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      1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
      2 // Use of this source code is governed by a BSD-style license that can be
      3 // found in the LICENSE file.
      4 
      5 #include "content/child/child_thread.h"
      6 
      7 #include <signal.h>
      8 
      9 #include <string>
     10 
     11 #include "base/allocator/allocator_extension.h"
     12 #include "base/base_switches.h"
     13 #include "base/basictypes.h"
     14 #include "base/command_line.h"
     15 #include "base/debug/leak_annotations.h"
     16 #include "base/lazy_instance.h"
     17 #include "base/logging.h"
     18 #include "base/message_loop/message_loop.h"
     19 #include "base/message_loop/timer_slack.h"
     20 #include "base/process/kill.h"
     21 #include "base/process/process_handle.h"
     22 #include "base/strings/string_number_conversions.h"
     23 #include "base/strings/string_util.h"
     24 #include "base/synchronization/condition_variable.h"
     25 #include "base/synchronization/lock.h"
     26 #include "base/threading/thread_local.h"
     27 #include "base/tracked_objects.h"
     28 #include "components/tracing/child_trace_message_filter.h"
     29 #include "content/child/child_histogram_message_filter.h"
     30 #include "content/child/child_process.h"
     31 #include "content/child/child_resource_message_filter.h"
     32 #include "content/child/child_shared_bitmap_manager.h"
     33 #include "content/child/fileapi/file_system_dispatcher.h"
     34 #include "content/child/fileapi/webfilesystem_impl.h"
     35 #include "content/child/mojo/mojo_application.h"
     36 #include "content/child/power_monitor_broadcast_source.h"
     37 #include "content/child/quota_dispatcher.h"
     38 #include "content/child/quota_message_filter.h"
     39 #include "content/child/resource_dispatcher.h"
     40 #include "content/child/service_worker/service_worker_dispatcher.h"
     41 #include "content/child/service_worker/service_worker_message_filter.h"
     42 #include "content/child/socket_stream_dispatcher.h"
     43 #include "content/child/thread_safe_sender.h"
     44 #include "content/child/websocket_dispatcher.h"
     45 #include "content/common/child_process_messages.h"
     46 #include "content/public/common/content_switches.h"
     47 #include "ipc/ipc_logging.h"
     48 #include "ipc/ipc_switches.h"
     49 #include "ipc/ipc_sync_channel.h"
     50 #include "ipc/ipc_sync_message_filter.h"
     51 
     52 #if defined(OS_WIN)
     53 #include "content/common/handle_enumerator_win.h"
     54 #endif
     55 
     56 #if defined(TCMALLOC_TRACE_MEMORY_SUPPORTED)
     57 #include "third_party/tcmalloc/chromium/src/gperftools/heap-profiler.h"
     58 #endif
     59 
     60 using tracked_objects::ThreadData;
     61 
     62 namespace content {
     63 namespace {
     64 
     65 // How long to wait for a connection to the browser process before giving up.
     66 const int kConnectionTimeoutS = 15;
     67 
     68 base::LazyInstance<base::ThreadLocalPointer<ChildThread> > g_lazy_tls =
     69     LAZY_INSTANCE_INITIALIZER;
     70 
     71 // This isn't needed on Windows because there the sandbox's job object
     72 // terminates child processes automatically. For unsandboxed processes (i.e.
     73 // plugins), PluginThread has EnsureTerminateMessageFilter.
     74 #if defined(OS_POSIX)
     75 
     76 // TODO(earthdok): Re-enable on CrOS http://crbug.com/360622
     77 #if (defined(ADDRESS_SANITIZER) || defined(LEAK_SANITIZER) || \
     78     defined(THREAD_SANITIZER)) && !defined(OS_CHROMEOS)
     79 // A thread delegate that waits for |duration| and then exits the process with
     80 // _exit(0).
     81 class WaitAndExitDelegate : public base::PlatformThread::Delegate {
     82  public:
     83   explicit WaitAndExitDelegate(base::TimeDelta duration)
     84       : duration_(duration) {}
     85   virtual ~WaitAndExitDelegate() OVERRIDE {}
     86 
     87   virtual void ThreadMain() OVERRIDE {
     88     base::PlatformThread::Sleep(duration_);
     89     _exit(0);
     90   }
     91 
     92  private:
     93   const base::TimeDelta duration_;
     94   DISALLOW_COPY_AND_ASSIGN(WaitAndExitDelegate);
     95 };
     96 
     97 bool CreateWaitAndExitThread(base::TimeDelta duration) {
     98   scoped_ptr<WaitAndExitDelegate> delegate(new WaitAndExitDelegate(duration));
     99 
    100   const bool thread_created =
    101       base::PlatformThread::CreateNonJoinable(0, delegate.get());
    102   if (!thread_created)
    103     return false;
    104 
    105   // A non joinable thread has been created. The thread will either terminate
    106   // the process or will be terminated by the process. Therefore, keep the
    107   // delegate object alive for the lifetime of the process.
