ipc/ipc_sync_channel.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 "ipc/ipc_sync_channel.h"
   6
   7 #include "base/bind.h"
   8 #include "base/lazy_instance.h"
   9 #include "base/location.h"
  10 #include "base/logging.h"
  11 #include "base/synchronization/waitable_event.h"
  12 #include "base/synchronization/waitable_event_watcher.h"
  13 #include "base/thread_task_runner_handle.h"
  14 #include "base/threading/thread_local.h"
  15 #include "base/trace_event/trace_event.h"
  16 #include "ipc/ipc_channel_factory.h"
  17 #include "ipc/ipc_logging.h"
  18 #include "ipc/ipc_message_macros.h"
  19 #include "ipc/ipc_sync_message.h"
  20
  21 using base::TimeDelta;
  22 using base::TimeTicks;
  23 using base::WaitableEvent;
  24
  25 namespace IPC {
  26 // When we're blocked in a Send(), we need to process incoming synchronous
  27 // messages right away because it could be blocking our reply (either
  28 // directly from the same object we're calling, or indirectly through one or
  29 // more other channels).  That means that in SyncContext's OnMessageReceived,
  30 // we need to process sync message right away if we're blocked.  However a
  31 // simple check isn't sufficient, because the listener thread can be in the
  32 // process of calling Send.
  33 // To work around this, when SyncChannel filters a sync message, it sets
  34 // an event that the listener thread waits on during its Send() call.  This
  35 // allows us to dispatch incoming sync messages when blocked.  The race
  36 // condition is handled because if Send is in the process of being called, it
  37 // will check the event.  In case the listener thread isn't sending a message,
  38 // we queue a task on the listener thread to dispatch the received messages.
  39 // The messages are stored in this queue object that's shared among all
  40 // SyncChannel objects on the same thread (since one object can receive a
  41 // sync message while another one is blocked).
  42
  43 class SyncChannel::ReceivedSyncMsgQueue :
  44     public base::RefCountedThreadSafe<ReceivedSyncMsgQueue> {
  45  public:
  46   // Returns the ReceivedSyncMsgQueue instance for this thread, creating one
  47   // if necessary.  Call RemoveContext on the same thread when done.
  48   static ReceivedSyncMsgQueue* AddContext() {
  49     // We want one ReceivedSyncMsgQueue per listener thread (i.e. since multiple
  50     // SyncChannel objects can block the same thread).
  51     ReceivedSyncMsgQueue* rv = lazy_tls_ptr_.Pointer()->Get();
  52     if (!rv) {
  53       rv = new ReceivedSyncMsgQueue();
  54       ReceivedSyncMsgQueue::lazy_tls_ptr_.Pointer()->Set(rv);
  55     }
  56     rv->listener_count_++;
  57     return rv;
  58   }
  59
  60   // Called on IPC thread when a synchronous message or reply arrives.
  61   void QueueMessage(const Message& msg, SyncChannel::SyncContext* context) {
  62     bool was_task_pending;
  63     {
  64       base::AutoLock auto_lock(message_lock_);
  65
  66       was_task_pending = task_pending_;
  67       task_pending_ = true;
  68
  69       // We set the event in case the listener thread is blocked (or is about
  70       // to). In case it's not, the PostTask dispatches the messages.
  71       message_queue_.push_back(QueuedMessage(new Message(msg), context));
  72       message_queue_version_++;
  73     }
  74
  75     dispatch_event_.Signal();
  76     if (!was_task_pending) {
  77       listener_task_runner_->PostTask(
  78           FROM_HERE, base::Bind(&ReceivedSyncMsgQueue::DispatchMessagesTask,
  79                                 this, scoped_refptr<SyncContext>(context)));
  80     }
  81   }
  82
  83   void QueueReply(const Message &msg, SyncChannel::SyncContext* context) {
  84     received_replies_.push_back(QueuedMessage(new Message(msg), context));
  85   }
  86
  87   // Called on the listener's thread to process any queues synchronous
  88   // messages.
