Avoid unnecessary copies by recv()ing data directly into the expat buffer

src/network/socket_handler.cpp

@@ -138,11 +138,16 @@ void SocketHandler::set_poller(Poller* poller)
   this->poller = poller;
 }
 
-void SocketHandler::on_recv(const size_t nb)
+void SocketHandler::on_recv()
 {
-  char buf[4096];
+  static constexpr size_t buf_size = 4096;
+  char buf[buf_size];
+  void* recv_buf = this->get_receive_buffer(buf_size);
 
-  ssize_t size = ::recv(this->socket, buf, nb, 0);
+  if (recv_buf == nullptr)
+    recv_buf = buf;
+
+  ssize_t size = ::recv(this->socket, recv_buf, buf_size, 0);
   if (0 == size)
     {
       this->on_connection_close();
@@ -156,8 +161,14 @@ void SocketHandler::on_recv(const size_t nb)
     }
   else
     {
-      this->in_buf += std::string(buf, size);
-      this->parse_in_buffer();
+      if (buf == recv_buf)
+        {
+          // data needs to be placed in the in_buf string, because no buffer
+          // was provided to receive that data directly. The in_buf buffer
+          // will be handled in parse_in_buffer()
+          this->in_buf += std::string(buf, size);
+        }
+      this->parse_in_buffer(size);
     }
 }
 
@@ -238,3 +249,8 @@ bool SocketHandler::is_connecting() const
 {
   return this->connecting;
 }
+
+void* SocketHandler::get_receive_buffer(const size_t) const
+{
+  return nullptr;
+}

src/network/socket_handler.hpp

@@ -41,7 +41,7 @@ public:
    * Reads data in our in_buf and the call parse_in_buf, for the implementor
    * to handle the data received so far.
    */
-  void on_recv(const size_t nb = 4096);
+  void on_recv();
   /**
    * Write as much data from out_buf as possible, in the socket.
    */
@@ -73,14 +73,28 @@ public:
    */
   virtual void on_connection_close() = 0;
   /**
-   * Handle/consume (some of) the data received so far. If some data is used, the in_buf
+   * Handle/consume (some of) the data received so far.  The data to handle
+   * may be in the in_buf buffer, or somewhere else, depending on what
+   * get_receive_buffer() returned.  If some data is used from in_buf, it
    * should be truncated, only the unused data should be left untouched.
+   *
+   * The size argument is the size of the last chunk of data that was added to the buffer.
    */
-  virtual void parse_in_buffer() = 0;
+  virtual void parse_in_buffer(const size_t size) = 0;
   bool is_connected() const;
   bool is_connecting() const;
 
 protected:
+  /**
+   * Provide a buffer in which data can be directly received. This can be
+   * used to avoid copying data into in_buf before using it. If no buffer
+   * can provided, nullptr is returned (the default implementation does
+   * that).
+   */
+  virtual void* get_receive_buffer(const size_t size) const;
+  /**
+   * The handled socket.
+   */
   socket_t socket;
   /**
    * Where data read from the socket is added until we can extract a full