Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 2027 insertions, 0 deletions

diff --git a/xpcom/base/CycleCollectedJSRuntime.cpp b/xpcom/base/CycleCollectedJSRuntime.cpp
new file mode 100644
index 0000000000..03af9e3074
--- /dev/null
+++ b/xpcom/base/CycleCollectedJSRuntime.cpp
@@ -0,0 +1,2027 @@
+/* -*- 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/. */
+
+// We're dividing JS objects into 3 categories:
+//
+// 1. "real" roots, held by the JS engine itself or rooted through the root
+//    and lock JS APIs. Roots from this category are considered black in the
+//    cycle collector, any cycle they participate in is uncollectable.
+//
+// 2. certain roots held by C++ objects that are guaranteed to be alive.
+//    Roots from this category are considered black in the cycle collector,
+//    and any cycle they participate in is uncollectable. These roots are
+//    traced from TraceNativeBlackRoots.
+//
+// 3. all other roots held by C++ objects that participate in cycle collection,
+//    held by us (see TraceNativeGrayRoots). Roots from this category are
+//    considered grey in the cycle collector; whether or not they are collected
+//    depends on the objects that hold them.
+//
+// Note that if a root is in multiple categories the fact that it is in
+// category 1 or 2 that takes precedence, so it will be considered black.
+//
+// During garbage collection we switch to an additional mark color (gray) when
+// tracing inside TraceNativeGrayRoots. This allows us to walk those roots later
+// on and add all objects reachable only from them to the cycle collector.
+//
+// Phases:
+//
+// 1. marking of the roots in category 1 by having the JS GC do its marking
+// 2. marking of the roots in category 2 by having the JS GC call us back
+//    (via JS_SetExtraGCRootsTracer) and running TraceNativeBlackRoots
+// 3. marking of the roots in category 3 by
+//    TraceNativeGrayRootsInCollectingZones using an additional color (gray).
+// 4. end of GC, GC can sweep its heap
+//
+// At some later point, when the cycle collector runs:
+//
+// 5. walk gray objects and add them to the cycle collector, cycle collect
+//
+// JS objects that are part of cycles the cycle collector breaks will be
+// collected by the next JS GC.
+//
+// If WantAllTraces() is false the cycle collector will not traverse roots
+// from category 1 or any JS objects held by them. Any JS objects they hold
+// will already be marked by the JS GC and will thus be colored black
+// themselves. Any C++ objects they hold will have a missing (untraversed)
+// edge from the JS object to the C++ object and so it will be marked black
+// too. This decreases the number of objects that the cycle collector has to
+// deal with.
+// To improve debugging, if WantAllTraces() is true all JS objects are
+// traversed.
+
+#include "mozilla/CycleCollectedJSRuntime.h"
+
+#include <algorithm>
+#include <utility>
+
+#include "js/Debug.h"
+#include "js/RealmOptions.h"
+#include "js/friend/DumpFunctions.h"  // js::DumpHeap
+#include "js/GCAPI.h"
+#include "js/HeapAPI.h"
+#include "js/Object.h"  // JS::GetClass, JS::GetCompartment, JS::GetPrivate
+#include "js/PropertyAndElement.h"  // JS_DefineProperty
+#include "js/Warnings.h"            // JS::SetWarningReporter
+#include "js/ShadowRealmCallbacks.h"
+#include "js/SliceBudget.h"
+#include "jsfriendapi.h"
+#include "mozilla/ArrayUtils.h"
+#include "mozilla/AutoRestore.h"
+#include "mozilla/CycleCollectedJSContext.h"
+#include "mozilla/DebuggerOnGCRunnable.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/PerfStats.h"
+#include "mozilla/ProfilerLabels.h"
+#include "mozilla/ProfilerMarkers.h"
+#include "mozilla/Sprintf.h"
+#include "mozilla/StaticPrefs_javascript.h"
+#include "mozilla/Telemetry.h"
+#include "mozilla/Unused.h"
+#include "mozilla/dom/AutoEntryScript.h"
+#include "mozilla/dom/DOMJSClass.h"
+#include "mozilla/dom/JSExecutionManager.h"
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/PromiseBinding.h"
+#include "mozilla/dom/PromiseDebugging.h"
+#include "mozilla/dom/ScriptSettings.h"
+#include "mozilla/dom/ShadowRealmGlobalScope.h"
+#include "mozilla/dom/RegisterShadowRealmBindings.h"
+#include "nsContentUtils.h"
+#include "nsCycleCollectionNoteRootCallback.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsCycleCollector.h"
+#include "nsDOMJSUtils.h"
+#include "nsExceptionHandler.h"
+#include "nsJSUtils.h"
+#include "nsStringBuffer.h"
+#include "nsWrapperCache.h"
+#include "prenv.h"
+
+#if defined(XP_MACOSX)
+#  include "nsMacUtilsImpl.h"
+#endif
+
+#include "nsThread.h"
+#include "nsThreadUtils.h"
+#include "xpcpublic.h"
+
+#ifdef NIGHTLY_BUILD
+// For performance reasons, we make the JS Dev Error Interceptor a Nightly-only
+// feature.
+#  define MOZ_JS_DEV_ERROR_INTERCEPTOR = 1
+#endif  // NIGHTLY_BUILD
+
+using namespace mozilla;
+using namespace mozilla::dom;
+
+namespace mozilla {
+
+struct DeferredFinalizeFunctionHolder {
+  DeferredFinalizeFunction run;
+  void* data;
+};
+
+class IncrementalFinalizeRunnable : public DiscardableRunnable {
+  typedef AutoTArray<DeferredFinalizeFunctionHolder, 16> DeferredFinalizeArray;
+  typedef CycleCollectedJSRuntime::DeferredFinalizerTable
+      DeferredFinalizerTable;
+
+  CycleCollectedJSRuntime* mRuntime;
+  DeferredFinalizeArray mDeferredFinalizeFunctions;
+  uint32_t mFinalizeFunctionToRun;
+  bool mReleasing;
+
+  static const PRTime SliceMillis = 5; /* ms */
+
+ public:
+  IncrementalFinalizeRunnable(CycleCollectedJSRuntime* aRt,
+                              DeferredFinalizerTable& aFinalizerTable);
+  virtual ~IncrementalFinalizeRunnable();
+
+  void ReleaseNow(bool aLimited);
+
+  NS_DECL_NSIRUNNABLE
+};
+
+}  // namespace mozilla
+
+struct NoteWeakMapChildrenTracer : public JS::CallbackTracer {
+  NoteWeakMapChildrenTracer(JSRuntime* aRt,
+                            nsCycleCollectionNoteRootCallback& aCb)
+      : JS::CallbackTracer(aRt, JS::TracerKind::Callback),
+        mCb(aCb),
+        mTracedAny(false),
+        mMap(nullptr),
+        mKey(nullptr),
+        mKeyDelegate(nullptr) {}
+  void onChild(JS::GCCellPtr aThing, const char* name) override;
+  nsCycleCollectionNoteRootCallback& mCb;
+  bool mTracedAny;
+  JSObject* mMap;
+  JS::GCCellPtr mKey;
+  JSObject* mKeyDelegate;
+};
+
+void NoteWeakMapChildrenTracer::onChild(JS::GCCellPtr aThing,
+                                        const char* name) {
+  if (aThing.is<JSString>()) {
+    return;
+  }
+
+  if (!JS::GCThingIsMarkedGrayInCC(aThing) && !mCb.WantAllTraces()) {
+    return;
+  }
+
+  if (JS::IsCCTraceKind(aThing.kind())) {
+    mCb.NoteWeakMapping(mMap, mKey, mKeyDelegate, aThing);
+    mTracedAny = true;
+  } else {
+    JS::TraceChildren(this, aThing);
+  }
+}
+
+struct NoteWeakMapsTracer : public js::WeakMapTracer {
+  NoteWeakMapsTracer(JSRuntime* aRt, nsCycleCollectionNoteRootCallback& aCccb)
+      : js::WeakMapTracer(aRt), mCb(aCccb), mChildTracer(aRt, aCccb) {}
+  void trace(JSObject* aMap, JS::GCCellPtr aKey, JS::GCCellPtr aValue) override;
+  nsCycleCollectionNoteRootCallback& mCb;
+  NoteWeakMapChildrenTracer mChildTracer;
+};
+
+void NoteWeakMapsTracer::trace(JSObject* aMap, JS::GCCellPtr aKey,
+                               JS::GCCellPtr aValue) {
+  // If nothing that could be held alive by this entry is marked gray, return.
+  if ((!aKey || !JS::GCThingIsMarkedGrayInCC(aKey)) &&
+      MOZ_LIKELY(!mCb.WantAllTraces())) {
+    if (!aValue || !JS::GCThingIsMarkedGrayInCC(aValue) ||
+        aValue.is<JSString>()) {
+      return;
+    }
+  }
+
+  // The cycle collector can only properly reason about weak maps if it can
+  // reason about the liveness of their keys, which in turn requires that
+  // the key can be represented in the cycle collector graph.  All existing
+  // uses of weak maps use either objects or scripts as keys, which are okay.
+  MOZ_ASSERT(JS::IsCCTraceKind(aKey.kind()));
+
+  // As an emergency fallback for non-debug builds, if the key is not
+  // representable in the cycle collector graph, we treat it as marked.  This
+  // can cause leaks, but is preferable to ignoring the binding, which could
+  // cause the cycle collector to free live objects.
+  if (!JS::IsCCTraceKind(aKey.kind())) {
+    aKey = nullptr;
+  }
+
+  JSObject* kdelegate = nullptr;
+  if (aKey.is<JSObject>()) {
+    kdelegate = js::UncheckedUnwrapWithoutExpose(&aKey.as<JSObject>());
+  }
+
+  if (JS::IsCCTraceKind(aValue.kind())) {
+    mCb.NoteWeakMapping(aMap, aKey, kdelegate, aValue);
+  } else {
+    mChildTracer.mTracedAny = false;
+    mChildTracer.mMap = aMap;
+    mChildTracer.mKey = aKey;
+    mChildTracer.mKeyDelegate = kdelegate;
+
+    if (!aValue.is<JSString>()) {
+      JS::TraceChildren(&mChildTracer, aValue);
+    }
+
+    // The delegate could hold alive the key, so report something to the CC
+    // if we haven't already.
+    if (!mChildTracer.mTracedAny && aKey && JS::GCThingIsMarkedGrayInCC(aKey) &&
+        kdelegate) {
+      mCb.NoteWeakMapping(aMap, aKey, kdelegate, nullptr);
+    }
+  }
+}
+
+// Report whether the key or value of a weak mapping entry are gray but need to
+// be marked black.
+static void ShouldWeakMappingEntryBeBlack(JSObject* aMap, JS::GCCellPtr aKey,
+                                          JS::GCCellPtr aValue,
+                                          bool* aKeyShouldBeBlack,
+                                          bool* aValueShouldBeBlack) {
+  *aKeyShouldBeBlack = false;
+  *aValueShouldBeBlack = false;
+
+  // If nothing that could be held alive by this entry is marked gray, return.
