1 files changed, 74 insertions, 0 deletions
diff --git a/xpcom/base/HoldDropJSObjects.h b/xpcom/base/HoldDropJSObjects.h
new file mode 100644
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+++ b/xpcom/base/HoldDropJSObjects.h
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+/* -*- 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_HoldDropJSObjects_h
+#define mozilla_HoldDropJSObjects_h
+
+#include <type_traits>
+#include "nsCycleCollectionNoteChild.h"
+
+class nsISupports;
+class nsScriptObjectTracer;
+
+namespace JS {
+class Zone;
+}
+
+// Only HoldJSObjects and DropJSObjects should be called directly.
+
+namespace mozilla {
+namespace cyclecollector {
+
+void HoldJSObjectsImpl(void* aHolder, nsScriptObjectTracer* aTracer,
+                       JS::Zone* aZone = nullptr);
+void HoldJSObjectsImpl(nsISupports* aHolder);
+void DropJSObjectsImpl(void* aHolder);
+void DropJSObjectsImpl(nsISupports* aHolder);
+
+}  // namespace cyclecollector
+
+template <class T, bool isISupports = std::is_base_of<nsISupports, T>::value>
+struct HoldDropJSObjectsHelper {
+  static void Hold(T* aHolder) {
+    cyclecollector::HoldJSObjectsImpl(aHolder,
+                                      NS_CYCLE_COLLECTION_PARTICIPANT(T));
+  }
+  static void Drop(T* aHolder) { cyclecollector::DropJSObjectsImpl(aHolder); }
+};
+
+template <class T>
+struct HoldDropJSObjectsHelper<T, true> {
+  static void Hold(T* aHolder) {
+    cyclecollector::HoldJSObjectsImpl(ToSupports(aHolder));
+  }
+  static void Drop(T* aHolder) {
+    cyclecollector::DropJSObjectsImpl(ToSupports(aHolder));
+  }
+};
+
+/**
+  Classes that hold strong references to JS GC things such as `JSObjects` and
+  `JS::Values` (e.g. `JS::Heap<JSObject*> mFoo;`) must use these, generally by
+  calling `HoldJSObjects(this)` and `DropJSObjects(this)` in the ctor and dtor
+  respectively.
+
+  For classes that are wrapper cached and hold no other strong references to JS
+  GC things, there's no need to call these; it will be taken care of
+  automatically by nsWrapperCache.
+**/
+template <class T>
+void HoldJSObjects(T* aHolder) {
+  HoldDropJSObjectsHelper<T>::Hold(aHolder);
+}
+
+template <class T>
+void DropJSObjects(T* aHolder) {
+  HoldDropJSObjectsHelper<T>::Drop(aHolder);
+}
+
+}  // namespace mozilla
+
+#endif  // mozilla_HoldDropJSObjects_h