  89   void DispatchMessagesTask(SyncContext* context) {
  90     {
  91       base::AutoLock auto_lock(message_lock_);
  92       task_pending_ = false;
  93     }
  94     context->DispatchMessages();
  95   }
  96
  97   void DispatchMessages(SyncContext* dispatching_context) {
  98     bool first_time = true;
  99     uint32 expected_version = 0;
 100     SyncMessageQueue::iterator it;
 101     while (true) {
 102       Message* message = NULL;
 103       scoped_refptr<SyncChannel::SyncContext> context;
 104       {
 105         base::AutoLock auto_lock(message_lock_);
 106         if (first_time || message_queue_version_ != expected_version) {
 107           it = message_queue_.begin();
 108           first_time = false;
 109         }
 110         for (; it != message_queue_.end(); it++) {
 111           int message_group = it->context->restrict_dispatch_group();
 112           if (message_group == kRestrictDispatchGroup_None ||
 113               message_group == dispatching_context->restrict_dispatch_group()) {
 114             message = it->message;
 115             context = it->context;
 116             it = message_queue_.erase(it);
 117             message_queue_version_++;
 118             expected_version = message_queue_version_;
 119             break;
 120           }
 121         }
 122       }
 123
 124       if (message == NULL)
 125         break;
 126       context->OnDispatchMessage(*message);
 127       delete message;
 128     }
 129   }
 130
 131   // SyncChannel calls this in its destructor.
 132   void RemoveContext(SyncContext* context) {
 133     base::AutoLock auto_lock(message_lock_);
 134
 135     SyncMessageQueue::iterator iter = message_queue_.begin();
 136     while (iter != message_queue_.end()) {
 137       if (iter->context.get() == context) {
 138         delete iter->message;
 139         iter = message_queue_.erase(iter);
 140         message_queue_version_++;
 141       } else {
 142         iter++;
 143       }
 144     }
 145
 146     if (--listener_count_ == 0) {
 147       DCHECK(lazy_tls_ptr_.Pointer()->Get());
 148       lazy_tls_ptr_.Pointer()->Set(NULL);
 149     }
 150   }
 151
 152   WaitableEvent* dispatch_event() { return &dispatch_event_; }
 153   base::SingleThreadTaskRunner* listener_task_runner() {
 154     return listener_task_runner_.get();
 155   }
 156
 157   // Holds a pointer to the per-thread ReceivedSyncMsgQueue object.
 158   static base::LazyInstance<base::ThreadLocalPointer<ReceivedSyncMsgQueue> >
 159       lazy_tls_ptr_;
 160
 161   // Called on the ipc thread to check if we can unblock any current Send()
 162   // calls based on a queued reply.
 163   void DispatchReplies() {
 164     for (size_t i = 0; i < received_replies_.size(); ++i) {
 165       Message* message = received_replies_[i].message;
 166       if (received_replies_[i].context->TryToUnblockListener(message)) {
 167         delete message;
 168         received_replies_.erase(received_replies_.begin() + i);
 169         return;
 170       }
 171     }
 172   }
 173
 174   base::WaitableEventWatcher* top_send_done_watcher() {
 175     return top_send_done_watcher_;
 176   }
 177
 178   void set_top_send_done_watcher(base::WaitableEventWatcher* watcher) {
 179     top_send_done_watcher_ = watcher;
 180   }
 181
 182  private:
 183   friend class base::RefCountedThreadSafe<ReceivedSyncMsgQueue>;
 184
 185   // See the comment in SyncChannel::SyncChannel for why this event is created
 186   // as manual reset.
 187   ReceivedSyncMsgQueue() :
 188       message_queue_version_(0),
 189       dispatch_event_(true, false),
 190       listener_task_runner_(base::ThreadTaskRunnerHandle::Get()),
 191       task_pending_(false),
 192       listener_count_(0),
 193       top_send_done_watcher_(NULL) {
 194   }
 195
 196   ~ReceivedSyncMsgQueue() {}
 197
 198   // Holds information about a queued synchronous message or reply.
 199   struct QueuedMessage {
 200     QueuedMessage(Message* m, SyncContext* c) : message(m), context(c) { }
 201     Message* message;
 202     scoped_refptr<SyncChannel::SyncContext> context;
 203   };
 204
 205   typedef std::list<QueuedMessage> SyncMessageQueue;
 206   SyncMessageQueue message_queue_;
 207   uint32 message_queue_version_;  // Used to signal DispatchMessages to rescan
 208
 209   std::vector<QueuedMessage> received_replies_;
 210
 211   // Set when we got a synchronous message that we must respond to as the
 212   // sender needs its reply before it can reply to our original synchronous
 213   // message.