    108   WaitAndExitDelegate* leaking_delegate = delegate.release();
    109   ANNOTATE_LEAKING_OBJECT_PTR(leaking_delegate);
    110   ignore_result(leaking_delegate);
    111   return true;
    112 }
    113 #endif
    114 
    115 class SuicideOnChannelErrorFilter : public IPC::MessageFilter {
    116  public:
    117   // IPC::MessageFilter
    118   virtual void OnChannelError() OVERRIDE {
    119     // For renderer/worker processes:
    120     // On POSIX, at least, one can install an unload handler which loops
    121     // forever and leave behind a renderer process which eats 100% CPU forever.
    122     //
    123     // This is because the terminate signals (ViewMsg_ShouldClose and the error
    124     // from the IPC sender) are routed to the main message loop but never
    125     // processed (because that message loop is stuck in V8).
    126     //
    127     // One could make the browser SIGKILL the renderers, but that leaves open a
    128     // large window where a browser failure (or a user, manually terminating
    129     // the browser because "it's stuck") will leave behind a process eating all
    130     // the CPU.
    131     //
    132     // So, we install a filter on the sender so that we can process this event
    133     // here and kill the process.
    134     // TODO(earthdok): Re-enable on CrOS http://crbug.com/360622
    135 #if (defined(ADDRESS_SANITIZER) || defined(LEAK_SANITIZER) || \
    136     defined(THREAD_SANITIZER)) && !defined(OS_CHROMEOS)
    137     // Some sanitizer tools rely on exit handlers (e.g. to run leak detection,
    138     // or dump code coverage data to disk). Instead of exiting the process
    139     // immediately, we give it 60 seconds to run exit handlers.
    140     CHECK(CreateWaitAndExitThread(base::TimeDelta::FromSeconds(60)));
    141 #if defined(LEAK_SANITIZER)
    142     // Invoke LeakSanitizer early to avoid detecting shutdown-only leaks. If
    143     // leaks are found, the process will exit here.
    144     __lsan_do_leak_check();
    145 #endif
    146 #else
    147     _exit(0);
    148 #endif
    149   }
    150 
    151  protected:
    152   virtual ~SuicideOnChannelErrorFilter() {}
    153 };
    154 
    155 #endif  // OS(POSIX)
    156 
    157 #if defined(OS_ANDROID)
    158 ChildThread* g_child_thread = NULL;
    159 
    160 // A lock protects g_child_thread.
    161 base::LazyInstance<base::Lock> g_lazy_child_thread_lock =
    162     LAZY_INSTANCE_INITIALIZER;
    163 
    164 // base::ConditionVariable has an explicit constructor that takes
    165 // a base::Lock pointer as parameter. The base::DefaultLazyInstanceTraits
    166 // doesn't handle the case. Thus, we need our own class here.
    167 struct CondVarLazyInstanceTraits {
    168   static const bool kRegisterOnExit = true;
    169 #ifndef NDEBUG
    170   static const bool kAllowedToAccessOnNonjoinableThread = false;
    171 #endif
    172 
    173   static base::ConditionVariable* New(void* instance) {
    174     return new (instance) base::ConditionVariable(
    175         g_lazy_child_thread_lock.Pointer());
    176   }
    177   static void Delete(base::ConditionVariable* instance) {
    178     instance->~ConditionVariable();
    179   }
    180 };
    181 
    182 // A condition variable that synchronize threads initializing and waiting
    183 // for g_child_thread.
