toBlob: If idle image encoding is postponed for too long, switch to main thread

third_party/WebKit/Source/core/html/canvas/CanvasAsyncBlobCreator.cpp

Index: third_party/WebKit/Source/core/html/canvas/CanvasAsyncBlobCreator.cpp
diff --git a/third_party/WebKit/Source/core/html/canvas/CanvasAsyncBlobCreator.cpp b/third_party/WebKit/Source/core/html/canvas/CanvasAsyncBlobCreator.cpp
index d0c082add4c2fb6da7454e845a3c9bd48435ef30..2e1415f3d71a7ff11178f96465dce446042c4aa1 100644
--- a/third_party/WebKit/Source/core/html/canvas/CanvasAsyncBlobCreator.cpp
+++ b/third_party/WebKit/Source/core/html/canvas/CanvasAsyncBlobCreator.cpp
@@ -5,7 +5,6 @@
 #include "CanvasAsyncBlobCreator.h"
 
 #include "core/fileapi/Blob.h"
-#include "platform/Task.h"
 #include "platform/ThreadSafeFunctional.h"
 #include "platform/graphics/ImageBuffer.h"
 #include "platform/heap/Handle.h"
@@ -28,6 +27,21 @@ const double SlackBeforeDeadline = 0.001; // a small slack period between deadli
 const int NumChannelsPng = 4;
 const int LongTaskImageSizeThreshold = 1000 * 1000; // The max image size we expect to encode in 14ms on Linux in PNG format
 
+// The encoding task is highly likely to switch from idle task to alternative
+// code path when the startTimeoutDelay is set to be below 150ms. As we want the
+// majority of encoding tasks to take the usual async idle task, we set a
+// lenient limit -- 200ms here.
+const double IdleTaskStartTimeoutDelay = 200.0;
+// We should be more lenient on completion timeout delay to ensure that the
+// switch from idle to main thread only happens to a minority of toBlob calls
+#if !OS(ANDROID)
+// Png image encoding on 4k by 4k canvas on Mac HDD takes 5.7+ seconds
+const double IdleTaskCompleteTimeoutDelay = 6700.0;
+#else
+// Png image encoding on 4k by 4k canvas on Android One takes 9.0+ seconds
+const double IdleTaskCompleteTimeoutDelay = 10000.0;
+#endif
+
 bool isDeadlineNearOrPassed(double deadlineSeconds)
 {
     return (deadlineSeconds - SlackBeforeDeadline - monotonicallyIncreasingTime() <= 0);
@@ -50,6 +64,7 @@ CanvasAsyncBlobCreator::CanvasAsyncBlobCreator(PassRefPtr<DOMUint8ClampedArray>
     ASSERT(m_data->length() == (unsigned) (size.height() * size.width() * 4));
     m_encodedImage = adoptPtr(new Vector<unsigned char>());
     m_pixelRowStride = size.width() * NumChannelsPng;
+    m_idleTaskStatus = IdleTaskStatus::Default;
     m_numRowsCompleted = 0;
 }
 
@@ -59,18 +74,23 @@ CanvasAsyncBlobCreator::~CanvasAsyncBlobCreator()
 
 void CanvasAsyncBlobCreator::scheduleAsyncBlobCreation(bool canUseIdlePeriodScheduling, double quality)
 {
-    // TODO: async blob creation should be supported in worker_pool threads as well. but right now blink does not have that
     ASSERT(isMainThread());
 
     // Make self-reference to keep this object alive until the final task completes
     m_selfRef = this;
 