 214   WaitableEvent dispatch_event_;
 215   scoped_refptr<base::SingleThreadTaskRunner> listener_task_runner_;
 216   base::Lock message_lock_;
 217   bool task_pending_;
 218   int listener_count_;
 219
 220   // The current send done event watcher for this thread. Used to maintain
 221   // a local global stack of send done watchers to ensure that nested sync
 222   // message loops complete correctly.
 223   base::WaitableEventWatcher* top_send_done_watcher_;
 224 };
 225
 226 base::LazyInstance<base::ThreadLocalPointer<SyncChannel::ReceivedSyncMsgQueue> >
 227     SyncChannel::ReceivedSyncMsgQueue::lazy_tls_ptr_ =
 228         LAZY_INSTANCE_INITIALIZER;
 229
 230 SyncChannel::SyncContext::SyncContext(
 231     Listener* listener,
 232     const scoped_refptr<base::SingleThreadTaskRunner>& ipc_task_runner,
 233     WaitableEvent* shutdown_event)
 234     : ChannelProxy::Context(listener, ipc_task_runner),
 235       received_sync_msgs_(ReceivedSyncMsgQueue::AddContext()),
 236       shutdown_event_(shutdown_event),
 237       restrict_dispatch_group_(kRestrictDispatchGroup_None) {
 238 }
 239
 240 SyncChannel::SyncContext::~SyncContext() {
 241   while (!deserializers_.empty())
 242     Pop();
 243 }
 244
 245 // Adds information about an outgoing sync message to the context so that
 246 // we know how to deserialize the reply.  Returns a handle that's set when
 247 // the reply has arrived.
 248 void SyncChannel::SyncContext::Push(SyncMessage* sync_msg) {
 249   // Create the tracking information for this message. This object is stored
 250   // by value since all members are pointers that are cheap to copy. These
 251   // pointers are cleaned up in the Pop() function.
 252   //
 253   // The event is created as manual reset because in between Signal and
 254   // OnObjectSignalled, another Send can happen which would stop the watcher
 255   // from being called.  The event would get watched later, when the nested
 256   // Send completes, so the event will need to remain set.
 257   PendingSyncMsg pending(SyncMessage::GetMessageId(*sync_msg),
 258                          sync_msg->GetReplyDeserializer(),
 259                          new WaitableEvent(true, false));
 260   base::AutoLock auto_lock(deserializers_lock_);
 261   deserializers_.push_back(pending);
 262 }
 263
 264 bool SyncChannel::SyncContext::Pop() {
 265   bool result;
 266   {
 267     base::AutoLock auto_lock(deserializers_lock_);
 268     PendingSyncMsg msg = deserializers_.back();
 269     delete msg.deserializer;
 270     delete msg.done_event;
 271     msg.done_event = NULL;
 272     deserializers_.pop_back();
 273     result = msg.send_result;
 274   }
 275
 276   // We got a reply to a synchronous Send() call that's blocking the listener
 277   // thread.  However, further down the call stack there could be another
 278   // blocking Send() call, whose reply we received after we made this last
 279   // Send() call.  So check if we have any queued replies available that
 280   // can now unblock the listener thread.
 281   ipc_task_runner()->PostTask(
 282       FROM_HERE, base::Bind(&ReceivedSyncMsgQueue::DispatchReplies,
 283                             received_sync_msgs_.get()));
 284
 285   return result;
 286 }
 287
 288 WaitableEvent* SyncChannel::SyncContext::GetSendDoneEvent() {
 289   base::AutoLock auto_lock(deserializers_lock_);
 290   return deserializers_.back().done_event;
 291 }
 292
 293 WaitableEvent* SyncChannel::SyncContext::GetDispatchEvent() {
 294   return received_sync_msgs_->dispatch_event();
 295 }
 296
 297 void SyncChannel::SyncContext::DispatchMessages() {
 298   received_sync_msgs_->DispatchMessages(this);
 299 }
 300
 301 bool SyncChannel::SyncContext::TryToUnblockListener(const Message* msg) {
 302   base::AutoLock auto_lock(deserializers_lock_);
 303   if (deserializers_.empty() ||
 304       !SyncMessage::IsMessageReplyTo(*msg, deserializers_.back().id)) {
 305     return false;
 306   }
 307
 308   // TODO(bauerb): Remove logging once investigation of http://crbug.com/141055
 309   // has finished.
