Bug 1079627 (Part 2) - Add SourceBuffer. r=tn

image/src/SourceBuffer.cpp

@@ -0,0 +1,470 @@
+/* -*- 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/. */
+
+#include "SourceBuffer.h"
+
+#include <algorithm>
+#include <cmath>
+#include <cstring>
+#include "mozilla/Likely.h"
+#include "MainThreadUtils.h"
+#include "SurfaceCache.h"
+
+using std::max;
+using std::min;
+
+namespace mozilla {
+namespace image {
+
+//////////////////////////////////////////////////////////////////////////////
+// SourceBufferIterator implementation.
+//////////////////////////////////////////////////////////////////////////////
+
+SourceBufferIterator::State
+SourceBufferIterator::AdvanceOrScheduleResume(IResumable* aConsumer)
+{
+  MOZ_ASSERT(mOwner);
+  return mOwner->AdvanceIteratorOrScheduleResume(*this, aConsumer);
+}
+
+bool
+SourceBufferIterator::RemainingBytesIsNoMoreThan(size_t aBytes) const
+{
+  MOZ_ASSERT(mOwner);
+  return mOwner->RemainingBytesIsNoMoreThan(*this, aBytes);
+}
+
+
+//////////////////////////////////////////////////////////////////////////////
+// SourceBuffer implementation.
+//////////////////////////////////////////////////////////////////////////////
+
+SourceBuffer::SourceBuffer()
+  : mMutex("image::SourceBuffer")
+{ }
+
+SourceBuffer::~SourceBuffer()
+{
+  // FallibleTArray doesn't call destructors so we have to manually release.
+  for (uint32_t i = 0 ; i < mChunks.Length() ; ++i) {
+    mChunks[i].Release();
+  }
+}
+
+nsresult
+SourceBuffer::AppendChunk(Maybe<Chunk>&& aChunk)
+{
+  mMutex.AssertCurrentThreadOwns();
+
+#ifdef DEBUG
+  if (mChunks.Length() > 0) {
+    NS_WARNING("Appending an extra chunk for SourceBuffer");
+  }
+#endif
+
+  if (MOZ_UNLIKELY(!aChunk)) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  if (MOZ_UNLIKELY(aChunk->AllocationFailed())) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  if (MOZ_UNLIKELY(!mChunks.AppendElement(Move(*aChunk)))) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  return NS_OK;
+}
+
+Maybe<SourceBuffer::Chunk>
+SourceBuffer::CreateChunk(size_t aCapacity)
+{
+  if (MOZ_UNLIKELY(aCapacity == 0)) {
+    MOZ_ASSERT_UNREACHABLE("Appending a chunk of zero size?");
+    return Nothing();
+  }
+
+  // Protect against overflow.
+  if (MOZ_UNLIKELY(SIZE_MAX - aCapacity < MIN_CHUNK_CAPACITY)) {
+    return Nothing();
+  }
+
+  // Round up to the next multiple of MIN_CHUNK_CAPACITY (which should be the
+  // size of a page).
+  size_t roundedCapacity =
+    (aCapacity + MIN_CHUNK_CAPACITY - 1) & ~(MIN_CHUNK_CAPACITY - 1);
+  MOZ_ASSERT(roundedCapacity >= aCapacity, "Bad math?");
+  MOZ_ASSERT(roundedCapacity - aCapacity < MIN_CHUNK_CAPACITY, "Bad math?");
+
+  // Use the size of the SurfaceCache as an additional heuristic to avoid
+  // allocating huge buffers. Generally images do not get smaller when decoded,
+  // so if we could store the source data in the SurfaceCache, we assume that
+  // there's no way we'll be able to store the decoded version.
