Use WaitableEvents to wake up sync IPC waiting

ipc/ipc_sync_channel.cc

Index: ipc/ipc_sync_channel.cc
diff --git a/ipc/ipc_sync_channel.cc b/ipc/ipc_sync_channel.cc
index 6852399bdfa4d8acdaba6bd7e766e715df085947..1dbf21e806452790d63d18d6fd76d0ba0fe5b140 100644
--- a/ipc/ipc_sync_channel.cc
+++ b/ipc/ipc_sync_channel.cc
@@ -24,9 +24,7 @@
 #include "ipc/ipc_logging.h"
 #include "ipc/ipc_message_macros.h"
 #include "ipc/ipc_sync_message.h"
-#include "ipc/mojo_event.h"
-#include "mojo/public/cpp/bindings/sync_handle_registry.h"
-#include "mojo/public/cpp/bindings/sync_handle_watcher.h"
+#include "mojo/public/cpp/bindings/sync_event_watcher.h"
 
 using base::WaitableEvent;
 
@@ -35,35 +33,13 @@ namespace IPC {
 namespace {
 
 // A generic callback used when watching handles synchronously. Sets |*signal|
-// to true. Also sets |*error| to true in case of an error.
-void OnSyncHandleReady(bool* signal, bool* error, MojoResult result) {
+// to true.
+void OnEventReady(bool* signal) {
   *signal = true;
-  *error = result != MOJO_RESULT_OK;
 }
 
-// A ReadyCallback for use with mojo::SimpleWatcher. Ignores the result
-// (DCHECKs, but is only used in cases where failure should be impossible) and
-// runs |callback|.
-void RunOnHandleReady(const base::Closure& callback, MojoResult result) {
-  DCHECK_EQ(result, MOJO_RESULT_OK);
-  callback.Run();
-}
-
-class PumpMessagesEvent {
- public:
-  PumpMessagesEvent() { event_.Signal(); }
-  ~PumpMessagesEvent() {}
-
-  const MojoEvent* event() const { return &event_; }
-
- private:
-  MojoEvent event_;
-
-  DISALLOW_COPY_AND_ASSIGN(PumpMessagesEvent);
-};
-
-base::LazyInstance<PumpMessagesEvent>::Leaky g_pump_messages_event =
-    LAZY_INSTANCE_INITIALIZER;
+base::LazyInstance<std::unique_ptr<base::WaitableEvent>>::Leaky
+    g_pump_messages_event = LAZY_INSTANCE_INITIALIZER;
 
 }  // namespace
 
@@ -208,7 +184,7 @@ class SyncChannel::ReceivedSyncMsgQueue :
     }
   }
 
-  MojoEvent* dispatch_event() { return &dispatch_event_; }
+  base::WaitableEvent* dispatch_event() { return &dispatch_event_; }
   base::SingleThreadTaskRunner* listener_task_runner() {
     return listener_task_runner_.get();
   }
@@ -230,12 +206,23 @@ class SyncChannel::ReceivedSyncMsgQueue :
     }
   }
 
-  mojo::SimpleWatcher* top_send_done_watcher() {
-    return top_send_done_watcher_;
-  }
-
-  void set_top_send_done_watcher(mojo::SimpleWatcher* watcher) {
+  // See SyncChannel::WaitForReplyWithNestedMessageLoop for details.
+  void SetTopSendDoneState(
+      base::WaitableEventWatcher* watcher,
+      base::WaitableEvent* event,
+      const base::WaitableEventWatcher::EventCallback& callback,
+      base::WaitableEventWatcher** outer_watcher,
+      base::WaitableEvent** outer_event,
+      base::WaitableEventWatcher::EventCallback* outer_callback) {
+    if (outer_watcher) {
+      DCHECK(outer_event && outer_callback);
+      *outer_watcher = top_send_done_watcher_;
+      *outer_event = top_send_done_event_;
+      *outer_callback = top_send_done_callback_;
+    }
     top_send_done_watcher_ = watcher;
+    top_send_done_event_ = event;
+    top_send_done_callback_ = callback;
   }
 
  private:
@@ -245,23 +232,19 @@ class SyncChannel::ReceivedSyncMsgQueue :
   // as manual reset.
   ReceivedSyncMsgQueue()
       : message_queue_version_(0),
+        dispatch_event_(base::WaitableEvent::ResetPolicy::MANUAL,
+                        base::WaitableEvent::InitialState::NOT_SIGNALED),
         listener_task_runner_(base::ThreadTaskRunnerHandle::Get()),
-        task_pending_(false),
-        listener_count_(0),
-        top_send_done_watcher_(nullptr) {
-    sync_dispatch_watcher_.reset(new mojo::SyncHandleWatcher(
-        dispatch_event_.GetHandle(), MOJO_HANDLE_SIGNAL_READABLE,
-        base::Bind(&ReceivedSyncMsgQueue::OnDispatchHandleReady,
-                   base::Unretained(this))));
+        sync_dispatch_watcher_(base::MakeUnique<mojo::SyncEventWatcher>(
+            &dispatch_event_,
+            base::Bind(&ReceivedSyncMsgQueue::OnDispatchEventReady,
+                       base::Unretained(this)))) {
     sync_dispatch_watcher_->AllowWokenUpBySyncWatchOnSameThread();
   }
 
