xpcom/threads/BackgroundHangMonitor.cpp
author Alessio Placitelli <alessio.placitelli@gmail.com>
Fri, 16 Oct 2015 07:04:00 +0200
changeset 296722 59e6a978773f8224f77f612947f79e94837e85a8
parent 264552 135c75e6fa1eb65189ebbebddffc5654b4d67c38
child 296724 6cabb1a43af6376dcef8757584f0e214298b58d1
permissions -rw-r--r--
Bug 1211411 - Limit the number of thread hang stats reported to Telemetry. r=vladan a=lizzard

/* -*- 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/ArrayUtils.h"
#include "mozilla/BackgroundHangMonitor.h"
#include "mozilla/LinkedList.h"
#include "mozilla/Monitor.h"
#include "mozilla/Move.h"
#include "mozilla/Preferences.h"
#include "mozilla/StaticPtr.h"
#include "mozilla/Telemetry.h"
#include "mozilla/ThreadHangStats.h"
#include "mozilla/ThreadLocal.h"
#ifdef MOZ_NUWA_PROCESS
#include "ipc/Nuwa.h"
#endif

#include "prinrval.h"
#include "prthread.h"
#include "ThreadStackHelper.h"
#include "nsIObserverService.h"
#include "nsIObserver.h"
#include "mozilla/Services.h"
#include "nsXULAppAPI.h"

#include <algorithm>

// Activate BHR only for one every BHR_BETA_MOD users.
#define BHR_BETA_MOD 100;

// Maximum depth of the call stack in the reported thread hangs. This value represents
// the 95th pecentile of the thread hangs stack depths reported by Telemetry.
static const size_t kMaxThreadHangStackDepth = 11;

// An utility comparator function used by std::unique to collapse "(* script)" entries in
// a vector representing a call stack.
bool StackScriptEntriesCollapser(const char* aStackEntry, const char *aAnotherStackEntry)
{
  return !strcmp(aStackEntry, aAnotherStackEntry) &&
         (!strcmp(aStackEntry, "(chrome script)") || !strcmp(aStackEntry, "(content script)"));
}

namespace mozilla {

/**
 * BackgroundHangManager is the global object that
 * manages all instances of BackgroundHangThread.
 */
class BackgroundHangManager : public nsIObserver
{
private:
  // Background hang monitor thread function
  static void MonitorThread(void* aData)
  {
    PR_SetCurrentThreadName("BgHangManager");

#ifdef MOZ_NUWA_PROCESS
    if (IsNuwaProcess()) {
      NuwaMarkCurrentThread(nullptr, nullptr);
    }
#endif

    /* We do not hold a reference to BackgroundHangManager here
       because the monitor thread only exists as long as the
       BackgroundHangManager instance exists. We stop the monitor
       thread in the BackgroundHangManager destructor, and we can
       only get to the destructor if we don't hold a reference here. */
    static_cast<BackgroundHangManager*>(aData)->RunMonitorThread();
  }

  // Hang monitor thread
  PRThread* mHangMonitorThread;
  // Stop hang monitoring
  bool mShutdown;

  BackgroundHangManager(const BackgroundHangManager&);
  BackgroundHangManager& operator=(const BackgroundHangManager&);
  void RunMonitorThread();

public:
  NS_DECL_THREADSAFE_ISUPPORTS
  NS_DECL_NSIOBSERVER
  static StaticRefPtr<BackgroundHangManager> sInstance;
  static bool sProhibited;
  static bool sDisabled;

  // Lock for access to members of this class
  Monitor mLock;
  // Current time as seen by hang monitors
  PRIntervalTime mIntervalNow;
  // List of BackgroundHangThread instances associated with each thread
  LinkedList<BackgroundHangThread> mHangThreads;

  void Shutdown()
  {
    MonitorAutoLock autoLock(mLock);
    mShutdown = true;
    autoLock.Notify();
  }

  void Wakeup()
  {
    // PR_CreateThread could have failed earlier
    if (mHangMonitorThread) {
      // Use PR_Interrupt to avoid potentially taking a lock
      PR_Interrupt(mHangMonitorThread);
    }
  }

  BackgroundHangManager();
private:
  virtual ~BackgroundHangManager();
};

NS_IMPL_ISUPPORTS(BackgroundHangManager, nsIObserver)

