/* -*- 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 <queue>

#include "mozilla/RefPtr.h"

#include "nsThreadUtils.h"

#include "nsIThreadManager.h"

#include "nsIThreadPool.h"

#include "nsISupports.h"

#include "nsISupportsImpl.h"

#include "nsCOMPtr.h"

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://dxr.mozilla.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_