    184 base::LazyInstance<base::ConditionVariable, CondVarLazyInstanceTraits>
    185     g_lazy_child_thread_cv = LAZY_INSTANCE_INITIALIZER;
    186 
    187 void QuitMainThreadMessageLoop() {
    188   base::MessageLoop::current()->Quit();
    189 }
    190 
    191 #endif
    192 
    193 }  // namespace
    194 
    195 ChildThread::ChildThreadMessageRouter::ChildThreadMessageRouter(
    196     IPC::Sender* sender)
    197     : sender_(sender) {}
    198 
    199 bool ChildThread::ChildThreadMessageRouter::Send(IPC::Message* msg) {
    200   return sender_->Send(msg);
    201 }
    202 
    203 ChildThread::ChildThread()
    204     : router_(this),
    205       channel_connected_factory_(this),
    206       in_browser_process_(false) {
    207   channel_name_ = CommandLine::ForCurrentProcess()->GetSwitchValueASCII(
    208       switches::kProcessChannelID);
    209   Init();
    210 }
    211 
    212 ChildThread::ChildThread(const std::string& channel_name)
    213     : channel_name_(channel_name),
    214       router_(this),
    215       channel_connected_factory_(this),
    216       in_browser_process_(true) {
    217   Init();
    218 }
    219 
    220 void ChildThread::Init() {
    221   g_lazy_tls.Pointer()->Set(this);
    222   on_channel_error_called_ = false;
    223   message_loop_ = base::MessageLoop::current();
    224 #ifdef IPC_MESSAGE_LOG_ENABLED
    225   // We must make sure to instantiate the IPC Logger *before* we create the
    226   // channel, otherwise we can get a callback on the IO thread which creates
    227   // the logger, and the logger does not like being created on the IO thread.
    228   IPC::Logging::GetInstance();
    229 #endif
    230   channel_ =
    231       IPC::SyncChannel::Create(channel_name_,
    232                                IPC::Channel::MODE_CLIENT,
    233                                this,
    234                                ChildProcess::current()->io_message_loop_proxy(),
    235                                true,
    236                                ChildProcess::current()->GetShutDownEvent());
    237 #ifdef IPC_MESSAGE_LOG_ENABLED
    238   if (!in_browser_process_)
    239     IPC::Logging::GetInstance()->SetIPCSender(this);
    240 #endif
    241 
    242   mojo_application_.reset(new MojoApplication(this));
    243 
    244   sync_message_filter_ =
    245       new IPC::SyncMessageFilter(ChildProcess::current()->GetShutDownEvent());
    246   thread_safe_sender_ = new ThreadSafeSender(
    247       base::MessageLoopProxy::current().get(), sync_message_filter_.get());
    248 
    249   resource_dispatcher_.reset(new ResourceDispatcher(this));
    250   socket_stream_dispatcher_.reset(new SocketStreamDispatcher());
    251   websocket_dispatcher_.reset(new WebSocketDispatcher);
    252   file_system_dispatcher_.reset(new FileSystemDispatcher());
    253 
    254   histogram_message_filter_ = new ChildHistogramMessageFilter();
    255   resource_message_filter_ =
    256       new ChildResourceMessageFilter(resource_dispatcher());
    257 
    258   service_worker_message_filter_ =
    259       new ServiceWorkerMessageFilter(thread_safe_sender_.get());
    260   service_worker_dispatcher_.reset(
    261       new ServiceWorkerDispatcher(thread_safe_sender_.get()));
    262 
    263   quota_message_filter_ =
    264       new QuotaMessageFilter(thread_safe_sender_.get());
    265   quota_dispatcher_.reset(new QuotaDispatcher(thread_safe_sender_.get(),
    266                                               quota_message_filter_.get()));
    267 
    268   channel_->AddFilter(histogram_message_filter_.get());
    269   channel_->AddFilter(sync_message_filter_.get());
    270   channel_->AddFilter(resource_message_filter_.get());
    271   channel_->AddFilter(quota_message_filter_->GetFilter());
    272   channel_->AddFilter(service_worker_message_filter_->GetFilter());
    273 
    274   if (!CommandLine::ForCurrentProcess()->HasSwitch(switches::kSingleProcess)) {
    275     // In single process mode, browser-side tracing will cover the whole
    276     // process including renderers.
    277     channel_->AddFilter(new tracing::ChildTraceMessageFilter(
    278         ChildProcess::current()->io_message_loop_proxy()));
    279   }
    280 
    281   // In single process mode we may already have a power monitor
    282   if (!base::PowerMonitor::Get()) {
    283     scoped_ptr<PowerMonitorBroadcastSource> power_monitor_source(
    284       new PowerMonitorBroadcastSource());
    285     channel_->AddFilter(power_monitor_source->GetMessageFilter());
    286 
    287     power_monitor_.reset(new base::PowerMonitor(
    288         power_monitor_source.PassAs<base::PowerMonitorSource>()));
    289   }
    290 
    291 #if defined(OS_POSIX)
    292   // Check that --process-type is specified so we don't do this in unit tests
    293   // and single-process mode.