NS_IMETHODIMP
BackgroundHangManager::Observe(nsISupports* aSubject, const char* aTopic, const char16_t* aData) {
  NS_ENSURE_TRUE(!strcmp(aTopic, "profile-after-change"), NS_ERROR_UNEXPECTED);
  BackgroundHangMonitor::DisableOnBeta();

  nsCOMPtr<nsIObserverService> observerService = mozilla::services::GetObserverService();
  MOZ_ASSERT(observerService);
  observerService->RemoveObserver(this, "profile-after-change");

  return NS_OK;
}

/**
 * BackgroundHangThread is a per-thread object that is used
 * by all instances of BackgroundHangMonitor to monitor hangs.
 */
class BackgroundHangThread : public LinkedListElement<BackgroundHangThread>
{
private:
  static ThreadLocal<BackgroundHangThread*> sTlsKey;

  BackgroundHangThread(const BackgroundHangThread&);
  BackgroundHangThread& operator=(const BackgroundHangThread&);
  ~BackgroundHangThread();

  /* Keep a reference to the manager, so we can keep going even
     after BackgroundHangManager::Shutdown is called. */
  const RefPtr<BackgroundHangManager> mManager;
  // Unique thread ID for identification
  const PRThread* mThreadID;

public:
  NS_INLINE_DECL_REFCOUNTING(BackgroundHangThread)
  static BackgroundHangThread* FindThread();

  static void Startup()
  {
    /* We can tolerate init() failing. */
    (void)!sTlsKey.init();
  }

  // Hang timeout in ticks
  const PRIntervalTime mTimeout;
  // PermaHang timeout in ticks
  const PRIntervalTime mMaxTimeout;
  // Time at last activity
  PRIntervalTime mInterval;
  // Time when a hang started
  PRIntervalTime mHangStart;
  // Is the thread in a hang
  bool mHanging;
  // Is the thread in a waiting state
  bool mWaiting;
  // Platform-specific helper to get hang stacks
  ThreadStackHelper mStackHelper;
  // Stack of current hang
  Telemetry::HangStack mHangStack;
  // Statistics for telemetry
  Telemetry::ThreadHangStats mStats;
  // Annotations for the current hang
  UniquePtr<HangMonitor::HangAnnotations> mAnnotations;
  // Annotators registered for this thread
  HangMonitor::Observer::Annotators mAnnotators;

  BackgroundHangThread(const char* aName,
                       uint32_t aTimeoutMs,
                       uint32_t aMaxTimeoutMs);

  // Report a hang; aManager->mLock IS locked
  Telemetry::HangHistogram& ReportHang(PRIntervalTime aHangTime);
  // Report a permanent hang; aManager->mLock IS locked
  void ReportPermaHang();
  // Called by BackgroundHangMonitor::NotifyActivity
  void NotifyActivity();
  // Called by BackgroundHangMonitor::NotifyWait
  void NotifyWait()
  {
    NotifyActivity();
    mWaiting = true;
  }
};


StaticRefPtr<BackgroundHangManager> BackgroundHangManager::sInstance;
bool BackgroundHangManager::sProhibited = false;
bool BackgroundHangManager::sDisabled = false;

ThreadLocal<BackgroundHangThread*> BackgroundHangThread::sTlsKey;


BackgroundHangManager::BackgroundHangManager()
  : mShutdown(false)
  , mLock("BackgroundHangManager")
  , mIntervalNow(0)
{
  // Lock so we don't race against the new monitor thread
  MonitorAutoLock autoLock(mLock);
  mHangMonitorThread = PR_CreateThread(
    PR_USER_THREAD, MonitorThread, this,
    PR_PRIORITY_LOW, PR_GLOBAL_THREAD, PR_JOINABLE_THREAD, 0);

  MOZ_ASSERT(mHangMonitorThread, "Failed to create monitor thread");
}

BackgroundHangManager::~BackgroundHangManager()
{
  MOZ_ASSERT(mShutdown, "Destruction without Shutdown call");
  MOZ_ASSERT(mHangThreads.isEmpty(), "Destruction with outstanding monitors");
  MOZ_ASSERT(mHangMonitorThread, "No monitor thread");

  // PR_CreateThread could have failed above due to resource limitation
  if (mHangMonitorThread) {
    // The monitor thread can only live as long as the instance lives
    PR_JoinThread(mHangMonitorThread);
  }
}

void
BackgroundHangManager::RunMonitorThread()
{
  // Keep us locked except when waiting
  MonitorAutoLock autoLock(mLock);

