content/child/child_thread.cc

   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_gpu_memory_buffer_manager.h"
  30 #include "content/child/child_histogram_message_filter.h"
  31 #include "content/child/child_process.h"
  32 #include "content/child/child_resource_message_filter.h"
  33 #include "content/child/child_shared_bitmap_manager.h"
  34 #include "content/child/fileapi/file_system_dispatcher.h"
  35 #include "content/child/fileapi/webfilesystem_impl.h"
  36 #include "content/child/geofencing/geofencing_message_filter.h"
  37 #include "content/child/mojo/mojo_application.h"
  38 #include "content/child/power_monitor_broadcast_source.h"
  39 #include "content/child/quota_dispatcher.h"
  40 #include "content/child/quota_message_filter.h"
  41 #include "content/child/resource_dispatcher.h"
  42 #include "content/child/service_worker/service_worker_message_filter.h"
  43 #include "content/child/socket_stream_dispatcher.h"
  44 #include "content/child/thread_safe_sender.h"
  45 #include "content/child/websocket_dispatcher.h"
  46 #include "content/common/child_process_messages.h"
  47 #include "content/public/common/content_switches.h"
  48 #include "ipc/ipc_logging.h"
  49 #include "ipc/ipc_switches.h"
  50 #include "ipc/ipc_sync_channel.h"
  51 #include "ipc/ipc_sync_message_filter.h"
  52 #include "ipc/mojo/ipc_channel_mojo.h"
  53
  54 #if defined(OS_WIN)
  55 #include "content/common/handle_enumerator_win.h"
  56 #endif
  57
  58 #if defined(TCMALLOC_TRACE_MEMORY_SUPPORTED)
  59 #include "third_party/tcmalloc/chromium/src/gperftools/heap-profiler.h"
  60 #endif
  61
  62 using tracked_objects::ThreadData;
  63
  64 namespace content {
  65 namespace {
  66
  67 // How long to wait for a connection to the browser process before giving up.
  68 const int kConnectionTimeoutS = 15;
  69
  70 base::LazyInstance<base::ThreadLocalPointer<ChildThread> > g_lazy_tls =
  71     LAZY_INSTANCE_INITIALIZER;
  72
  73 // This isn't needed on Windows because there the sandbox's job object
  74 // terminates child processes automatically. For unsandboxed processes (i.e.
  75 // plugins), PluginThread has EnsureTerminateMessageFilter.
  76 #if defined(OS_POSIX)
  77
  78 // TODO(earthdok): Re-enable on CrOS http://crbug.com/360622
  79 #if (defined(ADDRESS_SANITIZER) || defined(LEAK_SANITIZER) || \
  80     defined(THREAD_SANITIZER)) && !defined(OS_CHROMEOS)
  81 // A thread delegate that waits for |duration| and then exits the process with
  82 // _exit(0).
  83 class WaitAndExitDelegate : public base::PlatformThread::Delegate {
  84  public:
  85   explicit WaitAndExitDelegate(base::TimeDelta duration)
  86       : duration_(duration) {}
  87   virtual ~WaitAndExitDelegate() override {}
  88
  89   virtual void ThreadMain() override {
  90     base::PlatformThread::Sleep(duration_);
  91     _exit(0);
  92   }
  93
  94  private:
  95   const base::TimeDelta duration_;
  96   DISALLOW_COPY_AND_ASSIGN(WaitAndExitDelegate);
  97 };
  98
  99 bool CreateWaitAndExitThread(base::TimeDelta duration) {
 100   scoped_ptr<WaitAndExitDelegate> delegate(new WaitAndExitDelegate(duration));
 101
 102   const bool thread_created =
 103       base::PlatformThread::CreateNonJoinable(0, delegate.get());
 104   if (!thread_created)
 105     return false;
 106
 107   // A non joinable thread has been created. The thread will either terminate
 108   // the process or will be terminated by the process. Therefore, keep the
 109   // delegate object alive for the lifetime of the process.
 110   WaitAndExitDelegate* leaking_delegate = delegate.release();
 111   ANNOTATE_LEAKING_OBJECT_PTR(leaking_delegate);
 112   ignore_result(leaking_delegate);
 113   return true;
 114 }
 115 #endif
 116
 117 class SuicideOnChannelErrorFilter : public IPC::MessageFilter {
 118  public:
 119   // IPC::MessageFilter
 120   void OnChannelError() override {
 121     // For renderer/worker processes:
 122     // On POSIX, at least, one can install an unload handler which loops
 123     // forever and leave behind a renderer process which eats 100% CPU forever.
