gecko-dev/xpcom/threads/SharedThreadPool.h

/* -*- 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 SharedThreadPool_h_
#define SharedThreadPool_h_

#include <utility>
#include "mozilla/AlreadyAddRefed.h"
#include "mozilla/RefCountType.h"
#include "nsCOMPtr.h"
#include "nsID.h"
#include "nsIEventTarget.h"
#include "nsIThreadPool.h"
#include "nsString.h"
#include "nscore.h"

class nsIRunnable;

namespace mozilla {

// Wrapper that makes an nsIThreadPool a singleton, and provides a
// consistent threadsafe interface to get instances. Callers simply get a
// SharedThreadPool by the name of its nsIThreadPool. All get requests of
// the same name get the same SharedThreadPool. Users must store a reference
// to the pool, and when the last reference to a SharedThreadPool is dropped
// the pool is shutdown and deleted. Users aren't required to manually
// shutdown the pool, and can release references on any thread. This can make
// it significantly easier to use thread pools, because the caller doesn't need
// to worry about joining and tearing it down.
//
// On Windows all threads in the pool have MSCOM initialized with
// COINIT_MULTITHREADED. Note that not all users of MSCOM use this mode see [1],
// and mixing MSCOM objects between the two is terrible for performance, and can
// cause some functions to fail. So be careful when using Win32 APIs on a
// SharedThreadPool, and avoid sharing objects if at all possible.
//
// [1]
// https://searchfox.org/mozilla-central/search?q=coinitialize&redirect=false
class SharedThreadPool : public nsIThreadPool {
 public:
  // Gets (possibly creating) the shared thread pool singleton instance with
  // thread pool named aName.
  static already_AddRefed<SharedThreadPool> Get(const nsCString& aName,
                                                uint32_t aThreadLimit = 4);

  // We implement custom threadsafe AddRef/Release pair, that destroys the
  // the shared pool singleton when the refcount drops to 0. The addref/release
  // are implemented using locking, so it's not recommended that you use them
  // in a tight loop.
  NS_IMETHOD QueryInterface(REFNSIID aIID, void** aInstancePtr) override;
  NS_IMETHOD_(MozExternalRefCountType) AddRef(void) override;
  NS_IMETHOD_(MozExternalRefCountType) Release(void) override;

  // Forward behaviour to wrapped thread pool implementation.
  NS_FORWARD_SAFE_NSITHREADPOOL(mPool);

  // Call this when dispatching from an event on the same
  // threadpool that is about to complete. We should not create a new thread
  // in that case since a thread is about to become idle.
  nsresult DispatchFromEndOfTaskInThisPool(nsIRunnable* event) {
    return Dispatch(event, NS_DISPATCH_AT_END);
  }

  NS_IMETHOD DispatchFromScript(nsIRunnable* event, uint32_t flags) override {
    return Dispatch(event, flags);
  }

  NS_IMETHOD Dispatch(already_AddRefed<nsIRunnable> event,
                      uint32_t flags = NS_DISPATCH_NORMAL) override {
    return !mEventTarget ? NS_ERROR_NULL_POINTER
                         : mEventTarget->Dispatch(std::move(event), flags);
  }

  NS_IMETHOD DelayedDispatch(already_AddRefed<nsIRunnable>, uint32_t) override {
    return NS_ERROR_NOT_IMPLEMENTED;
  }

  using nsIEventTarget::Dispatch;

  NS_IMETHOD IsOnCurrentThread(bool* _retval) override {
    return !mEventTarget ? NS_ERROR_NULL_POINTER
                         : mEventTarget->IsOnCurrentThread(_retval);
  }

  NS_IMETHOD_(bool) IsOnCurrentThreadInfallible() override {
    return mEventTarget && mEventTarget->IsOnCurrentThread();
  }

  // Creates necessary statics. Called once at startup.
  static void InitStatics();

  // Spins the event loop until all thread pools are shutdown.
  // *Must* be called on the main thread.
  static void SpinUntilEmpty();

 private:
  // Returns whether there are no pools in existence at the moment.
  static bool IsEmpty();

  // Creates a singleton SharedThreadPool wrapper around aPool.
  // aName is the name of the aPool, and is used to lookup the
  // SharedThreadPool in the hash table of all created pools.
  SharedThreadPool(const nsCString& aName, nsIThreadPool* aPool);
  virtual ~SharedThreadPool();

  nsresult EnsureThreadLimitIsAtLeast(uint32_t aThreadLimit);

  // Name of mPool.
  const nsCString mName;

  // Thread pool being wrapped.
  nsCOMPtr<nsIThreadPool> mPool;

  // Refcount. We implement custom ref counting so that the thread pool is
  // shutdown in a threadsafe manner and singletonness is preserved.
  nsrefcnt mRefCnt;

  // mPool QI'd to nsIEventTarget. We cache this, so that we can use
  // NS_FORWARD_SAFE_NSIEVENTTARGET above.
  nsCOMPtr<nsIEventTarget> mEventTarget;
};

}  // namespace mozilla

#endif  // SharedThreadPool_h_