+  if (MOZ_UNLIKELY(!SurfaceCache::CanHold(roundedCapacity))) {
+    return Nothing();
+  }
+
+  return Some(Chunk(roundedCapacity));
+}
+
+size_t
+SourceBuffer::FibonacciCapacityWithMinimum(size_t aMinCapacity)
+{
+  mMutex.AssertCurrentThreadOwns();
+
+  // We grow the source buffer using a Fibonacci growth rate.
+
+  size_t length = mChunks.Length();
+
+  if (length == 0) {
+    return aMinCapacity;
+  }
+
+  if (length == 1) {
+    return max(2 * mChunks[0].Capacity(), aMinCapacity);
+  }
+
+  return max(mChunks[length - 1].Capacity() + mChunks[length - 2].Capacity(),
+             aMinCapacity);
+}
+
+void
+SourceBuffer::AddWaitingConsumer(IResumable* aConsumer)
+{
+  mMutex.AssertCurrentThreadOwns();
+
+  if (MOZ_UNLIKELY(NS_IsMainThread())) {
+    NS_WARNING("SourceBuffer consumer on the main thread needed to wait");
+  }
+
+  mWaitingConsumers.AppendElement(aConsumer);
+}
+
+void
+SourceBuffer::ResumeWaitingConsumers()
+{
+  mMutex.AssertCurrentThreadOwns();
+
+  if (mWaitingConsumers.Length() == 0) {
+    return;
+  }
+
+  for (uint32_t i = 0 ; i < mWaitingConsumers.Length() ; ++i) {
+    mWaitingConsumers[i]->Resume();
+    mWaitingConsumers[i] = nullptr;
+  }
+
+  mWaitingConsumers.Clear();
+}
+
+nsresult
+SourceBuffer::ExpectLength(size_t aExpectedLength)
+{
+  MOZ_ASSERT(aExpectedLength > 0, "Zero expected size?");
+
+  MutexAutoLock lock(mMutex);
+
+  if (MOZ_UNLIKELY(mStatus)) {
+    MOZ_ASSERT_UNREACHABLE("ExpectLength after SourceBuffer is complete");
+    return NS_OK;
+  }
+
+  if (MOZ_UNLIKELY(mChunks.Length() > 0)) {
+    MOZ_ASSERT_UNREACHABLE("Duplicate or post-Append call to ExpectLength");
+    return NS_OK;
+  }
+
+  if (MOZ_UNLIKELY(NS_FAILED(AppendChunk(CreateChunk(aExpectedLength))))) {
+    return HandleError(NS_ERROR_OUT_OF_MEMORY);
+  }
+
+  return NS_OK;
+}
+
+nsresult
+SourceBuffer::Append(const char* aData, size_t aLength)
+{
+  MOZ_ASSERT(aData, "Should have a buffer");
+  MOZ_ASSERT(aLength > 0, "Writing a zero-sized chunk");
+
+  size_t currentChunkCapacity = 0;
+  size_t currentChunkLength = 0;
+  char* currentChunkData = nullptr;
+  size_t currentChunkRemaining = 0;
+  size_t forCurrentChunk = 0;
+  size_t forNextChunk = 0;
+  size_t nextChunkCapacity = 0;
+
+  {
+    MutexAutoLock lock(mMutex);
+
+    if (MOZ_UNLIKELY(mStatus)) {
+      // This SourceBuffer is already complete; ignore further data.
+      return NS_ERROR_FAILURE;
+    }
+
+    if (MOZ_UNLIKELY(mChunks.Length() == 0)) {
+      if (MOZ_UNLIKELY(NS_FAILED(AppendChunk(CreateChunk(aLength))))) {
+        return HandleError(NS_ERROR_OUT_OF_MEMORY);
+      }
+    }
+
+    // Copy out the current chunk's information so we can release the lock.
+    // Note that this wouldn't be safe if multiple producers were allowed!
+    Chunk& currentChunk = mChunks.LastElement();
+    currentChunkCapacity = currentChunk.Capacity();
+    currentChunkLength = currentChunk.Length();
+    currentChunkData = currentChunk.Data();
+
+    // Partition this data between the current chunk and the next chunk.