 124     //
 125     // This is because the terminate signals (ViewMsg_ShouldClose and the error
 126     // from the IPC sender) are routed to the main message loop but never
 127     // processed (because that message loop is stuck in V8).
 128     //
 129     // One could make the browser SIGKILL the renderers, but that leaves open a
 130     // large window where a browser failure (or a user, manually terminating
 131     // the browser because "it's stuck") will leave behind a process eating all
 132     // the CPU.
 133     //
 134     // So, we install a filter on the sender so that we can process this event
 135     // here and kill the process.
 136     // TODO(earthdok): Re-enable on CrOS http://crbug.com/360622
 137 #if (defined(ADDRESS_SANITIZER) || defined(LEAK_SANITIZER) || \
 138     defined(THREAD_SANITIZER)) && !defined(OS_CHROMEOS)
 139     // Some sanitizer tools rely on exit handlers (e.g. to run leak detection,
 140     // or dump code coverage data to disk). Instead of exiting the process
 141     // immediately, we give it 60 seconds to run exit handlers.
 142     CHECK(CreateWaitAndExitThread(base::TimeDelta::FromSeconds(60)));
 143 #if defined(LEAK_SANITIZER)
 144     // Invoke LeakSanitizer early to avoid detecting shutdown-only leaks. If
 145     // leaks are found, the process will exit here.
 146     __lsan_do_leak_check();
 147 #endif
 148 #else
 149     _exit(0);
 150 #endif
 151   }
 152
 153  protected:
 154   ~SuicideOnChannelErrorFilter() override {}
 155 };
 156
 157 #endif  // OS(POSIX)
 158
 159 #if defined(OS_ANDROID)
 160 ChildThread* g_child_thread = NULL;
 161
 162 // A lock protects g_child_thread.
 163 base::LazyInstance<base::Lock> g_lazy_child_thread_lock =
 164     LAZY_INSTANCE_INITIALIZER;
 165
 166 // base::ConditionVariable has an explicit constructor that takes
 167 // a base::Lock pointer as parameter. The base::DefaultLazyInstanceTraits
 168 // doesn't handle the case. Thus, we need our own class here.
 169 struct CondVarLazyInstanceTraits {
 170   static const bool kRegisterOnExit = true;
 171 #ifndef NDEBUG
 172   static const bool kAllowedToAccessOnNonjoinableThread = false;
 173 #endif
 174
 175   static base::ConditionVariable* New(void* instance) {
 176     return new (instance) base::ConditionVariable(
 177         g_lazy_child_thread_lock.Pointer());
 178   }
 179   static void Delete(base::ConditionVariable* instance) {
 180     instance->~ConditionVariable();
 181   }
 182 };
 183
 184 // A condition variable that synchronize threads initializing and waiting
 185 // for g_child_thread.