   ~ReceivedSyncMsgQueue() {}
 
-  void OnDispatchHandleReady(MojoResult result) {
-    if (result != MOJO_RESULT_OK)
-      return;
-
+  void OnDispatchEventReady() {
     if (dispatch_flag_) {
       *dispatch_flag_ = true;
       return;
@@ -284,23 +267,27 @@ class SyncChannel::ReceivedSyncMsgQueue :
 
   typedef std::list<QueuedMessage> SyncMessageQueue;
   SyncMessageQueue message_queue_;
-  uint32_t message_queue_version_;  // Used to signal DispatchMessages to rescan
+
+  // Used to signal DispatchMessages to rescan
+  uint32_t message_queue_version_ = 0;
 
   std::vector<QueuedMessage> received_replies_;
 
   // Signaled when we get a synchronous message that we must respond to, as the
   // sender needs its reply before it can reply to our original synchronous
   // message.
-  MojoEvent dispatch_event_;
+  base::WaitableEvent dispatch_event_;
   scoped_refptr<base::SingleThreadTaskRunner> listener_task_runner_;
   base::Lock message_lock_;
-  bool task_pending_;
-  int listener_count_;
+  bool task_pending_ = false;
+  int listener_count_ = 0;
 
-  // The current send done handle watcher for this thread. Used to maintain
-  // a thread-local stack of send done watchers to ensure that nested sync
-  // message loops complete correctly.
-  mojo::SimpleWatcher* top_send_done_watcher_;
+  // The current send-done state for this thread. Used to maintain a thread-
+  // local stack of state to ensure that nested sync message loops complete
+  // correctly.
+  base::WaitableEventWatcher* top_send_done_watcher_ = nullptr;
+  base::WaitableEvent* top_send_done_event_ = nullptr;
+  base::WaitableEventWatcher::EventCallback top_send_done_callback_;
 
   // If not null, the address of a flag to set when the dispatch event signals,
   // in lieu of actually dispatching messages. This is used by
@@ -309,7 +296,7 @@ class SyncChannel::ReceivedSyncMsgQueue :
   bool* dispatch_flag_ = nullptr;
 
   // Watches |dispatch_event_| during all sync handle watches on this thread.
-  std::unique_ptr<mojo::SyncHandleWatcher> sync_dispatch_watcher_;
+  std::unique_ptr<mojo::SyncEventWatcher> sync_dispatch_watcher_;
 };
 
 base::LazyInstance<base::ThreadLocalPointer<
@@ -327,6 +314,13 @@ SyncChannel::SyncContext::SyncContext(
       restrict_dispatch_group_(kRestrictDispatchGroup_None) {
 }
 
+void SyncChannel::SyncContext::OnSendDoneEventSignaled(
+    base::RunLoop* nested_loop,
+    base::WaitableEvent* event) {
+  DCHECK_EQ(GetSendDoneEvent(), event);
+  nested_loop->Quit();
+}
+
 SyncChannel::SyncContext::~SyncContext() {
   while (!deserializers_.empty())
     Pop();
@@ -349,7 +343,8 @@ bool SyncChannel::SyncContext::Push(SyncMessage* sync_msg) {
     return false;
   PendingSyncMsg pending(
       SyncMessage::GetMessageId(*sync_msg), sync_msg->GetReplyDeserializer(),
-      new MojoEvent);
+      new base::WaitableEvent(base::WaitableEvent::ResetPolicy::MANUAL,
+                              base::WaitableEvent::InitialState::NOT_SIGNALED));
   deserializers_.push_back(pending);
   return true;
 }
@@ -378,12 +373,12 @@ bool SyncChannel::SyncContext::Pop() {
   return result;
 }
 
-MojoEvent* SyncChannel::SyncContext::GetSendDoneEvent() {
+base::WaitableEvent* SyncChannel::SyncContext::GetSendDoneEvent() {
   base::AutoLock auto_lock(deserializers_lock_);
   return deserializers_.back().done_event;
 }
 
-MojoEvent* SyncChannel::SyncContext::GetDispatchEvent() {
+base::WaitableEvent* SyncChannel::SyncContext::GetDispatchEvent() {
   return received_sync_msgs_->dispatch_event();
 }
 