-    // At the time being, progressive encoding is only applicable to png image format,
-    // and thus idle tasks scheduling can only be applied to png image format.
-    // TODO(xlai): Progressive encoding on jpeg and webp image formats (crbug.com/571398, crbug.com/571399)
     if (canUseIdlePeriodScheduling) {
+        // At the time being, progressive encoding is only applicable to png image format,
+        // and thus idle tasks scheduling can only be applied to png image format.
+        // TODO(xlai): Progressive encoding on jpeg and webp image formats (crbug.com/571398, crbug.com/571399)
         ASSERT(m_mimeType == "image/png");
-        Platform::current()->mainThread()->scheduler()->postIdleTask(BLINK_FROM_HERE, bind<double>(&CanvasAsyncBlobCreator::initiatePngEncoding, this));
+        m_idleTaskStatus = IdleTaskStatus::IdleTaskNotStarted;
+        m_alternativeSelfRef = this;
+
+        this->scheduleInitiatePngEncoding();
+        // We post the below task to check if the above idle task isn't late.
+        // There's no risk of concurrency as both tasks are on main thread.
+        this->postDelayedTaskToMainThread(BLINK_FROM_HERE, new Task(bind(&CanvasAsyncBlobCreator::idleTaskStartTimeoutEvent, this, quality)), IdleTaskStartTimeoutDelay);
     } else if (m_mimeType == "image/jpeg") {
         Platform::current()->mainThread()->taskRunner()->postTask(BLINK_FROM_HERE, bind(&CanvasAsyncBlobCreator::initiateJpegEncoding, this, quality));
     } else {
@@ -84,40 +104,55 @@ void CanvasAsyncBlobCreator::initiateJpegEncoding(const double& quality)
     m_jpegEncoderState = JPEGImageEncoderState::create(m_size, quality, m_encodedImage.get());
     if (!m_jpegEncoderState) {
         Platform::current()->mainThread()->taskRunner()->postTask(BLINK_FROM_HERE, bind(&BlobCallback::handleEvent, m_callback, nullptr));
-        m_selfRef.clear();
+        clearSelfReference();
         return;
     }
     BackgroundTaskRunner::TaskSize taskSize = (m_size.height() * m_size.width() >= LongTaskImageSizeThreshold) ? BackgroundTaskRunner::TaskSizeLongRunningTask : BackgroundTaskRunner::TaskSizeShortRunningTask;
     BackgroundTaskRunner::postOnBackgroundThread(BLINK_FROM_HERE, threadSafeBind(&CanvasAsyncBlobCreator::encodeImageOnEncoderThread, AllowCrossThreadAccess(this), quality), taskSize);
 }
 
+void CanvasAsyncBlobCreator::scheduleInitiatePngEncoding()
+{
+    Platform::current()->mainThread()->scheduler()->postIdleTask(BLINK_FROM_HERE, bind<double>(&CanvasAsyncBlobCreator::initiatePngEncoding, this));
+}
+
 void CanvasAsyncBlobCreator::initiatePngEncoding(double deadlineSeconds)
 {
     ASSERT(isMainThread());
-    m_pngEncoderState = PNGImageEncoderState::create(m_size, m_encodedImage.get());
-    if (!m_pngEncoderState) {
-        Platform::current()->mainThread()->taskRunner()->postTask(BLINK_FROM_HERE, bind(&BlobCallback::handleEvent, m_callback, nullptr));
-        m_selfRef.clear();
+    if (m_idleTaskStatus == IdleTaskStatus::IdleTaskSwitchedToMainThreadTask) {
+        // Clear the selfRef as the encoding task has already been carried out
+        // on alternative code path.
+        clearSelfReference();
         return;
     }
 
-    CanvasAsyncBlobCreator::idleEncodeRowsPng(deadlineSeconds);
-}
+    ASSERT(m_idleTaskStatus == IdleTaskStatus::IdleTaskNotStarted);
+    m_idleTaskStatus = IdleTaskStatus::IdleTaskStarted;
 
-void CanvasAsyncBlobCreator::scheduleIdleEncodeRowsPng()
-{
-    ASSERT(isMainThread());
-    Platform::current()->currentThread()->scheduler()->postIdleTask(BLINK_FROM_HERE, WTF::bind<double>(&CanvasAsyncBlobCreator::idleEncodeRowsPng, this));
+    if (!initializePngStruct()) {
+        m_idleTaskStatus = IdleTaskStatus::IdleTaskFailed;
+        // Clears the selfRef as the idle task has failed.
+        clearSelfReference();
+        return;
+    }
+    this->idleEncodeRowsPng(deadlineSeconds);
 }
 
 void CanvasAsyncBlobCreator::idleEncodeRowsPng(double deadlineSeconds)
 {
     ASSERT(isMainThread());
+    if (m_idleTaskStatus == IdleTaskStatus::IdleTaskSwitchedToMainThreadTask) {
+        // Clear the selfRef as the encoding task has already been carried out
+        // on alternative code path.
+        clearSelfReference();
+        return;
+    }
+
     unsigned char* inputPixels = m_data->data() + m_pixelRowStride * m_numRowsCompleted;
     for (int y = m_numRowsCompleted; y < m_size.height(); ++y) {
         if (isDeadlineNearOrPassed(deadlineSeconds)) {
             m_numRowsCompleted = y;
-            CanvasAsyncBlobCreator::scheduleIdleEncodeRowsPng();
+            Platform::current()->currentThread()->scheduler()->postIdleTask(BLINK_FROM_HERE, bind<double>(&CanvasAsyncBlobCreator::idleEncodeRowsPng, this));
             return;
         }
         PNGImageEncoder::writeOneRowToPng(inputPixels, m_pngEncoderState.get());
@@ -126,11 +161,33 @@ void CanvasAsyncBlobCreator::idleEncodeRowsPng(double deadlineSeconds)
     m_numRowsCompleted = m_size.height();
     PNGImageEncoder::finalizePng(m_pngEncoderState.get());
 