 186 base::LazyInstance<base::ConditionVariable, CondVarLazyInstanceTraits>
 187     g_lazy_child_thread_cv = LAZY_INSTANCE_INITIALIZER;
 188
 189 void QuitMainThreadMessageLoop() {
 190   base::MessageLoop::current()->Quit();
 191 }
 192
 193 #endif
 194
 195 }  // namespace
 196
 197 ChildThread::Options::Options()
 198     : channel_name(base::CommandLine::ForCurrentProcess()->GetSwitchValueASCII(
 199           switches::kProcessChannelID)),
 200       use_mojo_channel(false) {}
 201
 202 ChildThread::Options::Options(bool mojo)
 203     : channel_name(base::CommandLine::ForCurrentProcess()->GetSwitchValueASCII(
 204           switches::kProcessChannelID)),
 205       use_mojo_channel(mojo) {}
 206
 207
 208 ChildThread::ChildThreadMessageRouter::ChildThreadMessageRouter(
 209     IPC::Sender* sender)
 210     : sender_(sender) {}
 211
 212 bool ChildThread::ChildThreadMessageRouter::Send(IPC::Message* msg) {
 213   return sender_->Send(msg);
 214 }
 215
 216 ChildThread::ChildThread()
 217     : router_(this),
 218       in_browser_process_(false),
 219       channel_connected_factory_(this) {
 220   Init(Options());
 221 }
 222
 223 ChildThread::ChildThread(const Options& options)
 224     : router_(this),
 225       in_browser_process_(true),
 226       channel_connected_factory_(this) {
 227   Init(options);
 228 }
 229
 230 scoped_ptr<IPC::SyncChannel> ChildThread::CreateChannel(bool use_mojo_channel) {
 231   if (use_mojo_channel) {
 232     VLOG(1) << "Mojo is enabled on child";
 233     return IPC::SyncChannel::Create(
 234         IPC::ChannelMojo::CreateClientFactory(channel_name_),
 235         this,
 236         ChildProcess::current()->io_message_loop_proxy(),
 237         true,
 238         ChildProcess::current()->GetShutDownEvent());
 239   }
 240
 241   VLOG(1) << "Mojo is disabled on child";
 242   return IPC::SyncChannel::Create(
 243       channel_name_,
 244       IPC::Channel::MODE_CLIENT,
 245       this,
 246       ChildProcess::current()->io_message_loop_proxy(),
 247       true,
 248       ChildProcess::current()->GetShutDownEvent());
 249 }
 250
 251 void ChildThread::Init(const Options& options) {
 252   channel_name_ = options.channel_name;
 253
 254   g_lazy_tls.Pointer()->Set(this);
 255   on_channel_error_called_ = false;
 256   message_loop_ = base::MessageLoop::current();
 257 #ifdef IPC_MESSAGE_LOG_ENABLED
 258   // We must make sure to instantiate the IPC Logger *before* we create the
 259   // channel, otherwise we can get a callback on the IO thread which creates
 260   // the logger, and the logger does not like being created on the IO thread.
 261   IPC::Logging::GetInstance();
 262 #endif
 263   channel_ = CreateChannel(options.use_mojo_channel);
 264 #ifdef IPC_MESSAGE_LOG_ENABLED
 265   if (!in_browser_process_)
 266     IPC::Logging::GetInstance()->SetIPCSender(this);
 267 #endif
 268
 269   mojo_application_.reset(new MojoApplication);
 270
 271   sync_message_filter_ =
 272       new IPC::SyncMessageFilter(ChildProcess::current()->GetShutDownEvent());
 273   thread_safe_sender_ = new ThreadSafeSender(
 274       base::MessageLoopProxy::current().get(), sync_message_filter_.get());
 275
 276   resource_dispatcher_.reset(new ResourceDispatcher(this));
 277   socket_stream_dispatcher_.reset(new SocketStreamDispatcher());
 278   websocket_dispatcher_.reset(new WebSocketDispatcher);
 279   file_system_dispatcher_.reset(new FileSystemDispatcher());
 280
 281   histogram_message_filter_ = new ChildHistogramMessageFilter();
 282   resource_message_filter_ =
 283       new ChildResourceMessageFilter(resource_dispatcher());
 284
 285   service_worker_message_filter_ =
 286       new ServiceWorkerMessageFilter(thread_safe_sender_.get());
 287
 288   quota_message_filter_ =
 289       new QuotaMessageFilter(thread_safe_sender_.get());
 290   quota_dispatcher_.reset(new QuotaDispatcher(thread_safe_sender_.get(),
 291                                               quota_message_filter_.get()));
 292
 293   geofencing_message_filter_ =
 294       new GeofencingMessageFilter(thread_safe_sender_.get());
 295
 296   channel_->AddFilter(histogram_message_filter_.get());
 297   channel_->AddFilter(sync_message_filter_.get());
 298   channel_->AddFilter(resource_message_filter_.get());
 299   channel_->AddFilter(quota_message_filter_->GetFilter());
 300   channel_->AddFilter(service_worker_message_filter_->GetFilter());
 301   channel_->AddFilter(geofencing_message_filter_->GetFilter());
 302
 303   if (!base::CommandLine::ForCurrentProcess()->HasSwitch(
 304           switches::kSingleProcess)) {
 305     // In single process mode, browser-side tracing will cover the whole
 306     // process including renderers.
