diff -r 000000000000 -r 6474c204b198 xpcom/glue/nsEnumeratorUtils.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/glue/nsEnumeratorUtils.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,268 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* 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 "mozilla/Attributes.h"
+
+#include "nsEnumeratorUtils.h"
+
+#include "nsISimpleEnumerator.h"
+#include "nsIStringEnumerator.h"
+
+#include "nsCOMPtr.h"
+
+class EmptyEnumeratorImpl : public nsISimpleEnumerator,
+                            public nsIUTF8StringEnumerator,
+                            public nsIStringEnumerator
+{
+public:
+    EmptyEnumeratorImpl() {}
+    // nsISupports interface
+    NS_DECL_ISUPPORTS_INHERITED  // not really inherited, but no mRefCnt
+
+    // nsISimpleEnumerator
+    NS_DECL_NSISIMPLEENUMERATOR
+    NS_DECL_NSIUTF8STRINGENUMERATOR
+    // can't use NS_DECL_NSISTRINGENUMERATOR because they share the
+    // HasMore() signature
+    NS_IMETHOD GetNext(nsAString& aResult);
+
+    static EmptyEnumeratorImpl* GetInstance() {
+      static const EmptyEnumeratorImpl kInstance;
+      return const_cast<EmptyEnumeratorImpl*>(&kInstance);
+    }
+};
+
+// nsISupports interface
+NS_IMETHODIMP_(MozExternalRefCountType) EmptyEnumeratorImpl::AddRef(void)
+{
+    return 2;
+}
+
+NS_IMETHODIMP_(MozExternalRefCountType) EmptyEnumeratorImpl::Release(void)
+{
+    return 1;
+}
+
+NS_IMPL_QUERY_INTERFACE(EmptyEnumeratorImpl, nsISimpleEnumerator,
+                        nsIUTF8StringEnumerator, nsIStringEnumerator)
+
+// nsISimpleEnumerator interface
+NS_IMETHODIMP EmptyEnumeratorImpl::HasMoreElements(bool* aResult)
+{
+    *aResult = false;
+    return NS_OK;
+}
+
+NS_IMETHODIMP EmptyEnumeratorImpl::HasMore(bool* aResult)
+{
+    *aResult = false;
+    return NS_OK;
+}
+
+NS_IMETHODIMP EmptyEnumeratorImpl::GetNext(nsISupports** aResult)
+{
+    return NS_ERROR_UNEXPECTED;
+}
+
+NS_IMETHODIMP EmptyEnumeratorImpl::GetNext(nsACString& aResult)
+{
+    return NS_ERROR_UNEXPECTED;
+}
+
+NS_IMETHODIMP EmptyEnumeratorImpl::GetNext(nsAString& aResult)
+{
+    return NS_ERROR_UNEXPECTED;
+}
+
+nsresult
+NS_NewEmptyEnumerator(nsISimpleEnumerator** aResult)
+{
+    *aResult = EmptyEnumeratorImpl::GetInstance();
+    return NS_OK;
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+class nsSingletonEnumerator MOZ_FINAL : public nsISimpleEnumerator
+{
+public:
+    NS_DECL_ISUPPORTS
+
+    // nsISimpleEnumerator methods
+    NS_IMETHOD HasMoreElements(bool* aResult);
+    NS_IMETHOD GetNext(nsISupports** aResult);
+
+    nsSingletonEnumerator(nsISupports* aValue);
+
+private:
+    ~nsSingletonEnumerator();
+
+protected:
+    nsISupports* mValue;
+    bool mConsumed;
+};
+
+nsSingletonEnumerator::nsSingletonEnumerator(nsISupports* aValue)
+    : mValue(aValue)
+{
+    NS_IF_ADDREF(mValue);
+    mConsumed = (mValue ? false : true);
+}
+
+nsSingletonEnumerator::~nsSingletonEnumerator()
+{
+    NS_IF_RELEASE(mValue);
+}
+
+NS_IMPL_ISUPPORTS(nsSingletonEnumerator, nsISimpleEnumerator)
+
+NS_IMETHODIMP
+nsSingletonEnumerator::HasMoreElements(bool* aResult)
+{
+    NS_PRECONDITION(aResult != 0, "null ptr");
+    if (! aResult)
+        return NS_ERROR_NULL_POINTER;
+
+    *aResult = !mConsumed;
+    return NS_OK;
+}
+
+
+NS_IMETHODIMP
