SharedThreadPool.cpp

/* -*- 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/. */

#include "mozilla/SharedThreadPool.h"
#include "mozilla/Monitor.h"
#include "mozilla/ReentrantMonitor.h"
#include "mozilla/Services.h"
#include "mozilla/SpinEventLoopUntil.h"
#include "mozilla/StaticPtr.h"
#include "nsTHashMap.h"
#include "nsXPCOMCIDInternal.h"
#include "nsComponentManagerUtils.h"
#include "nsIObserver.h"
#include "nsIObserverService.h"
#include "nsIThreadManager.h"
#include "nsThreadPool.h"

namespace mozilla {

// Created and destroyed on the main thread.
static StaticAutoPtr<ReentrantMonitor> sMonitor;

// Hashtable, maps thread pool name to SharedThreadPool instance.
// Modified only on the main thread.
static StaticAutoPtr<nsTHashMap<nsCStringHashKey, SharedThreadPool*>> sPools;

static already_AddRefed<nsIThreadPool> CreateThreadPool(const nsCString& aName);

class SharedThreadPoolShutdownObserver : public nsIObserver {
 public:
  NS_DECL_ISUPPORTS
  NS_DECL_NSIOBSERVER
 protected:
  virtual ~SharedThreadPoolShutdownObserver() = default;
};

NS_IMPL_ISUPPORTS(SharedThreadPoolShutdownObserver, nsIObserver, nsISupports)

NS_IMETHODIMP
SharedThreadPoolShutdownObserver::Observe(nsISupports* aSubject,
                                          const char* aTopic,
                                          const char16_t* aData) {
  MOZ_RELEASE_ASSERT(!strcmp(aTopic, "xpcom-shutdown-threads"));
#ifdef EARLY_BETA_OR_EARLIER
  {
    ReentrantMonitorAutoEnter mon(*sMonitor);
    if (!sPools->IsEmpty()) {
      nsAutoCString str;
      for (const auto& key : sPools->Keys()) {
        str.AppendPrintf("\"%s\" ", nsAutoCString(key).get());
      }
      printf_stderr(
          "SharedThreadPool in xpcom-shutdown-threads. Waiting for "
          "pools %s\n",
          str.get());
    }
  }
#endif
  SharedThreadPool::SpinUntilEmpty();
  sMonitor = nullptr;
  sPools = nullptr;
  return NS_OK;
}

void SharedThreadPool::InitStatics() {
  MOZ_ASSERT(NS_IsMainThread());
  MOZ_ASSERT(!sMonitor && !sPools);
  sMonitor = new ReentrantMonitor("SharedThreadPool");
  sPools = new nsTHashMap<nsCStringHashKey, SharedThreadPool*>();
  nsCOMPtr<nsIObserverService> obsService =
      mozilla::services::GetObserverService();
  nsCOMPtr<nsIObserver> obs = new SharedThreadPoolShutdownObserver();
  obsService->AddObserver(obs, "xpcom-shutdown-threads", false);
}

/* static */
bool SharedThreadPool::IsEmpty() {
  ReentrantMonitorAutoEnter mon(*sMonitor);
  return !sPools->Count();
}

/* static */
void SharedThreadPool::SpinUntilEmpty() {
  MOZ_ASSERT(NS_IsMainThread());
  SpinEventLoopUntil("SharedThreadPool::SpinUntilEmpty"_ns, []() -> bool {
    sMonitor->AssertNotCurrentThreadIn();
    return IsEmpty();
  });
}

already_AddRefed<SharedThreadPool> SharedThreadPool::Get(
    const nsCString& aName, uint32_t aThreadLimit) {
  MOZ_ASSERT(sMonitor && sPools);
  ReentrantMonitorAutoEnter mon(*sMonitor);
  RefPtr<SharedThreadPool> pool;

  return sPools->WithEntryHandle(
      aName, [&](auto&& entry) -> already_AddRefed<SharedThreadPool> {
        if (entry) {
          pool = entry.Data();
          if (NS_FAILED(pool->EnsureThreadLimitIsAtLeast(aThreadLimit))) {
            NS_WARNING("Failed to set limits on thread pool");
          }
        } else {
          nsCOMPtr<nsIThreadPool> threadPool(CreateThreadPool(aName));
          if (NS_WARN_IF(!threadPool)) {
            sPools->Remove(aName);  // XXX entry.Remove()
            return nullptr;
          }
          pool = new SharedThreadPool(aName, threadPool);