+    // (Because we always allocate a chunk big enough to fit everything passed
+    // to Append, we'll never need more than those two chunks to store
+    // everything.)
+    currentChunkRemaining = currentChunkCapacity - currentChunkLength;
+    forCurrentChunk = min(aLength, currentChunkRemaining);
+    forNextChunk = aLength - forCurrentChunk;
+
+    // If we'll need another chunk, determine what its capacity should be while
+    // we still hold the lock.
+    nextChunkCapacity = forNextChunk > 0
+                      ? FibonacciCapacityWithMinimum(forNextChunk)
+                      : 0;
+  }
+
+  // Write everything we can fit into the current chunk.
+  MOZ_ASSERT(currentChunkLength + forCurrentChunk <= currentChunkCapacity);
+  memcpy(currentChunkData + currentChunkLength, aData, forCurrentChunk);
+
+  // If there's something left, create a new chunk and write it there.
+  Maybe<Chunk> nextChunk;
+  if (forNextChunk > 0) {
+    MOZ_ASSERT(nextChunkCapacity >= forNextChunk, "Next chunk too small?");
+    nextChunk = CreateChunk(nextChunkCapacity);
+    if (MOZ_LIKELY(nextChunk && !nextChunk->AllocationFailed())) {
+      memcpy(nextChunk->Data(), aData + forCurrentChunk, forNextChunk);
+      nextChunk->AddLength(forNextChunk);
+    }
+  }
+
+  // Update shared data structures.
+  {
+    MutexAutoLock lock(mMutex);
+
+    // Update the length of the current chunk.
+    Chunk& currentChunk = mChunks.LastElement();
+    MOZ_ASSERT(currentChunk.Data() == currentChunkData, "Multiple producers?");
+    MOZ_ASSERT(currentChunk.Length() == currentChunkLength,
+               "Multiple producers?");
+
+    currentChunk.AddLength(forCurrentChunk);
+
+    // If we created a new chunk, add it to the series.
+    if (forNextChunk > 0) {
+      if (MOZ_UNLIKELY(!nextChunk)) {
+        return HandleError(NS_ERROR_OUT_OF_MEMORY);
+      }
+
+      if (MOZ_UNLIKELY(NS_FAILED(AppendChunk(Move(nextChunk))))) {
+        return HandleError(NS_ERROR_OUT_OF_MEMORY);
+      }
+    }
+
+    // Resume any waiting readers now that there's new data.
+    ResumeWaitingConsumers();
+  }
+
+  return NS_OK;
+}
+
+void
+SourceBuffer::Complete(nsresult aStatus)
+{
+  MutexAutoLock lock(mMutex);
+
+  if (MOZ_UNLIKELY(mStatus)) {
+    MOZ_ASSERT_UNREACHABLE("Called Complete more than once");
+    return;
+  }
+
+  if (MOZ_UNLIKELY(NS_SUCCEEDED(aStatus) && IsEmpty())) {
+    // It's illegal to succeed without writing anything.
+    aStatus = NS_ERROR_FAILURE;
+  }
+
+  mStatus = Some(aStatus);
+
+  // Resume any waiting consumers now that we're complete.
+  ResumeWaitingConsumers();
+}
+
+bool
+SourceBuffer::IsComplete()
+{
+  MutexAutoLock lock(mMutex);
+  return bool(mStatus);
+}
+
+size_t
+SourceBuffer::SizeOfIncludingThisWithComputedFallback(MallocSizeOf
+                                                        aMallocSizeOf) const
+{
+  MutexAutoLock lock(mMutex);
+
+  size_t n = aMallocSizeOf(this);
+  n += mChunks.SizeOfExcludingThis(aMallocSizeOf);
+
+  for (uint32_t i = 0 ; i < mChunks.Length() ; ++i) {
+    size_t chunkSize = aMallocSizeOf(mChunks[i].Data());
+
+    if (chunkSize == 0) {
+      // We're on a platform where moz_malloc_size_of always returns 0.