 307     channel_->AddFilter(new tracing::ChildTraceMessageFilter(
 308         ChildProcess::current()->io_message_loop_proxy()));
 309   }
 310
 311   // In single process mode we may already have a power monitor
 312   if (!base::PowerMonitor::Get()) {
 313     scoped_ptr<PowerMonitorBroadcastSource> power_monitor_source(
 314       new PowerMonitorBroadcastSource());
 315     channel_->AddFilter(power_monitor_source->GetMessageFilter());
 316
 317     power_monitor_.reset(new base::PowerMonitor(
 318         power_monitor_source.Pass()));
 319   }
 320
 321 #if defined(OS_POSIX)
 322   // Check that --process-type is specified so we don't do this in unit tests
 323   // and single-process mode.
 324   if (base::CommandLine::ForCurrentProcess()->HasSwitch(switches::kProcessType))
 325     channel_->AddFilter(new SuicideOnChannelErrorFilter());
 326 #endif
 327
 328   int connection_timeout = kConnectionTimeoutS;
 329   std::string connection_override =
 330       base::CommandLine::ForCurrentProcess()->GetSwitchValueASCII(
 331           switches::kIPCConnectionTimeout);
 332   if (!connection_override.empty()) {
 333     int temp;
 334     if (base::StringToInt(connection_override, &temp))
 335       connection_timeout = temp;
 336   }
 337
 338   base::MessageLoop::current()->PostDelayedTask(
 339       FROM_HERE,
 340       base::Bind(&ChildThread::EnsureConnected,
 341                  channel_connected_factory_.GetWeakPtr()),
 342       base::TimeDelta::FromSeconds(connection_timeout));
 343
 344 #if defined(OS_ANDROID)
 345   {
 346     base::AutoLock lock(g_lazy_child_thread_lock.Get());
 347     g_child_thread = this;
 348   }
 349   // Signalling without locking is fine here because only
 350   // one thread can wait on the condition variable.
 351   g_lazy_child_thread_cv.Get().Signal();
 352 #endif
 353
 354 #if defined(TCMALLOC_TRACE_MEMORY_SUPPORTED)
 355   trace_memory_controller_.reset(new base::debug::TraceMemoryController(
 356       message_loop_->message_loop_proxy(),
 357       ::HeapProfilerWithPseudoStackStart,
 358       ::HeapProfilerStop,
 359       ::GetHeapProfile));
 360 #endif
 361
 362   shared_bitmap_manager_.reset(
 363       new ChildSharedBitmapManager(thread_safe_sender()));
 364
 365   gpu_memory_buffer_manager_.reset(
 366       new ChildGpuMemoryBufferManager(thread_safe_sender()));
 367 }
 368
 369 ChildThread::~ChildThread() {
 370 #ifdef IPC_MESSAGE_LOG_ENABLED
 371   IPC::Logging::GetInstance()->SetIPCSender(NULL);
 372 #endif
 373
 374   channel_->RemoveFilter(histogram_message_filter_.get());
 375   channel_->RemoveFilter(sync_message_filter_.get());
 376
 377   // The ChannelProxy object caches a pointer to the IPC thread, so need to
 378   // reset it as it's not guaranteed to outlive this object.
 379   // NOTE: this also has the side-effect of not closing the main IPC channel to
 380   // the browser process.  This is needed because this is the signal that the
 381   // browser uses to know that this process has died, so we need it to be alive
 382   // until this process is shut down, and the OS closes the handle
 383   // automatically.  We used to watch the object handle on Windows to do this,
 384   // but it wasn't possible to do so on POSIX.
 385   channel_->ClearIPCTaskRunner();
 386   g_lazy_tls.Pointer()->Set(NULL);
 387 }
 388
 389 void ChildThread::Shutdown() {
 390   // Delete objects that hold references to blink so derived classes can
 391   // safely shutdown blink in their Shutdown implementation.