  /* mIntervalNow is updated at various intervals determined by waitTime.
     However, if an update latency is too long (due to CPU scheduling, system
     sleep, etc.), we don't update mIntervalNow at all. This is done so that
     long latencies in our timing are not detected as hangs. systemTime is
     used to track PR_IntervalNow() and determine our latency. */

  PRIntervalTime systemTime = PR_IntervalNow();
  // Default values for the first iteration of thread loop
  PRIntervalTime waitTime = PR_INTERVAL_NO_WAIT;
  PRIntervalTime recheckTimeout = PR_INTERVAL_NO_WAIT;

  while (!mShutdown) {

    PR_ClearInterrupt();
    nsresult rv = autoLock.Wait(waitTime);

    PRIntervalTime newTime = PR_IntervalNow();
    PRIntervalTime systemInterval = newTime - systemTime;
    systemTime = newTime;

    /* waitTime is a quarter of the shortest timeout value; If our timing
       latency is low enough (less than half the shortest timeout value),
       we can update mIntervalNow. */
    if (MOZ_LIKELY(waitTime != PR_INTERVAL_NO_TIMEOUT &&
                   systemInterval < 2 * waitTime)) {
      mIntervalNow += systemInterval;
    }

    /* If it's before the next recheck timeout, and our wait did not
       get interrupted (either through Notify or PR_Interrupt), we can
       keep the current waitTime and skip iterating through hang monitors. */
    if (MOZ_LIKELY(systemInterval < recheckTimeout &&
                   systemInterval >= waitTime &&
                   rv == NS_OK)) {
      recheckTimeout -= systemInterval;
      continue;
    }

    /* We are in one of the following scenarios,
     - Hang or permahang recheck timeout
     - Thread added/removed
     - Thread wait or hang ended
       In all cases, we want to go through our list of hang
       monitors and update waitTime and recheckTimeout. */
    waitTime = PR_INTERVAL_NO_TIMEOUT;
    recheckTimeout = PR_INTERVAL_NO_TIMEOUT;

    // Locally hold mIntervalNow
    PRIntervalTime intervalNow = mIntervalNow;

    // iterate through hang monitors
    for (BackgroundHangThread* currentThread = mHangThreads.getFirst();
         currentThread; currentThread = currentThread->getNext()) {

      if (currentThread->mWaiting) {
        // Thread is waiting, not hanging
        continue;
      }
      PRIntervalTime interval = currentThread->mInterval;
      PRIntervalTime hangTime = intervalNow - interval;
      if (MOZ_UNLIKELY(hangTime >= currentThread->mMaxTimeout)) {
        // A permahang started
        // Skip subsequent iterations and tolerate a race on mWaiting here
        currentThread->mWaiting = true;
        currentThread->mHanging = false;
        currentThread->ReportPermaHang();
        continue;
      }

      if (MOZ_LIKELY(!currentThread->mHanging)) {
        if (MOZ_UNLIKELY(hangTime >= currentThread->mTimeout)) {
          // A hang started
          currentThread->mStackHelper.GetStack(currentThread->mHangStack);
          currentThread->mHangStart = interval;
          currentThread->mHanging = true;
          currentThread->mAnnotations =
            currentThread->mAnnotators.GatherAnnotations();
        }
      } else {
        if (MOZ_LIKELY(interval != currentThread->mHangStart)) {
          // A hang ended
          currentThread->ReportHang(intervalNow - currentThread->mHangStart);
          currentThread->mHanging = false;
        }
      }

      /* If we are hanging, the next time we check for hang status is when
         the hang turns into a permahang. If we're not hanging, the next
         recheck timeout is when we may be entering a hang. */
      PRIntervalTime nextRecheck;
      if (currentThread->mHanging) {
        nextRecheck = currentThread->mMaxTimeout;
      } else {
        nextRecheck = currentThread->mTimeout;
      }
      recheckTimeout = std::min(recheckTimeout, nextRecheck - hangTime);

      /* We wait for a quarter of the shortest timeout
         value to give mIntervalNow enough granularity. */
      waitTime = std::min(waitTime, currentThread->mTimeout / 4);
    }
  }

  /* We are shutting down now.
     Wait for all outstanding monitors to unregister. */
  while (!mHangThreads.isEmpty()) {
    autoLock.Wait(PR_INTERVAL_NO_TIMEOUT);
  }
}


