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diff --git a/xpcom/base/CycleCollectedJSContext.h b/xpcom/base/CycleCollectedJSContext.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_CycleCollectedJSContext_h
+#define mozilla_CycleCollectedJSContext_h
+
+#include <queue>
+
+#include "mozilla/Attributes.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/dom/AtomList.h"
+#include "mozilla/dom/Promise.h"
+#include "js/GCVector.h"
+#include "js/Promise.h"
+
+#include "nsCOMPtr.h"
+#include "nsRefPtrHashtable.h"
+#include "nsTArray.h"
+
+class nsCycleCollectionNoteRootCallback;
+class nsIRunnable;
+class nsThread;
+
+namespace mozilla {
+class AutoSlowOperation;
+
+class CycleCollectedJSContext;
+class CycleCollectedJSRuntime;
+
+namespace dom {
+class Exception;
+class WorkerJSContext;
+class WorkletJSContext;
+}  // namespace dom
+
+// Contains various stats about the cycle collection.
+struct CycleCollectorResults {
+  CycleCollectorResults() {
+    // Initialize here so when we increment mNumSlices the first time we're
+    // not using uninitialized memory.
+    Init();
+  }
+
+  void Init() {
+    mForcedGC = false;
+    mMergedZones = false;
+    mAnyManual = false;
+    mVisitedRefCounted = 0;
+    mVisitedGCed = 0;
+    mFreedRefCounted = 0;
+    mFreedGCed = 0;
+    mFreedJSZones = 0;
+    mNumSlices = 1;
+    // mNumSlices is initialized to one, because we call Init() after the
+    // per-slice increment of mNumSlices has already occurred.
+  }
+
+  bool mForcedGC;
+  bool mMergedZones;
+  // mAnyManual is true if any slice was manually triggered, and at shutdown.
+  bool mAnyManual;
+  uint32_t mVisitedRefCounted;
+  uint32_t mVisitedGCed;
+  uint32_t mFreedRefCounted;
+  uint32_t mFreedGCed;
+  uint32_t mFreedJSZones;
+  uint32_t mNumSlices;
+};
+
+class MicroTaskRunnable {
+ public:
+  MicroTaskRunnable() = default;
+  NS_INLINE_DECL_REFCOUNTING(MicroTaskRunnable)
+  MOZ_CAN_RUN_SCRIPT virtual void Run(AutoSlowOperation& aAso) = 0;
+  virtual bool Suppressed() { return false; }
+
+ protected:
+  virtual ~MicroTaskRunnable() = default;
+};
+
+// Support for JS FinalizationRegistry objects, which allow a JS callback to be
+// registered that is called when objects die.
+//
+// We keep a vector of functions that call back into the JS engine along
+// with their associated incumbent globals, one per FinalizationRegistry object
+// that has pending cleanup work. These are run in their own task.
+class FinalizationRegistryCleanup {
+ public:
+  explicit FinalizationRegistryCleanup(CycleCollectedJSContext* aContext);
+  void Init();
+  void Destroy();
+  void QueueCallback(JSFunction* aDoCleanup, JSObject* aIncumbentGlobal);
+  MOZ_CAN_RUN_SCRIPT void DoCleanup();
+
+ private:
+  static void QueueCallback(JSFunction* aDoCleanup, JSObject* aIncumbentGlobal,
+                            void* aData);
+
+  class CleanupRunnable;
+
+  struct Callback {
+    JSFunction* mCallbackFunction;
+    JSObject* mIncumbentGlobal;
+    void trace(JSTracer* trc);
+  };
+
+  // This object is part of CycleCollectedJSContext, so it's safe to have a raw
+  // pointer to its containing context here.