    294   if (CommandLine::ForCurrentProcess()->HasSwitch(switches::kProcessType))
    295     channel_->AddFilter(new SuicideOnChannelErrorFilter());
    296 #endif
    297 
    298   int connection_timeout = kConnectionTimeoutS;
    299   std::string connection_override =
    300       CommandLine::ForCurrentProcess()->GetSwitchValueASCII(
    301           switches::kIPCConnectionTimeout);
    302   if (!connection_override.empty()) {
    303     int temp;
    304     if (base::StringToInt(connection_override, &temp))
    305       connection_timeout = temp;
    306   }
    307 
    308   base::MessageLoop::current()->PostDelayedTask(
    309       FROM_HERE,
    310       base::Bind(&ChildThread::EnsureConnected,
    311                  channel_connected_factory_.GetWeakPtr()),
    312       base::TimeDelta::FromSeconds(connection_timeout));
    313 
    314 #if defined(OS_ANDROID)
    315   {
    316     base::AutoLock lock(g_lazy_child_thread_lock.Get());
    317     g_child_thread = this;
    318   }
    319   // Signalling without locking is fine here because only
    320   // one thread can wait on the condition variable.
    321   g_lazy_child_thread_cv.Get().Signal();
    322 #endif
    323 
    324 #if defined(TCMALLOC_TRACE_MEMORY_SUPPORTED)
    325   trace_memory_controller_.reset(new base::debug::TraceMemoryController(
    326       message_loop_->message_loop_proxy(),
    327       ::HeapProfilerWithPseudoStackStart,
    328       ::HeapProfilerStop,
    329       ::GetHeapProfile));
    330 #endif
    331 
    332   shared_bitmap_manager_.reset(
    333       new ChildSharedBitmapManager(thread_safe_sender()));
    334 }
    335 
    336 ChildThread::~ChildThread() {
    337 #ifdef IPC_MESSAGE_LOG_ENABLED
    338   IPC::Logging::GetInstance()->SetIPCSender(NULL);
    339 #endif
    340 
    341   channel_->RemoveFilter(histogram_message_filter_.get());
    342   channel_->RemoveFilter(sync_message_filter_.get());
    343 
    344   // The ChannelProxy object caches a pointer to the IPC thread, so need to
    345   // reset it as it's not guaranteed to outlive this object.
    346   // NOTE: this also has the side-effect of not closing the main IPC channel to
    347   // the browser process.  This is needed because this is the signal that the
    348   // browser uses to know that this process has died, so we need it to be alive
    349   // until this process is shut down, and the OS closes the handle
    350   // automatically.  We used to watch the object handle on Windows to do this,
    351   // but it wasn't possible to do so on POSIX.
    352   channel_->ClearIPCTaskRunner();
    353   g_lazy_tls.Pointer()->Set(NULL);
    354 }
    355 
    356 void ChildThread::Shutdown() {
    357   // Delete objects that hold references to blink so derived classes can
    358   // safely shutdown blink in their Shutdown implementation.
    359   file_system_dispatcher_.reset();
    360   quota_dispatcher_.reset();
    361   WebFileSystemImpl::DeleteThreadSpecificInstance();
    362 }
    363 
    364 void ChildThread::OnChannelConnected(int32 peer_pid) {
    365   channel_connected_factory_.InvalidateWeakPtrs();
    366 }
    367 
    368 void ChildThread::OnChannelError() {
    369   set_on_channel_error_called(true);
    370   base::MessageLoop::current()->Quit();
    371 }
    372 
    373 void ChildThread::ConnectToService(
    374     const mojo::String& service_url,
    375     const mojo::String& service_name,
    376     mojo::ScopedMessagePipeHandle message_pipe,
    377     const mojo::String& requestor_url) {
    378   // By default, we don't expect incoming connections.
