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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_message_filter.h"
     41 #include "content/child/socket_stream_dispatcher.h"
     42 #include "content/child/thread_safe_sender.h"
     43 #include "content/child/websocket_dispatcher.h"
     44 #include "content/common/child_process_messages.h"
     45 #include "content/public/common/content_switches.h"
     46 #include "ipc/ipc_logging.h"
     47 #include "ipc/ipc_switches.h"
     48 #include "ipc/ipc_sync_channel.h"
     49 #include "ipc/ipc_sync_message_filter.h"
     50 #include "ipc/mojo/ipc_channel_mojo.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::Options::Options()
    196     : channel_name(base::CommandLine::ForCurrentProcess()->GetSwitchValueASCII(
    197           switches::kProcessChannelID)),
    198       use_mojo_channel(false) {}
    199 
    200 ChildThread::Options::Options(bool mojo)
    201     : channel_name(base::CommandLine::ForCurrentProcess()->GetSwitchValueASCII(
    202           switches::kProcessChannelID)),
    203       use_mojo_channel(mojo) {}
    204 
    205 
    206 ChildThread::ChildThreadMessageRouter::ChildThreadMessageRouter(
    207     IPC::Sender* sender)
    208     : sender_(sender) {}
    209 
    210 bool ChildThread::ChildThreadMessageRouter::Send(IPC::Message* msg) {
    211   return sender_->Send(msg);
    212 }
    213 
    214 ChildThread::ChildThread()
    215     : router_(this),
    216       in_browser_process_(false),
    217       channel_connected_factory_(this) {
    218   Init(Options());
    219 }
    220 
    221 ChildThread::ChildThread(const Options& options)
    222     : router_(this),
    223       in_browser_process_(true),
    224       channel_connected_factory_(this) {
    225   Init(options);
    226 }
    227 
    228 scoped_ptr<IPC::SyncChannel> ChildThread::CreateChannel(bool use_mojo_channel) {
    229   if (use_mojo_channel) {
    230     VLOG(1) << "Mojo is enabled on child";
    231     return IPC::SyncChannel::Create(
    232         IPC::ChannelMojo::CreateClientFactory(channel_name_),
    233         this,
    234         ChildProcess::current()->io_message_loop_proxy(),
    235         true,
    236         ChildProcess::current()->GetShutDownEvent());
    237   }
    238 
    239   VLOG(1) << "Mojo is disabled on child";
    240   return IPC::SyncChannel::Create(
    241       channel_name_,
    242       IPC::Channel::MODE_CLIENT,
    243       this,
    244       ChildProcess::current()->io_message_loop_proxy(),
    245       true,
    246       ChildProcess::current()->GetShutDownEvent());
    247 }
    248 
    249 void ChildThread::Init(const Options& options) {
    250   channel_name_ = options.channel_name;
    251 
    252   g_lazy_tls.Pointer()->Set(this);
    253   on_channel_error_called_ = false;
    254   message_loop_ = base::MessageLoop::current();
    255 #ifdef IPC_MESSAGE_LOG_ENABLED
    256   // We must make sure to instantiate the IPC Logger *before* we create the
    257   // channel, otherwise we can get a callback on the IO thread which creates
    258   // the logger, and the logger does not like being created on the IO thread.
    259   IPC::Logging::GetInstance();
    260 #endif
    261   channel_ = CreateChannel(options.use_mojo_channel);
    262 #ifdef IPC_MESSAGE_LOG_ENABLED
    263   if (!in_browser_process_)
    264     IPC::Logging::GetInstance()->SetIPCSender(this);
    265 #endif
    266 
    267   mojo_application_.reset(new MojoApplication);
    268 
    269   sync_message_filter_ =
    270       new IPC::SyncMessageFilter(ChildProcess::current()->GetShutDownEvent());
    271   thread_safe_sender_ = new ThreadSafeSender(
    272       base::MessageLoopProxy::current().get(), sync_message_filter_.get());
    273 
    274   resource_dispatcher_.reset(new ResourceDispatcher(this));
    275   socket_stream_dispatcher_.reset(new SocketStreamDispatcher());
    276   websocket_dispatcher_.reset(new WebSocketDispatcher);
    277   file_system_dispatcher_.reset(new FileSystemDispatcher());
    278 
    279   histogram_message_filter_ = new ChildHistogramMessageFilter();
    280   resource_message_filter_ =
    281       new ChildResourceMessageFilter(resource_dispatcher());
    282 
    283   service_worker_message_filter_ =
    284       new ServiceWorkerMessageFilter(thread_safe_sender_.get());
    285 
    286   quota_message_filter_ =
    287       new QuotaMessageFilter(thread_safe_sender_.get());
    288   quota_dispatcher_.reset(new QuotaDispatcher(thread_safe_sender_.get(),
    289                                               quota_message_filter_.get()));
    290 
    291   channel_->AddFilter(histogram_message_filter_.get());
    292   channel_->AddFilter(sync_message_filter_.get());
    293   channel_->AddFilter(resource_message_filter_.get());
    294   channel_->AddFilter(quota_message_filter_->GetFilter());
    295   channel_->AddFilter(service_worker_message_filter_->GetFilter());
    296 
    297   if (!base::CommandLine::ForCurrentProcess()->HasSwitch(
    298           switches::kSingleProcess)) {
    299     // In single process mode, browser-side tracing will cover the whole
    300     // process including renderers.