+  bool keyMightNeedMarking = aKey && JS::GCThingIsMarkedGrayInCC(aKey);
+  bool valueMightNeedMarking = aValue && JS::GCThingIsMarkedGrayInCC(aValue) &&
+                               aValue.kind() != JS::TraceKind::String;
+  if (!keyMightNeedMarking && !valueMightNeedMarking) {
+    return;
+  }
+
+  if (!JS::IsCCTraceKind(aKey.kind())) {
+    aKey = nullptr;
+  }
+
+  if (keyMightNeedMarking && aKey.is<JSObject>()) {
+    JSObject* kdelegate =
+        js::UncheckedUnwrapWithoutExpose(&aKey.as<JSObject>());
+    if (kdelegate && !JS::ObjectIsMarkedGray(kdelegate) &&
+        (!aMap || !JS::ObjectIsMarkedGray(aMap))) {
+      *aKeyShouldBeBlack = true;
+    }
+  }
+
+  if (aValue && JS::GCThingIsMarkedGrayInCC(aValue) &&
+      (!aKey || !JS::GCThingIsMarkedGrayInCC(aKey)) &&
+      (!aMap || !JS::ObjectIsMarkedGray(aMap)) &&
+      aValue.kind() != JS::TraceKind::Shape) {
+    *aValueShouldBeBlack = true;
+  }
+}
+
+struct FixWeakMappingGrayBitsTracer : public js::WeakMapTracer {
+  explicit FixWeakMappingGrayBitsTracer(JSRuntime* aRt)
+      : js::WeakMapTracer(aRt) {}
+
+  void FixAll() {
+    do {
+      mAnyMarked = false;
+      js::TraceWeakMaps(this);
+    } while (mAnyMarked);
+  }
+
+  void trace(JSObject* aMap, JS::GCCellPtr aKey,
+             JS::GCCellPtr aValue) override {
+    bool keyShouldBeBlack;
+    bool valueShouldBeBlack;
+    ShouldWeakMappingEntryBeBlack(aMap, aKey, aValue, &keyShouldBeBlack,
+                                  &valueShouldBeBlack);
+    if (keyShouldBeBlack && JS::UnmarkGrayGCThingRecursively(aKey)) {
+      mAnyMarked = true;
+    }
+
+    if (valueShouldBeBlack && JS::UnmarkGrayGCThingRecursively(aValue)) {
+      mAnyMarked = true;
+    }
+  }
+
+  MOZ_INIT_OUTSIDE_CTOR bool mAnyMarked;
+};
+
+#ifdef DEBUG
+// Check whether weak maps are marked correctly according to the logic above.
+struct CheckWeakMappingGrayBitsTracer : public js::WeakMapTracer {
+  explicit CheckWeakMappingGrayBitsTracer(JSRuntime* aRt)
+      : js::WeakMapTracer(aRt), mFailed(false) {}
+
+  static bool Check(JSRuntime* aRt) {
+    CheckWeakMappingGrayBitsTracer tracer(aRt);
+    js::TraceWeakMaps(&tracer);
+    return !tracer.mFailed;
+  }
+
+  void trace(JSObject* aMap, JS::GCCellPtr aKey,
+             JS::GCCellPtr aValue) override {
+    bool keyShouldBeBlack;
+    bool valueShouldBeBlack;
+    ShouldWeakMappingEntryBeBlack(aMap, aKey, aValue, &keyShouldBeBlack,
+                                  &valueShouldBeBlack);
+
+    if (keyShouldBeBlack) {
+      fprintf(stderr, "Weak mapping key %p of map %p should be black\n",
+              aKey.asCell(), aMap);
+      mFailed = true;
+    }
+
+    if (valueShouldBeBlack) {
+      fprintf(stderr, "Weak mapping value %p of map %p should be black\n",
+              aValue.asCell(), aMap);
+      mFailed = true;
+    }
+  }
+
+  bool mFailed;
+};
+#endif  // DEBUG
+
+static void CheckParticipatesInCycleCollection(JS::GCCellPtr aThing,
+                                               const char* aName,
+                                               void* aClosure) {
+  bool* cycleCollectionEnabled = static_cast<bool*>(aClosure);
+
+  if (*cycleCollectionEnabled) {
+    return;
+  }
+
+  if (JS::IsCCTraceKind(aThing.kind()) && JS::GCThingIsMarkedGrayInCC(aThing)) {
+    *cycleCollectionEnabled = true;
+  }
+}
+
+NS_IMETHODIMP
+JSGCThingParticipant::TraverseNative(void* aPtr,
+                                     nsCycleCollectionTraversalCallback& aCb) {
+  auto runtime = reinterpret_cast<CycleCollectedJSRuntime*>(
+      reinterpret_cast<char*>(this) -
+      offsetof(CycleCollectedJSRuntime, mGCThingCycleCollectorGlobal));
+
+  JS::GCCellPtr cellPtr(aPtr, JS::GCThingTraceKind(aPtr));
+  runtime->TraverseGCThing(CycleCollectedJSRuntime::TRAVERSE_FULL, cellPtr,
+                           aCb);
+  return NS_OK;
+}
+
+// NB: This is only used to initialize the participant in
+// CycleCollectedJSRuntime. It should never be used directly.
+static JSGCThingParticipant sGCThingCycleCollectorGlobal;
+
+NS_IMETHODIMP
+JSZoneParticipant::TraverseNative(void* aPtr,
+                                  nsCycleCollectionTraversalCallback& aCb) {
+  auto runtime = reinterpret_cast<CycleCollectedJSRuntime*>(
+      reinterpret_cast<char*>(this) -
+      offsetof(CycleCollectedJSRuntime, mJSZoneCycleCollectorGlobal));
+
+  MOZ_ASSERT(!aCb.WantAllTraces());
+  JS::Zone* zone = static_cast<JS::Zone*>(aPtr);
+
+  runtime->TraverseZone(zone, aCb);
+  return NS_OK;
+}
+
+struct TraversalTracer : public JS::CallbackTracer {
+  TraversalTracer(JSRuntime* aRt, nsCycleCollectionTraversalCallback& aCb)
+      : JS::CallbackTracer(aRt, JS::TracerKind::Callback,
+                           JS::TraceOptions(JS::WeakMapTraceAction::Skip,
+                                            JS::WeakEdgeTraceAction::Trace)),
+        mCb(aCb) {}
+  void onChild(JS::GCCellPtr aThing, const char* name) override;
+  nsCycleCollectionTraversalCallback& mCb;
+};
+
+void TraversalTracer::onChild(JS::GCCellPtr aThing, const char* name) {
+  // Checking strings and symbols for being gray is rather slow, and we don't
+  // need either of them for the cycle collector.
+  if (aThing.is<JSString>() || aThing.is<JS::Symbol>()) {
+    return;
+  }
+
+  // Don't traverse non-gray objects, unless we want all traces.
+  if (!JS::GCThingIsMarkedGrayInCC(aThing) && !mCb.WantAllTraces()) {
+    return;
+  }
+
+  /*
+   * This function needs to be careful to avoid stack overflow. Normally, when
+   * IsCCTraceKind is true, the recursion terminates immediately as we just add
+   * |thing| to the CC graph. So overflow is only possible when there are long
+   * or cyclic chains of non-IsCCTraceKind GC things. Places where this can
+   * occur use special APIs to handle such chains iteratively.
+   */
+  if (JS::IsCCTraceKind(aThing.kind())) {
+    if (MOZ_UNLIKELY(mCb.WantDebugInfo())) {
+      char buffer[200];
+      context().getEdgeName(name, buffer, sizeof(buffer));
+      mCb.NoteNextEdgeName(buffer);
+    }
+    mCb.NoteJSChild(aThing);
+    return;
+  }
+
+  // Allow re-use of this tracer inside trace callback.
+  JS::AutoClearTracingContext actc(this);
+
+  if (aThing.is<js::Shape>()) {
+    // The maximum depth of traversal when tracing a Shape is unbounded, due to
+    // the parent pointers on the shape.
+    JS_TraceShapeCycleCollectorChildren(this, aThing);
+  } else {
+    JS::TraceChildren(this, aThing);
+  }
+}
+
+/*
+ * The cycle collection participant for a Zone is intended to produce the same
+ * results as if all of the gray GCthings in a zone were merged into a single
+ * node, except for self-edges. This avoids the overhead of representing all of
+ * the GCthings in the zone in the cycle collector graph, which should be much
+ * faster if many of the GCthings in the zone are gray.
+ *
+ * Zone merging should not always be used, because it is a conservative
+ * approximation of the true cycle collector graph that can incorrectly identify
+ * some garbage objects as being live. For instance, consider two cycles that
+ * pass through a zone, where one is garbage and the other is live. If we merge
+ * the entire zone, the cycle collector will think that both are alive.
+ *
+ * We don't have to worry about losing track of a garbage cycle, because any
+ * such garbage cycle incorrectly identified as live must contain at least one
+ * C++ to JS edge, and XPConnect will always add the C++ object to the CC graph.
+ * (This is in contrast to pure C++ garbage cycles, which must always be
+ * properly identified, because we clear the purple buffer during every CC,
+ * which may contain the last reference to a garbage cycle.)
+ */
+
+// NB: This is only used to initialize the participant in
+// CycleCollectedJSRuntime. It should never be used directly.
+static const JSZoneParticipant sJSZoneCycleCollectorGlobal;
+
+static void JSObjectsTenuredCb(JSContext* aContext, void* aData) {
+  static_cast<CycleCollectedJSRuntime*>(aData)->JSObjectsTenured();
+}
+
+static void MozCrashWarningReporter(JSContext*, JSErrorReport*) {
+  MOZ_CRASH("Why is someone touching JSAPI without an AutoJSAPI?");
+}
+
+JSHolderMap::Entry::Entry() : Entry(nullptr, nullptr, nullptr) {}
+
+JSHolderMap::Entry::Entry(void* aHolder, nsScriptObjectTracer* aTracer,
+                          JS::Zone* aZone)
+    : mHolder(aHolder),
+      mTracer(aTracer)
+#ifdef DEBUG
+      ,
+      mZone(aZone)
+#endif
+{
+}
+
+void JSHolderMap::EntryVectorIter::Settle() {
+  if (Done()) {
+    return;
+  }
+
+  Entry* entry = &mIter.Get();
+
+  // If the entry has been cleared, remove it and shrink the vector.
+  if (!entry->mHolder && !mHolderMap.RemoveEntry(mVector, entry)) {
+    // We removed the last entry, so reset the iterator to an empty one.
