Add FileHandleFileWriter

util/file/file_writer.h

Index: util/file/file_writer.h
diff --git a/util/file/file_writer.h b/util/file/file_writer.h
index 7b9524fc537caa22cbf057e66181993093d99029..234211159378c8e11a98603c4b0e49fc51ee0fb1 100644
--- a/util/file/file_writer.h
+++ b/util/file/file_writer.h
@@ -75,6 +75,48 @@ class FileWriterInterface {
   ~FileWriterInterface() {}
 };
 
+//! \brief A file writer backed by a FileHandle.
+//!
+//! Like FileWriter, this class may write to a filesystem-based file, but unlike
+//! FileWriter, this class is not responsible for creating or closing the file.
+//! Users of this class must ensure that the file handle is closed appropriately
+//! elsewhere. This class may also be used to write to file handles not
+//! associated with disk-based files, although special attention should be paid
+//! to the Seek() method, which may not function on file handles that do not
+//! refer to disk-based files.
+//!
+//! This class is expected to be used when other code is responsible for
+//! creating files and already provides file handles.
+class FileHandleFileWriter : public FileWriterInterface {
+ public:
+  explicit FileHandleFileWriter(FileHandle file_handle);
+  ~FileHandleFileWriter();
+
+  // FileWriterInterface:
+  bool Write(const void* data, size_t size) override;
+  bool WriteIoVec(std::vector<WritableIoVec>* iovecs) override;
+
+  //! \copydoc FileWriterInterface::Seek()
+  //!
+  //! \note This method is only guaranteed to function on file handles referring
+  //!     to disk-based files.
+  FileOffset Seek(FileOffset offset, int whence) override;
+
+ private:
+  void set_file_handle(FileHandle file_handle) { file_handle_ = file_handle; }
+
+  FileHandle file_handle_;  // weak
+
+  // FileWriter uses this class as its internal implementation, and it needs to
+  // be able to call set_file_handle(). FileWriter cannot initialize an
+  // FileHandleFileWriter  with a correct file descriptor at the time of

[scottmg, 2015/02/04 21:14:10]

extra space here

+  // construction because no file descriptor will be available until
+  // FileWriter::Open() is called.
+  friend class FileWriter;
+
+  DISALLOW_COPY_AND_ASSIGN(FileHandleFileWriter);
+};
+
 //! \brief A file writer implementation that wraps traditional system file
 //!     operations on files accessed through the filesystem.
 class FileWriter : public FileWriterInterface {
@@ -122,6 +164,7 @@ class FileWriter : public FileWriterInterface {
 
  private:
   ScopedFileHandle file_;
+  FileHandleFileWriter file_handle_file_writer_;
 
   DISALLOW_COPY_AND_ASSIGN(FileWriter);
 };