+    m_idleTaskStatus = IdleTaskStatus::IdleTaskCompleted;
+
     if (isDeadlineNearOrPassed(deadlineSeconds)) {
         Platform::current()->mainThread()->taskRunner()->postTask(BLINK_FROM_HERE, bind(&CanvasAsyncBlobCreator::createBlobAndCall, this));
     } else {
         this->createBlobAndCall();
     }
+
+    // Clears the selfRef as the idle task has come to a completion.
+    Platform::current()->mainThread()->taskRunner()->postTask(BLINK_FROM_HERE, bind(&CanvasAsyncBlobCreator::clearSelfReference, this));
+}
+
+void CanvasAsyncBlobCreator::encodeRowsPngOnMainThread()
+{
+    ASSERT(m_idleTaskStatus == IdleTaskStatus::IdleTaskSwitchedToMainThreadTask);
+
+    // Continue encoding from the last completed row
+    unsigned char* inputPixels = m_data->data() + m_pixelRowStride * m_numRowsCompleted;
+    for (int y = m_numRowsCompleted; y < m_size.height(); ++y) {
+        PNGImageEncoder::writeOneRowToPng(inputPixels, m_pngEncoderState.get());
+        inputPixels += m_pixelRowStride;
+    }
+    PNGImageEncoder::finalizePng(m_pngEncoderState.get());
+    this->createBlobAndCall();
+
+    // Clears alternative selfRef as the alternative code path has completed.
+    this->clearAlternativeSelfReference();
 }
 
 void CanvasAsyncBlobCreator::createBlobAndCall()
@@ -138,20 +195,43 @@ void CanvasAsyncBlobCreator::createBlobAndCall()
     ASSERT(isMainThread());
     Blob* resultBlob = Blob::create(m_encodedImage->data(), m_encodedImage->size(), m_mimeType);
     Platform::current()->mainThread()->taskRunner()->postTask(BLINK_FROM_HERE, bind(&BlobCallback::handleEvent, m_callback, resultBlob));
-    clearSelfReference(); // self-destruct once job is done.
+}
+
+void CanvasAsyncBlobCreator::createNullptrAndCall()
+{
+    ASSERT(isMainThread());
+    Platform::current()->mainThread()->taskRunner()->postTask(BLINK_FROM_HERE, bind(&BlobCallback::handleEvent, m_callback, nullptr));
 }
 
 void CanvasAsyncBlobCreator::encodeImageOnEncoderThread(double quality)
 {
     ASSERT(!isMainThread());
 
+    bool encodeSuccess;
     if (m_mimeType == "image/jpeg") {
         JPEGImageEncoder::encodeWithPreInitializedState(m_jpegEncoderState.get(), m_data->data());
-    } else if (!ImageDataBuffer(m_size, m_data->data()).encodeImage(m_mimeType, quality, m_encodedImage.get())) {
-        scheduleCreateNullptrAndCallOnMainThread();
+        encodeSuccess = true;
+    } else {
+        encodeSuccess = ImageDataBuffer(m_size, m_data->data()).encodeImage(m_mimeType, quality, m_encodedImage.get());
+    }
+
+    if (encodeSuccess) {
+        Platform::current()->mainThread()->taskRunner()->postTask(BLINK_FROM_HERE, threadSafeBind(&CanvasAsyncBlobCreator::createBlobAndCall, AllowCrossThreadAccess(this)));
+    } else {
+        Platform::current()->mainThread()->taskRunner()->postTask(BLINK_FROM_HERE, threadSafeBind(&BlobCallback::handleEvent, m_callback.get(), nullptr));
     }
 