    301     channel_->AddFilter(new tracing::ChildTraceMessageFilter(
    302         ChildProcess::current()->io_message_loop_proxy()));
    303   }
    304 
    305   // In single process mode we may already have a power monitor
    306   if (!base::PowerMonitor::Get()) {
    307     scoped_ptr<PowerMonitorBroadcastSource> power_monitor_source(
    308       new PowerMonitorBroadcastSource());
    309     channel_->AddFilter(power_monitor_source->GetMessageFilter());
    310 
    311     power_monitor_.reset(new base::PowerMonitor(
    312         power_monitor_source.PassAs<base::PowerMonitorSource>()));
    313   }
    314 
    315 #if defined(OS_POSIX)
    316   // Check that --process-type is specified so we don't do this in unit tests
    317   // and single-process mode.
    318   if (base::CommandLine::ForCurrentProcess()->HasSwitch(switches::kProcessType))
    319     channel_->AddFilter(new SuicideOnChannelErrorFilter());
    320 #endif
    321 
    322   int connection_timeout = kConnectionTimeoutS;
    323   std::string connection_override =
    324       base::CommandLine::ForCurrentProcess()->GetSwitchValueASCII(
    325           switches::kIPCConnectionTimeout);
    326   if (!connection_override.empty()) {
    327     int temp;
    328     if (base::StringToInt(connection_override, &temp))
    329       connection_timeout = temp;
    330   }
    331 
    332   base::MessageLoop::current()->PostDelayedTask(
    333       FROM_HERE,
    334       base::Bind(&ChildThread::EnsureConnected,
    335                  channel_connected_factory_.GetWeakPtr()),
    336       base::TimeDelta::FromSeconds(connection_timeout));
    337 
    338 #if defined(OS_ANDROID)
    339   {
    340     base::AutoLock lock(g_lazy_child_thread_lock.Get());
    341     g_child_thread = this;
    342   }
    343   // Signalling without locking is fine here because only
    344   // one thread can wait on the condition variable.
    345   g_lazy_child_thread_cv.Get().Signal();
    346 #endif
    347 
    348 #if defined(TCMALLOC_TRACE_MEMORY_SUPPORTED)
    349   trace_memory_controller_.reset(new base::debug::TraceMemoryController(
    350       message_loop_->message_loop_proxy(),
    351       ::HeapProfilerWithPseudoStackStart,
    352       ::HeapProfilerStop,
    353       ::GetHeapProfile));
    354 #endif
    355 
    356   shared_bitmap_manager_.reset(
    357       new ChildSharedBitmapManager(thread_safe_sender()));
    358 }
    359 
    360 ChildThread::~ChildThread() {
    361 #ifdef IPC_MESSAGE_LOG_ENABLED
    362   IPC::Logging::GetInstance()->SetIPCSender(NULL);
    363 #endif
    364 
    365   channel_->RemoveFilter(histogram_message_filter_.get());
    366   channel_->RemoveFilter(sync_message_filter_.get());
    367 
    368   // The ChannelProxy object caches a pointer to the IPC thread, so need to
    369   // reset it as it's not guaranteed to outlive this object.
    370   // NOTE: this also has the side-effect of not closing the main IPC channel to
    371   // the browser process.  This is needed because this is the signal that the
    372   // browser uses to know that this process has died, so we need it to be alive
    373   // until this process is shut down, and the OS closes the handle
    374   // automatically.  We used to watch the object handle on Windows to do this,
    375   // but it wasn't possible to do so on POSIX.
    376   channel_->ClearIPCTaskRunner();
    377   g_lazy_tls.Pointer()->Set(NULL);
    378 }
    379 
    380 void ChildThread::Shutdown() {
    381   // Delete objects that hold references to blink so derived classes can
    382   // safely shutdown blink in their Shutdown implementation.