 310   if (!msg->is_reply_error()) {
 311     bool send_result = deserializers_.back().deserializer->
 312         SerializeOutputParameters(*msg);
 313     deserializers_.back().send_result = send_result;
 314     VLOG_IF(1, !send_result) << "Couldn't deserialize reply message";
 315   } else {
 316     VLOG(1) << "Received error reply";
 317   }
 318   deserializers_.back().done_event->Signal();
 319
 320   return true;
 321 }
 322
 323 void SyncChannel::SyncContext::Clear() {
 324   CancelPendingSends();
 325   received_sync_msgs_->RemoveContext(this);
 326   Context::Clear();
 327 }
 328
 329 bool SyncChannel::SyncContext::OnMessageReceived(const Message& msg) {
 330   // Give the filters a chance at processing this message.
 331   if (TryFilters(msg))
 332     return true;
 333
 334   if (TryToUnblockListener(&msg))
 335     return true;
 336
 337   if (msg.is_reply()) {
 338     received_sync_msgs_->QueueReply(msg, this);
 339     return true;
 340   }
 341
 342   if (msg.should_unblock()) {
 343     received_sync_msgs_->QueueMessage(msg, this);
 344     return true;
 345   }
 346
 347   return Context::OnMessageReceivedNoFilter(msg);
 348 }
 349
 350 void SyncChannel::SyncContext::OnChannelError() {
 351   CancelPendingSends();
 352   shutdown_watcher_.StopWatching();
 353   Context::OnChannelError();
 354 }
 355
 356 void SyncChannel::SyncContext::OnChannelOpened() {
 357   shutdown_watcher_.StartWatching(
 358       shutdown_event_,
 359       base::Bind(&SyncChannel::SyncContext::OnWaitableEventSignaled,
 360                  base::Unretained(this)));
 361   Context::OnChannelOpened();
 362 }
 363
 364 void SyncChannel::SyncContext::OnChannelClosed() {
 365   CancelPendingSends();
 366   shutdown_watcher_.StopWatching();
 367   Context::OnChannelClosed();
 368 }
 369
 370 void SyncChannel::SyncContext::OnSendTimeout(int message_id) {
 371   base::AutoLock auto_lock(deserializers_lock_);
 372   PendingSyncMessageQueue::iterator iter;
 373   VLOG(1) << "Send timeout";
 374   for (iter = deserializers_.begin(); iter != deserializers_.end(); iter++) {
 375     if (iter->id == message_id) {
 376       iter->done_event->Signal();
 377       break;
 378     }
 379   }
 380 }
 381
 382 void SyncChannel::SyncContext::CancelPendingSends() {
 383   base::AutoLock auto_lock(deserializers_lock_);
 384   PendingSyncMessageQueue::iterator iter;
 385   // TODO(bauerb): Remove once http://crbug/141055 is fixed.
 386   VLOG(1) << "Canceling pending sends";
 387   for (iter = deserializers_.begin(); iter != deserializers_.end(); iter++)
 388     iter->done_event->Signal();
 389 }
 390
 391 void SyncChannel::SyncContext::OnWaitableEventSignaled(WaitableEvent* event) {
 392   if (event == shutdown_event_) {
 393     // Process shut down before we can get a reply to a synchronous message.
 394     // Cancel pending Send calls, which will end up setting the send done event.
 395     CancelPendingSends();
 396   } else {
 397     // We got the reply, timed out or the process shutdown.
