diff -r 000000000000 -r 6474c204b198 js/xpconnect/src/XPCString.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/xpconnect/src/XPCString.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,108 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* vim: set ts=8 sts=4 et sw=4 tw=99: */
+/* 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/. */
+
+/*
+ * Infrastructure for sharing DOMString data with JSStrings.
+ *
+ * Importing an nsAString into JS:
+ * If possible (GetSharedBufferHandle works) use the external string support in
+ * JS to create a JSString that points to the readable's buffer.  We keep a
+ * reference to the buffer handle until the JSString is finalized.
+ *
+ * Exporting a JSString as an nsAReadable:
+ * Wrap the JSString with a root-holding XPCJSReadableStringWrapper, which roots
+ * the string and exposes its buffer via the nsAString interface, as
+ * well as providing refcounting support.
+ */
+
+#include "nscore.h"
+#include "nsString.h"
+#include "nsStringBuffer.h"
+#include "jsapi.h"
+#include "xpcpublic.h"
+
+using namespace JS;
+
+// static
+void
+XPCStringConvert::FreeZoneCache(JS::Zone *zone)
+{
+    // Put the zone user data into an AutoPtr (which will do the cleanup for us),
+    // and null out the user data (which may already be null).
+    nsAutoPtr<ZoneStringCache> cache(static_cast<ZoneStringCache*>(JS_GetZoneUserData(zone)));
+    JS_SetZoneUserData(zone, nullptr);
+}
+
+// static
+void
+XPCStringConvert::ClearZoneCache(JS::Zone *zone)
+{
+    ZoneStringCache *cache = static_cast<ZoneStringCache*>(JS_GetZoneUserData(zone));
+    if (cache) {
+        cache->mBuffer = nullptr;
+        cache->mString = nullptr;
+    }
+}
+
+// static
+void
+XPCStringConvert::FinalizeLiteral(const JSStringFinalizer *fin, jschar *chars)
+{
+}
+
+const JSStringFinalizer XPCStringConvert::sLiteralFinalizer =
+    { XPCStringConvert::FinalizeLiteral };
+
+// static
+void
+XPCStringConvert::FinalizeDOMString(const JSStringFinalizer *fin, jschar *chars)
+{
+    nsStringBuffer* buf = nsStringBuffer::FromData(chars);
+    buf->Release();
+}
+
+const JSStringFinalizer XPCStringConvert::sDOMStringFinalizer =
+    { XPCStringConvert::FinalizeDOMString };
+
+// convert a readable to a JSString, copying string data
+// static
+bool
+XPCStringConvert::ReadableToJSVal(JSContext *cx,
+                                  const nsAString &readable,
+                                  nsStringBuffer** sharedBuffer,
+                                  MutableHandleValue vp)
+{
+    *sharedBuffer = nullptr;
+
+    uint32_t length = readable.Length();
+
+    if (readable.IsLiteral()) {
+        JSString *str = JS_NewExternalString(cx,
+                                             static_cast<const jschar*>(readable.BeginReading()),
+                                             length, &sLiteralFinalizer);
+        if (!str)
+            return false;
+        vp.setString(str);
+        return true;
+    }
+
+    nsStringBuffer *buf = nsStringBuffer::FromString(readable);
+    if (buf) {
+        bool shared;
+        if (!StringBufferToJSVal(cx, buf, length, vp, &shared))
+            return false;
+        if (shared)
+            *sharedBuffer = buf;
+        return true;
+    }
+
+    // blech, have to copy.
+    JSString *str = JS_NewUCStringCopyN(cx, readable.BeginReading(), length);
+    if (!str)
+        return false;
+    vp.setString(str);
+    return true;
+}