Convert WaitableEvent::EventCallback to OnceCallback

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 0d61e08ce945b01e27fd3005b138c1bfbc5c7eee..842e04e74c6f33038a7415c205af3df1c8c53137 100644
--- a/ipc/ipc_sync_channel.cc
+++ b/ipc/ipc_sync_channel.cc
@@ -88,9 +88,9 @@ class SyncChannel::ReceivedSyncMsgQueue :
           outer_state_(sync_msg_queue_->top_send_done_event_watcher_),
           event_(context->GetSendDoneEvent()),
           callback_(
-              base::Bind(&SyncChannel::SyncContext::OnSendDoneEventSignaled,
-                         context,
-                         run_loop)) {
+              base::BindOnce(&SyncChannel::SyncContext::OnSendDoneEventSignaled,
+                             context,
+                             run_loop)) {
       sync_msg_queue_->top_send_done_event_watcher_ = this;
       if (outer_state_)
         outer_state_->StopWatching();
@@ -104,14 +104,23 @@ class SyncChannel::ReceivedSyncMsgQueue :
     }
 
    private:
-    void StartWatching() { watcher_.StartWatching(event_, callback_); }
+    void Run(WaitableEvent* event) {
+      DCHECK(callback_);
+      std::move(callback_).Run(event);
+    }
+
+    void StartWatching() {
+      watcher_.StartWatching(event_, base::BindOnce(&NestedSendDoneWatcher::Run,
+                                                    base::Unretained(this)));

[dcheng, 2017/04/14 21:56:59]

Out of curiosity, why do we need this intermediate OnceCallback? It seems like
we could just std::move(callback_) here.

[tzik, 2017/04/17 08:28:34]

StartWatching/StopWatching may be called more than once by other
NestedSendDoneWatcher instances (as |outer_state_| of the other instance), so we
can not consume |callback_| here.

+    }
+
     void StopWatching() { watcher_.StopWatching(); }
 
     ReceivedSyncMsgQueue* const sync_msg_queue_;
     NestedSendDoneWatcher* const outer_state_;
 
     base::WaitableEvent* const event_;
-    const base::WaitableEventWatcher::EventCallback callback_;
+    base::WaitableEventWatcher::EventCallback callback_;
     base::WaitableEventWatcher watcher_;
 
     DISALLOW_COPY_AND_ASSIGN(NestedSendDoneWatcher);