data_format(0)};
   else if (size == -2)
     throw Client_exception{PQerrorMessage(connection().conn())};
 
diff --git a/src/pgfe/copier.hpp b/src/pgfe/copier.hpp
index 6fe17e7..e8f049d 100644
--- a/src/pgfe/copier.hpp
+++ b/src/pgfe/copier.hpp
@@ -27,6 +27,8 @@
 #include "dll.hpp"
 #include "response.hpp"
 
+#include <memory>
+
 namespace dmitigr::pgfe {
 
 /**
@@ -124,14 +126,14 @@ class Copier final : public Response {
    * `data_direction() == Data_direction::from_server`.
    *
    * @returns Returns:
-   *   -# `nullptr` if the `COPY` command is done;
-   *   -# the empty data to indicate that the COPY is undone, but no row is yet
-   *   available (this is only possible when `wait` is `false`);
-   *   -# the non-empty data received from the server.
+   *   -# invalid instance if the `COPY` command is done;
+   *   -# the empty instance to indicate that the COPY is undone, but no row is
+   *   yet available (this is only possible when `wait` is `false`);
+   *   -# the non-empty instance received from the server.
    *
    *  @remarks The format of returned data is equals to `data_format(0)`.
    */
-  DMITIGR_PGFE_API std::unique_ptr<Data> receive(bool wait = true) const;
+  DMITIGR_PGFE_API Data_view receive(bool wait = true) const;
 
   /**
    * @returns The underlying connection instance.
@@ -149,11 +151,13 @@ class Copier final : public Response {
 
   Connection* connection_{};
   detail::pq::Result pq_result_;
+  mutable std::unique_ptr<char, void(*)(void*)> buffer_{nullptr, &dummy_free};
 
   /// The constructor.
   explicit DMITIGR_PGFE_API Copier(Connection& connection,
     detail::pq::Result&& pq_result) noexcept;
 
+  static inline void dummy_free(void*)noexcept{};
   void check_send() const;
   void check_receive() const;
 };
diff --git a/test/pgfe/pgfe-unit-copier.cpp b/test/pgfe/pgfe-unit-copier.cpp
index 475b1b3..57c2888 100644
--- a/test/pgfe/pgfe-unit-copier.cpp
+++ b/test/pgfe/pgfe-unit-copier.cpp
@@ -66,7 +66,7 @@ try {
   ASSERT(copier.data_direction() == pgfe::Data_direction::from_server);
   int i{};
   while (const auto data = copier.receive()) {
-    const char* const bytes = static_cast<const char*>(data->bytes()); // with '\n'
+    const char* const bytes = static_cast<const char*>(data.bytes()); // with '\n'
     ASSERT(expected[i++] == bytes);
     std::cout << bytes;
   }