Add File::Duplicate to duplicate a file handle.

base/files/file.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_FILES_FILE_H_
 #define BASE_FILES_FILE_H_
 
 #include "build/build_config.h"
 #if defined(OS_WIN)
 #include <windows.h>
@@ skipping 269 matching lines @@
   // POSIX-specific semantics:
   //  * Locks are advisory only.
   //  * Within a process, locking the same file (by the same or new handle)
   //    will succeed.
   //  * Closing any descriptor on a given file releases the lock.
   Error Lock();
 
   // Unlock a file previously locked.
   Error Unlock();
 
+  // Returns a new handle to this file for use within the current process. On

[rvargas (doing something else), 2015/03/18 20:56:42]

nit: handle -> object

[grt (UTC plus 2), 2015/03/19 03:31:02]

Done.

+  // POSIX, only the original File will obey FLAG_DELETE_ON_CLOSE.

[rvargas (doing something else), 2015/03/18 20:56:42]

I don't think this behavior is posix exclusive... or rather, the comment is kind
of misleading about what happens. How about saying that on posix that flag is
not propagated (if we have to say something here)

[grt (UTC plus 2), 2015/03/18 21:06:24]

I singled out POSIX here because "delete on close" is implemented by base::File
there, whereas on Windows the OS does it and the order of closes doesn't matter:
the deletion takes place when all are gone (at least, this is my understanding).
How does this sound:

Handling of FLAG_DELETE_ON_CLOSE varies by OS. On POSIX, the File object that
was created or initialized with this flag will unlink the underlying file on
close. On Windows, the underlying file is deleted when the last handle to it is
closed.

[rvargas (doing something else), 2015/03/18 21:29:58]

sounds good

[grt (UTC plus 2), 2015/03/19 03:31:02]

Done.

+  File Duplicate();
+
   bool async() const { return async_; }
 
 #if defined(OS_WIN)
   static Error OSErrorToFileError(DWORD last_error);
 #elif defined(OS_POSIX)
   static Error OSErrorToFileError(int saved_errno);
 #endif
 
   // Converts an error value to a human-readable form. Used for logging.
   static std::string ErrorToString(Error error);
@@ skipping 55 matching lines @@
 #endif
 
   Error error_details_;
   bool created_;
   bool async_;
 };
 
 }  // namespace base
 
 #endif  // BASE_FILES_FILE_H_