    379   NOTREACHED();
    380 }
    381 
    382 bool ChildThread::Send(IPC::Message* msg) {
    383   DCHECK(base::MessageLoop::current() == message_loop());
    384   if (!channel_) {
    385     delete msg;
    386     return false;
    387   }
    388 
    389   return channel_->Send(msg);
    390 }
    391 
    392 MessageRouter* ChildThread::GetRouter() {
    393   DCHECK(base::MessageLoop::current() == message_loop());
    394   return &router_;
    395 }
    396 
    397 base::SharedMemory* ChildThread::AllocateSharedMemory(size_t buf_size) {
    398   return AllocateSharedMemory(buf_size, this);
    399 }
    400 
    401 // static
    402 base::SharedMemory* ChildThread::AllocateSharedMemory(
    403     size_t buf_size,
    404     IPC::Sender* sender) {
    405   scoped_ptr<base::SharedMemory> shared_buf;
    406 #if defined(OS_WIN)
    407   shared_buf.reset(new base::SharedMemory);
    408   if (!shared_buf->CreateAndMapAnonymous(buf_size)) {
    409     NOTREACHED();
    410     return NULL;
    411   }
    412 #else
    413   // On POSIX, we need to ask the browser to create the shared memory for us,
    414   // since this is blocked by the sandbox.
    415   base::SharedMemoryHandle shared_mem_handle;
    416   if (sender->Send(new ChildProcessHostMsg_SyncAllocateSharedMemory(
    417                            buf_size, &shared_mem_handle))) {
    418     if (base::SharedMemory::IsHandleValid(shared_mem_handle)) {
    419       shared_buf.reset(new base::SharedMemory(shared_mem_handle, false));
    420       if (!shared_buf->Map(buf_size)) {
    421         NOTREACHED() << "Map failed";
    422         return NULL;
    423       }
    424     } else {
    425       NOTREACHED() << "Browser failed to allocate shared memory";
    426       return NULL;
    427     }
    428   } else {
    429     NOTREACHED() << "Browser allocation request message failed";
    430     return NULL;
    431   }
    432 #endif
    433   return shared_buf.release();
    434 }
    435 
    436 bool ChildThread::OnMessageReceived(const IPC::Message& msg) {
    437   if (mojo_application_->OnMessageReceived(msg))
    438     return true;
    439 
    440   // Resource responses are sent to the resource dispatcher.
    441   if (resource_dispatcher_->OnMessageReceived(msg))
    442     return true;
    443   if (socket_stream_dispatcher_->OnMessageReceived(msg))
    444     return true;
    445   if (websocket_dispatcher_->OnMessageReceived(msg))
    446     return true;
    447   if (file_system_dispatcher_->OnMessageReceived(msg))
    448     return true;
    449 
    450   bool handled = true;
    451   IPC_BEGIN_MESSAGE_MAP(ChildThread, msg)
    452     IPC_MESSAGE_HANDLER(ChildProcessMsg_Shutdown, OnShutdown)
    453 #if defined(IPC_MESSAGE_LOG_ENABLED)
    454     IPC_MESSAGE_HANDLER(ChildProcessMsg_SetIPCLoggingEnabled,
    455                         OnSetIPCLoggingEnabled)
    456 #endif
    457     IPC_MESSAGE_HANDLER(ChildProcessMsg_SetProfilerStatus,
    458                         OnSetProfilerStatus)
    459     IPC_MESSAGE_HANDLER(ChildProcessMsg_GetChildProfilerData,
    460                         OnGetChildProfilerData)
    461     IPC_MESSAGE_HANDLER(ChildProcessMsg_DumpHandles, OnDumpHandles)
    462     IPC_MESSAGE_HANDLER(ChildProcessMsg_SetProcessBackgrounded,
    463                         OnProcessBackgrounded)
    464 #if defined(USE_TCMALLOC)
    465     IPC_MESSAGE_HANDLER(ChildProcessMsg_GetTcmallocStats, OnGetTcmallocStats)
    466 #endif
    467     IPC_MESSAGE_UNHANDLED(handled = false)
    468   IPC_END_MESSAGE_MAP()
    469 
    470   if (handled)
    471     return true;
    472 
    473   if (msg.routing_id() == MSG_ROUTING_CONTROL)
    474     return OnControlMessageReceived(msg);
    475 
    476   return router_.OnMessageReceived(msg);
    477 }
    478 
    479 bool ChildThread::OnControlMessageReceived(const IPC::Message& msg) {
    480   return false;
    481 }
    482 
    483 void ChildThread::OnShutdown() {
    484   base::MessageLoop::current()->Quit();
    485 }
    486 
    487 #if defined(IPC_MESSAGE_LOG_ENABLED)