+    mIter = EntryVector().Iter();
+    MOZ_ASSERT(Done());
+  }
+}
+
+JSHolderMap::Iter::Iter(JSHolderMap& aMap, WhichHolders aWhich)
+    : mHolderMap(aMap), mIter(aMap, aMap.mAnyZoneJSHolders) {
+  MOZ_RELEASE_ASSERT(!mHolderMap.mHasIterator);
+  mHolderMap.mHasIterator = true;
+
+  // Populate vector of zones to iterate after the any-zone holders.
+  for (auto i = aMap.mPerZoneJSHolders.iter(); !i.done(); i.next()) {
+    JS::Zone* zone = i.get().key();
+    if (aWhich == AllHolders || JS::NeedGrayRootsForZone(i.get().key())) {
+      MOZ_ALWAYS_TRUE(mZones.append(zone));
+    }
+  }
+
+  Settle();
+}
+
+void JSHolderMap::Iter::Settle() {
+  while (mIter.Done()) {
+    if (mZone && mIter.Vector().IsEmpty()) {
+      mHolderMap.mPerZoneJSHolders.remove(mZone);
+    }
+
+    mZone = nullptr;
+    if (mZones.empty()) {
+      break;
+    }
+
+    mZone = mZones.popCopy();
+    EntryVector& vector = *mHolderMap.mPerZoneJSHolders.lookup(mZone)->value();
+    new (&mIter) EntryVectorIter(mHolderMap, vector);
+  }
+}
+
+void JSHolderMap::Iter::UpdateForRemovals() {
+  mIter.Settle();
+  Settle();
+}
+
+JSHolderMap::JSHolderMap() : mJSHolderMap(256) {}
+
+bool JSHolderMap::RemoveEntry(EntryVector& aJSHolders, Entry* aEntry) {
+  MOZ_ASSERT(aEntry);
+  MOZ_ASSERT(!aEntry->mHolder);
+
+  // Remove all dead entries from the end of the vector.
+  while (!aJSHolders.GetLast().mHolder && &aJSHolders.GetLast() != aEntry) {
+    aJSHolders.PopLast();
+  }
+
+  // Swap the element we want to remove with the last one and update the hash
+  // table.
+  Entry* lastEntry = &aJSHolders.GetLast();
+  if (aEntry != lastEntry) {
+    MOZ_ASSERT(lastEntry->mHolder);
+    *aEntry = *lastEntry;
+    MOZ_ASSERT(mJSHolderMap.has(aEntry->mHolder));
+    MOZ_ALWAYS_TRUE(mJSHolderMap.put(aEntry->mHolder, aEntry));
+  }
+
+  aJSHolders.PopLast();
+
+  // Return whether aEntry is still in the vector.
+  return aEntry != lastEntry;
+}
+
+bool JSHolderMap::Has(void* aHolder) const { return mJSHolderMap.has(aHolder); }
+
+nsScriptObjectTracer* JSHolderMap::Get(void* aHolder) const {
+  auto ptr = mJSHolderMap.lookup(aHolder);
+  if (!ptr) {
+    return nullptr;
+  }
+
+  Entry* entry = ptr->value();
+  MOZ_ASSERT(entry->mHolder == aHolder);
+  return entry->mTracer;
+}
+
+nsScriptObjectTracer* JSHolderMap::Extract(void* aHolder) {
+  MOZ_ASSERT(aHolder);
+
+  auto ptr = mJSHolderMap.lookup(aHolder);
+  if (!ptr) {
+    return nullptr;
+  }
+
+  Entry* entry = ptr->value();
+  MOZ_ASSERT(entry->mHolder == aHolder);
+  nsScriptObjectTracer* tracer = entry->mTracer;
+
+  // Clear the entry's contents. It will be removed the next time iteration
+  // visits this entry.
+  *entry = Entry();
+
+  mJSHolderMap.remove(ptr);
+
+  return tracer;
+}
+
+void JSHolderMap::Put(void* aHolder, nsScriptObjectTracer* aTracer,
+                      JS::Zone* aZone) {
+  MOZ_ASSERT(aHolder);
+  MOZ_ASSERT(aTracer);
+
+  // Don't associate multi-zone holders with a zone, even if one is supplied.
+  if (!aTracer->IsSingleZoneJSHolder()) {
+    aZone = nullptr;
+  }
+
+  auto ptr = mJSHolderMap.lookupForAdd(aHolder);
+  if (ptr) {
+    Entry* entry = ptr->value();
+#ifdef DEBUG
+    MOZ_ASSERT(entry->mHolder == aHolder);
+    MOZ_ASSERT(entry->mTracer == aTracer,
+               "Don't call HoldJSObjects in superclass ctors");
+    if (aZone) {
+      if (entry->mZone) {
+        MOZ_ASSERT(entry->mZone == aZone);
+      } else {
+        entry->mZone = aZone;
+      }
+    }
+#endif
+    entry->mTracer = aTracer;
+    return;
+  }
+
+  EntryVector* vector = &mAnyZoneJSHolders;
+  if (aZone) {
+    auto ptr = mPerZoneJSHolders.lookupForAdd(aZone);
+    if (!ptr) {
+      MOZ_ALWAYS_TRUE(
+          mPerZoneJSHolders.add(ptr, aZone, MakeUnique<EntryVector>()));
+    }
+    vector = ptr->value().get();
+  }
+
+  vector->InfallibleAppend(Entry{aHolder, aTracer, aZone});
+  MOZ_ALWAYS_TRUE(mJSHolderMap.add(ptr, aHolder, &vector->GetLast()));
+}
+
+size_t JSHolderMap::SizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const {
+  size_t n = 0;
+
+  // We're deliberately not measuring anything hanging off the entries in
+  // mJSHolders.
+  n += mJSHolderMap.shallowSizeOfExcludingThis(aMallocSizeOf);
+  n += mAnyZoneJSHolders.SizeOfExcludingThis(aMallocSizeOf);
+  n += mPerZoneJSHolders.shallowSizeOfExcludingThis(aMallocSizeOf);
+  for (auto i = mPerZoneJSHolders.iter(); !i.done(); i.next()) {
+    n += i.get().value()->SizeOfExcludingThis(aMallocSizeOf);
+  }
+
+  return n;
+}
+
+static bool InitializeShadowRealm(JSContext* aCx,
+                                  JS::Handle<JSObject*> aGlobal) {
+  MOZ_ASSERT(StaticPrefs::javascript_options_experimental_shadow_realms());
+
+  JSAutoRealm ar(aCx, aGlobal);
+  return dom::RegisterShadowRealmBindings(aCx, aGlobal);
+}
+
+CycleCollectedJSRuntime::CycleCollectedJSRuntime(JSContext* aCx)
+    : mContext(nullptr),
+      mGCThingCycleCollectorGlobal(sGCThingCycleCollectorGlobal),
+      mJSZoneCycleCollectorGlobal(sJSZoneCycleCollectorGlobal),
+      mJSRuntime(JS_GetRuntime(aCx)),
+      mHasPendingIdleGCTask(false),
+      mPrevGCSliceCallback(nullptr),
+      mOutOfMemoryState(OOMState::OK),
+      mLargeAllocationFailureState(OOMState::OK)
+#ifdef DEBUG
+      ,
+      mShutdownCalled(false)
+#endif
+{
+  MOZ_COUNT_CTOR(CycleCollectedJSRuntime);
+  MOZ_ASSERT(aCx);
+  MOZ_ASSERT(mJSRuntime);
+
+#if defined(XP_MACOSX)
+  if (!XRE_IsParentProcess()) {
+    nsMacUtilsImpl::EnableTCSMIfAvailable();
+  }
+#endif
+
+  if (!JS_AddExtraGCRootsTracer(aCx, TraceBlackJS, this)) {
+    MOZ_CRASH("JS_AddExtraGCRootsTracer failed");
+  }
+  JS_SetGrayGCRootsTracer(aCx, TraceGrayJS, this);
+  JS_SetGCCallback(aCx, GCCallback, this);
+  mPrevGCSliceCallback = JS::SetGCSliceCallback(aCx, GCSliceCallback);
+
+  if (NS_IsMainThread()) {
+    // We would like to support all threads here, but the way timeline consumers
+    // are set up currently, you can either add a marker for one specific
+    // docshell, or for every consumer globally. We would like to add a marker
+    // for every consumer observing anything on this thread, but that is not
+    // currently possible. For now, add global markers only when we are on the
+    // main thread, since the UI for this tracing data only displays data
+    // relevant to the main-thread.
+    JS::AddGCNurseryCollectionCallback(aCx, GCNurseryCollectionCallback,
+                                       nullptr);
+  }
+
+  JS_SetObjectsTenuredCallback(aCx, JSObjectsTenuredCb, this);
+  JS::SetOutOfMemoryCallback(aCx, OutOfMemoryCallback, this);
+  JS::SetWaitCallback(mJSRuntime, BeforeWaitCallback, AfterWaitCallback,
+                      sizeof(dom::AutoYieldJSThreadExecution));
+  JS::SetWarningReporter(aCx, MozCrashWarningReporter);
+  JS::SetShadowRealmInitializeGlobalCallback(aCx, InitializeShadowRealm);
+  JS::SetShadowRealmGlobalCreationCallback(aCx, dom::NewShadowRealmGlobal);
+
+  js::AutoEnterOOMUnsafeRegion::setAnnotateOOMAllocationSizeCallback(
+      CrashReporter::AnnotateOOMAllocationSize);
+
+  static js::DOMCallbacks DOMcallbacks = {InstanceClassHasProtoAtDepth};
+  SetDOMCallbacks(aCx, &DOMcallbacks);
+  js::SetScriptEnvironmentPreparer(aCx, &mEnvironmentPreparer);
+
+  JS::dbg::SetDebuggerMallocSizeOf(aCx, moz_malloc_size_of);
+
+#ifdef MOZ_JS_DEV_ERROR_INTERCEPTOR
+  JS_SetErrorInterceptorCallback(mJSRuntime, &mErrorInterceptor);
+#endif  // MOZ_JS_DEV_ERROR_INTERCEPTOR
+
+  JS_SetDestroyZoneCallback(aCx, OnZoneDestroyed);
+}
+
+#ifdef NS_BUILD_REFCNT_LOGGING
+class JSLeakTracer : public JS::CallbackTracer {
+ public:
+  explicit JSLeakTracer(JSRuntime* aRuntime)
+      : JS::CallbackTracer(aRuntime, JS::TracerKind::Callback,
+                           JS::WeakMapTraceAction::TraceKeysAndValues) {}
+
+ private:
+  void onChild(JS::GCCellPtr thing, const char* name) override {
+    const char* kindName = JS::GCTraceKindToAscii(thing.kind());
+    size_t size = JS::GCTraceKindSize(thing.kind());
+    MOZ_LOG_CTOR(thing.asCell(), kindName, size);
+  }
+};
+#endif
+
+void CycleCollectedJSRuntime::Shutdown(JSContext* aCx) {
+#ifdef MOZ_JS_DEV_ERROR_INTERCEPTOR
+  mErrorInterceptor.Shutdown(mJSRuntime);
+#endif  // MOZ_JS_DEV_ERROR_INTERCEPTOR
+
+  // There should not be any roots left to trace at this point. Ensure any that
+  // remain are flagged as leaks.
