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diff --git a/dom/workers/WorkerRunnable.h b/dom/workers/WorkerRunnable.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_dom_workers_workerrunnable_h__
+#define mozilla_dom_workers_workerrunnable_h__
+
+#include "Workers.h"
+
+#include "nsICancelableRunnable.h"
+
+#include "mozilla/Atomics.h"
+#include "nsISupportsImpl.h"
+#include "nsThreadUtils.h" /* nsRunnable */
+
+struct JSContext;
+class nsIEventTarget;
+
+namespace mozilla {
+class ErrorResult;
+} // namespace mozilla
+
+BEGIN_WORKERS_NAMESPACE
+
+class WorkerPrivate;
+
+// Use this runnable to communicate from the worker to its parent or vice-versa.
+// The busy count must be taken into consideration and declared at construction
+// time.
+class WorkerRunnable : public nsIRunnable,
+                       public nsICancelableRunnable
+{
+public:
+  enum TargetAndBusyBehavior {
+    // Target the main thread for top-level workers, otherwise target the
+    // WorkerThread of the worker's parent. No change to the busy count.
+    ParentThreadUnchangedBusyCount,
+
+    // Target the thread where the worker event loop runs. The busy count will
+    // be incremented before dispatching and decremented (asynchronously) after
+    // running.
+    WorkerThreadModifyBusyCount,
+
+    // Target the thread where the worker event loop runs. The busy count will
+    // not be modified in any way. Besides worker-internal runnables this is
+    // almost always the wrong choice.
+    WorkerThreadUnchangedBusyCount
+  };
+
+protected:
+  // The WorkerPrivate that this runnable is associated with.
+  WorkerPrivate* mWorkerPrivate;
+
+  // See above.
+  TargetAndBusyBehavior mBehavior;
+
+  // It's unclear whether or not Cancel() is supposed to work when called on any
+  // thread. To be safe we're using an atomic but it's likely overkill.
+  Atomic<uint32_t> mCanceled;
+
+private:
+  // Whether or not Cancel() is currently being called from inside the Run()
+  // method. Avoids infinite recursion when a subclass calls Run() from inside
+  // Cancel(). Only checked and modified on the target thread.
+  bool mCallingCancelWithinRun;
+
+public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+
+  // If you override Cancel() then you'll need to either call the base class
+  // Cancel() method or override IsCanceled() so that the Run() method bails out
+  // appropriately.
+  nsresult
+  Cancel() override;
+
+  // The return value is true if and only if both PreDispatch and
+  // DispatchInternal return true.
+  bool
+  Dispatch();
+
+  // See above note about Cancel().
+  virtual bool
+  IsCanceled() const
+  {
+    return mCanceled != 0;
+  }
+
+  static WorkerRunnable*
+  FromRunnable(nsIRunnable* aRunnable);
+
+protected:
+  WorkerRunnable(WorkerPrivate* aWorkerPrivate,
+                 TargetAndBusyBehavior aBehavior = WorkerThreadModifyBusyCount)
+#ifdef DEBUG
+  ;
+#else
+  : mWorkerPrivate(aWorkerPrivate), mBehavior(aBehavior), mCanceled(0),
+    mCallingCancelWithinRun(false)
+  { }
+#endif
+
+  // This class is reference counted.
+  virtual ~WorkerRunnable()
+  { }
+
+  // Returns true if this runnable should be dispatched to the debugger queue,
+  // and false otherwise.
+  virtual bool
+  IsDebuggerRunnable() const;
+
+  nsIGlobalObject*
+  DefaultGlobalObject() const;
+
+  // By default asserts that Dispatch() is being called on the right thread
+  // (ParentThread if |mTarget| is WorkerThread, or WorkerThread otherwise).
+  // Also increments the busy count of |mWorkerPrivate| if targeting the
+  // WorkerThread.
+  virtual bool
+  PreDispatch(WorkerPrivate* aWorkerPrivate);
+
+  // By default asserts that Dispatch() is being called on the right thread
+  // (ParentThread if |mTarget| is WorkerThread, or WorkerThread otherwise).
+  virtual void
+  PostDispatch(WorkerPrivate* aWorkerPrivate, bool aDispatchResult);
+
+  // May be implemented by subclasses if desired if they need to do some sort of
+  // setup before we try to set up our JSContext and compartment for real.
+  // Typically the only thing that should go in here is creation of the worker's
+  // global.
+  //
+  // If false is returned, WorkerRun will not be called at all.  PostRun will
+  // still be called, with false passed for aRunResult.
