AbstractThread.h - mozsearch

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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 XPCOM_THREADS_ABSTRACTTHREAD_H_

#define XPCOM_THREADS_ABSTRACTTHREAD_H_

#include "mozilla/AlreadyAddRefed.h"

#include "mozilla/ThreadLocal.h"

#include "nscore.h"

#include "nsISerialEventTarget.h"

#include "nsISupports.h"

class nsIEventTarget;

class nsIRunnable;

class nsIThread;

namespace mozilla {

class TaskDispatcher;

/*

 * We often want to run tasks on a target that guarantees that events will never

 * run in parallel. There are various target types that achieve this - namely

 * nsIThread and TaskQueue. Note that nsIThreadPool (which implements

 * nsIEventTarget) does not have this property, so we do not want to use

 * nsIEventTarget for this purpose. This class encapsulates the specifics of

 * the structures we might use here and provides a consistent interface.

 * At present, the supported AbstractThread implementations are TaskQueue,

 * AbstractThread::MainThread() and XPCOMThreadWrapper which can wrap any

 * nsThread.

 * The primary use of XPCOMThreadWrapper is to allow any threads to provide

 * Direct Task dispatching which is similar (but not identical to) the microtask

 * semantics of JS promises. Instantiating a XPCOMThreadWrapper on the current

 * nsThread is sufficient to enable direct task dispatching.

 * You shouldn't use pointers when comparing AbstractThread or nsIThread to

 * determine if you are currently on the thread, but instead use the

 * nsISerialEventTarget::IsOnCurrentThread() method.

*/

class AbstractThread : public nsISerialEventTarget {

 public:

  // Returns the AbstractThread that the caller is currently running in, or null

  // if the caller is not running in an AbstractThread.

  static AbstractThread* GetCurrent() { return sCurrentThreadTLS.get(); }

  AbstractThread(bool aSupportsTailDispatch)

      : mSupportsTailDispatch(aSupportsTailDispatch) {}

  // We don't use NS_DECL_NSIEVENTTARGET so that we can remove the default

  // |flags| parameter from Dispatch. Otherwise, a single-argument Dispatch call

  // would be ambiguous.

  using nsISerialEventTarget::IsOnCurrentThread;

  NS_IMETHOD_(bool) IsOnCurrentThreadInfallible(void) override;

  NS_IMETHOD IsOnCurrentThread(bool* _retval) override;

  NS_IMETHOD Dispatch(already_AddRefed<nsIRunnable> event,

                      uint32_t flags) override;

  NS_IMETHOD DispatchFromScript(nsIRunnable* event, uint32_t flags) override;

  NS_IMETHOD DelayedDispatch(already_AddRefed<nsIRunnable> event,

                             uint32_t delay) override;

  enum DispatchReason { NormalDispatch, TailDispatch };

  virtual nsresult Dispatch(already_AddRefed<nsIRunnable> aRunnable,

                            DispatchReason aReason = NormalDispatch) = 0;

  virtual bool IsCurrentThreadIn() const = 0;

  // Returns a TaskDispatcher that will dispatch its tasks when the currently-

  // running tasks pops off the stack.

//

  // May only be called when running within the it is invoked up, and only on

  // threads which support it.

  virtual TaskDispatcher& TailDispatcher() = 0;

  // Returns true if we have tail tasks scheduled, or if this isn't known.

  // Returns false if we definitely don't have any tail tasks.

  virtual bool MightHaveTailTasks() { return true; }

  // Returns true if the tail dispatcher is available. In certain edge cases

  // like shutdown, it might not be.

  virtual bool IsTailDispatcherAvailable() { return true; }

  // Helper functions for methods on the tail TasklDispatcher. These check

  // HasTailTasks to avoid allocating a TailDispatcher if it isn't

  // needed.

  nsresult TailDispatchTasksFor(AbstractThread* aThread);

  bool HasTailTasksFor(AbstractThread* aThread);

  // Returns true if this supports the tail dispatcher.

  bool SupportsTailDispatch() const { return mSupportsTailDispatch; }

  // Returns true if this thread requires all dispatches originating from

  // aThread go through the tail dispatcher.

  bool RequiresTailDispatch(AbstractThread* aThread) const;

  bool RequiresTailDispatchFromCurrentThread() const;

  virtual nsIEventTarget* AsEventTarget() { MOZ_CRASH("Not an event target!"); }

  // Returns the non-DocGroup version of AbstractThread on the main thread.

  // A DocGroup-versioned one is available in

  // DispatcherTrait::AbstractThreadFor(). Note:

  // DispatcherTrait::AbstractThreadFor() SHALL be used when possible.

  static AbstractThread* MainThread();

  // Must be called exactly once during startup.

  static void InitTLS();

  static void InitMainThread();

  static void ShutdownMainThread();

  void DispatchStateChange(already_AddRefed<nsIRunnable> aRunnable);

  static void DispatchDirectTask(already_AddRefed<nsIRunnable> aRunnable);

 protected:

  virtual ~AbstractThread() = default;

  static MOZ_THREAD_LOCAL(AbstractThread*) sCurrentThreadTLS;

  // True if we want to require that every task dispatched from tasks running in

  // this queue go through our queue's tail dispatcher.

  const bool mSupportsTailDispatch;

};

}  // namespace mozilla

#endif