-    scheduleCreateBlobAndCallOnMainThread();
+    Platform::current()->mainThread()->taskRunner()->postTask(BLINK_FROM_HERE, threadSafeBind(&CanvasAsyncBlobCreator::clearSelfReference, AllowCrossThreadAccess(this)));
+}
+
+bool CanvasAsyncBlobCreator::initializePngStruct()
+{
+    m_pngEncoderState = PNGImageEncoderState::create(m_size, m_encodedImage.get());
+    if (!m_pngEncoderState) {
+        this->createNullptrAndCall();
+        return false;
+    }
+    return true;
 }
 
 void CanvasAsyncBlobCreator::clearSelfReference()
@@ -162,17 +242,55 @@ void CanvasAsyncBlobCreator::clearSelfReference()
     m_selfRef.clear();
 }
 
-void CanvasAsyncBlobCreator::scheduleCreateBlobAndCallOnMainThread()
+void CanvasAsyncBlobCreator::clearAlternativeSelfReference()
 {
-    ASSERT(!isMainThread());
-    Platform::current()->mainThread()->taskRunner()->postTask(BLINK_FROM_HERE, threadSafeBind(&CanvasAsyncBlobCreator::createBlobAndCall, AllowCrossThreadAccess(this)));
+    ASSERT(isMainThread());
+    m_alternativeSelfRef.clear();
 }
 
-void CanvasAsyncBlobCreator::scheduleCreateNullptrAndCallOnMainThread()
+void CanvasAsyncBlobCreator::idleTaskStartTimeoutEvent(double quality)
 {
-    ASSERT(!isMainThread());
-    Platform::current()->mainThread()->taskRunner()->postTask(BLINK_FROM_HERE, threadSafeBind(&BlobCallback::handleEvent, m_callback.get(), nullptr));
-    Platform::current()->mainThread()->taskRunner()->postTask(BLINK_FROM_HERE, threadSafeBind(&CanvasAsyncBlobCreator::clearSelfReference, AllowCrossThreadAccess(this)));
+    if (m_idleTaskStatus == IdleTaskStatus::IdleTaskStarted) {
+        // Even if the task started quickly, we still want to ensure completion
+        this->postDelayedTaskToMainThread(BLINK_FROM_HERE, new Task(bind(&CanvasAsyncBlobCreator::idleTaskCompleteTimeoutEvent, this)), IdleTaskCompleteTimeoutDelay);
+    } else if (m_idleTaskStatus == IdleTaskStatus::IdleTaskNotStarted) {
+        // If the idle task does not start after a delay threshold, we will
+        // force it to happen on main thread (even though it may cause more
+        // janks) to prevent toBlob being postponed forever in extreme cases.
+        m_idleTaskStatus = IdleTaskStatus::IdleTaskSwitchedToMainThreadTask;
+        signalTaskSwitchInStartTimeoutEventForTesting();
+        if (initializePngStruct()) {
+            Platform::current()->mainThread()->taskRunner()->postTask(BLINK_FROM_HERE, bind(&CanvasAsyncBlobCreator::encodeRowsPngOnMainThread, this));
+        } else {
+            this->clearAlternativeSelfReference(); // As it fails on alternative path
+        }
+    } else {
+        // No need to hold the alternativeSelfRef as the alternative code path
+        // will not occur for IdleTaskFailed and IdleTaskCompleted
+        this->clearAlternativeSelfReference();
+    }
+
+}
+
+void CanvasAsyncBlobCreator::idleTaskCompleteTimeoutEvent()
+{
+    ASSERT(m_idleTaskStatus != IdleTaskStatus::IdleTaskNotStarted);
+
+    if (m_idleTaskStatus == IdleTaskStatus::IdleTaskStarted) {
+        // It has taken too long to complete for the idle task.
+        m_idleTaskStatus = IdleTaskStatus::IdleTaskSwitchedToMainThreadTask;
+        signalTaskSwitchInCompleteTimeoutEventForTesting();
+        Platform::current()->mainThread()->taskRunner()->postTask(BLINK_FROM_HERE, bind(&CanvasAsyncBlobCreator::encodeRowsPngOnMainThread, this));
+    } else {
+        // No need to hold the alternativeSelfRef as the alternative code path
+        // will not occur for IdleTaskFailed and IdleTaskCompleted
+        this->clearAlternativeSelfReference();
+    }
+}
+
+void CanvasAsyncBlobCreator::postDelayedTaskToMainThread(const WebTraceLocation& location, Task* task, double delayMs)
+{
+    Platform::current()->mainThread()->taskRunner()->postDelayedTask(location, task, delayMs);
 }
 
 } // namespace blink