+  CycleCollectedJSContext* mContext;
+
+  using CallbackVector = JS::GCVector<Callback, 0, InfallibleAllocPolicy>;
+  JS::PersistentRooted<CallbackVector> mCallbacks;
+};
+
+class CycleCollectedJSContext : dom::PerThreadAtomCache, private JS::JobQueue {
+  friend class CycleCollectedJSRuntime;
+
+ protected:
+  CycleCollectedJSContext();
+  virtual ~CycleCollectedJSContext();
+
+  MOZ_IS_CLASS_INIT
+  nsresult Initialize(JSRuntime* aParentRuntime, uint32_t aMaxBytes);
+
+  virtual CycleCollectedJSRuntime* CreateRuntime(JSContext* aCx) = 0;
+
+  size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
+
+ private:
+  static JSObject* GetIncumbentGlobalCallback(JSContext* aCx);
+  static bool EnqueuePromiseJobCallback(JSContext* aCx,
+                                        JS::HandleObject aPromise,
+                                        JS::HandleObject aJob,
+                                        JS::HandleObject aAllocationSite,
+                                        JS::HandleObject aIncumbentGlobal,
+                                        void* aData);
+  static void PromiseRejectionTrackerCallback(
+      JSContext* aCx, bool aMutedErrors, JS::HandleObject aPromise,
+      JS::PromiseRejectionHandlingState state, void* aData);
+
+  void AfterProcessMicrotasks();
+
+ public:
+  void ProcessStableStateQueue();
+
+ private:
+  void CleanupIDBTransactions(uint32_t aRecursionDepth);
+
+ public:
+  enum DeferredFinalizeType {
+    FinalizeIncrementally,
+    FinalizeNow,
+  };
+
+  virtual dom::WorkerJSContext* GetAsWorkerJSContext() { return nullptr; }
+  virtual dom::WorkletJSContext* GetAsWorkletJSContext() { return nullptr; }
+
+  CycleCollectedJSRuntime* Runtime() const {
+    MOZ_ASSERT(mRuntime);
+    return mRuntime;
+  }
+
+  already_AddRefed<dom::Exception> GetPendingException() const;
+  void SetPendingException(dom::Exception* aException);
+
+  std::queue<RefPtr<MicroTaskRunnable>>& GetMicroTaskQueue();
+  std::queue<RefPtr<MicroTaskRunnable>>& GetDebuggerMicroTaskQueue();
+
+  JSContext* Context() const {
+    MOZ_ASSERT(mJSContext);
+    return mJSContext;
+  }
+
+  JS::RootingContext* RootingCx() const {
+    MOZ_ASSERT(mJSContext);
+    return JS::RootingContext::get(mJSContext);
+  }
+
+  void SetTargetedMicroTaskRecursionDepth(uint32_t aDepth) {
+    mTargetedMicroTaskRecursionDepth = aDepth;
+  }
+
+ protected:
+  JSContext* MaybeContext() const { return mJSContext; }
+
+ public:
+  // nsThread entrypoints
+  //
+  // MOZ_CAN_RUN_SCRIPT_BOUNDARY so we don't need to annotate
+  // nsThread::ProcessNextEvent and all its callers MOZ_CAN_RUN_SCRIPT for now.
+  // But we really should!
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY
+  virtual void BeforeProcessTask(bool aMightBlock);
+  // MOZ_CAN_RUN_SCRIPT_BOUNDARY so we don't need to annotate
+  // nsThread::ProcessNextEvent and all its callers MOZ_CAN_RUN_SCRIPT for now.
+  // But we really should!
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY
+  virtual void AfterProcessTask(uint32_t aRecursionDepth);
+
+  // Check whether we need an idle GC task.
+  void IsIdleGCTaskNeeded() const;
+
+  uint32_t RecursionDepth() const;
+
+  // Run in stable state (call through nsContentUtils)
+  void RunInStableState(already_AddRefed<nsIRunnable>&& aRunnable);
+
+  void AddPendingIDBTransaction(already_AddRefed<nsIRunnable>&& aTransaction);
+
+  // Get the CycleCollectedJSContext for a JSContext.
