Add FileHandleFileWriter

util/file/file_writer.h

 // Copyright 2014 The Crashpad Authors. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
@@ skipping 57 matching lines @@
   //!     with identical semantics.
   //!
   //! \return The return value of LoggingFileSeek(). `-1` on failure,
   //!     with an error message logged.
   virtual FileOffset Seek(FileOffset offset, int whence) = 0;
 
  protected:
   ~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 {
  public:
   FileWriter();
   ~FileWriter();
 
   //! \brief Wraps LoggingOpenFileForWrite().
   //!
   //! \return `true` if the operation succeeded, `false` if it failed, with an
@@ skipping 27 matching lines @@
   bool WriteIoVec(std::vector<WritableIoVec>* iovecs) override;
 
   //! \copydoc FileWriterInterface::Seek()
   //!
   //! \note It is only valid to call this method between a successful Open() and
   //!     a Close().
   FileOffset Seek(FileOffset offset, int whence) override;
 
  private:
   ScopedFileHandle file_;
+  WeakFileHandleFileWriter weak_file_handle_file_writer_;
 
   DISALLOW_COPY_AND_ASSIGN(FileWriter);
 };
 
 }  // namespace crashpad
 
 #endif  // CRASHPAD_UTIL_FILE_FILE_WRITER_H_