BackgroundHangThread::BackgroundHangThread(const char* aName,
                                           uint32_t aTimeoutMs,
                                           uint32_t aMaxTimeoutMs)
  : mManager(BackgroundHangManager::sInstance)
  , mThreadID(PR_GetCurrentThread())
  , mTimeout(aTimeoutMs == BackgroundHangMonitor::kNoTimeout
             ? PR_INTERVAL_NO_TIMEOUT
             : PR_MillisecondsToInterval(aTimeoutMs))
  , mMaxTimeout(aMaxTimeoutMs == BackgroundHangMonitor::kNoTimeout
                ? PR_INTERVAL_NO_TIMEOUT
                : PR_MillisecondsToInterval(aMaxTimeoutMs))
  , mInterval(mManager->mIntervalNow)
  , mHangStart(mInterval)
  , mHanging(false)
  , mWaiting(true)
  , mStats(aName)
{
  if (sTlsKey.initialized()) {
    sTlsKey.set(this);
  }
  // Lock here because LinkedList is not thread-safe
  MonitorAutoLock autoLock(mManager->mLock);
  // Add to thread list
  mManager->mHangThreads.insertBack(this);
  // Wake up monitor thread to process new thread
  autoLock.Notify();
}

BackgroundHangThread::~BackgroundHangThread()
{
  // Lock here because LinkedList is not thread-safe
  MonitorAutoLock autoLock(mManager->mLock);
  // Remove from thread list
  remove();
  // Wake up monitor thread to process removed thread
  autoLock.Notify();

  // We no longer have a thread
  if (sTlsKey.initialized()) {
    sTlsKey.set(nullptr);
  }

  // Move our copy of ThreadHangStats to Telemetry storage
  Telemetry::RecordThreadHangStats(mStats);
}

Telemetry::HangHistogram&
BackgroundHangThread::ReportHang(PRIntervalTime aHangTime)
{
  // Recovered from a hang; called on the monitor thread
  // mManager->mLock IS locked

  // Remove unwanted "js::RunScript" frame from the stack
  for (const char** f = &mHangStack.back(); f >= mHangStack.begin(); f--) {
    if (!mHangStack.IsInBuffer(*f) && !strcmp(*f, "js::RunScript")) {
      mHangStack.erase(f);
    }
  }

  // Collapse duplicated "(chrome script)" and "(content script)" entries in the stack.
  auto it = std::unique(mHangStack.begin(), mHangStack.end(), StackScriptEntriesCollapser);
  mHangStack.erase(it, mHangStack.end());

  // Limit the depth of the reported stack if greater than our limit. Only keep its
  // last entries, since the most recent frames are at the end of the vector.
  if (mHangStack.length() > kMaxThreadHangStackDepth) {
    const int elementsToRemove = mHangStack.length() - kMaxThreadHangStackDepth;
    // Replace the oldest frame with a known label so that we can tell this stack
    // was limited.
    mHangStack[0] = "(reduced stack)";
    mHangStack.erase(mHangStack.begin() + 1, mHangStack.begin() + elementsToRemove);
  }

  Telemetry::HangHistogram newHistogram(Move(mHangStack));
  for (Telemetry::HangHistogram* oldHistogram = mStats.mHangs.begin();
       oldHistogram != mStats.mHangs.end(); oldHistogram++) {
    if (newHistogram == *oldHistogram) {
      // New histogram matches old one
      oldHistogram->Add(aHangTime, Move(mAnnotations));
      return *oldHistogram;
    }
  }
  // Add new histogram
  newHistogram.Add(aHangTime, Move(mAnnotations));
  mStats.mHangs.append(Move(newHistogram));
  return mStats.mHangs.back();
}

void
BackgroundHangThread::ReportPermaHang()
{
  // Permanently hanged; called on the monitor thread
  // mManager->mLock IS locked

  Telemetry::HangHistogram& hang = ReportHang(mMaxTimeout);
  Telemetry::HangStack& stack = hang.GetNativeStack();
  if (stack.empty()) {
    mStackHelper.GetNativeStack(stack);
  }
}