 398     DCHECK_EQ(GetSendDoneEvent(), event);
 399     base::MessageLoop::current()->QuitNow();
 400   }
 401 }
 402
 403 base::WaitableEventWatcher::EventCallback
 404     SyncChannel::SyncContext::MakeWaitableEventCallback() {
 405   return base::Bind(&SyncChannel::SyncContext::OnWaitableEventSignaled, this);
 406 }
 407
 408 // static
 409 scoped_ptr<SyncChannel> SyncChannel::Create(
 410     const IPC::ChannelHandle& channel_handle,
 411     Channel::Mode mode,
 412     Listener* listener,
 413     const scoped_refptr<base::SingleThreadTaskRunner>& ipc_task_runner,
 414     bool create_pipe_now,
 415     base::WaitableEvent* shutdown_event,
 416     AttachmentBroker* broker) {
 417   scoped_ptr<SyncChannel> channel =
 418       Create(listener, ipc_task_runner, shutdown_event);
 419   channel->Init(channel_handle, mode, create_pipe_now, broker);
 420   return channel.Pass();
 421 }
 422
 423 // static
 424 scoped_ptr<SyncChannel> SyncChannel::Create(
 425     scoped_ptr<ChannelFactory> factory,
 426     Listener* listener,
 427     const scoped_refptr<base::SingleThreadTaskRunner>& ipc_task_runner,
 428     bool create_pipe_now,
 429     base::WaitableEvent* shutdown_event) {
 430   scoped_ptr<SyncChannel> channel =
 431       Create(listener, ipc_task_runner, shutdown_event);
 432   channel->Init(factory.Pass(), create_pipe_now);
 433   return channel.Pass();
 434 }
 435
 436 // static
 437 scoped_ptr<SyncChannel> SyncChannel::Create(
 438     Listener* listener,
 439     const scoped_refptr<base::SingleThreadTaskRunner>& ipc_task_runner,
 440     WaitableEvent* shutdown_event) {
 441   return make_scoped_ptr(
 442       new SyncChannel(listener, ipc_task_runner, shutdown_event));
 443 }
 444
 445 SyncChannel::SyncChannel(
 446     Listener* listener,
 447     const scoped_refptr<base::SingleThreadTaskRunner>& ipc_task_runner,
 448     WaitableEvent* shutdown_event)
 449     : ChannelProxy(new SyncContext(listener, ipc_task_runner, shutdown_event)) {
 450   // The current (listener) thread must be distinct from the IPC thread, or else
 451   // sending synchronous messages will deadlock.
 452   DCHECK_NE(ipc_task_runner.get(), base::ThreadTaskRunnerHandle::Get().get());
 453   StartWatching();
 454 }
 455
 456 SyncChannel::~SyncChannel() {
 457 }
 458
 459 void SyncChannel::SetRestrictDispatchChannelGroup(int group) {
 460   sync_context()->set_restrict_dispatch_group(group);
 461 }
 462
 463 scoped_refptr<SyncMessageFilter> SyncChannel::CreateSyncMessageFilter() {
 464   scoped_refptr<SyncMessageFilter> filter = new SyncMessageFilter(
 465       sync_context()->shutdown_event(),
 466       sync_context()->IsChannelSendThreadSafe());
 467   AddFilter(filter.get());
 468   if (!did_init())
 469     pre_init_sync_message_filters_.push_back(filter);
 470   return filter;
 471 }
 472
 473 bool SyncChannel::Send(Message* message) {
 474 #ifdef IPC_MESSAGE_LOG_ENABLED
 475   std::string name;
 476   Logging::GetInstance()->GetMessageText(message->type(), &name, message, NULL);
 477   TRACE_EVENT1("ipc", "SyncChannel::Send", "name", name);
 478 #else
 479   TRACE_EVENT2("ipc", "SyncChannel::Send",
 480                "class", IPC_MESSAGE_ID_CLASS(message->type()),
 481                "line", IPC_MESSAGE_ID_LINE(message->type()));
 482 #endif
 483   if (!message->is_sync()) {
 484     ChannelProxy::Send(message);
 485     return true;
 486   }
 487
 488   // *this* might get deleted in WaitForReply.
 489   scoped_refptr<SyncContext> context(sync_context());
 490   if (context->shutdown_event()->IsSignaled()) {
 491     VLOG(1) << "shutdown event is signaled";
 492     delete message;
 493     return false;
 494   }
 495
 496   SyncMessage* sync_msg = static_cast<SyncMessage*>(message);
 497   context->Push(sync_msg);
 498   WaitableEvent* pump_messages_event = sync_msg->pump_messages_event();
 499
 500   ChannelProxy::Send(message);
 501
 502   // Wait for reply, or for any other incoming synchronous messages.
 503   // *this* might get deleted, so only call static functions at this point.