+      chunkSize = mChunks[i].Capacity();
+    }
+
+    n += chunkSize;
+  }
+
+  return n;
+}
+
+bool
+SourceBuffer::RemainingBytesIsNoMoreThan(const SourceBufferIterator& aIterator,
+                                         size_t aBytes) const
+{
+  MutexAutoLock lock(mMutex);
+
+  // If we're not complete, we always say no.
+  if (!mStatus) {
+    return false;
+  }
+
+  // If the iterator's at the end, the answer is trivial.
+  if (!aIterator.HasMore()) {
+    return true;
+  }
+
+  uint32_t iteratorChunk = aIterator.mData.mIterating.mChunk;
+  size_t iteratorOffset = aIterator.mData.mIterating.mOffset;
+  size_t iteratorLength = aIterator.mData.mIterating.mLength;
+
+  // Include the bytes the iterator is currently pointing to in the limit, so
+  // that the current chunk doesn't have to be a special case.
+  size_t bytes = aBytes + iteratorOffset + iteratorLength;
+
+  // Count the length over all of our chunks, starting with the one that the
+  // iterator is currently pointing to. (This is O(N), but N is expected to be
+  // ~1, so it doesn't seem worth caching the length separately.)
+  size_t lengthSoFar = 0;
+  for (uint32_t i = iteratorChunk ; i < mChunks.Length() ; ++i) {
+    lengthSoFar += mChunks[i].Length();
+    if (lengthSoFar > bytes) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+SourceBufferIterator::State
+SourceBuffer::AdvanceIteratorOrScheduleResume(SourceBufferIterator& aIterator,
+                                              IResumable* aConsumer)
+{
+  MutexAutoLock lock(mMutex);
+
+  if (MOZ_UNLIKELY(!aIterator.HasMore())) {
+    MOZ_ASSERT_UNREACHABLE("Should not advance a completed iterator");
+    return SourceBufferIterator::COMPLETE;
+  }
+
+  if (MOZ_UNLIKELY(mStatus && NS_FAILED(*mStatus))) {
+    // This SourceBuffer is complete due to an error; all reads fail.
+    return aIterator.SetComplete(*mStatus);
+  }
+
+  if (MOZ_UNLIKELY(mChunks.Length() == 0)) {
+    // We haven't gotten an initial chunk yet.
+    AddWaitingConsumer(aConsumer);
+    return aIterator.SetWaiting();
+  }
+
+  uint32_t iteratorChunkIdx = aIterator.mData.mIterating.mChunk;
+  MOZ_ASSERT(iteratorChunkIdx < mChunks.Length());
+
+  const Chunk& currentChunk = mChunks[iteratorChunkIdx];
+  size_t iteratorEnd = aIterator.mData.mIterating.mOffset +
+                       aIterator.mData.mIterating.mLength;
+  MOZ_ASSERT(iteratorEnd <= currentChunk.Length());
+  MOZ_ASSERT(iteratorEnd <= currentChunk.Capacity());
+
+  if (iteratorEnd < currentChunk.Length()) {
+    // There's more data in the current chunk.
+    return aIterator.SetReady(iteratorChunkIdx, currentChunk.Data(),
+                              iteratorEnd, currentChunk.Length() - iteratorEnd);
+  }
+
+  if (iteratorEnd == currentChunk.Capacity() &&
+      !IsLastChunk(iteratorChunkIdx)) {
+    // Advance to the next chunk.
+    const Chunk& nextChunk = mChunks[iteratorChunkIdx + 1];
+    return aIterator.SetReady(iteratorChunkIdx + 1, nextChunk.Data(), 0,
+                              nextChunk.Length());
+  }
+
+  MOZ_ASSERT(IsLastChunk(iteratorChunkIdx), "Should've advanced");
+
+  if (mStatus) {
+    // There's no more data and this SourceBuffer completed successfully.
