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..62fddf5f37180ef39c78c3c6ecfeda726d6d30aa 100644
--- a/util/file/file_writer.h
+++ b/util/file/file_writer.h
@@ -75,6 +75,50 @@ class FileWriterInterface {
   ~FileWriterInterface() {}
 };
 
+//! \brief A file writer backed by a FileHandle.
+//!
+//! FileWriter requires users to provide a FilePath to open, but this class
+//! accepts an already-open FileHandle instead. 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. Objects of
+//! this class may be used to write to file handles not associated with
+//! filesystem-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 WeakFileHandleFileWriter : public FileWriterInterface {
+ public:
+  explicit WeakFileHandleFileWriter(FileHandle file_handle);
+  ~WeakFileHandleFileWriter();
+
+  // 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
+  // WeakFileHandleFileWriter with a correct file descriptor at the time of
+  // construction because no file descriptor will be available until
+  // FileWriter::Open() is called.
+  friend class FileWriter;
+
+  DISALLOW_COPY_AND_ASSIGN(WeakFileHandleFileWriter);
+};
+
 //! \brief A file writer implementation that wraps traditional system file
 //!     operations on files accessed through the filesystem.
 class FileWriter : public FileWriterInterface {
@@ -122,6 +166,7 @@ class FileWriter : public FileWriterInterface {
 
  private:
   ScopedFileHandle file_;
+  WeakFileHandleFileWriter weak_file_handle_file_writer_;
 
   DISALLOW_COPY_AND_ASSIGN(FileWriter);
 };