Bug 1119158 - Retarget OnDataAvailable to a new I/O thread instead of the image decoding thread pool. r=sworkman

image/src/DecodePool.cpp

@@ -12,6 +12,7 @@
 #include "nsCOMPtr.h"
 #include "nsIObserverService.h"
 #include "nsIThreadPool.h"
+#include "nsThreadUtils.h"
 #include "nsXPCOMCIDInternal.h"
 #include "prsystem.h"
 
@@ -180,7 +181,7 @@ DecodePool::Singleton()
 }
 
 DecodePool::DecodePool()
-  : mThreadPoolMutex("Thread Pool")
+  : mMutex("image::DecodePool")
 {
   mThreadPool = do_CreateInstance(NS_THREADPOOL_CONTRACTID);
   MOZ_RELEASE_ASSERT(mThreadPool,
@@ -204,6 +205,11 @@ DecodePool::DecodePool()
   }
 #endif
 
+  // Initialize the I/O thread.
+  nsresult rv = NS_NewNamedThread("ImageIO", getter_AddRefs(mIOThread));
+  MOZ_RELEASE_ASSERT(NS_SUCCEEDED(rv) && mIOThread,
+                     "Should successfully create image I/O thread");
+
   nsCOMPtr<nsIObserverService> obsSvc = services::GetObserverService();
   if (obsSvc) {
     obsSvc->AddObserver(this, "xpcom-shutdown-threads", false);
@@ -221,17 +227,22 @@ DecodePool::Observe(nsISupports*, const char* aTopic, const char16_t*)
   MOZ_ASSERT(strcmp(aTopic, "xpcom-shutdown-threads") == 0, "Unexpected topic");
 
   nsCOMPtr<nsIThreadPool> threadPool;
+  nsCOMPtr<nsIThread> ioThread;
 
   {
-    MutexAutoLock threadPoolLock(mThreadPoolMutex);
-    threadPool = mThreadPool;
-    mThreadPool = nullptr;
+    MutexAutoLock lock(mMutex);
+    threadPool.swap(mThreadPool);
+    ioThread.swap(mIOThread);
   }
 
   if (threadPool) {
     threadPool->Shutdown();
   }
 
+  if (ioThread) {
+    ioThread->Shutdown();
+  }
+
   return NS_OK;
 }
 
@@ -244,7 +255,7 @@ DecodePool::AsyncDecode(Decoder* aDecoder)
 
   // Dispatch to the thread pool if it exists. If it doesn't, we're currently
   // shutting down, so it's OK to just drop the job on the floor.
-  MutexAutoLock threadPoolLock(mThreadPoolMutex);
+  MutexAutoLock threadPoolLock(mMutex);
   if (mThreadPool) {
     mThreadPool->Dispatch(worker, nsIEventTarget::DISPATCH_NORMAL);
   }
@@ -274,11 +285,19 @@ DecodePool::SyncDecodeIfPossible(Decoder* aDecoder)
 already_AddRefed<nsIEventTarget>
 DecodePool::GetEventTarget()
 {
-  MutexAutoLock threadPoolLock(mThreadPoolMutex);
+  MutexAutoLock threadPoolLock(mMutex);
   nsCOMPtr<nsIEventTarget> target = do_QueryInterface(mThreadPool);
   return target.forget();
 }
 
+already_AddRefed<nsIEventTarget>
+DecodePool::GetIOEventTarget()
+{
+  MutexAutoLock threadPoolLock(mMutex);
+  nsCOMPtr<nsIEventTarget> target = do_QueryInterface(mIOThread);
+  return target.forget();
+}
+
 already_AddRefed<nsIRunnable>
 DecodePool::CreateDecodeWorker(Decoder* aDecoder)
 {

image/src/DecodePool.h

@@ -17,6 +17,7 @@
 #include "nsIEventTarget.h"
 #include "nsIObserver.h"
 
+class nsIThread;
 class nsIThreadPool;
 
 namespace mozilla {
@@ -69,6 +70,15 @@ public:
    */
   already_AddRefed<nsIEventTarget> GetEventTarget();
 
+  /**
+   * Returns an event target interface to the DecodePool's I/O thread. Callers
+   * who want to deliver data to workers on the DecodePool can use this event
+   * target.
+   *
+   * @return An nsIEventTarget interface to the thread pool's I/O thread.
+   */
+  already_AddRefed<nsIEventTarget> GetIOEventTarget();
+
   /**
    * Creates a worker which can be used to attempt further decoding using the
    * provided decoder.
@@ -91,11 +101,10 @@ private:
 
   static StaticRefPtr<DecodePool> sSingleton;
 
-  // mThreadPoolMutex protects mThreadPool. For all RasterImages R,
-  // R::mDecodingMonitor must be acquired before mThreadPoolMutex
-  // if both are acquired; the other order may cause deadlock.
-  Mutex                     mThreadPoolMutex;
+  // mMutex protects mThreadPool and mIOThread.
+  Mutex                     mMutex;
   nsCOMPtr<nsIThreadPool>   mThreadPool;
+  nsCOMPtr<nsIThread>       mIOThread;
 };
 
 } // namespace image

image/src/imgRequest.cpp

@@ -699,10 +699,9 @@ NS_IMETHODIMP imgRequest::OnStartRequest(nsIRequest *aRequest, nsISupports *ctxt
     nsAutoCString mimeType;
     nsresult rv = httpChannel->GetContentType(mimeType);
     if (NS_SUCCEEDED(rv) && !mimeType.EqualsLiteral(IMAGE_SVG_XML)) {
-      // Image object not created until OnDataAvailable, so forward to static
-      // DecodePool directly.
+      // Retarget OnDataAvailable to the DecodePool's IO thread.
       nsCOMPtr<nsIEventTarget> target =
-        DecodePool::Singleton()->GetEventTarget();
+        DecodePool::Singleton()->GetIOEventTarget();
       rv = retargetable->RetargetDeliveryTo(target);
     }
     PR_LOG(GetImgLog(), PR_LOG_WARNING,