          // Set the thread and idle limits. Note that we don't rely on the
          // EnsureThreadLimitIsAtLeast() call below, as the default thread
          // limit is 4, and if aThreadLimit is less than 4 we'll end up with a
          // pool with 4 threads rather than what we expected; so we'll have
          // unexpected behaviour.
          nsresult rv = pool->SetThreadLimit(aThreadLimit);
          if (NS_WARN_IF(NS_FAILED(rv))) {
            sPools->Remove(aName);  // XXX entry.Remove()
            return nullptr;
          }

          rv = pool->SetIdleThreadLimit(aThreadLimit);
          if (NS_WARN_IF(NS_FAILED(rv))) {
            sPools->Remove(aName);  // XXX entry.Remove()
            return nullptr;
          }

          entry.Insert(pool.get());
        }

        return pool.forget();
      });
}

NS_IMETHODIMP_(MozExternalRefCountType) SharedThreadPool::AddRef(void) {
  MOZ_ASSERT(sMonitor);
  ReentrantMonitorAutoEnter mon(*sMonitor);
  MOZ_ASSERT(int32_t(mRefCnt) >= 0, "illegal refcnt");
  nsrefcnt count = ++mRefCnt;
  NS_LOG_ADDREF(this, count, "SharedThreadPool", sizeof(*this));
  return count;
}

NS_IMETHODIMP_(MozExternalRefCountType) SharedThreadPool::Release(void) {
  MOZ_ASSERT(sMonitor);
  ReentrantMonitorAutoEnter mon(*sMonitor);
  nsrefcnt count = --mRefCnt;
  NS_LOG_RELEASE(this, count, "SharedThreadPool");
  if (count) {
    return count;
  }

  // Remove SharedThreadPool from table of pools.
  sPools->Remove(mName);
  MOZ_ASSERT(!sPools->Get(mName));

  // Dispatch an event to the main thread to call Shutdown() on
  // the nsIThreadPool. The Runnable here will add a refcount to the pool,
  // and when the Runnable releases the nsIThreadPool it will be deleted.
  NS_DispatchToMainThread(NewRunnableMethod("nsIThreadPool::Shutdown", mPool,
                                            &nsIThreadPool::Shutdown));

  // Stabilize refcount, so that if something in the dtor QIs, it won't explode.
  mRefCnt = 1;
  delete this;
  return 0;
}

NS_IMPL_QUERY_INTERFACE(SharedThreadPool, nsIThreadPool, nsIEventTarget)

SharedThreadPool::SharedThreadPool(const nsCString& aName, nsIThreadPool* aPool)
    : mName(aName), mPool(aPool), mRefCnt(0) {}

SharedThreadPool::~SharedThreadPool() = default;

nsresult SharedThreadPool::EnsureThreadLimitIsAtLeast(uint32_t aLimit) {
  // We limit the number of threads that we use. Note that we
  // set the thread limit to the same as the idle limit so that we're not
  // constantly creating and destroying threads (see Bug 881954). When the
  // thread pool threads shutdown they dispatch an event to the main thread
  // to call nsIThread::Shutdown(), and if we're very busy that can take a
  // while to run, and we end up with dozens of extra threads. Note that
  // threads that are idle for 60 seconds are shutdown naturally.
  uint32_t existingLimit = 0;
  nsresult rv;

  rv = mPool->GetThreadLimit(&existingLimit);
  NS_ENSURE_SUCCESS(rv, rv);
  if (aLimit > existingLimit) {
    rv = mPool->SetThreadLimit(aLimit);
    NS_ENSURE_SUCCESS(rv, rv);
  }

  rv = mPool->GetIdleThreadLimit(&existingLimit);
  NS_ENSURE_SUCCESS(rv, rv);
  if (aLimit > existingLimit) {
    rv = mPool->SetIdleThreadLimit(aLimit);
    NS_ENSURE_SUCCESS(rv, rv);
  }

  return NS_OK;
}

static already_AddRefed<nsIThreadPool> CreateThreadPool(
    const nsCString& aName) {
  nsCOMPtr<nsIThreadPool> pool = new nsThreadPool();

  nsresult rv = pool->SetName(aName);
  NS_ENSURE_SUCCESS(rv, nullptr);

  rv = pool->SetThreadStackSize(nsIThreadManager::kThreadPoolStackSize);
  NS_ENSURE_SUCCESS(rv, nullptr);

  return pool.forget();
}

}  // namespace mozilla