+  // Returns null only if Initialize() has not completed on or during
+  // destruction of the CycleCollectedJSContext.
+  static CycleCollectedJSContext* GetFor(JSContext* aCx);
+
+  // Get the current thread's CycleCollectedJSContext.  Returns null if there
+  // isn't one.
+  static CycleCollectedJSContext* Get();
+
+  // Queue an async microtask to the current main or worker thread.
+  virtual void DispatchToMicroTask(
+      already_AddRefed<MicroTaskRunnable> aRunnable);
+
+  // Call EnterMicroTask when you're entering JS execution.
+  // Usually the best way to do this is to use nsAutoMicroTask.
+  void EnterMicroTask() { ++mMicroTaskLevel; }
+
+  MOZ_CAN_RUN_SCRIPT
+  void LeaveMicroTask() {
+    if (--mMicroTaskLevel == 0) {
+      PerformMicroTaskCheckPoint();
+    }
+  }
+
+  bool IsInMicroTask() const { return mMicroTaskLevel != 0; }
+
+  uint32_t MicroTaskLevel() const { return mMicroTaskLevel; }
+
+  void SetMicroTaskLevel(uint32_t aLevel) { mMicroTaskLevel = aLevel; }
+
+  MOZ_CAN_RUN_SCRIPT
+  bool PerformMicroTaskCheckPoint(bool aForce = false);
+
+  MOZ_CAN_RUN_SCRIPT
+  void PerformDebuggerMicroTaskCheckpoint();
+
+  bool IsInStableOrMetaStableState() const { return mDoingStableStates; }
+
+  // Storage for watching rejected promises waiting for some client to
+  // consume their rejection.
+  // Promises in this list have been rejected in the last turn of the
+  // event loop without the rejection being handled.
+  // Note that this can contain nullptrs in place of promises removed because
+  // they're consumed before it'd be reported.
+  JS::PersistentRooted<JS::GCVector<JSObject*, 0, js::SystemAllocPolicy>>
+      mUncaughtRejections;
+
+  // Promises in this list have previously been reported as rejected
+  // (because they were in the above list), but the rejection was handled
+  // in the last turn of the event loop.
+  JS::PersistentRooted<JS::GCVector<JSObject*, 0, js::SystemAllocPolicy>>
+      mConsumedRejections;
+  nsTArray<nsCOMPtr<nsISupports /* UncaughtRejectionObserver */>>
+      mUncaughtRejectionObservers;
+
+  virtual bool IsSystemCaller() const = 0;
+
+  // Unused on main thread.  Used by AutoJSAPI on Worker and Worklet threads.
+  virtual void ReportError(JSErrorReport* aReport,
+                           JS::ConstUTF8CharsZ aToStringResult) {
+    MOZ_ASSERT_UNREACHABLE("Not supported");
+  }
+
+ private:
+  // JS::JobQueue implementation: see js/public/Promise.h.
+  // SpiderMonkey uses some of these methods to enqueue promise resolution jobs.
+  // Others protect the debuggee microtask queue from the debugger's
+  // interruptions; see the comments on JS::AutoDebuggerJobQueueInterruption for
+  // details.
+  JSObject* getIncumbentGlobal(JSContext* cx) override;
+  bool enqueuePromiseJob(JSContext* cx, JS::HandleObject promise,
+                         JS::HandleObject job, JS::HandleObject allocationSite,
+                         JS::HandleObject incumbentGlobal) override;
+  // MOZ_CAN_RUN_SCRIPT_BOUNDARY for now so we don't have to change SpiderMonkey
+  // headers.  The caller presumably knows this can run script (like everything
+  // in SpiderMonkey!) and will deal.
