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

third_party/WebKit/Source/modules/compositorworker/CompositorWorkerThread.cpp

Index: third_party/WebKit/Source/modules/compositorworker/CompositorWorkerThread.cpp
diff --git a/third_party/WebKit/Source/modules/compositorworker/CompositorWorkerThread.cpp b/third_party/WebKit/Source/modules/compositorworker/CompositorWorkerThread.cpp
index 78f1ed6beb27b0a324c26fab5deda377eb353b22..39383d21734c8a107183a88521a024eb23fe26fd 100644
--- a/third_party/WebKit/Source/modules/compositorworker/CompositorWorkerThread.cpp
+++ b/third_party/WebKit/Source/modules/compositorworker/CompositorWorkerThread.cpp
@@ -18,10 +18,12 @@ namespace blink {
 
 namespace {
 
+// Tracks whether we have instantiated a BackingThreadHolder.
+static bool s_hasBackingThreadHolder = false;
+
 // This is a singleton class holding the compositor worker thread in this
-// renderrer process. BackingThreadHolst::m_thread will never be cleared,
-// but Oilpan and V8 are detached from the thread when the last compositor
-// worker thread is gone.
+// renderer process. BackingThreadHolder::m_thread is cleared by
+// ModulesInitializer::shutdown.
 // See WorkerThread::terminateAndWaitForAllWorkers for the process shutdown
 // case.
 class BackingThreadHolder {
@@ -29,10 +31,18 @@ public:
     static BackingThreadHolder& instance()
     {
         DEFINE_THREAD_SAFE_STATIC_LOCAL(BackingThreadHolder, holder, new BackingThreadHolder);
+        s_hasBackingThreadHolder = true;

[yhirano, 2016/05/11 02:21:02]

You need to protect the boolean with a mutex.

[flackr, 2016/05/18 05:26:50]

Why does this need to be protected, aren't instance() and clear() always
accessed on the main thread?

         return holder;
     }
 
     WorkerBackingThread* thread() { return m_thread.get(); }
+    static void clear()
+    {
+        // Calling instance() if it has not already been called would create

[yhirano, 2016/05/11 02:21:02]

ditto

+        // a backing thread so check if we have an instance first.
+        if (s_hasBackingThreadHolder)
+            instance().m_thread = nullptr;
+    }
     void resetForTest()
     {
         ASSERT(!m_thread || (m_thread->workerScriptCount() == 0));
@@ -77,6 +87,16 @@ WorkerGlobalScope*CompositorWorkerThread::createWorkerGlobalScope(PassOwnPtr<Wor
     return CompositorWorkerGlobalScope::create(this, std::move(startupData), m_timeOrigin);
 }
 
+void CompositorWorkerThread::ensureSharedBackingThread()
+{
+    BackingThreadHolder::instance();
+}
+
+void CompositorWorkerThread::clearSharedBackingThread()
+{
+    BackingThreadHolder::clear();
+}
+
 void CompositorWorkerThread::resetSharedBackingThreadForTest()
 {
     BackingThreadHolder::instance().resetForTest();