+#ifdef NS_BUILD_REFCNT_LOGGING
+  JSLeakTracer tracer(Runtime());
+  TraceNativeBlackRoots(&tracer);
+  TraceAllNativeGrayRoots(&tracer);
+#endif
+
+#ifdef DEBUG
+  mShutdownCalled = true;
+#endif
+
+  JS_SetDestroyZoneCallback(aCx, nullptr);
+
+  if (NS_IsMainThread()) {
+    JS::RemoveGCNurseryCollectionCallback(aCx, GCNurseryCollectionCallback,
+                                          nullptr);
+  }
+}
+
+CycleCollectedJSRuntime::~CycleCollectedJSRuntime() {
+  MOZ_COUNT_DTOR(CycleCollectedJSRuntime);
+  MOZ_ASSERT(!mDeferredFinalizerTable.Count());
+  MOZ_ASSERT(!mFinalizeRunnable);
+  MOZ_ASSERT(mShutdownCalled);
+}
+
+void CycleCollectedJSRuntime::SetContext(CycleCollectedJSContext* aContext) {
+  MOZ_ASSERT(!mContext || !aContext, "Don't replace the context!");
+  mContext = aContext;
+}
+
+size_t CycleCollectedJSRuntime::SizeOfExcludingThis(
+    MallocSizeOf aMallocSizeOf) const {
+  return mJSHolders.SizeOfExcludingThis(aMallocSizeOf);
+}
+
+void CycleCollectedJSRuntime::UnmarkSkippableJSHolders() {
+  for (JSHolderMap::Iter entry(mJSHolders); !entry.Done(); entry.Next()) {
+    entry->mTracer->CanSkip(entry->mHolder, true);
+  }
+}
+
+void CycleCollectedJSRuntime::DescribeGCThing(
+    bool aIsMarked, JS::GCCellPtr aThing,
+    nsCycleCollectionTraversalCallback& aCb) const {
+  if (!aCb.WantDebugInfo()) {
+    aCb.DescribeGCedNode(aIsMarked, "JS Object");
+    return;
+  }
+
+  char name[72];
+  uint64_t compartmentAddress = 0;
+  if (aThing.is<JSObject>()) {
+    JSObject* obj = &aThing.as<JSObject>();
+    compartmentAddress = (uint64_t)JS::GetCompartment(obj);
+    const JSClass* clasp = JS::GetClass(obj);
+
+    // Give the subclass a chance to do something
+    if (DescribeCustomObjects(obj, clasp, name)) {
+      // Nothing else to do!
+    } else if (js::IsFunctionObject(obj)) {
+      JSFunction* fun = JS_GetObjectFunction(obj);
+      JSString* str = JS_GetMaybePartialFunctionDisplayId(fun);
+      if (str) {
+        JSLinearString* linear = JS_ASSERT_STRING_IS_LINEAR(str);
+        nsAutoString chars;
+        AssignJSLinearString(chars, linear);
+        NS_ConvertUTF16toUTF8 fname(chars);
+        SprintfLiteral(name, "JS Object (Function - %s)", fname.get());
+      } else {
+        SprintfLiteral(name, "JS Object (Function)");
+      }
+    } else {
+      SprintfLiteral(name, "JS Object (%s)", clasp->name);
+    }
+  } else {
+    SprintfLiteral(name, "%s", JS::GCTraceKindToAscii(aThing.kind()));
+  }
+
+  // Disable printing global for objects while we figure out ObjShrink fallout.
+  aCb.DescribeGCedNode(aIsMarked, name, compartmentAddress);
+}
+
+void CycleCollectedJSRuntime::NoteGCThingJSChildren(
+    JS::GCCellPtr aThing, nsCycleCollectionTraversalCallback& aCb) const {
+  TraversalTracer trc(mJSRuntime, aCb);
+  JS::TraceChildren(&trc, aThing);
+}
+
+void CycleCollectedJSRuntime::NoteGCThingXPCOMChildren(
+    const JSClass* aClasp, JSObject* aObj,
+    nsCycleCollectionTraversalCallback& aCb) const {
+  MOZ_ASSERT(aClasp);
+  MOZ_ASSERT(aClasp == JS::GetClass(aObj));
+
+  JS::Rooted<JSObject*> obj(RootingCx(), aObj);
+
+  if (NoteCustomGCThingXPCOMChildren(aClasp, obj, aCb)) {
+    // Nothing else to do!
+    return;
+  }
+
+  // XXX This test does seem fragile, we should probably allowlist classes
+  //     that do hold a strong reference, but that might not be possible.
+  if (aClasp->slot0IsISupports()) {
+    NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(aCb, "JS::GetObjectISupports(obj)");
+    aCb.NoteXPCOMChild(JS::GetObjectISupports<nsISupports>(obj));
+    return;
+  }
+
+  const DOMJSClass* domClass = GetDOMClass(aClasp);
+  if (domClass) {
+    NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(aCb, "UnwrapDOMObject(obj)");
+    // It's possible that our object is an unforgeable holder object, in
+    // which case it doesn't actually have a C++ DOM object associated with
+    // it.  Use UnwrapPossiblyNotInitializedDOMObject, which produces null in
+    // that case, since NoteXPCOMChild/NoteNativeChild are null-safe.
+    if (domClass->mDOMObjectIsISupports) {
+      aCb.NoteXPCOMChild(
+          UnwrapPossiblyNotInitializedDOMObject<nsISupports>(obj));
+    } else if (domClass->mParticipant) {
+      aCb.NoteNativeChild(UnwrapPossiblyNotInitializedDOMObject<void>(obj),
+                          domClass->mParticipant);
+    }
+    return;
+  }
+
+  if (IsRemoteObjectProxy(obj)) {
+    auto handler =
+        static_cast<const RemoteObjectProxyBase*>(js::GetProxyHandler(obj));
+    return handler->NoteChildren(obj, aCb);
+  }
+
+  JS::Value value = js::MaybeGetScriptPrivate(obj);
+  if (!value.isUndefined()) {
+    aCb.NoteXPCOMChild(static_cast<nsISupports*>(value.toPrivate()));
+  }
+}
+
+void CycleCollectedJSRuntime::TraverseGCThing(
+    TraverseSelect aTs, JS::GCCellPtr aThing,
+    nsCycleCollectionTraversalCallback& aCb) {
+  bool isMarkedGray = JS::GCThingIsMarkedGrayInCC(aThing);
+
+  if (aTs == TRAVERSE_FULL) {
+    DescribeGCThing(!isMarkedGray, aThing, aCb);
+  }
+
+  // If this object is alive, then all of its children are alive. For JS
+  // objects, the black-gray invariant ensures the children are also marked
+  // black. For C++ objects, the ref count from this object will keep them
+  // alive. Thus we don't need to trace our children, unless we are debugging
+  // using WantAllTraces.
+  if (!isMarkedGray && !aCb.WantAllTraces()) {
+    return;
+  }
+
+  if (aTs == TRAVERSE_FULL) {
+    NoteGCThingJSChildren(aThing, aCb);
+  }
+
+  if (aThing.is<JSObject>()) {
+    JSObject* obj = &aThing.as<JSObject>();
+    NoteGCThingXPCOMChildren(JS::GetClass(obj), obj, aCb);
+  }
+}
+
+struct TraverseObjectShimClosure {
+  nsCycleCollectionTraversalCallback& cb;
+  CycleCollectedJSRuntime* self;
+};
+
+void CycleCollectedJSRuntime::TraverseZone(
+    JS::Zone* aZone, nsCycleCollectionTraversalCallback& aCb) {
+  /*
+   * We treat the zone as being gray. We handle non-gray GCthings in the
+   * zone by not reporting their children to the CC. The black-gray invariant
+   * ensures that any JS children will also be non-gray, and thus don't need to
+   * be added to the graph. For C++ children, not representing the edge from the
+   * non-gray JS GCthings to the C++ object will keep the child alive.
+   *
+   * We don't allow zone merging in a WantAllTraces CC, because then these
+   * assumptions don't hold.
+   */
+  aCb.DescribeGCedNode(false, "JS Zone");
+
+  /*
+   * Every JS child of everything in the zone is either in the zone
+   * or is a cross-compartment wrapper. In the former case, we don't need to
+   * represent these edges in the CC graph because JS objects are not ref
+   * counted. In the latter case, the JS engine keeps a map of these wrappers,
+   * which we iterate over. Edges between compartments in the same zone will add
+   * unnecessary loop edges to the graph (bug 842137).
+   */
+  TraversalTracer trc(mJSRuntime, aCb);
+  js::TraceGrayWrapperTargets(&trc, aZone);
+
+  /*
+   * To find C++ children of things in the zone, we scan every JS Object in
+   * the zone. Only JS Objects can have C++ children.
+   */
+  TraverseObjectShimClosure closure = {aCb, this};
+  js::IterateGrayObjects(aZone, TraverseObjectShim, &closure);
+}
+
+/* static */
+void CycleCollectedJSRuntime::TraverseObjectShim(
+    void* aData, JS::GCCellPtr aThing, const JS::AutoRequireNoGC& nogc) {
+  TraverseObjectShimClosure* closure =
+      static_cast<TraverseObjectShimClosure*>(aData);
+
+  MOZ_ASSERT(aThing.is<JSObject>());
+  closure->self->TraverseGCThing(CycleCollectedJSRuntime::TRAVERSE_CPP, aThing,
+                                 closure->cb);
+}
+
+void CycleCollectedJSRuntime::TraverseNativeRoots(
+    nsCycleCollectionNoteRootCallback& aCb) {
+  // NB: This is here just to preserve the existing XPConnect order. I doubt it
+  // would hurt to do this after the JS holders.
+  TraverseAdditionalNativeRoots(aCb);
+
+  for (JSHolderMap::Iter entry(mJSHolders); !entry.Done(); entry.Next()) {
+    void* holder = entry->mHolder;
+    nsScriptObjectTracer* tracer = entry->mTracer;
+
+    bool noteRoot = false;
+    if (MOZ_UNLIKELY(aCb.WantAllTraces())) {
+      noteRoot = true;
+    } else {
+      tracer->Trace(holder,
+                    TraceCallbackFunc(CheckParticipatesInCycleCollection),
+                    &noteRoot);
+    }
+
+    if (noteRoot) {
+      aCb.NoteNativeRoot(holder, tracer);
+    }
+  }
+}
+
+/* static */
+void CycleCollectedJSRuntime::TraceBlackJS(JSTracer* aTracer, void* aData) {
+  CycleCollectedJSRuntime* self = static_cast<CycleCollectedJSRuntime*>(aData);
+
+  self->TraceNativeBlackRoots(aTracer);
+}
+
+/* static */
+bool CycleCollectedJSRuntime::TraceGrayJS(JSTracer* aTracer,
+                                          js::SliceBudget& budget,
+                                          void* aData) {
+  CycleCollectedJSRuntime* self = static_cast<CycleCollectedJSRuntime*>(aData);
+
+  // Mark these roots as gray so the CC can walk them later.
