compositor-worker: Keep worker backing thread alive for the lifetime of the compositor thread.

third_party/WebKit/Source/core/workers/WorkerBackingThread.cpp

Index: third_party/WebKit/Source/core/workers/WorkerBackingThread.cpp
diff --git a/third_party/WebKit/Source/core/workers/WorkerBackingThread.cpp b/third_party/WebKit/Source/core/workers/WorkerBackingThread.cpp
index 040ce92ea79208362ca3f48cfceb67291d092614..5e8e3914ddcffe06ae1b8ddec3443c7527dd866f 100644
--- a/third_party/WebKit/Source/core/workers/WorkerBackingThread.cpp
+++ b/third_party/WebKit/Source/core/workers/WorkerBackingThread.cpp
@@ -11,6 +11,7 @@
 #include "bindings/core/v8/V8PerIsolateData.h"
 #include "platform/RuntimeEnabledFeatures.h"
 #include "platform/ThreadSafeFunctional.h"
+#include "platform/WaitableEvent.h"
 #include "platform/WebThreadSupportingGC.h"
 #include "public/platform/Platform.h"
 #include "public/platform/WebTraceLocation.h"
@@ -29,10 +30,13 @@ WorkerBackingThread::WorkerBackingThread(WebThread* thread, bool shouldCallGCOnS
     , m_isOwningThread(false)
     , m_shouldCallGCOnShutdown(shouldCallGCOnShutdown)
 {
+    thread->getWebTaskRunner()->postTask(BLINK_FROM_HERE, threadSafeBind(&WorkerBackingThread::initialize, AllowCrossThreadAccess(this)));

[haraken, 2016/05/09 02:00:53]

Would you elaborate on why you need to post a task?

[flackr, 2016/05/10 18:07:39]

Added a comment - we create WorkerBackingThread on main and initialize needs to
be run on the thread to attach that thread.

 }
 
 WorkerBackingThread::~WorkerBackingThread()
 {
+    if (!m_isOwningThread)
+        signalShutdownAndWait();

[haraken, 2016/05/09 02:00:53]

Would you help me understand why you need to call this in WorkerBackingThread's
destructor? (i.e., isn't there any more explicit place you can shutdown the
backing thrad?)

In general, it's not a good idea to do complicated stuff in destructors (like
posting tasks).

[flackr, 2016/05/10 18:07:39]

Similar to initialization, the shutdown must be done on the thread. The
WorkerBackingThread object must still exist in order to run the shutdown task,
otherwise we try to access this destructed object. We could call
signalShutdownAndWait (indirectly) from
CompositorWorkerThread::clearSharedBackingThread to be explicit about it and
just assert that it has been shut down in the destructor.

 #if DCHECK_IS_ON()
     MutexLocker locker(m_mutex);
     DCHECK_EQ(0u, m_workerScriptCount);
@@ -46,7 +50,8 @@ void WorkerBackingThread::attach()
         if (++m_workerScriptCount > 1)
             return;
     }
-    initialize();
+    if (m_isOwningThread)
+        initialize();
 }
 
 void WorkerBackingThread::detach()
@@ -56,7 +61,8 @@ void WorkerBackingThread::detach()
         if (--m_workerScriptCount > 0)
             return;
     }
-    shutdown();
+    if (m_isOwningThread)
+        shutdown();
 }
 
 void WorkerBackingThread::initialize()
@@ -75,7 +81,7 @@ void WorkerBackingThread::initialize()
         Platform::current()->didStartWorkerThread();
 }
 
-void WorkerBackingThread::shutdown()
+void WorkerBackingThread::shutdown(WaitableEvent* doneEvent)
 {
     if (m_isOwningThread)
         Platform::current()->willStopWorkerThread();
@@ -90,6 +96,15 @@ void WorkerBackingThread::shutdown()
 
     V8PerIsolateData::destroy(m_isolate);
     m_isolate = nullptr;
+    if (doneEvent)
+        doneEvent->signal();
+}
+
+void WorkerBackingThread::signalShutdownAndWait()
+{
+    WaitableEvent doneEvent;
+    m_backingThread->platformThread().getWebTaskRunner()->postTask(BLINK_FROM_HERE, threadSafeBind(&WorkerBackingThread::shutdown, AllowCrossThreadAccess(this), AllowCrossThreadAccess(&doneEvent)));
+    doneEvent.wait();
 }
 
 } // namespace blink