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY
+  void runJobs(JSContext* cx) override;
+  bool empty() const override;
+  class SavedMicroTaskQueue;
+  js::UniquePtr<SavedJobQueue> saveJobQueue(JSContext*) override;
+
+ private:
+  CycleCollectedJSRuntime* mRuntime;
+
+  JSContext* mJSContext;
+
+  nsCOMPtr<dom::Exception> mPendingException;
+  nsThread* mOwningThread;  // Manual refcounting to avoid include hell.
+
+  struct PendingIDBTransactionData {
+    nsCOMPtr<nsIRunnable> mTransaction;
+    uint32_t mRecursionDepth;
+  };
+
+  nsTArray<nsCOMPtr<nsIRunnable>> mStableStateEvents;
+  nsTArray<PendingIDBTransactionData> mPendingIDBTransactions;
+  uint32_t mBaseRecursionDepth;
+  bool mDoingStableStates;
+
+  // If set to none 0, microtasks will be processed only when recursion depth
+  // is the set value.
+  uint32_t mTargetedMicroTaskRecursionDepth;
+
+  uint32_t mMicroTaskLevel;
+
+  std::queue<RefPtr<MicroTaskRunnable>> mPendingMicroTaskRunnables;
+  std::queue<RefPtr<MicroTaskRunnable>> mDebuggerMicroTaskQueue;
+
+  // How many times the debugger has interrupted execution, possibly creating
+  // microtask checkpoints in places that they would not normally occur.
+  uint32_t mDebuggerRecursionDepth;
+
+  uint32_t mMicroTaskRecursionDepth;
+
+  // This implements about-to-be-notified rejected promises list in the spec.
+  // https://html.spec.whatwg.org/multipage/webappapis.html#about-to-be-notified-rejected-promises-list
+  typedef nsTArray<RefPtr<dom::Promise>> PromiseArray;
+  PromiseArray mAboutToBeNotifiedRejectedPromises;
+
+  // This is for the "outstanding rejected promises weak set" in the spec,
+  // https://html.spec.whatwg.org/multipage/webappapis.html#outstanding-rejected-promises-weak-set
+  // We use different data structure and opposite logic here to achieve the same
+  // effect. Basically this is used for tracking the rejected promise that does
+  // NOT need firing a rejectionhandled event. We will check the table to see if
+  // firing rejectionhandled event is required when a rejected promise is being
+  // handled.
+  //
+  // The rejected promise will be stored in the table if
+  // - it is unhandled, and
+  // - The unhandledrejection is not yet fired.
+  //
+  // And be removed when
+  // - it is handled, or
+  // - A unhandledrejection is fired and it isn't being handled in event
+  // handler.
+  typedef nsRefPtrHashtable<nsUint64HashKey, dom::Promise> PromiseHashtable;
+  PromiseHashtable mPendingUnhandledRejections;
+
+  class NotifyUnhandledRejections final : public CancelableRunnable {
+   public:
+    NotifyUnhandledRejections(CycleCollectedJSContext* aCx,
+                              PromiseArray&& aPromises)
+        : CancelableRunnable("NotifyUnhandledRejections"),
+          mCx(aCx),
+          mUnhandledRejections(std::move(aPromises)) {}
+
+    NS_IMETHOD Run() final;
+
+    nsresult Cancel() final;
+
+   private:
+    CycleCollectedJSContext* mCx;
+    PromiseArray mUnhandledRejections;
+  };
+
+  FinalizationRegistryCleanup mFinalizationRegistryCleanup;
+};
+
+class MOZ_STACK_CLASS nsAutoMicroTask {
+ public:
+  nsAutoMicroTask() {
+    CycleCollectedJSContext* ccjs = CycleCollectedJSContext::Get();
+    if (ccjs) {
+      ccjs->EnterMicroTask();
+    }
+  }
+  MOZ_CAN_RUN_SCRIPT ~nsAutoMicroTask() {
+    CycleCollectedJSContext* ccjs = CycleCollectedJSContext::Get();
+    if (ccjs) {
+      ccjs->LeaveMicroTask();
+    }
+  }
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_CycleCollectedJSContext_h