+
+  JSHolderMap::WhichHolders which = JSHolderMap::AllHolders;
+
+  // Only trace holders in collecting zones when marking, except if we are
+  // collecting the atoms zone since any holder may point into that zone.
+  if (aTracer->isMarkingTracer() &&
+      !JS::AtomsZoneIsCollecting(self->Runtime())) {
+    which = JSHolderMap::HoldersRequiredForGrayMarking;
+  }
+
+  return self->TraceNativeGrayRoots(aTracer, which, budget);
+}
+
+/* static */
+void CycleCollectedJSRuntime::GCCallback(JSContext* aContext,
+                                         JSGCStatus aStatus,
+                                         JS::GCReason aReason, void* aData) {
+  CycleCollectedJSRuntime* self = static_cast<CycleCollectedJSRuntime*>(aData);
+
+  MOZ_ASSERT(CycleCollectedJSContext::Get()->Context() == aContext);
+  MOZ_ASSERT(CycleCollectedJSContext::Get()->Runtime() == self);
+
+  self->OnGC(aContext, aStatus, aReason);
+}
+
+struct GCMajorMarker : public BaseMarkerType<GCMajorMarker> {
+  static constexpr const char* Name = "GCMajor";
+  static constexpr const char* Description =
+      "Summary data for an entire major GC, encompassing a set of "
+      "incremental slices. The main thread is not blocked for the "
+      "entire major GC interval, only for the individual slices.";
+
+  using MS = MarkerSchema;
+  static constexpr MS::PayloadField PayloadFields[] = {
+      {"timings", MS::InputType::String, "GC timings"}};
+
+  static constexpr MS::Location Locations[] = {MS::Location::MarkerChart,
+                                               MS::Location::MarkerTable,
+                                               MS::Location::TimelineMemory};
+  static constexpr MS::ETWMarkerGroup Group = MS::ETWMarkerGroup::Memory;
+
+  static void StreamJSONMarkerData(
+      mozilla::baseprofiler::SpliceableJSONWriter& aWriter,
+      const mozilla::ProfilerString8View& aTimingJSON) {
+    if (aTimingJSON.Length() != 0) {
+      aWriter.SplicedJSONProperty("timings", aTimingJSON);
+    } else {
+      aWriter.NullProperty("timings");
+    }
+  }
+};
+
+/* static */
+void CycleCollectedJSRuntime::GCSliceCallback(JSContext* aContext,
+                                              JS::GCProgress aProgress,
+                                              const JS::GCDescription& aDesc) {
+  CycleCollectedJSRuntime* self = CycleCollectedJSRuntime::Get();
+  MOZ_ASSERT(CycleCollectedJSContext::Get()->Context() == aContext);
+
+  if (profiler_thread_is_being_profiled_for_markers()) {
+    if (aProgress == JS::GC_CYCLE_END) {
+      profiler_add_marker("GCMajor", baseprofiler::category::GCCC,
+                          MarkerTiming::Interval(aDesc.startTime(aContext),
+                                                 aDesc.endTime(aContext)),
+                          GCMajorMarker{},
+                          ProfilerString8View::WrapNullTerminatedString(
+                              aDesc.formatJSONProfiler(aContext).get()));
+    } else if (aProgress == JS::GC_SLICE_END) {
+      struct GCSliceMarker {
+        static constexpr mozilla::Span<const char> MarkerTypeName() {
+          return mozilla::MakeStringSpan("GCSlice");
+        }
+        static void StreamJSONMarkerData(
+            mozilla::baseprofiler::SpliceableJSONWriter& aWriter,
+            const mozilla::ProfilerString8View& aTimingJSON) {
+          if (aTimingJSON.Length() != 0) {
+            aWriter.SplicedJSONProperty("timings", aTimingJSON);
+          } else {
+            aWriter.NullProperty("timings");
+          }
+        }
+        static mozilla::MarkerSchema MarkerTypeDisplay() {
+          using MS = mozilla::MarkerSchema;
+          MS schema{MS::Location::MarkerChart, MS::Location::MarkerTable,
+                    MS::Location::TimelineMemory};
+          schema.AddStaticLabelValue(
+              "Description",
+              "One slice of an incremental garbage collection (GC). The main "
+              "thread is blocked during this time.");
+          // No display instructions here, there is special handling in the
+          // front-end.
+          return schema;
+        }
+      };
+
+      profiler_add_marker("GCSlice", baseprofiler::category::GCCC,
+                          MarkerTiming::Interval(aDesc.lastSliceStart(aContext),
+                                                 aDesc.lastSliceEnd(aContext)),
+                          GCSliceMarker{},
+                          ProfilerString8View::WrapNullTerminatedString(
+                              aDesc.sliceToJSONProfiler(aContext).get()));
+    }
+  }
+
+  if (aProgress == JS::GC_CYCLE_END &&
+      JS::dbg::FireOnGarbageCollectionHookRequired(aContext)) {
+    JS::GCReason reason = aDesc.reason_;
+    Unused << NS_WARN_IF(
+        NS_FAILED(DebuggerOnGCRunnable::Enqueue(aContext, aDesc)) &&
+        reason != JS::GCReason::SHUTDOWN_CC &&
+        reason != JS::GCReason::DESTROY_RUNTIME &&
+        reason != JS::GCReason::XPCONNECT_SHUTDOWN);
+  }
+
+  if (self->mPrevGCSliceCallback) {
+    self->mPrevGCSliceCallback(aContext, aProgress, aDesc);
+  }
+}
+
+/* static */
+void CycleCollectedJSRuntime::GCNurseryCollectionCallback(
+    JSContext* aContext, JS::GCNurseryProgress aProgress, JS::GCReason aReason,
+    void* data) {
+  CycleCollectedJSRuntime* self = CycleCollectedJSRuntime::Get();
+  MOZ_ASSERT(CycleCollectedJSContext::Get()->Context() == aContext);
+  MOZ_ASSERT(NS_IsMainThread());
+
+  TimeStamp now = TimeStamp::Now();
+  if (aProgress == JS::GCNurseryProgress::GC_NURSERY_COLLECTION_START) {
+    self->mLatestNurseryCollectionStart = now;
+  } else if (aProgress == JS::GCNurseryProgress::GC_NURSERY_COLLECTION_END) {
+    PerfStats::RecordMeasurement(PerfStats::Metric::MinorGC,
+                                 now - self->mLatestNurseryCollectionStart);
+  }
+
+  if (aProgress == JS::GCNurseryProgress::GC_NURSERY_COLLECTION_END &&
+      profiler_thread_is_being_profiled_for_markers()) {
+    struct GCMinorMarker {
+      static constexpr mozilla::Span<const char> MarkerTypeName() {
+        return mozilla::MakeStringSpan("GCMinor");
+      }
+      static void StreamJSONMarkerData(
+          mozilla::baseprofiler::SpliceableJSONWriter& aWriter,
+          const mozilla::ProfilerString8View& aTimingJSON) {
+        if (aTimingJSON.Length() != 0) {
+          aWriter.SplicedJSONProperty("nursery", aTimingJSON);
+        } else {
+          aWriter.NullProperty("nursery");
+        }
+      }
+      static mozilla::MarkerSchema MarkerTypeDisplay() {
+        using MS = mozilla::MarkerSchema;
+        MS schema{MS::Location::MarkerChart, MS::Location::MarkerTable,
+                  MS::Location::TimelineMemory};
+        schema.AddStaticLabelValue(
+            "Description",
+            "A minor GC (aka nursery collection) to clear out the buffer used "
+            "for recent allocations and move surviving data to the tenured "
+            "(long-lived) heap.");
+        // No display instructions here, there is special handling in the
+        // front-end.
+        return schema;
+      }
+    };
+
+    profiler_add_marker(
+        "GCMinor", baseprofiler::category::GCCC,
+        MarkerTiming::Interval(self->mLatestNurseryCollectionStart, now),
+        GCMinorMarker{},
+        ProfilerString8View::WrapNullTerminatedString(
+            JS::MinorGcToJSON(aContext).get()));
+  }
+}
+
+/* static */
+void CycleCollectedJSRuntime::OutOfMemoryCallback(JSContext* aContext,
+                                                  void* aData) {
+  CycleCollectedJSRuntime* self = static_cast<CycleCollectedJSRuntime*>(aData);
+
+  MOZ_ASSERT(CycleCollectedJSContext::Get()->Context() == aContext);
+  MOZ_ASSERT(CycleCollectedJSContext::Get()->Runtime() == self);
+
+  self->OnOutOfMemory();
+}
+
+/* static */
+void* CycleCollectedJSRuntime::BeforeWaitCallback(uint8_t* aMemory) {
+  MOZ_ASSERT(aMemory);
+
+  // aMemory is stack allocated memory to contain our RAII object. This allows
+  // for us to avoid allocations on the heap during this callback.
+  return new (aMemory) dom::AutoYieldJSThreadExecution;
+}
+
+/* static */
+void CycleCollectedJSRuntime::AfterWaitCallback(void* aCookie) {
+  MOZ_ASSERT(aCookie);
+  static_cast<dom::AutoYieldJSThreadExecution*>(aCookie)
+      ->~AutoYieldJSThreadExecution();
+}
+
+struct JsGcTracer : public TraceCallbacks {
+  virtual void Trace(JS::Heap<JS::Value>* aPtr, const char* aName,
+                     void* aClosure) const override {
+    JS::TraceEdge(static_cast<JSTracer*>(aClosure), aPtr, aName);
+  }
+  virtual void Trace(JS::Heap<jsid>* aPtr, const char* aName,
+                     void* aClosure) const override {
+    JS::TraceEdge(static_cast<JSTracer*>(aClosure), aPtr, aName);
+  }
+  virtual void Trace(JS::Heap<JSObject*>* aPtr, const char* aName,
+                     void* aClosure) const override {
+    JS::TraceEdge(static_cast<JSTracer*>(aClosure), aPtr, aName);
+  }
+  virtual void Trace(nsWrapperCache* aPtr, const char* aName,
+                     void* aClosure) const override {
+    aPtr->TraceWrapper(static_cast<JSTracer*>(aClosure), aName);
+  }
+  virtual void Trace(JS::TenuredHeap<JSObject*>* aPtr, const char* aName,
+                     void* aClosure) const override {
+    JS::TraceEdge(static_cast<JSTracer*>(aClosure), aPtr, aName);
+  }
+  virtual void Trace(JS::Heap<JSString*>* aPtr, const char* aName,
+                     void* aClosure) const override {
+    JS::TraceEdge(static_cast<JSTracer*>(aClosure), aPtr, aName);
+  }
+  virtual void Trace(JS::Heap<JSScript*>* aPtr, const char* aName,
+                     void* aClosure) const override {
+    JS::TraceEdge(static_cast<JSTracer*>(aClosure), aPtr, aName);
+  }
+  virtual void Trace(JS::Heap<JSFunction*>* aPtr, const char* aName,
+                     void* aClosure) const override {
+    JS::TraceEdge(static_cast<JSTracer*>(aClosure), aPtr, aName);
+  }
+};
+
+void mozilla::TraceScriptHolder(nsISupports* aHolder, JSTracer* aTracer) {
+  nsXPCOMCycleCollectionParticipant* participant = nullptr;
+  CallQueryInterface(aHolder, &participant);
+  participant->Trace(aHolder, JsGcTracer(), aTracer);
+}
+
+#if defined(NIGHTLY_BUILD) || defined(MOZ_DEV_EDITION) || defined(DEBUG)
+#  define CHECK_SINGLE_ZONE_JS_HOLDERS
+#endif
+
+#ifdef CHECK_SINGLE_ZONE_JS_HOLDERS
+
+// A tracer that checks that a JS holder only holds JS GC things in a single
+// JS::Zone.
