Add File::Duplicate to duplicate a file handle.

base/files/file_win.cc

 // 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.
 
 #include "base/files/file.h"
 
 #include <io.h>
 
 #include "base/files/file_path.h"
 #include "base/logging.h"
@@ skipping 291 matching lines @@
 }
 
 File::Error File::Unlock() {
   DCHECK(IsValid());
   BOOL result = UnlockFile(file_.Get(), 0, 0, MAXDWORD, MAXDWORD);
   if (!result)
     return OSErrorToFileError(GetLastError());
   return FILE_OK;
 }
 
+File File::Duplicate() {
+  if (!IsValid())
+    return File();
+
+  HANDLE other_handle = nullptr;
+
+  if (!::DuplicateHandle(GetCurrentProcess(),  // hSourceProcessHandle
+                         GetPlatformFile(),
+                         GetCurrentProcess(),  // hTargetProcessHandle
+                         &other_handle,
+                         0,  // dwDesiredAccess ignored due to SAME_ACCESS
+                         FALSE,  // !bInheritHandle
+                         DUPLICATE_SAME_ACCESS)) {
+    return File(OSErrorToFileError(GetLastError()));
+  }
+
+  File other(other_handle);
+  if (async())
+    other.async_ = true;
+  return other.Pass();
+}
+
 // Static.
 File::Error File::OSErrorToFileError(DWORD last_error) {
   switch (last_error) {
     case ERROR_SHARING_VIOLATION:
       return FILE_ERROR_IN_USE;
     case ERROR_FILE_EXISTS:
       return FILE_ERROR_EXISTS;
     case ERROR_FILE_NOT_FOUND:
     case ERROR_PATH_NOT_FOUND:
       return FILE_ERROR_NOT_FOUND;
@@ skipping 22 matching lines @@
                                   last_error);
       return FILE_ERROR_FAILED;
   }
 }
 
 void File::SetPlatformFile(PlatformFile file) {
   file_.Set(file);
 }
 
 }  // namespace base