diff -r 000000000000 -r 6474c204b198 xpcom/io/FileUtilsWin.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/io/FileUtilsWin.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,63 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "FileUtilsWin.h"
+
+#include <windows.h>
+#include <psapi.h>
+
+#include "nsWindowsHelpers.h"
+
+namespace {
+
+// Scoped type used by HandleToFilename
+struct ScopedMappedViewTraits
+{
+  typedef void* type;
+  static void* empty() { return nullptr; }
+  static void release(void* ptr) { UnmapViewOfFile(ptr); }
+};
+typedef mozilla::Scoped<ScopedMappedViewTraits> ScopedMappedView;
+
+} // anonymous namespace
+
+namespace mozilla {
+
+bool
+HandleToFilename(HANDLE aHandle, const LARGE_INTEGER& aOffset,
+                 nsAString& aFilename)
+{
+  aFilename.Truncate();
+  // This implementation is nice because it uses fully documented APIs that
+  // are available on all Windows versions that we support.
+  nsAutoHandle fileMapping(CreateFileMapping(aHandle, nullptr, PAGE_READONLY,
+                                             0, 1, nullptr));
+  if (!fileMapping) {
+    return false;
+  }
+  ScopedMappedView view(MapViewOfFile(fileMapping, FILE_MAP_READ,
+                                      aOffset.HighPart, aOffset.LowPart, 1));
+  if (!view) {
+    return false;
+  }
+  nsAutoString mappedFilename;
+  DWORD len = 0;
+  SetLastError(ERROR_SUCCESS);
+  do {
+    mappedFilename.SetLength(mappedFilename.Length() + MAX_PATH);
+    len = GetMappedFileNameW(GetCurrentProcess(), view,
+                             wwc(mappedFilename.BeginWriting()),
+                             mappedFilename.Length());
+  } while (!len && GetLastError() == ERROR_INSUFFICIENT_BUFFER);
+  if (!len) {
+    return false;
+  }
+  mappedFilename.Truncate(len);
+  return NtPathToDosPath(mappedFilename, aFilename);
+}
+
+} // namespace mozilla
+