+struct CheckZoneTracer : public TraceCallbacks {
+  const char* mClassName;
+  mutable JS::Zone* mZone;
+
+  explicit CheckZoneTracer(const char* aClassName, JS::Zone* aZone = nullptr)
+      : mClassName(aClassName), mZone(aZone) {}
+
+  void checkZone(JS::Zone* aZone, const char* aName) const {
+    if (JS::IsAtomsZone(aZone)) {
+      // Any holder may contain pointers into the atoms zone.
+      return;
+    }
+
+    if (!mZone) {
+      mZone = aZone;
+      return;
+    }
+
+    if (aZone == mZone) {
+      return;
+    }
+
+    // Most JS holders only contain pointers to GC things in a single zone. We
+    // group holders by referent zone where possible, allowing us to improve GC
+    // performance by only tracing holders for zones that are being collected.
+    //
+    // Additionally, pointers from any holder into the atoms zone are allowed
+    // since all holders are traced when we collect the atoms zone.
+    //
+    // If you added a holder that has pointers into multiple zones do not
+    // use NS_IMPL_CYCLE_COLLECTION_SINGLE_ZONE_SCRIPT_HOLDER_CLASS.
+    MOZ_CRASH_UNSAFE_PRINTF(
+        "JS holder %s contains pointers to GC things in more than one zone ("
+        "found in %s)\n",
+        mClassName, aName);
+  }
+
+  virtual void Trace(JS::Heap<JS::Value>* aPtr, const char* aName,
+                     void* aClosure) const override {
+    JS::Value value = aPtr->unbarrieredGet();
+    if (value.isGCThing()) {
+      checkZone(JS::GetGCThingZone(value.toGCCellPtr()), aName);
+    }
+  }
+  virtual void Trace(JS::Heap<jsid>* aPtr, const char* aName,
+                     void* aClosure) const override {
+    jsid id = aPtr->unbarrieredGet();
+    if (id.isGCThing()) {
+      MOZ_ASSERT(JS::IsAtomsZone(JS::GetTenuredGCThingZone(id.toGCCellPtr())));
+    }
+  }
+  virtual void Trace(JS::Heap<JSObject*>* aPtr, const char* aName,
+                     void* aClosure) const override {
+    JSObject* obj = aPtr->unbarrieredGet();
+    if (obj) {
+      checkZone(js::GetObjectZoneFromAnyThread(obj), aName);
+    }
+  }
+  virtual void Trace(nsWrapperCache* aPtr, const char* aName,
+                     void* aClosure) const override {
+    JSObject* obj = aPtr->GetWrapperPreserveColor();
+    if (obj) {
+      checkZone(js::GetObjectZoneFromAnyThread(obj), aName);
+    }
+  }
+  virtual void Trace(JS::TenuredHeap<JSObject*>* aPtr, const char* aName,
+                     void* aClosure) const override {
+    JSObject* obj = aPtr->unbarrieredGetPtr();
+    if (obj) {
+      checkZone(js::GetObjectZoneFromAnyThread(obj), aName);
+    }
+  }
+  virtual void Trace(JS::Heap<JSString*>* aPtr, const char* aName,
+                     void* aClosure) const override {
+    JSString* str = aPtr->unbarrieredGet();
+    if (str) {
+      checkZone(JS::GetStringZone(str), aName);
+    }
+  }
+  virtual void Trace(JS::Heap<JSScript*>* aPtr, const char* aName,
+                     void* aClosure) const override {
+    JSScript* script = aPtr->unbarrieredGet();
+    if (script) {
+      checkZone(JS::GetTenuredGCThingZone(JS::GCCellPtr(script)), aName);
+    }
+  }
+  virtual void Trace(JS::Heap<JSFunction*>* aPtr, const char* aName,
+                     void* aClosure) const override {
+    JSFunction* fun = aPtr->unbarrieredGet();
+    if (fun) {
+      checkZone(js::GetObjectZoneFromAnyThread(JS_GetFunctionObject(fun)),
+                aName);
+    }
+  }
+};
+
+static inline void CheckHolderIsSingleZone(
+    void* aHolder, nsCycleCollectionParticipant* aParticipant,
+    JS::Zone* aZone) {
+  CheckZoneTracer tracer(aParticipant->ClassName(), aZone);
+  aParticipant->Trace(aHolder, tracer, nullptr);
+}
+
+#endif
+
+static inline bool ShouldCheckSingleZoneHolders() {
+#if defined(DEBUG)
+  return true;
+#elif defined(NIGHTLY_BUILD) || defined(MOZ_DEV_EDITION)
+  // Don't check every time to avoid performance impact.
+  return rand() % 256 == 0;
+#else
+  return false;
+#endif
+}
+
+#ifdef NS_BUILD_REFCNT_LOGGING
+void CycleCollectedJSRuntime::TraceAllNativeGrayRoots(JSTracer* aTracer) {
+  MOZ_RELEASE_ASSERT(mHolderIter.isNothing());
+  js::SliceBudget budget = js::SliceBudget::unlimited();
+  MOZ_ALWAYS_TRUE(
+      TraceNativeGrayRoots(aTracer, JSHolderMap::AllHolders, budget));
+}
+#endif
+
+bool CycleCollectedJSRuntime::TraceNativeGrayRoots(
+    JSTracer* aTracer, JSHolderMap::WhichHolders aWhich,
+    js::SliceBudget& aBudget) {
+  if (!mHolderIter) {
+    // NB: This is here just to preserve the existing XPConnect order. I doubt
+    // it would hurt to do this after the JS holders.
+    TraceAdditionalNativeGrayRoots(aTracer);
+
+    mHolderIter.emplace(mJSHolders, aWhich);
+    aBudget.forceCheck();
+  } else {
+    // Holders may have been removed between slices, so we may need to update
+    // the iterator.
+    mHolderIter->UpdateForRemovals();
+  }
+
+  bool finished = TraceJSHolders(aTracer, *mHolderIter, aBudget);
+  if (finished) {
+    mHolderIter.reset();
+  }
+
+  return finished;
+}
+
+bool CycleCollectedJSRuntime::TraceJSHolders(JSTracer* aTracer,
+                                             JSHolderMap::Iter& aIter,
+                                             js::SliceBudget& aBudget) {
+  bool checkSingleZoneHolders = ShouldCheckSingleZoneHolders();
+
+  while (!aIter.Done() && !aBudget.isOverBudget()) {
+    void* holder = aIter->mHolder;
+    nsScriptObjectTracer* tracer = aIter->mTracer;
+
+#ifdef CHECK_SINGLE_ZONE_JS_HOLDERS
+    if (checkSingleZoneHolders && tracer->IsSingleZoneJSHolder()) {
+      CheckHolderIsSingleZone(holder, tracer, aIter.Zone());
+    }
+#else
+    Unused << checkSingleZoneHolders;
+#endif
+
+    tracer->Trace(holder, JsGcTracer(), aTracer);
+
+    aIter.Next();
+    aBudget.step();
+  }
+
+  return aIter.Done();
+}
+
+void CycleCollectedJSRuntime::AddJSHolder(void* aHolder,
+                                          nsScriptObjectTracer* aTracer,
+                                          JS::Zone* aZone) {
+  mJSHolders.Put(aHolder, aTracer, aZone);
+}
+
+struct ClearJSHolder : public TraceCallbacks {
+  virtual void Trace(JS::Heap<JS::Value>* aPtr, const char*,
+                     void*) const override {
+    aPtr->setUndefined();
+  }
+
+  virtual void Trace(JS::Heap<jsid>* aPtr, const char*, void*) const override {
+    *aPtr = JS::PropertyKey::Void();
+  }
+
+  virtual void Trace(JS::Heap<JSObject*>* aPtr, const char*,
+                     void*) const override {
+    *aPtr = nullptr;
+  }
+
+  virtual void Trace(nsWrapperCache* aPtr, const char* aName,
+                     void* aClosure) const override {
+    aPtr->ClearWrapper();
+  }
+
+  virtual void Trace(JS::TenuredHeap<JSObject*>* aPtr, const char*,
+                     void*) const override {
+    *aPtr = nullptr;
+  }
+
+  virtual void Trace(JS::Heap<JSString*>* aPtr, const char*,
+                     void*) const override {
+    *aPtr = nullptr;
+  }
+
+  virtual void Trace(JS::Heap<JSScript*>* aPtr, const char*,
+                     void*) const override {
+    *aPtr = nullptr;
+  }
+
+  virtual void Trace(JS::Heap<JSFunction*>* aPtr, const char*,
+                     void*) const override {
+    *aPtr = nullptr;
+  }
+};
+
+void CycleCollectedJSRuntime::RemoveJSHolder(void* aHolder) {
+  nsScriptObjectTracer* tracer = mJSHolders.Extract(aHolder);
+  if (tracer) {
+    // Bug 1531951: The analysis can't see through the virtual call but we know
+    // that the ClearJSHolder tracer will never GC.