+nsSingletonEnumerator::GetNext(nsISupports** aResult)
+{
+    NS_PRECONDITION(aResult != 0, "null ptr");
+    if (! aResult)
+        return NS_ERROR_NULL_POINTER;
+
+    if (mConsumed)
+        return NS_ERROR_UNEXPECTED;
+
+    mConsumed = true;
+
+    *aResult = mValue;
+    NS_ADDREF(*aResult);
+    return NS_OK;
+}
+
+nsresult
+NS_NewSingletonEnumerator(nsISimpleEnumerator* *result,
+                          nsISupports* singleton)
+{
+    nsSingletonEnumerator* enumer = new nsSingletonEnumerator(singleton);
+    if (enumer == nullptr)
+        return NS_ERROR_OUT_OF_MEMORY;
+    *result = enumer; 
+    NS_ADDREF(*result);
+    return NS_OK;
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+class nsUnionEnumerator MOZ_FINAL : public nsISimpleEnumerator
+{
+public:
+    NS_DECL_ISUPPORTS
+
+    // nsISimpleEnumerator methods
+    NS_IMETHOD HasMoreElements(bool* aResult);
+    NS_IMETHOD GetNext(nsISupports** aResult);
+
+    nsUnionEnumerator(nsISimpleEnumerator* firstEnumerator,
+                      nsISimpleEnumerator* secondEnumerator);
+
+private:
+    ~nsUnionEnumerator();
+
+protected:
+    nsCOMPtr<nsISimpleEnumerator> mFirstEnumerator, mSecondEnumerator;
+    bool mConsumed;
+    bool mAtSecond;
+};
+
+nsUnionEnumerator::nsUnionEnumerator(nsISimpleEnumerator* firstEnumerator,
+                                     nsISimpleEnumerator* secondEnumerator)
+    : mFirstEnumerator(firstEnumerator),
+      mSecondEnumerator(secondEnumerator),
+      mConsumed(false), mAtSecond(false)
+{
+}
+
+nsUnionEnumerator::~nsUnionEnumerator()
+{
+}
+
+NS_IMPL_ISUPPORTS(nsUnionEnumerator, nsISimpleEnumerator)
+
+NS_IMETHODIMP
+nsUnionEnumerator::HasMoreElements(bool* aResult)
+{
+    NS_PRECONDITION(aResult != 0, "null ptr");
+    if (! aResult)
+        return NS_ERROR_NULL_POINTER;
+
+    nsresult rv;
+
+    if (mConsumed) {
+        *aResult = false;
+        return NS_OK;
+    }
+
+    if (! mAtSecond) {
+        rv = mFirstEnumerator->HasMoreElements(aResult);
+        if (NS_FAILED(rv)) return rv;
+
+        if (*aResult)
+            return NS_OK;
+
+        mAtSecond = true;
+    }
+
+    rv = mSecondEnumerator->HasMoreElements(aResult);
+    if (NS_FAILED(rv)) return rv;
+
+    if (*aResult)
+        return NS_OK;
+
+    *aResult = false;
+    mConsumed = true;
+    return NS_OK;
+}
+
+NS_IMETHODIMP
+nsUnionEnumerator::GetNext(nsISupports** aResult)
+{
+    NS_PRECONDITION(aResult != 0, "null ptr");
+    if (! aResult)
+        return NS_ERROR_NULL_POINTER;
+
+    if (mConsumed)
+        return NS_ERROR_UNEXPECTED;
+
+    if (! mAtSecond)
+        return mFirstEnumerator->GetNext(aResult);
+
+    return mSecondEnumerator->GetNext(aResult);
+}
+
+nsresult
+NS_NewUnionEnumerator(nsISimpleEnumerator* *result,
+                      nsISimpleEnumerator* firstEnumerator,
+                      nsISimpleEnumerator* secondEnumerator)
+{
+    *result = nullptr;
+    if (! firstEnumerator) {
+        *result = secondEnumerator;
+    } else if (! secondEnumerator) {
+        *result = firstEnumerator;
+    } else {
+        nsUnionEnumerator* enumer = new nsUnionEnumerator(firstEnumerator, secondEnumerator);
+        if (enumer == nullptr)
+            return NS_ERROR_OUT_OF_MEMORY;
+        *result = enumer; 
+    }
+    NS_ADDREF(*result);
+    return NS_OK;
+}