 392   file_system_dispatcher_.reset();
 393   quota_dispatcher_.reset();
 394   WebFileSystemImpl::DeleteThreadSpecificInstance();
 395 }
 396
 397 void ChildThread::OnChannelConnected(int32 peer_pid) {
 398   channel_connected_factory_.InvalidateWeakPtrs();
 399 }
 400
 401 void ChildThread::OnChannelError() {
 402   set_on_channel_error_called(true);
 403   base::MessageLoop::current()->Quit();
 404 }
 405
 406 bool ChildThread::Send(IPC::Message* msg) {
 407   DCHECK(base::MessageLoop::current() == message_loop());
 408   if (!channel_) {
 409     delete msg;
 410     return false;
 411   }
 412
 413   return channel_->Send(msg);
 414 }
 415
 416 MessageRouter* ChildThread::GetRouter() {
 417   DCHECK(base::MessageLoop::current() == message_loop());
 418   return &router_;
 419 }
 420
 421 base::SharedMemory* ChildThread::AllocateSharedMemory(size_t buf_size) {
 422   return AllocateSharedMemory(buf_size, this);
 423 }
 424
 425 // static
 426 base::SharedMemory* ChildThread::AllocateSharedMemory(
 427     size_t buf_size,
 428     IPC::Sender* sender) {
 429   scoped_ptr<base::SharedMemory> shared_buf;
 430 #if defined(OS_WIN)
 431   shared_buf.reset(new base::SharedMemory);
 432   if (!shared_buf->CreateAndMapAnonymous(buf_size)) {
 433     NOTREACHED();
 434     return NULL;
 435   }
 436 #else
 437   // On POSIX, we need to ask the browser to create the shared memory for us,
 438   // since this is blocked by the sandbox.
 439   base::SharedMemoryHandle shared_mem_handle;
 440   if (sender->Send(new ChildProcessHostMsg_SyncAllocateSharedMemory(
 441                            buf_size, &shared_mem_handle))) {
 442     if (base::SharedMemory::IsHandleValid(shared_mem_handle)) {
 443       shared_buf.reset(new base::SharedMemory(shared_mem_handle, false));
 444       if (!shared_buf->Map(buf_size)) {
 445         NOTREACHED() << "Map failed";
 446         return NULL;
 447       }
 448     } else {
 449       NOTREACHED() << "Browser failed to allocate shared memory";
 450       return NULL;
 451     }
 452   } else {
 453     NOTREACHED() << "Browser allocation request message failed";
 454     return NULL;
 455   }
 456 #endif
 457   return shared_buf.release();
 458 }
 459
 460 bool ChildThread::OnMessageReceived(const IPC::Message& msg) {
 461   if (mojo_application_->OnMessageReceived(msg))
 462     return true;
 463
 464   // Resource responses are sent to the resource dispatcher.
 465   if (resource_dispatcher_->OnMessageReceived(msg))
 466     return true;
 467   if (socket_stream_dispatcher_->OnMessageReceived(msg))
 468     return true;
 469   if (websocket_dispatcher_->OnMessageReceived(msg))
 470     return true;
 471   if (file_system_dispatcher_->OnMessageReceived(msg))
 472     return true;
 473
 474   bool handled = true;
 475   IPC_BEGIN_MESSAGE_MAP(ChildThread, msg)
 476     IPC_MESSAGE_HANDLER(ChildProcessMsg_Shutdown, OnShutdown)
 477 #if defined(IPC_MESSAGE_LOG_ENABLED)
 478     IPC_MESSAGE_HANDLER(ChildProcessMsg_SetIPCLoggingEnabled,
 479                         OnSetIPCLoggingEnabled)
 480 #endif
 481     IPC_MESSAGE_HANDLER(ChildProcessMsg_SetProfilerStatus,
 482                         OnSetProfilerStatus)
 483     IPC_MESSAGE_HANDLER(ChildProcessMsg_GetChildProfilerData,
 484                         OnGetChildProfilerData)
 485     IPC_MESSAGE_HANDLER(ChildProcessMsg_DumpHandles, OnDumpHandles)
 486     IPC_MESSAGE_HANDLER(ChildProcessMsg_SetProcessBackgrounded,
 487                         OnProcessBackgrounded)
 488 #if defined(USE_TCMALLOC)
 489     IPC_MESSAGE_HANDLER(ChildProcessMsg_GetTcmallocStats, OnGetTcmallocStats)
 490 #endif
 491     IPC_MESSAGE_UNHANDLED(handled = false)
 492   IPC_END_MESSAGE_MAP()
 493
 494   if (handled)
 495     return true;
 496
 497   if (msg.routing_id() == MSG_ROUTING_CONTROL)