@@ -407,7 +402,7 @@ bool SyncChannel::SyncContext::TryToUnblockListener(const Message* msg) {
     DVLOG(1) << "Received error reply";
   }
 
-  MojoEvent* done_event = deserializers_.back().done_event;
+  base::WaitableEvent* done_event = deserializers_.back().done_event;
   TRACE_EVENT_FLOW_BEGIN0(
       TRACE_DISABLED_BY_DEFAULT("ipc.flow"),
       "SyncChannel::SyncContext::TryToUnblockListener", done_event);
@@ -526,9 +521,7 @@ SyncChannel::SyncChannel(
     const scoped_refptr<base::SingleThreadTaskRunner>& ipc_task_runner,
     WaitableEvent* shutdown_event)
     : ChannelProxy(new SyncContext(listener, ipc_task_runner, shutdown_event)),
-      sync_handle_registry_(mojo::SyncHandleRegistry::current()),
-      dispatch_watcher_(FROM_HERE,
-                        mojo::SimpleWatcher::ArmingPolicy::AUTOMATIC) {
+      sync_handle_registry_(mojo::SyncHandleRegistry::current()) {
   // The current (listener) thread must be distinct from the IPC thread, or else
   // sending synchronous messages will deadlock.
   DCHECK_NE(ipc_task_runner.get(), base::ThreadTaskRunnerHandle::Get().get());
@@ -596,24 +589,28 @@ void SyncChannel::WaitForReply(mojo::SyncHandleRegistry* registry,
                                bool pump_messages) {
   context->DispatchMessages();
 
-  const MojoEvent* pump_messages_event = nullptr;
-  if (pump_messages)
-    pump_messages_event = g_pump_messages_event.Get().event();
+  base::WaitableEvent* pump_messages_event = nullptr;
+  if (pump_messages) {
+    if (!g_pump_messages_event.Get()) {
+      g_pump_messages_event.Get() = base::MakeUnique<base::WaitableEvent>(
+          base::WaitableEvent::ResetPolicy::MANUAL,
+          base::WaitableEvent::InitialState::SIGNALED);
+    }
+    pump_messages_event = g_pump_messages_event.Get().get();
+  }
 
   while (true) {
     bool dispatch = false;
     bool send_done = false;
     bool should_pump_messages = false;
-    bool error = false;
-    bool registered = registry->RegisterHandle(
-        context->GetSendDoneEvent()->GetHandle(),
-        MOJO_HANDLE_SIGNAL_READABLE,
-        base::Bind(&OnSyncHandleReady, &send_done, &error));
+    bool registered = registry->RegisterEvent(
+        context->GetSendDoneEvent(), base::Bind(&OnEventReady, &send_done));
     DCHECK(registered);
+
     if (pump_messages_event) {
-      registered = registry->RegisterHandle(
-          pump_messages_event->GetHandle(), MOJO_HANDLE_SIGNAL_READABLE,
-          base::Bind(&OnSyncHandleReady, &should_pump_messages, &error));
+      registered = registry->RegisterEvent(
+          pump_messages_event,
+          base::Bind(&OnEventReady, &should_pump_messages));
       DCHECK(registered);
     }
 
@@ -621,11 +618,10 @@ void SyncChannel::WaitForReply(mojo::SyncHandleRegistry* registry,
     context->received_sync_msgs()->BlockDispatch(&dispatch);
     registry->WatchAllHandles(stop_flags, 3);
     context->received_sync_msgs()->UnblockDispatch();
-    DCHECK(!error);
 
-    registry->UnregisterHandle(context->GetSendDoneEvent()->GetHandle());
+    registry->UnregisterEvent(context->GetSendDoneEvent());
     if (pump_messages_event)
-      registry->UnregisterHandle(pump_messages_event->GetHandle());
+      registry->UnregisterEvent(pump_messages_event);
 
     if (dispatch) {
       // We're waiting for a reply, but we received a blocking synchronous call.
@@ -643,46 +639,55 @@ void SyncChannel::WaitForReply(mojo::SyncHandleRegistry* registry,
 }
 
 void SyncChannel::WaitForReplyWithNestedMessageLoop(SyncContext* context) {
-  mojo::SimpleWatcher send_done_watcher(
-      FROM_HERE, mojo::SimpleWatcher::ArmingPolicy::AUTOMATIC);
-
   ReceivedSyncMsgQueue* sync_msg_queue = context->received_sync_msgs();
-  DCHECK_NE(sync_msg_queue, nullptr);
-
-  mojo::SimpleWatcher* old_watcher = sync_msg_queue->top_send_done_watcher();
-  mojo::Handle old_handle(mojo::kInvalidHandleValue);
-  mojo::SimpleWatcher::ReadyCallback old_callback;
-
-  // Maintain a thread-local stack of watchers to ensure nested calls complete
-  // in the correct sequence, i.e. the outermost call completes first, etc.
-  if (old_watcher) {
-    old_callback = old_watcher->ready_callback();
-    old_handle = old_watcher->handle();
-    old_watcher->Cancel();
-  }
+  DCHECK(sync_msg_queue);
 
