b=865241 make HRTFDatabaseLoader ref-counting thread-safe r=ehsan

--HG--
extra : rebase_source : 8cab96807c111eb103dbe931d14a86a8b97a824e

content/media/webaudio/blink/HRTFDatabaseLoader.cpp

@@ -65,7 +65,8 @@ TemporaryRef<HRTFDatabaseLoader> HRTFDatabaseLoader::createAndLoadAsynchronously
 }
 
 HRTFDatabaseLoader::HRTFDatabaseLoader(float sampleRate)
-    : m_threadLock("HRTFDatabaseLoader")
+    : m_refCnt(0)
+    , m_threadLock("HRTFDatabaseLoader")
     , m_databaseLoaderThread(nullptr)
     , m_databaseSampleRate(sampleRate)
 {
@@ -87,6 +88,47 @@ HRTFDatabaseLoader::~HRTFDatabaseLoader()
     }
 }
 
+class HRTFDatabaseLoader::ProxyReleaseEvent MOZ_FINAL : public nsRunnable {
+public:
+    explicit ProxyReleaseEvent(HRTFDatabaseLoader* loader) : mLoader(loader) {}
+    NS_IMETHOD Run() MOZ_OVERRIDE
+    {
+        mLoader->MainThreadRelease();
+        return NS_OK;
+    }
+private:
+    HRTFDatabaseLoader* mLoader;
+};
+
+void HRTFDatabaseLoader::ProxyRelease()
+{
+    nsCOMPtr<nsIThread> mainThread = do_GetMainThread();
+    if (MOZ_LIKELY(mainThread)) {
+        nsRefPtr<ProxyReleaseEvent> event = new ProxyReleaseEvent(this);
+        DebugOnly<nsresult> rv =
+            mainThread->Dispatch(event, NS_DISPATCH_NORMAL);
+        MOZ_ASSERT(NS_SUCCEEDED(rv), "Failed to dispatch release event");
+    } else {
+        // Should be in XPCOM shutdown.
+        MOZ_ASSERT(NS_IsMainThread(),
+                   "Main thread is not available for dispatch.");
+        MainThreadRelease();
+    }
+}
+
+void HRTFDatabaseLoader::MainThreadRelease()
+{
+    MOZ_ASSERT(NS_IsMainThread());
+    int count = --m_refCnt;
+    MOZ_ASSERT(count >= 0, "extra release");
+    NS_LOG_RELEASE(this, count, "HRTFDatabaseLoader");
+    if (count == 0) {
+        // It is safe to delete here as the first reference can only be added
+        // on this (main) thread.
+        delete this;
+    }
+}
+
 // Asynchronously load the database in this thread.
 static void databaseLoaderEntry(void* threadData)
 {

content/media/webaudio/blink/HRTFDatabaseLoader.h

@@ -39,7 +39,7 @@ namespace WebCore {
 
 // HRTFDatabaseLoader will asynchronously load the default HRTFDatabase in a new thread.
 
-class HRTFDatabaseLoader : public mozilla::RefCounted<HRTFDatabaseLoader> {
+class HRTFDatabaseLoader {
 public:
     // Lazily creates a HRTFDatabaseLoader (if not already created) for the given sample-rate
     // and starts loading asynchronously (when created the first time).
@@ -47,9 +47,34 @@ public:
     // Must be called from the main thread.
     static mozilla::TemporaryRef<HRTFDatabaseLoader> createAndLoadAsynchronouslyIfNecessary(float sampleRate);
 
-    // Both constructor and destructor must be called from the main thread.
-    ~HRTFDatabaseLoader();
-    
+    // AddRef and Release may be called from any thread.
+    void AddRef()
+    {
+#if defined(DEBUG) || defined(NS_BUILD_REFCNT_LOGGING)
+        int count =
+#endif
+          ++m_refCnt;
+        MOZ_ASSERT(count > 0, "invalid ref count");
+        NS_LOG_ADDREF(this, count, "HRTFDatabaseLoader", sizeof(*this));
+    }
+
+    void Release()
+    {
+        // The last reference can't be removed on a non-main thread because
+        // the object can be accessed on the main thread from the hash
+        // table via createAndLoadAsynchronouslyIfNecessary().
+        int count = m_refCnt;
+        MOZ_ASSERT(count > 0, "extra release");
+        // Optimization attempt to possibly skip proxying the release to the
+        // main thread.
+        if (count != 1 && m_refCnt.compareExchange(count, count - 1)) {
+            NS_LOG_RELEASE(this, count - 1, "HRTFDatabaseLoader");
+            return;
+        }
+
+        ProxyRelease();
+    }
+
     // Returns true once the default database has been completely loaded.
     bool isLoaded() const;
 
@@ -67,7 +92,12 @@ public:
 private:
     // Both constructor and destructor must be called from the main thread.
     explicit HRTFDatabaseLoader(float sampleRate);
+    ~HRTFDatabaseLoader();
     
+    void ProxyRelease(); // any thread
+    void MainThreadRelease(); // main thread only
+    class ProxyReleaseEvent;
+
     // If it hasn't already been loaded, creates a new thread and initiates asynchronous loading of the default database.
     // This must be called from the main thread.
     void loadAsynchronously();
@@ -85,6 +115,8 @@ private:
     // Keeps track of loaders on a per-sample-rate basis.
     static nsTHashtable<LoaderByRateEntry> *s_loaderMap; // singleton
 
+    mozilla::Atomic<int> m_refCnt;
+
     nsAutoRef<HRTFDatabase> m_hrtfDatabase;
 
     // Holding a m_threadLock is required when accessing m_databaseLoaderThread.