 498     return OnControlMessageReceived(msg);
 499
 500   return router_.OnMessageReceived(msg);
 501 }
 502
 503 bool ChildThread::OnControlMessageReceived(const IPC::Message& msg) {
 504   return false;
 505 }
 506
 507 void ChildThread::OnShutdown() {
 508   base::MessageLoop::current()->Quit();
 509 }
 510
 511 #if defined(IPC_MESSAGE_LOG_ENABLED)
 512 void ChildThread::OnSetIPCLoggingEnabled(bool enable) {
 513   if (enable)
 514     IPC::Logging::GetInstance()->Enable();
 515   else
 516     IPC::Logging::GetInstance()->Disable();
 517 }
 518 #endif  //  IPC_MESSAGE_LOG_ENABLED
 519
 520 void ChildThread::OnSetProfilerStatus(ThreadData::Status status) {
 521   ThreadData::InitializeAndSetTrackingStatus(status);
 522 }
 523
 524 void ChildThread::OnGetChildProfilerData(int sequence_number) {
 525   tracked_objects::ProcessDataSnapshot process_data;
 526   ThreadData::Snapshot(false, &process_data);
 527
 528   Send(new ChildProcessHostMsg_ChildProfilerData(sequence_number,
 529                                                  process_data));
 530 }
 531
 532 void ChildThread::OnDumpHandles() {
 533 #if defined(OS_WIN)
 534   scoped_refptr<HandleEnumerator> handle_enum(
 535       new HandleEnumerator(
 536           base::CommandLine::ForCurrentProcess()->HasSwitch(
 537               switches::kAuditAllHandles)));
 538   handle_enum->EnumerateHandles();
 539   Send(new ChildProcessHostMsg_DumpHandlesDone);
 540 #else
 541   NOTIMPLEMENTED();
 542 #endif
 543 }
 544
 545 #if defined(USE_TCMALLOC)
 546 void ChildThread::OnGetTcmallocStats() {
 547   std::string result;
 548   char buffer[1024 * 32];
 549   base::allocator::GetStats(buffer, sizeof(buffer));
 550   result.append(buffer);
 551   Send(new ChildProcessHostMsg_TcmallocStats(result));
 552 }
 553 #endif
 554
 555 ChildThread* ChildThread::current() {
 556   return g_lazy_tls.Pointer()->Get();
 557 }
 558
 559 #if defined(OS_ANDROID)
 560 // The method must NOT be called on the child thread itself.
 561 // It may block the child thread if so.
 562 void ChildThread::ShutdownThread() {
 563   DCHECK(!ChildThread::current()) <<
 564       "this method should NOT be called from child thread itself";
 565   {
 566     base::AutoLock lock(g_lazy_child_thread_lock.Get());
 567     while (!g_child_thread)
 568       g_lazy_child_thread_cv.Get().Wait();
 569   }
 570   DCHECK_NE(base::MessageLoop::current(), g_child_thread->message_loop());
 571   g_child_thread->message_loop()->PostTask(
 572       FROM_HERE, base::Bind(&QuitMainThreadMessageLoop));
 573 }
 574 #endif
 575
 576 void ChildThread::OnProcessFinalRelease() {
 577   if (on_channel_error_called_) {
 578     base::MessageLoop::current()->Quit();
 579     return;
 580   }
 581
 582   // The child process shutdown sequence is a request response based mechanism,
 583   // where we send out an initial feeler request to the child process host
 584   // instance in the browser to verify if it's ok to shutdown the child process.
 585   // The browser then sends back a response if it's ok to shutdown. This avoids
 586   // race conditions if the process refcount is 0 but there's an IPC message
 587   // inflight that would addref it.
 588   Send(new ChildProcessHostMsg_ShutdownRequest);
 589 }
 590
 591 void ChildThread::EnsureConnected() {
 592   VLOG(0) << "ChildThread::EnsureConnected()";
 593   base::KillProcess(base::GetCurrentProcessHandle(), 0, false);
 594 }
 595
 596 void ChildThread::OnProcessBackgrounded(bool background) {
 597   // Set timer slack to maximum on main thread when in background.
 598   base::TimerSlack timer_slack = base::TIMER_SLACK_NONE;
 599   if (background)
 600     timer_slack = base::TIMER_SLACK_MAXIMUM;
 601   base::MessageLoop::current()->SetTimerSlack(timer_slack);
 602 }
 603
 604 }  // namespace content