 504   WaitForReply(context.get(), pump_messages_event);
 505
 506   return context->Pop();
 507 }
 508
 509 void SyncChannel::WaitForReply(
 510     SyncContext* context, WaitableEvent* pump_messages_event) {
 511   context->DispatchMessages();
 512   while (true) {
 513     WaitableEvent* objects[] = {
 514       context->GetDispatchEvent(),
 515       context->GetSendDoneEvent(),
 516       pump_messages_event
 517     };
 518
 519     unsigned count = pump_messages_event ? 3: 2;
 520     size_t result = WaitableEvent::WaitMany(objects, count);
 521     if (result == 0 /* dispatch event */) {
 522       // We're waiting for a reply, but we received a blocking synchronous
 523       // call.  We must process it or otherwise a deadlock might occur.
 524       context->GetDispatchEvent()->Reset();
 525       context->DispatchMessages();
 526       continue;
 527     }
 528
 529     if (result == 2 /* pump_messages_event */)
 530       WaitForReplyWithNestedMessageLoop(context);  // Run a nested message loop.
 531
 532     break;
 533   }
 534 }
 535
 536 void SyncChannel::WaitForReplyWithNestedMessageLoop(SyncContext* context) {
 537   base::WaitableEventWatcher send_done_watcher;
 538
 539   ReceivedSyncMsgQueue* sync_msg_queue = context->received_sync_msgs();
 540   DCHECK(sync_msg_queue != NULL);
 541
 542   base::WaitableEventWatcher* old_send_done_event_watcher =
 543       sync_msg_queue->top_send_done_watcher();
 544
 545   base::WaitableEventWatcher::EventCallback old_callback;
 546   base::WaitableEvent* old_event = NULL;
 547
 548   // Maintain a local global stack of send done delegates to ensure that
 549   // nested sync calls complete in the correct sequence, i.e. the
 550   // outermost call completes first, etc.
 551   if (old_send_done_event_watcher) {
 552     old_callback = old_send_done_event_watcher->callback();
 553     old_event = old_send_done_event_watcher->GetWatchedEvent();
 554     old_send_done_event_watcher->StopWatching();
 555   }
 556
 557   sync_msg_queue->set_top_send_done_watcher(&send_done_watcher);
 558
 559   send_done_watcher.StartWatching(context->GetSendDoneEvent(),
 560                                   context->MakeWaitableEventCallback());
 561
 562   {
 563     base::MessageLoop::ScopedNestableTaskAllower allow(
 564         base::MessageLoop::current());
 565     base::MessageLoop::current()->Run();
 566   }
 567
 568   sync_msg_queue->set_top_send_done_watcher(old_send_done_event_watcher);
 569   if (old_send_done_event_watcher && old_event) {
 570     old_send_done_event_watcher->StartWatching(old_event, old_callback);
 571   }
 572 }
 573
 574 void SyncChannel::OnWaitableEventSignaled(WaitableEvent* event) {
 575   DCHECK(event == sync_context()->GetDispatchEvent());
 576   // The call to DispatchMessages might delete this object, so reregister
 577   // the object watcher first.
 578   event->Reset();
 579   dispatch_watcher_.StartWatching(event, dispatch_watcher_callback_);
 580   sync_context()->DispatchMessages();
 581 }
 582
 583 void SyncChannel::StartWatching() {
 584   // Ideally we only want to watch this object when running a nested message
 585   // loop.  However, we don't know when it exits if there's another nested
 586   // message loop running under it or not, so we wouldn't know whether to
 587   // stop or keep watching.  So we always watch it, and create the event as
 588   // manual reset since the object watcher might otherwise reset the event
 589   // when we're doing a WaitMany.
 590   dispatch_watcher_callback_ =
 591       base::Bind(&SyncChannel::OnWaitableEventSignaled,
 592                   base::Unretained(this));
 593   dispatch_watcher_.StartWatching(sync_context()->GetDispatchEvent(),
 594                                   dispatch_watcher_callback_);
 595 }
 596
 597 void SyncChannel::OnChannelInit() {
 598   for (const auto& filter : pre_init_sync_message_filters_) {
 599     filter->set_is_channel_send_thread_safe(
 600         context()->IsChannelSendThreadSafe());
 601   }
 602   pre_init_sync_message_filters_.clear();
 603 }
 604
 605 }  // namespace IPC