    383   file_system_dispatcher_.reset();
    384   quota_dispatcher_.reset();
    385   WebFileSystemImpl::DeleteThreadSpecificInstance();
    386 }
    387 
    388 void ChildThread::OnChannelConnected(int32 peer_pid) {
    389   channel_connected_factory_.InvalidateWeakPtrs();
    390 }
    391 
    392 void ChildThread::OnChannelError() {
    393   set_on_channel_error_called(true);
    394   base::MessageLoop::current()->Quit();
    395 }
    396 
    397 bool ChildThread::Send(IPC::Message* msg) {
    398   DCHECK(base::MessageLoop::current() == message_loop());
    399   if (!channel_) {
    400     delete msg;
    401     return false;
    402   }
    403 
    404   return channel_->Send(msg);
    405 }
    406 
    407 MessageRouter* ChildThread::GetRouter() {
    408   DCHECK(base::MessageLoop::current() == message_loop());
    409   return &router_;
    410 }
    411 
    412 base::SharedMemory* ChildThread::AllocateSharedMemory(size_t buf_size) {
    413   return AllocateSharedMemory(buf_size, this);
    414 }
    415 
    416 // static
    417 base::SharedMemory* ChildThread::AllocateSharedMemory(
    418     size_t buf_size,
    419     IPC::Sender* sender) {
    420   scoped_ptr<base::SharedMemory> shared_buf;
    421 #if defined(OS_WIN)
    422   shared_buf.reset(new base::SharedMemory);
    423   if (!shared_buf->CreateAndMapAnonymous(buf_size)) {
    424     NOTREACHED();
    425     return NULL;
    426   }
    427 #else
    428   // On POSIX, we need to ask the browser to create the shared memory for us,
    429   // since this is blocked by the sandbox.
    430   base::SharedMemoryHandle shared_mem_handle;
    431   if (sender->Send(new ChildProcessHostMsg_SyncAllocateSharedMemory(
    432                            buf_size, &shared_mem_handle))) {
    433     if (base::SharedMemory::IsHandleValid(shared_mem_handle)) {
    434       shared_buf.reset(new base::SharedMemory(shared_mem_handle, false));
    435       if (!shared_buf->Map(buf_size)) {
    436         NOTREACHED() << "Map failed";
    437         return NULL;
    438       }
    439     } else {
    440       NOTREACHED() << "Browser failed to allocate shared memory";
    441       return NULL;
    442     }
    443   } else {
    444     NOTREACHED() << "Browser allocation request message failed";
    445     return NULL;
    446   }
    447 #endif
    448   return shared_buf.release();
    449 }
    450 
    451 bool ChildThread::OnMessageReceived(const IPC::Message& msg) {
    452   if (mojo_application_->OnMessageReceived(msg))
    453     return true;
    454 
    455   // Resource responses are sent to the resource dispatcher.
    456   if (resource_dispatcher_->OnMessageReceived(msg))
    457     return true;
    458   if (socket_stream_dispatcher_->OnMessageReceived(msg))
    459     return true;
    460   if (websocket_dispatcher_->OnMessageReceived(msg))
    461     return true;
    462   if (file_system_dispatcher_->OnMessageReceived(msg))
    463     return true;
    464 
    465   bool handled = true;
    466   IPC_BEGIN_MESSAGE_MAP(ChildThread, msg)
    467     IPC_MESSAGE_HANDLER(ChildProcessMsg_Shutdown, OnShutdown)
    468 #if defined(IPC_MESSAGE_LOG_ENABLED)
    469     IPC_MESSAGE_HANDLER(ChildProcessMsg_SetIPCLoggingEnabled,
    470                         OnSetIPCLoggingEnabled)
    471 #endif
    472     IPC_MESSAGE_HANDLER(ChildProcessMsg_SetProfilerStatus,
    473                         OnSetProfilerStatus)
    474     IPC_MESSAGE_HANDLER(ChildProcessMsg_GetChildProfilerData,
    475                         OnGetChildProfilerData)
    476     IPC_MESSAGE_HANDLER(ChildProcessMsg_DumpHandles, OnDumpHandles)
    477     IPC_MESSAGE_HANDLER(ChildProcessMsg_SetProcessBackgrounded,
    478                         OnProcessBackgrounded)
    479 #if defined(USE_TCMALLOC)
    480     IPC_MESSAGE_HANDLER(ChildProcessMsg_GetTcmallocStats, OnGetTcmallocStats)
    481 #endif
    482     IPC_MESSAGE_UNHANDLED(handled = false)
    483   IPC_END_MESSAGE_MAP()
    484 
    485   if (handled)
    486     return true;
    487 
    488   if (msg.routing_id() == MSG_ROUTING_CONTROL)
