diff -r 000000000000 -r 6474c204b198 xpcom/base/nsGZFileWriter.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/base/nsGZFileWriter.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,98 @@
+/* -*- 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 "nsGZFileWriter.h"
+#include "nsIFile.h"
+#include "nsString.h"
+#include "zlib.h"
+
+#ifdef XP_WIN
+#include <io.h>
+#define _dup dup
+#else
+#include <unistd.h>
+#endif
+
+NS_IMPL_ISUPPORTS(nsGZFileWriter, nsIGZFileWriter)
+
+nsGZFileWriter::nsGZFileWriter()
+  : mInitialized(false)
+  , mFinished(false)
+{}
+
+nsGZFileWriter::~nsGZFileWriter()
+{
+  if (mInitialized && !mFinished) {
+    Finish();
+  }
+}
+
+NS_IMETHODIMP
+nsGZFileWriter::Init(nsIFile* aFile)
+{
+  if (NS_WARN_IF(mInitialized) ||
+      NS_WARN_IF(mFinished))
+    return NS_ERROR_FAILURE;
+
+  // Get a FILE out of our nsIFile.  Convert that into a file descriptor which
+  // gzip can own.  Then close our FILE, leaving only gzip's fd open.
+
+  FILE* file;
+  nsresult rv = aFile->OpenANSIFileDesc("wb", &file);
+  if (NS_WARN_IF(NS_FAILED(rv)))
+    return rv;
+
+  mGZFile = gzdopen(dup(fileno(file)), "wb");
+  fclose(file);
+
+  // gzdopen returns nullptr on error.
+  if (NS_WARN_IF(!mGZFile))
+    return NS_ERROR_FAILURE;
+
+  mInitialized = true;
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsGZFileWriter::Write(const nsACString& aStr)
+{
+  if (NS_WARN_IF(!mInitialized) ||
+      NS_WARN_IF(mFinished))
+    return NS_ERROR_FAILURE;
+
+  // gzwrite uses a return value of 0 to indicate failure.  Otherwise, it
+  // returns the number of uncompressed bytes written.  To ensure we can
+  // distinguish between success and failure, don't call gzwrite when we have 0
+  // bytes to write.
+  if (aStr.IsEmpty()) {
+    return NS_OK;
+  }
+
+  // gzwrite never does a short write -- that is, the return value should
+  // always be either 0 or aStr.Length(), and we shouldn't have to call it
+  // multiple times in order to get it to read the whole buffer.
+  int rv = gzwrite(mGZFile, aStr.BeginReading(), aStr.Length());
+  if (NS_WARN_IF(rv != static_cast<int>(aStr.Length())))
+    return NS_ERROR_FAILURE;
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsGZFileWriter::Finish()
+{
+  if (NS_WARN_IF(!mInitialized) ||
+      NS_WARN_IF(mFinished))
+    return NS_ERROR_FAILURE;
+
+  mFinished = true;
+  gzclose(mGZFile);
+
+  // Ignore errors from gzclose; it's not like there's anything we can do about
+  // it, at this point!
+  return NS_OK;
+}