Fix a race where backgroud page might ack an event before we're ready

extensions/browser/event_router.cc

Index: extensions/browser/event_router.cc
diff --git a/extensions/browser/event_router.cc b/extensions/browser/event_router.cc
index 365cd2f15153234e62d2c675a7e0b04387f8d8d8..a82ed24d2c6e168231f1532ec53ec9f36cdda4bd 100644
--- a/extensions/browser/event_router.cc
+++ b/extensions/browser/event_router.cc
@@ -155,14 +155,12 @@ void EventRouter::DispatchEvent(IPC::Sender* ipc_sender,
                                 const EventFilteringInfo& info) {
   int event_id = g_extension_event_id.GetNext();
 
+  IncrementInFlightEventsOnUI(browser_context_id, extension_id, event_id,
+                              event_name);
+
   DispatchExtensionMessage(ipc_sender, browser_context_id, extension_id,
                            event_id, event_name, event_args.get(), user_gesture,
                            info);
-
-  BrowserThread::PostTask(
-      BrowserThread::UI, FROM_HERE,
-      base::Bind(&EventRouter::IncrementInFlightEventsOnUI, browser_context_id,
-                 extension_id, event_id, event_name));
 }
 
 EventRouter::EventRouter(BrowserContext* browser_context,
@@ -664,6 +662,14 @@ void EventRouter::IncrementInFlightEventsOnUI(void* browser_context_id,
                                               const std::string& extension_id,
                                               int event_id,
                                               const std::string& event_name) {
+  if (!BrowserThread::CurrentlyOn(BrowserThread::UI)) {

[not at google - send to devlin, 2015/05/15 21:18:07]

For a function called "...OnUI" this line is misleading. Put the checkup at the
call site, so that it's obvious - and we should figure out what to do in the
case where it's *not* on the UI thread. We can't just ignore that?

[lazyboy, 2015/05/15 21:29:53]

I've moved this check to call site.
Put the checkup at the
For non-UI thread, I'm still PostTasking to UI to keep the behavior same in this
CL. I need to investigate more on the keep alive semantics before I'm
comfortable /w ignoring Increment... call on non-UI.
This means this change currently fixes race only when ::DispatchEvent() is
called from UI thread.

+    BrowserThread::PostTask(
+        BrowserThread::UI, FROM_HERE,
+        base::Bind(&EventRouter::IncrementInFlightEventsOnUI,
+                   browser_context_id, extension_id, event_id, event_name));
+    return;
+  }
+
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
   BrowserContext* browser_context =
       reinterpret_cast<BrowserContext*>(browser_context_id);