+    JS::AutoSuppressGCAnalysis nogc;
+    tracer->Trace(aHolder, ClearJSHolder(), nullptr);
+  }
+}
+
+#ifdef DEBUG
+static void AssertNoGcThing(JS::GCCellPtr aGCThing, const char* aName,
+                            void* aClosure) {
+  MOZ_ASSERT(!aGCThing);
+}
+
+void CycleCollectedJSRuntime::AssertNoObjectsToTrace(void* aPossibleJSHolder) {
+  nsScriptObjectTracer* tracer = mJSHolders.Get(aPossibleJSHolder);
+  if (tracer) {
+    tracer->Trace(aPossibleJSHolder, TraceCallbackFunc(AssertNoGcThing),
+                  nullptr);
+  }
+}
+#endif
+
+nsCycleCollectionParticipant* CycleCollectedJSRuntime::GCThingParticipant() {
+  return &mGCThingCycleCollectorGlobal;
+}
+
+nsCycleCollectionParticipant* CycleCollectedJSRuntime::ZoneParticipant() {
+  return &mJSZoneCycleCollectorGlobal;
+}
+
+nsresult CycleCollectedJSRuntime::TraverseRoots(
+    nsCycleCollectionNoteRootCallback& aCb) {
+  TraverseNativeRoots(aCb);
+
+  NoteWeakMapsTracer trc(mJSRuntime, aCb);
+  js::TraceWeakMaps(&trc);
+
+  return NS_OK;
+}
+
+bool CycleCollectedJSRuntime::UsefulToMergeZones() const { return false; }
+
+void CycleCollectedJSRuntime::FixWeakMappingGrayBits() const {
+  MOZ_ASSERT(!JS::IsIncrementalGCInProgress(mJSRuntime),
+             "Don't call FixWeakMappingGrayBits during a GC.");
+  FixWeakMappingGrayBitsTracer fixer(mJSRuntime);
+  fixer.FixAll();
+}
+
+void CycleCollectedJSRuntime::CheckGrayBits() const {
+  MOZ_ASSERT(!JS::IsIncrementalGCInProgress(mJSRuntime),
+             "Don't call CheckGrayBits during a GC.");
+
+#ifndef ANDROID
+  // Bug 1346874 - The gray state check is expensive. Android tests are already
+  // slow enough that this check can easily push them over the threshold to a
+  // timeout.
+
+  MOZ_ASSERT(js::CheckGrayMarkingState(mJSRuntime));
+  MOZ_ASSERT(CheckWeakMappingGrayBitsTracer::Check(mJSRuntime));
+#endif
+}
+
+bool CycleCollectedJSRuntime::AreGCGrayBitsValid() const {
+  return js::AreGCGrayBitsValid(mJSRuntime);
+}
+
+void CycleCollectedJSRuntime::GarbageCollect(JS::GCOptions aOptions,
+                                             JS::GCReason aReason) const {
+  JSContext* cx = CycleCollectedJSContext::Get()->Context();
+  JS::PrepareForFullGC(cx);
+  JS::NonIncrementalGC(cx, aOptions, aReason);
+}
+
+void CycleCollectedJSRuntime::JSObjectsTenured() {
+  JSContext* cx = CycleCollectedJSContext::Get()->Context();
+  for (auto iter = mNurseryObjects.Iter(); !iter.Done(); iter.Next()) {
+    nsWrapperCache* cache = iter.Get();
+    JSObject* wrapper = cache->GetWrapperMaybeDead();
+    MOZ_DIAGNOSTIC_ASSERT(wrapper);
+    if (!JS::ObjectIsTenured(wrapper)) {
+      MOZ_ASSERT(!cache->PreservingWrapper());
+      js::gc::FinalizeDeadNurseryObject(cx, wrapper);
+    }
+  }
+
+  mNurseryObjects.Clear();
+}
+
+void CycleCollectedJSRuntime::NurseryWrapperAdded(nsWrapperCache* aCache) {
+  MOZ_ASSERT(aCache);
+  MOZ_ASSERT(aCache->GetWrapperMaybeDead());
+  MOZ_ASSERT(!JS::ObjectIsTenured(aCache->GetWrapperMaybeDead()));
+  mNurseryObjects.InfallibleAppend(aCache);
+}
+
+void CycleCollectedJSRuntime::DeferredFinalize(
+    DeferredFinalizeAppendFunction aAppendFunc, DeferredFinalizeFunction aFunc,
+    void* aThing) {
+  // Tell the analysis that the function pointers will not GC.
+  JS::AutoSuppressGCAnalysis suppress;
+  mDeferredFinalizerTable.WithEntryHandle(aFunc, [&](auto&& entry) {
+    if (entry) {
+      aAppendFunc(entry.Data(), aThing);
+    } else {
+      entry.Insert(aAppendFunc(nullptr, aThing));
+    }
+  });
+}
+
+void CycleCollectedJSRuntime::DeferredFinalize(nsISupports* aSupports) {
+  typedef DeferredFinalizerImpl<nsISupports> Impl;
+  DeferredFinalize(Impl::AppendDeferredFinalizePointer, Impl::DeferredFinalize,
+                   aSupports);
+}
+
+void CycleCollectedJSRuntime::DumpJSHeap(FILE* aFile) {
+  JSContext* cx = CycleCollectedJSContext::Get()->Context();
+
+  mozilla::MallocSizeOf mallocSizeOf =
+      PR_GetEnv("MOZ_GC_LOG_SIZE") ? moz_malloc_size_of : nullptr;
+  js::DumpHeap(cx, aFile, js::CollectNurseryBeforeDump, mallocSizeOf);
+}
+
+IncrementalFinalizeRunnable::IncrementalFinalizeRunnable(
+    CycleCollectedJSRuntime* aRt, DeferredFinalizerTable& aFinalizers)
+    : DiscardableRunnable("IncrementalFinalizeRunnable"),
+      mRuntime(aRt),
+      mFinalizeFunctionToRun(0),
+      mReleasing(false) {
+  for (auto iter = aFinalizers.Iter(); !iter.Done(); iter.Next()) {
+    DeferredFinalizeFunction& function = iter.Key();
+    void*& data = iter.Data();
+
+    DeferredFinalizeFunctionHolder* holder =
+        mDeferredFinalizeFunctions.AppendElement();
+    holder->run = function;
+    holder->data = data;
+
+    iter.Remove();
+  }
+  MOZ_ASSERT(mDeferredFinalizeFunctions.Length());
+}
+
+IncrementalFinalizeRunnable::~IncrementalFinalizeRunnable() {
+  MOZ_ASSERT(!mDeferredFinalizeFunctions.Length());
+  MOZ_ASSERT(!mRuntime);
+}
+
+void IncrementalFinalizeRunnable::ReleaseNow(bool aLimited) {
+  if (mReleasing) {
+    NS_WARNING("Re-entering ReleaseNow");
+    return;
+  }
+  {
+    AUTO_PROFILER_LABEL("IncrementalFinalizeRunnable::ReleaseNow",
+                        GCCC_Finalize);
+
+    mozilla::AutoRestore<bool> ar(mReleasing);
+    mReleasing = true;
+    MOZ_ASSERT(mDeferredFinalizeFunctions.Length() != 0,
+               "We should have at least ReleaseSliceNow to run");
+    MOZ_ASSERT(mFinalizeFunctionToRun < mDeferredFinalizeFunctions.Length(),
+               "No more finalizers to run?");
+
+    TimeDuration sliceTime = TimeDuration::FromMilliseconds(SliceMillis);
+    TimeStamp started = aLimited ? TimeStamp::Now() : TimeStamp();
+    bool timeout = false;
+    do {
+      const DeferredFinalizeFunctionHolder& function =
+          mDeferredFinalizeFunctions[mFinalizeFunctionToRun];
+      if (aLimited) {
+        bool done = false;
+        while (!timeout && !done) {
+          /*
+           * We don't want to read the clock too often, so we try to
+           * release slices of 100 items.
+           */
+          done = function.run(100, function.data);
+          timeout = TimeStamp::Now() - started >= sliceTime;
+        }
+        if (done) {
+          ++mFinalizeFunctionToRun;
+        }
+        if (timeout) {
+          break;
+        }
+      } else {
+        while (!function.run(UINT32_MAX, function.data))
+          ;
+        ++mFinalizeFunctionToRun;
+      }
+    } while (mFinalizeFunctionToRun < mDeferredFinalizeFunctions.Length());
+  }
+
+  if (mFinalizeFunctionToRun == mDeferredFinalizeFunctions.Length()) {
+    MOZ_ASSERT(mRuntime->mFinalizeRunnable == this);
+    mDeferredFinalizeFunctions.Clear();
+    CycleCollectedJSRuntime* runtime = mRuntime;
+    mRuntime = nullptr;
+    // NB: This may delete this!
+    runtime->mFinalizeRunnable = nullptr;
+  }
+}
+
+NS_IMETHODIMP
+IncrementalFinalizeRunnable::Run() {
+  if (!mDeferredFinalizeFunctions.Length()) {
+    /* These items were already processed synchronously in JSGC_END. */
+    MOZ_ASSERT(!mRuntime);
+    return NS_OK;
+  }
+
+  MOZ_ASSERT(mRuntime->mFinalizeRunnable == this);
+  TimeStamp start = TimeStamp::Now();
+  ReleaseNow(true);
+
+  if (mDeferredFinalizeFunctions.Length()) {
+    nsresult rv = NS_DispatchToCurrentThread(this);
+    if (NS_FAILED(rv)) {
+      ReleaseNow(false);
+    }
+  } else {
+    MOZ_ASSERT(!mRuntime);
+  }
+
+  uint32_t duration = (uint32_t)((TimeStamp::Now() - start).ToMilliseconds());
+  Telemetry::Accumulate(Telemetry::DEFERRED_FINALIZE_ASYNC, duration);
+
+  return NS_OK;
+}
+
+void CycleCollectedJSRuntime::FinalizeDeferredThings(
+    DeferredFinalizeType aType) {
+  // If mFinalizeRunnable isn't null, we didn't finalize everything from the
+  // previous GC.
+  if (mFinalizeRunnable) {
+    if (aType == FinalizeLater) {
+      // We need to defer all finalization until we return to the event loop,
+      // so leave things alone. Any new objects to be finalized from the current
+      // GC will be handled by the existing mFinalizeRunnable.
+      return;
+    }
+    MOZ_ASSERT(aType == FinalizeIncrementally || aType == FinalizeNow);
+    // If we're finalizing incrementally, we don't want finalizers to build up,
+    // so try to finish them off now.
+    // If we're finalizing synchronously, also go ahead and clear them out,
+    // so we make sure as much as possible is freed.
+    mFinalizeRunnable->ReleaseNow(false);
+    if (mFinalizeRunnable) {
+      // If we re-entered ReleaseNow, we couldn't delete mFinalizeRunnable and
+      // we need to just continue processing it.
+      return;
+    }
+  }
+
+  // If there's nothing to finalize, don't create a new runnable.
+  if (mDeferredFinalizerTable.Count() == 0) {
+    return;
+  }
+
+  mFinalizeRunnable =
+      new IncrementalFinalizeRunnable(this, mDeferredFinalizerTable);
+
+  // Everything should be gone now.