    488 void ChildThread::OnSetIPCLoggingEnabled(bool enable) {
    489   if (enable)
    490     IPC::Logging::GetInstance()->Enable();
    491   else
    492     IPC::Logging::GetInstance()->Disable();
    493 }
    494 #endif  //  IPC_MESSAGE_LOG_ENABLED
    495 
    496 void ChildThread::OnSetProfilerStatus(ThreadData::Status status) {
    497   ThreadData::InitializeAndSetTrackingStatus(status);
    498 }
    499 
    500 void ChildThread::OnGetChildProfilerData(int sequence_number) {
    501   tracked_objects::ProcessDataSnapshot process_data;
    502   ThreadData::Snapshot(false, &process_data);
    503 
    504   Send(new ChildProcessHostMsg_ChildProfilerData(sequence_number,
    505                                                  process_data));
    506 }
    507 
    508 void ChildThread::OnDumpHandles() {
    509 #if defined(OS_WIN)
    510   scoped_refptr<HandleEnumerator> handle_enum(
    511       new HandleEnumerator(
    512           CommandLine::ForCurrentProcess()->HasSwitch(
    513               switches::kAuditAllHandles)));
    514   handle_enum->EnumerateHandles();
    515   Send(new ChildProcessHostMsg_DumpHandlesDone);
    516 #else
    517   NOTIMPLEMENTED();
    518 #endif
    519 }
    520 
    521 #if defined(USE_TCMALLOC)
    522 void ChildThread::OnGetTcmallocStats() {
    523   std::string result;
    524   char buffer[1024 * 32];
    525   base::allocator::GetStats(buffer, sizeof(buffer));
    526   result.append(buffer);
    527   Send(new ChildProcessHostMsg_TcmallocStats(result));
    528 }
    529 #endif
    530 
    531 ChildThread* ChildThread::current() {
    532   return g_lazy_tls.Pointer()->Get();
    533 }
    534 
    535 #if defined(OS_ANDROID)
    536 // The method must NOT be called on the child thread itself.
    537 // It may block the child thread if so.
    538 void ChildThread::ShutdownThread() {
    539   DCHECK(!ChildThread::current()) <<
    540       "this method should NOT be called from child thread itself";
    541   {
    542     base::AutoLock lock(g_lazy_child_thread_lock.Get());
    543     while (!g_child_thread)
    544       g_lazy_child_thread_cv.Get().Wait();
    545   }
    546   DCHECK_NE(base::MessageLoop::current(), g_child_thread->message_loop());
    547   g_child_thread->message_loop()->PostTask(
    548       FROM_HERE, base::Bind(&QuitMainThreadMessageLoop));
    549 }
    550 #endif
    551 
    552 void ChildThread::OnProcessFinalRelease() {
    553   if (on_channel_error_called_) {
    554     base::MessageLoop::current()->Quit();
    555     return;
    556   }
    557 
    558   // The child process shutdown sequence is a request response based mechanism,
    559   // where we send out an initial feeler request to the child process host
    560   // instance in the browser to verify if it's ok to shutdown the child process.
    561   // The browser then sends back a response if it's ok to shutdown. This avoids
    562   // race conditions if the process refcount is 0 but there's an IPC message
    563   // inflight that would addref it.
    564   Send(new ChildProcessHostMsg_ShutdownRequest);
    565 }
    566 
    567 void ChildThread::EnsureConnected() {
    568   VLOG(0) << "ChildThread::EnsureConnected()";
    569   base::KillProcess(base::GetCurrentProcessHandle(), 0, false);
    570 }
    571 
    572 void ChildThread::OnProcessBackgrounded(bool background) {
    573   // Set timer slack to maximum on main thread when in background.
    574   base::TimerSlack timer_slack = base::TIMER_SLACK_NONE;
    575   if (background)
    576     timer_slack = base::TIMER_SLACK_MAXIMUM;
    577   base::MessageLoop::current()->SetTimerSlack(timer_slack);
    578 
    579 #ifdef OS_WIN
    580   // Windows Vista+ has a fancy process backgrounding mode that can only be set
    581   // from within the process.
    582   // TODO(wfh) Do not set background from within process until the issue with
    583   // white tabs is resolved.  See http://crbug.com/398103.
    584   // base::Process::Current().SetProcessBackgrounded(background);
    585 #endif  // OS_WIN
    586 }
    587 
    588 }  // namespace content
    589