diff -r 000000000000 -r 6474c204b198 xpcom/io/FileUtilsWin.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/io/FileUtilsWin.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,101 @@
+/* -*- 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/. */
+
+#ifndef mozilla_FileUtilsWin_h
+#define mozilla_FileUtilsWin_h
+
+#include <windows.h>
+
+#include "mozilla/Scoped.h"
+#include "nsStringGlue.h"
+
+namespace mozilla {
+
+inline bool
+NtPathToDosPath(const nsAString& aNtPath, nsAString& aDosPath)
+{
+  aDosPath.Truncate();
+  if (aNtPath.IsEmpty()) {
+    return true;
+  }
+  NS_NAMED_LITERAL_STRING(symLinkPrefix, "\\??\\");
+  uint32_t ntPathLen = aNtPath.Length();
+  uint32_t symLinkPrefixLen = symLinkPrefix.Length();
+  if (ntPathLen >= 6 && aNtPath.CharAt(5) == L':' &&
+      ntPathLen >= symLinkPrefixLen &&
+      Substring(aNtPath, 0, symLinkPrefixLen).Equals(symLinkPrefix)) {
+    // Symbolic link for DOS device. Just strip off the prefix.
+    aDosPath = aNtPath;
+    aDosPath.Cut(0, 4);
+    return true;
+  }
+  nsAutoString logicalDrives;
+  DWORD len = 0;
+  while(true) {
+    len = GetLogicalDriveStringsW(len, reinterpret_cast<wchar_t*>(logicalDrives.BeginWriting()));
+    if (!len) {
+      return false;
+    } else if (len > logicalDrives.Length()) {
+      logicalDrives.SetLength(len);
+    } else {
+      break;
+    }
+  }
+  const char16_t* cur = logicalDrives.BeginReading();
+  const char16_t* end = logicalDrives.EndReading();
+  nsString targetPath;
+  targetPath.SetLength(MAX_PATH);
+  wchar_t driveTemplate[] = L" :";
+  do {
+    // Unfortunately QueryDosDevice doesn't support the idiom for querying the
+    // output buffer size, so it may require retries.
+    driveTemplate[0] = *cur;
+    DWORD targetPathLen = 0;
+    SetLastError(ERROR_SUCCESS);
+    while (true) {
+      targetPathLen = QueryDosDeviceW(driveTemplate, reinterpret_cast<wchar_t*>(targetPath.BeginWriting()),
+                                      targetPath.Length());
+      if (targetPathLen || GetLastError() != ERROR_INSUFFICIENT_BUFFER) {
+        break;
+      }
+      targetPath.SetLength(targetPath.Length() * 2);
+    }
+    if (targetPathLen) {
+      // Need to use wcslen here because targetPath contains embedded NULL chars
+      size_t firstTargetPathLen = wcslen(targetPath.get());
+      const char16_t* pathComponent = aNtPath.BeginReading() +
+                                      firstTargetPathLen;
+      bool found = _wcsnicmp(char16ptr_t(aNtPath.BeginReading()), targetPath.get(),
+                             firstTargetPathLen) == 0 &&
+                   *pathComponent == L'\\';
+      if (found) {
+        aDosPath = driveTemplate;
+        aDosPath += pathComponent;
+        return true;
+      }
+    }
+    // Advance to the next NUL character in logicalDrives
+    while (*cur++);
+  } while (cur != end);
+  // Code for handling UNC paths would go here, if eventually required.
+#if defined(DEBUG)
+  NS_NAMED_LITERAL_STRING(deviceMupPrefix, "\\Device\\Mup\\");
+  uint32_t deviceMupPrefixLen = deviceMupPrefix.Length();
+  if (ntPathLen >= deviceMupPrefixLen &&
+      Substring(aNtPath, 0, deviceMupPrefixLen).Equals(deviceMupPrefix)) {
+    NS_WARNING("UNC paths not yet supported in NtPathToDosPath");
+  }
+#endif // defined(DEBUG)
+  return false;
+}
+
+bool
+HandleToFilename(HANDLE aHandle, const LARGE_INTEGER& aOffset,
+                 nsAString& aFilename);
+
+} // namespace mozilla
+
+#endif // mozilla_FileUtilsWin_h