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diff --git a/image/IDecodingTask.h b/image/IDecodingTask.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * An interface for tasks which can execute on the ImageLib DecodePool, and
+ * various implementations.
+ */
+
+#ifndef mozilla_image_IDecodingTask_h
+#define mozilla_image_IDecodingTask_h
+
+#include "mozilla/NotNull.h"
+#include "mozilla/RefPtr.h"
+
+#include "imgFrame.h"
+#include "SourceBuffer.h"
+
+namespace mozilla {
+namespace image {
+
+class Decoder;
+class RasterImage;
+
+/// A priority hint that DecodePool can use when scheduling an IDecodingTask.
+enum class TaskPriority : uint8_t
+{
+  eLow,
+  eHigh
+};
+
+/**
+ * An interface for tasks which can execute on the ImageLib DecodePool.
+ */
+class IDecodingTask : public IResumable
+{
+public:
+  /// Run the task.
+  virtual void Run() = 0;
+
+  /// @return true if, given the option, this task prefers to run synchronously.
+  virtual bool ShouldPreferSyncRun() const = 0;
+
+  /// @return a priority hint that DecodePool can use when scheduling this task.
+  virtual TaskPriority Priority() const = 0;
+
+  /// A default implementation of IResumable which resubmits the task to the
+  /// DecodePool. Subclasses can override this if they need different behavior.
+  void Resume() override;
+
+protected:
+  /// Notify @aImage of @aDecoder's progress.
+  static void NotifyProgress(NotNull<RasterImage*> aImage,
+                             NotNull<Decoder*> aDecoder);
+
+  /// Notify @aImage that @aDecoder has finished.
+  static void NotifyDecodeComplete(NotNull<RasterImage*> aImage,
+                                   NotNull<Decoder*> aDecoder);
+
+  virtual ~IDecodingTask() { }
+};
+
+
+/**
+ * An IDecodingTask implementation for metadata decodes of images.
+ */
+class MetadataDecodingTask final : public IDecodingTask
+{
+public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(MetadataDecodingTask, override)
+
+  explicit MetadataDecodingTask(NotNull<Decoder*> aDecoder);
+
+  void Run() override;
+
+  // Metadata decodes are very fast (since they only need to examine an image's
+  // header) so there's no reason to refuse to run them synchronously if the
+  // caller will allow us to.
+  bool ShouldPreferSyncRun() const override { return true; }
+
+  // Metadata decodes run at the highest priority because they block layout and
+  // page load.
+  TaskPriority Priority() const override { return TaskPriority::eHigh; }
+
+private:
+  virtual ~MetadataDecodingTask() { }
+
+  /// Mutex protecting access to mDecoder.
+  Mutex mMutex;
+
+  NotNull<RefPtr<Decoder>> mDecoder;
+};
+
+
+/**
+ * An IDecodingTask implementation for anonymous decoders - that is, decoders
+ * with no associated Image object.
+ */
+class AnonymousDecodingTask final : public IDecodingTask
+{
+public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(AnonymousDecodingTask, override)
+
+  explicit AnonymousDecodingTask(NotNull<Decoder*> aDecoder);
+
+  void Run() override;
+
+  bool ShouldPreferSyncRun() const override { return true; }
+  TaskPriority Priority() const override { return TaskPriority::eLow; }
+
+  // Anonymous decoders normally get all their data at once. We have tests where
+  // they don't; in these situations, the test re-runs them manually. So no
+  // matter what, we don't want to resume by posting a task to the DecodePool.
+  void Resume() override { }
+
+private:
+  virtual ~AnonymousDecodingTask() { }
+
+  NotNull<RefPtr<Decoder>> mDecoder;
+};
+
+} // namespace image
+} // namespace mozilla
+
+#endif // mozilla_image_IDecodingTask_h