Support response data streaming and add an example file downloader.

webcc/body.h

@@ -147,8 +147,18 @@ private:
 // the memory.
 class FileBody : public Body {
 public:
+  // For message to be sent out.
   FileBody(const Path& path, std::size_t chunk_size);
 
+  // For message received.
+  // No |chunk_size| is needed since you don't iterate the payload of a
+  // received message.
+  // If |auto_delete| is true, the file will be deleted on destructor unless it
+  // is moved to another path (see Move()).
+  FileBody(const Path& path, bool auto_delete = false);
+
+  ~FileBody() override;
+
   std::size_t GetSize() const override {
     return size_;
   }
@@ -159,13 +169,26 @@ public:
 
   void Dump(std::ostream& os, const std::string& prefix) const override;
 
+  // Move (or rename) the file.
+  // Used to move the streamed file of the received message to a new place.
+  // Applicable to both client and server.
+  // After move, the original path will be reset to empty.
+  // If |new_path| and |path_| resolve to the same file, do nothing and just
+  // return false.
+  // If |new_path| resolves to an existing non-directory file, it is removed.
+  // If |new_path| resolves to an existing directory, it is removed if empty
+  // on ISO/IEC 9945 but is an error on Windows.
+  // See boost::filesystem::rename() for more details.
+  bool Move(const Path& new_path);
+
 private:
   Path path_;
   std::size_t chunk_size_;
+  bool auto_delete_;
 
   std::size_t size_;  // File size in bytes
 
-  boost::filesystem::ifstream stream_;
+  boost::filesystem::ifstream ifstream_;
   std::string chunk_;
 };