+  virtual bool
+  PreRun(WorkerPrivate* aWorkerPrivate);
+
+  // Must be implemented by subclasses. Called on the target thread.  The return
+  // value will be passed to PostRun().  The JSContext passed in here comes from
+  // an AutoJSAPI (or AutoEntryScript) that we set up on the stack.  If
+  // mBehavior is ParentThreadUnchangedBusyCount, it is in the compartment of
+  // mWorkerPrivate's reflector (i.e. the worker object in the parent thread),
+  // unless that reflector is null, in which case it's in the compartment of the
+  // parent global (which is the compartment reflector would have been in), or
+  // in the null compartment if there is no parent global.  For other mBehavior
+  // values, we're running on the worker thread and aCx is in whatever
+  // compartment GetCurrentThreadJSContext() was in when nsIRunnable::Run() got
+  // called.  This is actually important for cases when a runnable spins a
+  // syncloop and wants everything that happens during the syncloop to happen in
+  // the compartment that runnable set up (which may, for example, be a debugger
+  // sandbox compartment!).  If aCx wasn't in a compartment to start with, aCx
+  // will be in either the debugger global's compartment or the worker's
+  // global's compartment depending on whether IsDebuggerRunnable() is true.
+  //
+  // Immediately after WorkerRun returns, the caller will assert that either it
+  // returns false or there is no exception pending on aCx.  Then it will report
+  // any pending exceptions on aCx.
+  virtual bool
+  WorkerRun(JSContext* aCx, WorkerPrivate* aWorkerPrivate) = 0;
+
+  // By default asserts that Run() (and WorkerRun()) were called on the correct
+  // thread.  Also sends an asynchronous message to the ParentThread if the
+  // busy count was previously modified in PreDispatch().
+  //
+  // The aCx passed here is the same one as was passed to WorkerRun and is
+  // still in the same compartment.  PostRun implementations must NOT leave an
+  // exception on the JSContext and must not run script, because the incoming
+  // JSContext may be in the null compartment.
+  virtual void
+  PostRun(JSContext* aCx, WorkerPrivate* aWorkerPrivate, bool aRunResult);
+
+  virtual bool
+  DispatchInternal();
+
+  // Calling Run() directly is not supported. Just call Dispatch() and
+  // WorkerRun() will be called on the correct thread automatically.
+  NS_DECL_NSIRUNNABLE
+};
+
+// This runnable is used to send a message to a worker debugger.
+class WorkerDebuggerRunnable : public WorkerRunnable
+{
+protected:
+  explicit WorkerDebuggerRunnable(WorkerPrivate* aWorkerPrivate)
+  : WorkerRunnable(aWorkerPrivate, WorkerThreadUnchangedBusyCount)
+  {
+  }
+
+  virtual ~WorkerDebuggerRunnable()
+  { }
+
+private:
+  virtual bool
+  IsDebuggerRunnable() const override
+  {
+    return true;
+  }
+
+  virtual bool
+  PreDispatch(WorkerPrivate* aWorkerPrivate) override final
+  {
+    AssertIsOnMainThread();
+
+    return true;
+  }
+
+  virtual void
+  PostDispatch(WorkerPrivate* aWorkerPrivate, bool aDispatchResult) override;
+};
+
+// This runnable is used to send a message directly to a worker's sync loop.
+class WorkerSyncRunnable : public WorkerRunnable
+{
+protected:
+  nsCOMPtr<nsIEventTarget> mSyncLoopTarget;
+
+  // Passing null for aSyncLoopTarget is allowed and will result in the behavior
+  // of a normal WorkerRunnable.
+  WorkerSyncRunnable(WorkerPrivate* aWorkerPrivate,
+                     nsIEventTarget* aSyncLoopTarget);
+
+  WorkerSyncRunnable(WorkerPrivate* aWorkerPrivate,
+                     already_AddRefed<nsIEventTarget>&& aSyncLoopTarget);
+
+  virtual ~WorkerSyncRunnable();
+
+  virtual bool
+  DispatchInternal() override;
+};
+
+// This runnable is identical to WorkerSyncRunnable except it is meant to be
+// created on and dispatched from the main thread only.  Its WorkerRun/PostRun
+// will run on the worker thread.
+class MainThreadWorkerSyncRunnable : public WorkerSyncRunnable
+{
+protected:
+  // Passing null for aSyncLoopTarget is allowed and will result in the behavior
+  // of a normal WorkerRunnable.