    489     return OnControlMessageReceived(msg);
    490 
    491   return router_.OnMessageReceived(msg);
    492 }
    493 
    494 bool ChildThread::OnControlMessageReceived(const IPC::Message& msg) {
    495   return false;
    496 }
    497 
    498 void ChildThread::OnShutdown() {
    499   base::MessageLoop::current()->Quit();
    500 }
    501 
    502 #if defined(IPC_MESSAGE_LOG_ENABLED)
    503 void ChildThread::OnSetIPCLoggingEnabled(bool enable) {
    504   if (enable)
    505     IPC::Logging::GetInstance()->Enable();
    506   else
    507     IPC::Logging::GetInstance()->Disable();
    508 }
    509 #endif  //  IPC_MESSAGE_LOG_ENABLED
    510 
    511 void ChildThread::OnSetProfilerStatus(ThreadData::Status status) {
    512   ThreadData::InitializeAndSetTrackingStatus(status);
    513 }
    514 
    515 void ChildThread::OnGetChildProfilerData(int sequence_number) {
    516   tracked_objects::ProcessDataSnapshot process_data;
    517   ThreadData::Snapshot(false, &process_data);
    518 
    519   Send(new ChildProcessHostMsg_ChildProfilerData(sequence_number,
    520                                                  process_data));
    521 }
    522 
    523 void ChildThread::OnDumpHandles() {
    524 #if defined(OS_WIN)
    525   scoped_refptr<HandleEnumerator> handle_enum(
    526       new HandleEnumerator(
    527           base::CommandLine::ForCurrentProcess()->HasSwitch(
    528               switches::kAuditAllHandles)));
    529   handle_enum->EnumerateHandles();
    530   Send(new ChildProcessHostMsg_DumpHandlesDone);
    531 #else
    532   NOTIMPLEMENTED();
    533 #endif
    534 }
    535 
    536 #if defined(USE_TCMALLOC)
    537 void ChildThread::OnGetTcmallocStats() {
    538   std::string result;
    539   char buffer[1024 * 32];
    540   base::allocator::GetStats(buffer, sizeof(buffer));
    541   result.append(buffer);
    542   Send(new ChildProcessHostMsg_TcmallocStats(result));
    543 }
    544 #endif
    545 
    546 ChildThread* ChildThread::current() {
    547   return g_lazy_tls.Pointer()->Get();
    548 }
    549 
    550 #if defined(OS_ANDROID)
    551 // The method must NOT be called on the child thread itself.
    552 // It may block the child thread if so.
    553 void ChildThread::ShutdownThread() {
    554   DCHECK(!ChildThread::current()) <<
    555       "this method should NOT be called from child thread itself";
    556   {
    557     base::AutoLock lock(g_lazy_child_thread_lock.Get());
    558     while (!g_child_thread)
    559       g_lazy_child_thread_cv.Get().Wait();
    560   }
    561   DCHECK_NE(base::MessageLoop::current(), g_child_thread->message_loop());
    562   g_child_thread->message_loop()->PostTask(
    563       FROM_HERE, base::Bind(&QuitMainThreadMessageLoop));
    564 }
    565 #endif
    566 
    567 void ChildThread::OnProcessFinalRelease() {
    568   if (on_channel_error_called_) {
    569     base::MessageLoop::current()->Quit();
    570     return;
    571   }
    572 
    573   // The child process shutdown sequence is a request response based mechanism,
    574   // where we send out an initial feeler request to the child process host
    575   // instance in the browser to verify if it's ok to shutdown the child process.
    576   // The browser then sends back a response if it's ok to shutdown. This avoids
    577   // race conditions if the process refcount is 0 but there's an IPC message
    578   // inflight that would addref it.
    579   Send(new ChildProcessHostMsg_ShutdownRequest);
    580 }
    581 
    582 void ChildThread::EnsureConnected() {
    583   VLOG(0) << "ChildThread::EnsureConnected()";
    584   base::KillProcess(base::GetCurrentProcessHandle(), 0, false);
    585 }
    586 
    587 void ChildThread::OnProcessBackgrounded(bool background) {
    588   // Set timer slack to maximum on main thread when in background.
    589   base::TimerSlack timer_slack = base::TIMER_SLACK_NONE;
    590   if (background)
    591     timer_slack = base::TIMER_SLACK_MAXIMUM;
    592   base::MessageLoop::current()->SetTimerSlack(timer_slack);
    593 }
    594 
    595 }  // namespace content
    596