Add File::Duplicate to duplicate a file handle.

base/files/file_posix.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 <errno.h>
 #include <fcntl.h>
 #include <sys/stat.h>
 #include <unistd.h>
@@ skipping 445 matching lines @@
 }
 
 File::Error File::Lock() {
   return CallFctnlFlock(file_.get(), true);
 }
 
 File::Error File::Unlock() {
   return CallFctnlFlock(file_.get(), false);
 }
 
+File File::Duplicate() {
+  if (!IsValid())
+    return File();
+
+  PlatformFile other_fd = dup(GetPlatformFile());
+  if (other_fd == -1)
+    return File(OSErrorToFileError(errno));
+
+  File other(other_fd);
+  if (async())
+    other.async_ = true;
+  return other.Pass();
+}
+
 // Static.
 File::Error File::OSErrorToFileError(int saved_errno) {
   switch (saved_errno) {
     case EACCES:
     case EISDIR:
     case EROFS:
     case EPERM:
       return FILE_ERROR_ACCESS_DENIED;
     case EBUSY:
 #if !defined(OS_NACL)  // ETXTBSY not defined by NaCl.
@@ skipping 67 matching lines @@
 }
 
 void File::SetPlatformFile(PlatformFile file) {
   CHECK(!file_.is_valid());
   file_.reset(file);
   if (file_.is_valid())
     ProtectFileDescriptor(GetPlatformFile());
 }
 
 }  // namespace base