+  MainThreadWorkerSyncRunnable(WorkerPrivate* aWorkerPrivate,
+                               nsIEventTarget* aSyncLoopTarget)
+  : WorkerSyncRunnable(aWorkerPrivate, aSyncLoopTarget)
+  {
+    AssertIsOnMainThread();
+  }
+
+  MainThreadWorkerSyncRunnable(WorkerPrivate* aWorkerPrivate,
+                               already_AddRefed<nsIEventTarget>&& aSyncLoopTarget)
+  : WorkerSyncRunnable(aWorkerPrivate, Move(aSyncLoopTarget))
+  {
+    AssertIsOnMainThread();
+  }
+
+  virtual ~MainThreadWorkerSyncRunnable()
+  { }
+
+private:
+  virtual bool
+  PreDispatch(WorkerPrivate* aWorkerPrivate) override
+  {
+    AssertIsOnMainThread();
+    return true;
+  }
+
+  virtual void
+  PostDispatch(WorkerPrivate* aWorkerPrivate, bool aDispatchResult) override;
+};
+
+// This runnable is processed as soon as it is received by the worker,
+// potentially running before previously queued runnables and perhaps even with
+// other JS code executing on the stack. These runnables must not alter the
+// state of the JS runtime and should only twiddle state values. The busy count
+// is never modified.
+class WorkerControlRunnable : public WorkerRunnable
+{
+  friend class WorkerPrivate;
+
+protected:
+  WorkerControlRunnable(WorkerPrivate* aWorkerPrivate,
+                        TargetAndBusyBehavior aBehavior = WorkerThreadModifyBusyCount)
+#ifdef DEBUG
+  ;
+#else
+  : WorkerRunnable(aWorkerPrivate, aBehavior)
+  { }
+#endif
+
+  virtual ~WorkerControlRunnable()
+  { }
+
+  nsresult
+  Cancel() override;
+
+public:
+  NS_DECL_ISUPPORTS_INHERITED
+
+private:
+  virtual bool
+  DispatchInternal() override;
+
+  // Should only be called by WorkerPrivate::DoRunLoop.
+  using WorkerRunnable::Cancel;
+};
+
+// A convenience class for WorkerRunnables that are originated on the main
+// thread.
+class MainThreadWorkerRunnable : public WorkerRunnable
+{
+protected:
+  explicit MainThreadWorkerRunnable(WorkerPrivate* aWorkerPrivate)
+  : WorkerRunnable(aWorkerPrivate, WorkerThreadUnchangedBusyCount)
+  {
+    AssertIsOnMainThread();
+  }
+
+  virtual ~MainThreadWorkerRunnable()
+  {}
+
+  virtual bool
+  PreDispatch(WorkerPrivate* aWorkerPrivate) override
+  {
+    AssertIsOnMainThread();
+    return true;
+  }
+
+  virtual void
+  PostDispatch(WorkerPrivate* aWorkerPrivate,
+               bool aDispatchResult) override
+  {
+    AssertIsOnMainThread();
+  }
+};
+
+// A convenience class for WorkerControlRunnables that originate on the main
+// thread.
+class MainThreadWorkerControlRunnable : public WorkerControlRunnable
+{
+protected:
+  explicit MainThreadWorkerControlRunnable(WorkerPrivate* aWorkerPrivate)
+  : WorkerControlRunnable(aWorkerPrivate, WorkerThreadUnchangedBusyCount)
+  { }
+
+  virtual ~MainThreadWorkerControlRunnable()
+  { }
+
+  virtual bool
+  PreDispatch(WorkerPrivate* aWorkerPrivate) override
+  {
+    AssertIsOnMainThread();
+    return true;
+  }
+
+  virtual void
+  PostDispatch(WorkerPrivate* aWorkerPrivate, bool aDispatchResult) override
+  {
+    AssertIsOnMainThread();
+  }
+};
+
+// A WorkerRunnable that should be dispatched from the worker to itself for
+// async tasks. This will increment the busy count PostDispatch() (only if
+// dispatch was successful) and decrement it in PostRun().
+//
+// Async tasks will almost always want to use this since
+// a WorkerSameThreadRunnable keeps the Worker from being GCed.
+class WorkerSameThreadRunnable : public WorkerRunnable
+{
+protected:
+  explicit WorkerSameThreadRunnable(WorkerPrivate* aWorkerPrivate)
+  : WorkerRunnable(aWorkerPrivate, WorkerThreadModifyBusyCount)
+  { }
+
+  virtual ~WorkerSameThreadRunnable()
+  { }
+
+  virtual bool
+  PreDispatch(WorkerPrivate* aWorkerPrivate) override;
+
+  virtual void
+  PostDispatch(WorkerPrivate* aWorkerPrivate, bool aDispatchResult) override;
+
+  // We just delegate PostRun to WorkerRunnable, since it does exactly
+  // what we want.