MOZ_ALWAYS_INLINE void
BackgroundHangThread::NotifyActivity()
{
  PRIntervalTime intervalNow = mManager->mIntervalNow;
  if (mWaiting) {
    mInterval = intervalNow;
    mWaiting = false;
    /* We have to wake up the manager thread because when all threads
       are waiting, the manager thread waits indefinitely as well. */
    mManager->Wakeup();
  } else {
    PRIntervalTime duration = intervalNow - mInterval;
    mStats.mActivity.Add(duration);
    if (MOZ_UNLIKELY(duration >= mTimeout)) {
      /* Wake up the manager thread to tell it that a hang ended */
      mManager->Wakeup();
    }
    mInterval = intervalNow;
  }
}

BackgroundHangThread*
BackgroundHangThread::FindThread()
{
#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
  if (BackgroundHangManager::sInstance == nullptr) {
    MOZ_ASSERT(BackgroundHangManager::sProhibited || BackgroundHangManager::sDisabled,
               "BackgroundHandleManager is not initialized");
    return nullptr;
  }

  if (sTlsKey.initialized()) {
    // Use TLS if available
    MOZ_ASSERT(!BackgroundHangManager::sProhibited,
               "BackgroundHandleManager is not initialized");
    return sTlsKey.get();
  }
  // If TLS is unavailable, we can search through the thread list
  RefPtr<BackgroundHangManager> manager(BackgroundHangManager::sInstance);
  MOZ_ASSERT(manager, "Creating BackgroundHangMonitor after shutdown");

  PRThread* threadID = PR_GetCurrentThread();
  // Lock thread list for traversal
  MonitorAutoLock autoLock(manager->mLock);
  for (BackgroundHangThread* thread = manager->mHangThreads.getFirst();
       thread; thread = thread->getNext()) {
    if (thread->mThreadID == threadID) {
      return thread;
    }
  }
#endif
  // Current thread is not initialized
  return nullptr;
}

bool
BackgroundHangMonitor::ShouldDisableOnBeta(const nsCString &clientID) {
  MOZ_ASSERT(clientID.Length() == 36, "clientID is invalid");
  const char *suffix = clientID.get() + clientID.Length() - 4;
  return strtol(suffix, NULL, 16) % BHR_BETA_MOD;
}

bool
BackgroundHangMonitor::IsDisabled() {
#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
  return BackgroundHangManager::sDisabled;
#else
  return true;
#endif
}

bool
BackgroundHangMonitor::DisableOnBeta() {
  nsAdoptingCString clientID = Preferences::GetCString("toolkit.telemetry.cachedClientID");
  bool telemetryEnabled = Preferences::GetBool("toolkit.telemetry.enabled");

  if (!telemetryEnabled || !clientID || BackgroundHangMonitor::ShouldDisableOnBeta(clientID)) {
    if (XRE_IsParentProcess()) {
      BackgroundHangMonitor::Shutdown();
    } else {
      BackgroundHangManager::sDisabled = true;
    }
    return true;
  }

  return false;
}

void
BackgroundHangMonitor::Startup()
{
#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
  MOZ_ASSERT(!BackgroundHangManager::sProhibited, "Prohibited");
  MOZ_ASSERT(!BackgroundHangManager::sInstance, "Already initialized");

  if (!strcmp(NS_STRINGIFY(MOZ_UPDATE_CHANNEL), "beta")) {
    if (XRE_IsParentProcess()) { // cached ClientID hasn't been read yet
      ThreadStackHelper::Startup();
      BackgroundHangThread::Startup();
      BackgroundHangManager::sInstance = new BackgroundHangManager();

      nsCOMPtr<nsIObserverService> observerService = mozilla::services::GetObserverService();
      MOZ_ASSERT(observerService);

      observerService->AddObserver(BackgroundHangManager::sInstance, "profile-after-change", false);
      return;
    } else if(DisableOnBeta()){
      return;
    }
  }

  ThreadStackHelper::Startup();
  BackgroundHangThread::Startup();
  BackgroundHangManager::sInstance = new BackgroundHangManager();
#endif
}

void
BackgroundHangMonitor::Shutdown()
{
#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
  if (BackgroundHangManager::sDisabled) {
    MOZ_ASSERT(!BackgroundHangManager::sInstance, "Initialized");
    return;
  }