+  MOZ_ASSERT(mDeferredFinalizerTable.Count() == 0);
+
+  if (aType == FinalizeNow) {
+    mFinalizeRunnable->ReleaseNow(false);
+    MOZ_ASSERT(!mFinalizeRunnable);
+  } else {
+    MOZ_ASSERT(aType == FinalizeIncrementally || aType == FinalizeLater);
+    NS_DispatchToCurrentThreadQueue(do_AddRef(mFinalizeRunnable), 2500,
+                                    EventQueuePriority::Idle);
+  }
+}
+
+const char* CycleCollectedJSRuntime::OOMStateToString(
+    const OOMState aOomState) const {
+  switch (aOomState) {
+    case OOMState::OK:
+      return "OK";
+    case OOMState::Reporting:
+      return "Reporting";
+    case OOMState::Reported:
+      return "Reported";
+    case OOMState::Recovered:
+      return "Recovered";
+    default:
+      MOZ_ASSERT_UNREACHABLE("OOMState holds an invalid value");
+      return "Unknown";
+  }
+}
+
+bool CycleCollectedJSRuntime::OOMReported() {
+  return mOutOfMemoryState == OOMState::Reported;
+}
+
+void CycleCollectedJSRuntime::AnnotateAndSetOutOfMemory(OOMState* aStatePtr,
+                                                        OOMState aNewState) {
+  *aStatePtr = aNewState;
+  CrashReporter::Annotation annotation =
+      (aStatePtr == &mOutOfMemoryState)
+          ? CrashReporter::Annotation::JSOutOfMemory
+          : CrashReporter::Annotation::JSLargeAllocationFailure;
+
+  CrashReporter::AnnotateCrashReport(
+      annotation, nsDependentCString(OOMStateToString(aNewState)));
+}
+
+void CycleCollectedJSRuntime::OnGC(JSContext* aContext, JSGCStatus aStatus,
+                                   JS::GCReason aReason) {
+  switch (aStatus) {
+    case JSGC_BEGIN:
+      MOZ_RELEASE_ASSERT(mHolderIter.isNothing());
+      nsCycleCollector_prepareForGarbageCollection();
+      PrepareWaitingZonesForGC();
+      break;
+    case JSGC_END: {
+      MOZ_RELEASE_ASSERT(mHolderIter.isNothing());
+      if (mOutOfMemoryState == OOMState::Reported) {
+        AnnotateAndSetOutOfMemory(&mOutOfMemoryState, OOMState::Recovered);
+      }
+      if (mLargeAllocationFailureState == OOMState::Reported) {
+        AnnotateAndSetOutOfMemory(&mLargeAllocationFailureState,
+                                  OOMState::Recovered);
+      }
+
+      DeferredFinalizeType finalizeType;
+      if (JS_IsExceptionPending(aContext)) {
+        // There is a pending exception. The finalizers are not set up to run
+        // in that state, so don't run the finalizer until we've returned to the
+        // event loop.
+        finalizeType = FinalizeLater;
+      } else if (JS::InternalGCReason(aReason)) {
+        if (aReason == JS::GCReason::DESTROY_RUNTIME) {
+          // We're shutting down, so we need to destroy things immediately.
+          finalizeType = FinalizeNow;
+        } else {
+          // We may be in the middle of running some code that the JIT has
+          // assumed can't have certain kinds of side effects. Finalizers can do
+          // all sorts of things, such as run JS, so we want to run them later,
+          // after we've returned to the event loop.
+          finalizeType = FinalizeLater;
+        }
+      } else if (JS::WasIncrementalGC(mJSRuntime)) {
+        // The GC was incremental, so we probably care about pauses. Try to
+        // break up finalization, but it is okay if we do some now.
+        finalizeType = FinalizeIncrementally;
+      } else {
+        // If we're running a synchronous GC, we probably want to free things as
+        // quickly as possible. This can happen during testing or if memory is
+        // low.
+        finalizeType = FinalizeNow;
+      }
+      FinalizeDeferredThings(finalizeType);
+
+      break;
+    }
+    default:
+      MOZ_CRASH();
+  }
+
+  CustomGCCallback(aStatus);
+}
+
+void CycleCollectedJSRuntime::OnOutOfMemory() {
+  AnnotateAndSetOutOfMemory(&mOutOfMemoryState, OOMState::Reporting);
+  CustomOutOfMemoryCallback();
+  AnnotateAndSetOutOfMemory(&mOutOfMemoryState, OOMState::Reported);
+}
+
+void CycleCollectedJSRuntime::SetLargeAllocationFailure(OOMState aNewState) {
+  AnnotateAndSetOutOfMemory(&mLargeAllocationFailureState, aNewState);
+}
+
+void CycleCollectedJSRuntime::PrepareWaitingZonesForGC() {
+  JSContext* cx = CycleCollectedJSContext::Get()->Context();
+  if (mZonesWaitingForGC.Count() == 0) {
+    JS::PrepareForFullGC(cx);
+  } else {
+    for (const auto& key : mZonesWaitingForGC) {
+      JS::PrepareZoneForGC(cx, key);
+    }
+    mZonesWaitingForGC.Clear();
+  }
+}
+
+/* static */
+void CycleCollectedJSRuntime::OnZoneDestroyed(JS::GCContext* aGcx,
+                                              JS::Zone* aZone) {
+  // Remove the zone from the set of zones waiting for GC, if present. This can
+  // happen if a zone is added to the set during an incremental GC in which it
+  // is later destroyed.
+  CycleCollectedJSRuntime* runtime = Get();
+  runtime->mZonesWaitingForGC.Remove(aZone);
+}
+
+void CycleCollectedJSRuntime::EnvironmentPreparer::invoke(
+    JS::HandleObject global, js::ScriptEnvironmentPreparer::Closure& closure) {
+  MOZ_ASSERT(JS_IsGlobalObject(global));
+  nsIGlobalObject* nativeGlobal = xpc::NativeGlobal(global);
+
+  // Not much we can do if we simply don't have a usable global here...
+  NS_ENSURE_TRUE_VOID(nativeGlobal && nativeGlobal->HasJSGlobal());
+
+  AutoEntryScript aes(nativeGlobal, "JS-engine-initiated execution");
+
+  MOZ_ASSERT(!JS_IsExceptionPending(aes.cx()));
+
+  DebugOnly<bool> ok = closure(aes.cx());
+
+  MOZ_ASSERT_IF(ok, !JS_IsExceptionPending(aes.cx()));
+
+  // The AutoEntryScript will check for JS_IsExceptionPending on the
+  // JSContext and report it as needed as it comes off the stack.
+}
+
+/* static */
+CycleCollectedJSRuntime* CycleCollectedJSRuntime::Get() {
+  auto context = CycleCollectedJSContext::Get();
+  if (context) {
+    return context->Runtime();
+  }
+  return nullptr;
+}
+
+#ifdef MOZ_JS_DEV_ERROR_INTERCEPTOR
+
+namespace js {
+extern void DumpValue(const JS::Value& val);
+}
+
+void CycleCollectedJSRuntime::ErrorInterceptor::Shutdown(JSRuntime* rt) {
+  JS_SetErrorInterceptorCallback(rt, nullptr);
+  mThrownError.reset();
+}
+
+/* virtual */
+void CycleCollectedJSRuntime::ErrorInterceptor::interceptError(
+    JSContext* cx, JS::HandleValue exn) {
+  if (mThrownError) {
+    // We already have an error, we don't need anything more.
+    return;
+  }
+
+  if (!nsContentUtils::ThreadsafeIsSystemCaller(cx)) {
+    // We are only interested in chrome code.
+    return;
+  }
+
+  const auto type = JS_GetErrorType(exn);
+  if (!type) {
+    // This is not one of the primitive error types.
+    return;
+  }
+
+  switch (*type) {
+    case JSExnType::JSEXN_REFERENCEERR:
+    case JSExnType::JSEXN_SYNTAXERR:
+      break;
+    default:
+      // Not one of the errors we are interested in.
+      // Note that we are not interested in instances of `TypeError`
+      // for the time being, as DOM (ab)uses this constructor to represent
+      // all sorts of errors that are not even remotely related to type
+      // errors (e.g. some network errors).
+      // If we ever have a mechanism to differentiate between DOM-thrown
+      // and SpiderMonkey-thrown instances of `TypeError`, we should
+      // consider watching for `TypeError` here.
+      return;
+  }
+
+  // Now copy the details of the exception locally.
+  // While copying the details of an exception could be expensive, in most runs,
+  // this will be done at most once during the execution of the process, so the
+  // total cost should be reasonable.
+
+  ErrorDetails details;
+  details.mType = *type;
+  // If `exn` isn't an exception object, `ExtractErrorValues` could end up
+  // calling `toString()`, which could in turn end up throwing an error. While
+  // this should work, we want to avoid that complex use case. Fortunately, we
+  // have already checked above that `exn` is an exception object, so nothing
+  // such should happen.
+  nsContentUtils::ExtractErrorValues(cx, exn, details.mFilename, &details.mLine,
+                                     &details.mColumn, details.mMessage);
+
+  JS::UniqueChars buf =
+      JS::FormatStackDump(cx, /* showArgs = */ false, /* showLocals = */ false,
+                          /* showThisProps = */ false);
+  CopyUTF8toUTF16(mozilla::MakeStringSpan(buf.get()), details.mStack);
+
+  mThrownError.emplace(std::move(details));
+}
+
+void CycleCollectedJSRuntime::ClearRecentDevError() {
+  mErrorInterceptor.mThrownError.reset();
+}
+
+bool CycleCollectedJSRuntime::GetRecentDevError(
+    JSContext* cx, JS::MutableHandle<JS::Value> error) {
+  if (!mErrorInterceptor.mThrownError) {
+    return true;
+  }
+
+  // Create a copy of the exception.
+  JS::RootedObject obj(cx, JS_NewPlainObject(cx));
+  if (!obj) {
+    return false;
+  }
+
+  JS::RootedValue message(cx);
+  JS::RootedValue filename(cx);
+  JS::RootedValue stack(cx);
+  if (!ToJSValue(cx, mErrorInterceptor.mThrownError->mMessage, &message) ||
+      !ToJSValue(cx, mErrorInterceptor.mThrownError->mFilename, &filename) ||
+      !ToJSValue(cx, mErrorInterceptor.mThrownError->mStack, &stack)) {
+    return false;
+  }
+
+  // Build the object.
+  const auto FLAGS = JSPROP_READONLY | JSPROP_ENUMERATE | JSPROP_PERMANENT;
+  if (!JS_DefineProperty(cx, obj, "message", message, FLAGS) ||
+      !JS_DefineProperty(cx, obj, "fileName", filename, FLAGS) ||
+      !JS_DefineProperty(cx, obj, "lineNumber",
+                         mErrorInterceptor.mThrownError->mLine, FLAGS) ||
+      !JS_DefineProperty(cx, obj, "stack", stack, FLAGS)) {
+    return false;
+  }
+
+  // Pass the result.
+  error.setObject(*obj);
+  return true;
+}
+#endif  // MOZ_JS_DEV_ERROR_INTERCEPTOR
+
+#undef MOZ_JS_DEV_ERROR_INTERCEPTOR