diff -r 000000000000 -r 6474c204b198 js/xpconnect/src/XPCThrower.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/xpconnect/src/XPCThrower.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,173 @@
+/* -*- 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/. */
+
+/* Code for throwing errors into JavaScript. */
+
+#include "xpcprivate.h"
+#include "XPCWrapper.h"
+#include "jsprf.h"
+#include "mozilla/dom/BindingUtils.h"
+#include "mozilla/dom/Exceptions.h"
+
+using namespace mozilla;
+using namespace mozilla::dom;
+
+bool XPCThrower::sVerbose = true;
+
+// static
+void
+XPCThrower::Throw(nsresult rv, JSContext* cx)
+{
+    const char* format;
+    if (JS_IsExceptionPending(cx))
+        return;
+    if (!nsXPCException::NameAndFormatForNSResult(rv, nullptr, &format))
+        format = "";
+    dom::Throw(cx, rv, format);
+}
+
+namespace xpc {
+
+bool
+Throw(JSContext *cx, nsresult rv)
+{
+    XPCThrower::Throw(rv, cx);
+    return false;
+}
+
+} // namespace xpc
+
+/*
+ * If there has already been an exception thrown, see if we're throwing the
+ * same sort of exception, and if we are, don't clobber the old one. ccx
+ * should be the current call context.
+ */
+// static
+bool
+XPCThrower::CheckForPendingException(nsresult result, JSContext *cx)
+{
+    nsCOMPtr<nsIException> e = XPCJSRuntime::Get()->GetPendingException();
+    if (!e)
+        return false;
+    XPCJSRuntime::Get()->SetPendingException(nullptr);
+
+    nsresult e_result;
+    if (NS_FAILED(e->GetResult(&e_result)) || e_result != result)
+        return false;
+
+    if (!ThrowExceptionObject(cx, e))
+        JS_ReportOutOfMemory(cx);
+    return true;
+}
+
+// static
+void
+XPCThrower::Throw(nsresult rv, XPCCallContext& ccx)
+{
+    char* sz;
+    const char* format;
+
+    if (CheckForPendingException(rv, ccx))
+        return;
+
+    if (!nsXPCException::NameAndFormatForNSResult(rv, nullptr, &format))
+        format = "";
+
+    sz = (char*) format;
+
+    if (sz && sVerbose)
+        Verbosify(ccx, &sz, false);
+
+    dom::Throw(ccx, rv, sz);
+
+    if (sz && sz != format)
+        JS_smprintf_free(sz);
+}
+
+
+// static
+void
+XPCThrower::ThrowBadResult(nsresult rv, nsresult result, XPCCallContext& ccx)
+{
+    char* sz;
+    const char* format;
+    const char* name;
+
+    /*
+    *  If there is a pending exception when the native call returns and
+    *  it has the same error result as returned by the native call, then
+    *  the native call may be passing through an error from a previous JS
+    *  call. So we'll just throw that exception into our JS.
+    */
+
+    if (CheckForPendingException(result, ccx))
+        return;
+
+    // else...
+
+    if (!nsXPCException::NameAndFormatForNSResult(rv, nullptr, &format) || !format)
+        format = "";
+
+    if (nsXPCException::NameAndFormatForNSResult(result, &name, nullptr) && name)
+        sz = JS_smprintf("%s 0x%x (%s)", format, result, name);
+    else
+        sz = JS_smprintf("%s 0x%x", format, result);
+
+    if (sz && sVerbose)
+        Verbosify(ccx, &sz, true);
+
+    dom::Throw(ccx, result, sz);
+
+    if (sz)
+        JS_smprintf_free(sz);
+}
+
+// static
+void
+XPCThrower::ThrowBadParam(nsresult rv, unsigned paramNum, XPCCallContext& ccx)
+{
+    char* sz;
+    const char* format;
+
+    if (!nsXPCException::NameAndFormatForNSResult(rv, nullptr, &format))
+        format = "";
+
+    sz = JS_smprintf("%s arg %d", format, paramNum);
+
+    if (sz && sVerbose)
+        Verbosify(ccx, &sz, true);
+
+    dom::Throw(ccx, rv, sz);
+
+    if (sz)
+        JS_smprintf_free(sz);
+}
+
+
+// static
+void
+XPCThrower::Verbosify(XPCCallContext& ccx,
+                      char** psz, bool own)
+{
+    char* sz = nullptr;
+
+    if (ccx.HasInterfaceAndMember()) {
+        XPCNativeInterface* iface = ccx.GetInterface();
+        jsid id = ccx.GetMember()->GetName();
+        JSAutoByteString bytes;
+        const char *name = JSID_IS_VOID(id) ? "Unknown" : bytes.encodeLatin1(ccx, JSID_TO_STRING(id));
+        if (!name) {
+            name = "";
+        }
+        sz = JS_smprintf("%s [%s.%s]", *psz, iface->GetNameString(), name);
+    }
+
+    if (sz) {
+        if (own)
+            JS_smprintf_free(*psz);
+        *psz = sz;
+    }
+}