  MOZ_ASSERT(!BackgroundHangManager::sProhibited, "Prohibited");
  MOZ_ASSERT(BackgroundHangManager::sInstance, "Not initialized");
  /* Scope our lock inside Shutdown() because the sInstance object can
     be destroyed as soon as we set sInstance to nullptr below, and
     we don't want to hold the lock when it's being destroyed. */
  BackgroundHangManager::sInstance->Shutdown();
  BackgroundHangManager::sInstance = nullptr;
  ThreadStackHelper::Shutdown();
  BackgroundHangManager::sDisabled = true;
#endif
}

BackgroundHangMonitor::BackgroundHangMonitor(const char* aName,
                                             uint32_t aTimeoutMs,
                                             uint32_t aMaxTimeoutMs)
  : mThread(BackgroundHangThread::FindThread())
{
#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
  if (!BackgroundHangManager::sDisabled && !BackgroundHangManager::sProhibited && !mThread) {
    // If sProhibit is true, mThread would be null, and no monitoring.
    mThread = new BackgroundHangThread(aName, aTimeoutMs, aMaxTimeoutMs);
  }
#endif
}

BackgroundHangMonitor::BackgroundHangMonitor()
  : mThread(BackgroundHangThread::FindThread())
{
#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
  if (BackgroundHangManager::sDisabled) {
    return;
  }

  MOZ_ASSERT(!BackgroundHangManager::sProhibited || mThread,
             "This thread is not initialized for hang monitoring");
#endif
}

BackgroundHangMonitor::~BackgroundHangMonitor()
{
}

void
BackgroundHangMonitor::NotifyActivity()
{
#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
  if (mThread == nullptr) {
    MOZ_ASSERT(BackgroundHangManager::sProhibited ||
               BackgroundHangManager::sDisabled,
               "This thread is not initialized for hang monitoring");
    return;
  }

  if (Telemetry::CanRecordExtended()) {
    mThread->NotifyActivity();
  }
#endif
}

void
BackgroundHangMonitor::NotifyWait()
{
#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
  if (mThread == nullptr) {
    MOZ_ASSERT(BackgroundHangManager::sProhibited ||
               BackgroundHangManager::sDisabled,
               "This thread is not initialized for hang monitoring");
    return;
  }

  if (Telemetry::CanRecordExtended()) {
    mThread->NotifyWait();
  }
#endif
}

void
BackgroundHangMonitor::Prohibit()
{
#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
  MOZ_ASSERT(BackgroundHangManager::sInstance == nullptr,
             "The background hang monitor is already initialized");
  BackgroundHangManager::sProhibited = true;
#endif
}

void
BackgroundHangMonitor::Allow()
{
#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
  MOZ_ASSERT(BackgroundHangManager::sInstance == nullptr,
             "The background hang monitor is already initialized");
  BackgroundHangManager::sProhibited = false;
#endif
}

bool
BackgroundHangMonitor::RegisterAnnotator(HangMonitor::Annotator& aAnnotator)
{
#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
  BackgroundHangThread* thisThread = BackgroundHangThread::FindThread();
  if (!thisThread) {
    return false;
  }
  return thisThread->mAnnotators.Register(aAnnotator);
#else
  return false;
#endif
}

bool
BackgroundHangMonitor::UnregisterAnnotator(HangMonitor::Annotator& aAnnotator)
{
#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
  BackgroundHangThread* thisThread = BackgroundHangThread::FindThread();
  if (!thisThread) {
    return false;
  }
  return thisThread->mAnnotators.Unregister(aAnnotator);
#else
  return false;
#endif
}

/* Because we are iterating through the BackgroundHangThread linked list,
   we need to take a lock. Using MonitorAutoLock as a base class makes
   sure all of that is taken care of for us. */
BackgroundHangMonitor::ThreadHangStatsIterator::ThreadHangStatsIterator()
  : MonitorAutoLock(BackgroundHangManager::sInstance->mLock)
  , mThread(BackgroundHangManager::sInstance ?
            BackgroundHangManager::sInstance->mHangThreads.getFirst() :
            nullptr)
{
#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
  MOZ_ASSERT(BackgroundHangManager::sInstance ||
             BackgroundHangManager::sProhibited ||
             BackgroundHangManager::sDisabled,
             "Inconsistent state");
#endif
}

Telemetry::ThreadHangStats*
BackgroundHangMonitor::ThreadHangStatsIterator::GetNext()
{
  if (!mThread) {
    return nullptr;
  }
  Telemetry::ThreadHangStats* stats = &mThread->mStats;
  mThread = mThread->getNext();
  return stats;
}

} // namespace mozilla