+};
+
+// Base class for the runnable objects, which makes a synchronous call to
+// dispatch the tasks from the worker thread to the main thread.
+//
+// Note that the derived class must override MainThreadRun.
+class WorkerMainThreadRunnable : public Runnable
+{
+protected:
+  WorkerPrivate* mWorkerPrivate;
+  nsCOMPtr<nsIEventTarget> mSyncLoopTarget;
+  const nsCString mTelemetryKey;
+
+  explicit WorkerMainThreadRunnable(WorkerPrivate* aWorkerPrivate,
+                                    const nsACString& aTelemetryKey);
+  ~WorkerMainThreadRunnable() {}
+
+  virtual bool MainThreadRun() = 0;
+
+public:
+  // Dispatch the runnable to the main thread.  If dispatch to main thread
+  // fails, or if the worker is shut down while dispatching, an error will be
+  // reported on aRv.  In that case the error MUST be propagated out to script.
+  void Dispatch(ErrorResult& aRv);
+
+private:
+  NS_IMETHOD Run() override;
+};
+
+// This runnable is an helper class for dispatching something from a worker
+// thread to the main-thread and back to the worker-thread. During this
+// operation, this class will keep the worker alive.
+class WorkerProxyToMainThreadRunnable : public Runnable
+{
+protected:
+  explicit WorkerProxyToMainThreadRunnable(WorkerPrivate* aWorkerPrivate);
+
+  virtual ~WorkerProxyToMainThreadRunnable();
+
+  // First this method is called on the main-thread.
+  virtual void RunOnMainThread() = 0;
+
+  // After this second method is called on the worker-thread.
+  virtual void RunBackOnWorkerThread() = 0;
+
+public:
+  bool Dispatch();
+
+private:
+  NS_IMETHOD Run() override;
+
+  void PostDispatchOnMainThread();
+
+  bool HoldWorker();
+  void ReleaseWorker();
+
+protected:
+  WorkerPrivate* mWorkerPrivate;
+  UniquePtr<WorkerHolder> mWorkerHolder;
+};
+
+// Class for checking API exposure.  This totally violates the "MUST" in the
+// comments on WorkerMainThreadRunnable::Dispatch, because API exposure checks
+// can't throw.  Maybe we should change it so they _could_ throw.  But for now
+// we are bad people and should be ashamed of ourselves.  Let's hope none of
+// them happen while a worker is shutting down.
+//
+// Do NOT copy what this class is doing elsewhere.  Just don't.
+class WorkerCheckAPIExposureOnMainThreadRunnable
+  : public WorkerMainThreadRunnable
+{
+public:
+  explicit
+  WorkerCheckAPIExposureOnMainThreadRunnable(WorkerPrivate* aWorkerPrivate);
+  virtual
+  ~WorkerCheckAPIExposureOnMainThreadRunnable();
+
+  // Returns whether the dispatch succeeded.  If this returns false, the API
+  // should not be exposed.
+  bool Dispatch();
+};
+
+// This runnable is used to stop a sync loop and it's meant to be used on the
+// main-thread only. As sync loops keep the busy count incremented as long as
+// they run this runnable does not modify the busy count
+// in any way.
+class MainThreadStopSyncLoopRunnable : public WorkerSyncRunnable
+{
+  bool mResult;
+
+public:
+  // Passing null for aSyncLoopTarget is not allowed.
+  MainThreadStopSyncLoopRunnable(
+                               WorkerPrivate* aWorkerPrivate,
+                               already_AddRefed<nsIEventTarget>&& aSyncLoopTarget,
+                               bool aResult);
+
+  // By default StopSyncLoopRunnables cannot be canceled since they could leave
+  // a sync loop spinning forever.
+  nsresult
+  Cancel() override;
+
+protected:
+  virtual ~MainThreadStopSyncLoopRunnable()
+  { }
+
+private:
+  virtual bool
+  PreDispatch(WorkerPrivate* aWorkerPrivate) override final
+  {
+    AssertIsOnMainThread();
+    return true;
+  }
+
+  virtual void
+  PostDispatch(WorkerPrivate* aWorkerPrivate, bool aDispatchResult) override;
+
+  virtual bool
+  WorkerRun(JSContext* aCx, WorkerPrivate* aWorkerPrivate) override;
+
+  virtual bool
+  DispatchInternal() override final;
+};
+
+END_WORKERS_NAMESPACE
+
+#endif // mozilla_dom_workers_workerrunnable_h__