-  sync_msg_queue->set_top_send_done_watcher(&send_done_watcher);
+  base::RunLoop nested_loop;
 
-  {
-    base::RunLoop nested_loop;
-    send_done_watcher.Watch(
-        context->GetSendDoneEvent()->GetHandle(), MOJO_HANDLE_SIGNAL_READABLE,
-        base::Bind(&RunOnHandleReady, nested_loop.QuitClosure()));
-
-    base::MessageLoop::ScopedNestableTaskAllower allow(
-        base::MessageLoop::current());
-    nested_loop.Run();
-    send_done_watcher.Cancel();
-  }
-
-  sync_msg_queue->set_top_send_done_watcher(old_watcher);
-  if (old_watcher)
-    old_watcher->Watch(old_handle, MOJO_HANDLE_SIGNAL_READABLE, old_callback);
+  // WaitForReplyWithNestedMessageLoop may be re-entered, i.e. we can nest
+  // waiting message loops arbitrarily deep on the SyncChannel's thread. Every
+  // such operation has a corresponding WaitableEvent to be watched which, when
+  // signalled for IPC completion, breaks out of the loop. The innermost (i.e.
+  // topmost) such event is stored in |sync_msg_queue| state.
+  //
+  // Here we preserve the current top-of-stack event state (if any) within the
+  // local stack frame. We then replace the event state in |sync_msg_queue| with
+  // our own and run a nested loop. If a subsequent nested loop is started
+  // therein the process is repeated in that stack frame, and so on.
+  //
+  // Once the innermost nested loop is broken, the locally preserved event state
+  // is swapped back into |sync_msg_queue| before unwinding the stack.
+  base::WaitableEventWatcher* outer_watcher = nullptr;

[yzshen1, 2017/03/23 20:15:50]

Does it make sense to group these three things into a struct, so it is easier to
init or pass around?

[Ken Rockot(use gerrit already), 2017/03/23 22:04:20]

Sure, that sounds nice. In fact I've gone and moved all of this logic into a
helper class and I think it's much easier to understand now.

+  base::WaitableEvent* outer_event = nullptr;
+  base::WaitableEventWatcher::EventCallback outer_callback;
+  base::WaitableEventWatcher send_done_watcher;
+  base::WaitableEvent* event = context->GetSendDoneEvent();
+  const base::WaitableEventWatcher::EventCallback callback =
+      base::Bind(&SyncContext::OnSendDoneEventSignaled, context, &nested_loop);
+  sync_msg_queue->SetTopSendDoneState(&send_done_watcher, event, callback,
+                                      &outer_watcher, &outer_event,
+                                      &outer_callback);
+  if (outer_watcher)
+    outer_watcher->StopWatching();
+  send_done_watcher.StartWatching(event, callback);
+
+  base::MessageLoop::ScopedNestableTaskAllower allow(
+      base::MessageLoop::current());
+  nested_loop.Run();
+
+  sync_msg_queue->SetTopSendDoneState(
+      outer_watcher, outer_event, outer_callback, nullptr, nullptr, nullptr);
+  if (outer_watcher)
+    outer_watcher->StartWatching(outer_event, outer_callback);
 }
 
-void SyncChannel::OnDispatchHandleReady(MojoResult result) {
-  DCHECK_EQ(result, MOJO_RESULT_OK);
+void SyncChannel::OnDispatchEventSignaled(base::WaitableEvent* event) {
+  DCHECK_EQ(sync_context()->GetDispatchEvent(), event);
   sync_context()->GetDispatchEvent()->Reset();
+
+  StartWatching();
+
+  // NOTE: May delete |this|.
   sync_context()->DispatchMessages();
 }
 
@@ -691,10 +696,10 @@ void SyncChannel::StartWatching() {
   // messages once the listener thread is unblocked and pumping its task queue.
   // The ReceivedSyncMsgQueue also watches this event and may dispatch
   // immediately if woken up by a message which it's allowed to dispatch.
-  dispatch_watcher_.Watch(
-      sync_context()->GetDispatchEvent()->GetHandle(),
-      MOJO_HANDLE_SIGNAL_READABLE,
-      base::Bind(&SyncChannel::OnDispatchHandleReady, base::Unretained(this)));
+  dispatch_watcher_.StartWatching(
+      sync_context()->GetDispatchEvent(),
+      base::Bind(&SyncChannel::OnDispatchEventSignaled,
+                 base::Unretained(this)));
 }
 
 void SyncChannel::OnChannelInit() {