+    MOZ_ASSERT(NS_SUCCEEDED(*mStatus), "Handled failures earlier");
+    return aIterator.SetComplete(*mStatus);
+  }
+
+  // We're not complete, but there's no more data right now. Arrange to wake up
+  // the consumer when we get more data.
+  AddWaitingConsumer(aConsumer);
+  return aIterator.SetWaiting();
+}
+
+nsresult
+SourceBuffer::HandleError(nsresult aError)
+{
+  MOZ_ASSERT(NS_FAILED(aError), "Should have an error here");
+  MOZ_ASSERT(aError == NS_ERROR_OUT_OF_MEMORY,
+             "Unexpected error; may want to notify waiting readers, which "
+             "HandleError currently doesn't do");
+
+  mMutex.AssertCurrentThreadOwns();
+
+  NS_WARNING("SourceBuffer encountered an unrecoverable error");
+
+  // Record the error.
+  mStatus = Some(aError);
+
+  // Drop our references to waiting readers.
+  for (uint32_t i = 0 ; i < mWaitingConsumers.Length() ; ++i) {
+    mWaitingConsumers[i] = nullptr;
+  }
+  mWaitingConsumers.Clear();
+
+  return *mStatus;
+}
+
+bool
+SourceBuffer::IsEmpty()
+{
+  mMutex.AssertCurrentThreadOwns();
+  return mChunks.Length() == 0 ||
+         mChunks[0].Length() == 0;
+}
+
+bool
+SourceBuffer::IsLastChunk(uint32_t aChunk)
+{
+  mMutex.AssertCurrentThreadOwns();
+  return aChunk + 1 == mChunks.Length();
+}
+
+} // namespace image
+} // namespace mozilla

image/src/SourceBuffer.h

@@ -0,0 +1,371 @@
+/* -*- 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/. */
+
+/**
+ * SourceBuffer is a single producer, multiple consumer data structure used for
+ * storing image source (compressed) data.
+ */
+
+#ifndef MOZILLA_IMAGELIB_SOURCEBUFFER_H_
+#define MOZILLA_IMAGELIB_SOURCEBUFFER_H_
+
+#include "mozilla/Maybe.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/Move.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/UniquePtr.h"
+#include "nsRefPtr.h"
+#include "nsTArray.h"
+
+namespace mozilla {
+namespace image {
+
+class SourceBuffer;
+
+/**
+ * IResumable is an interface for classes that can schedule themselves to resume
+ * their work later. An implementation of IResumable generally should post a
+ * runnable to some event target which continues the work of the task.
+ */
+struct IResumable
+{
+  MOZ_DECLARE_REFCOUNTED_TYPENAME(IResumable)
+
+  // Subclasses may or may not be XPCOM classes, so we just require that they
+  // implement AddRef and Release.
+  NS_IMETHOD_(MozExternalRefCountType) AddRef(void) = 0;
+  NS_IMETHOD_(MozExternalRefCountType) Release(void) = 0;
+
+  virtual void Resume() = 0;
+
+protected:
+  virtual ~IResumable() { }
+};
+
+/**
+ * SourceBufferIterator is a class that allows consumers of image source data to
+ * read the contents of a SourceBuffer sequentially.
+ *
+ * Consumers can advance through the SourceBuffer by calling
+ * AdvanceOrScheduleResume() repeatedly. After every advance, they should call
+ * check the return value, which will tell them the iterator's new state.
+ *
+ * If WAITING is returned, AdvanceOrScheduleResume() has arranged
+ * to call the consumer's Resume() method later, so the consumer should save its
+ * state if needed and stop running.
+ *
+ * If the iterator's new state is READY, then the consumer can call Data() and
+ * Length() to read new data from the SourceBuffer.
+ *
+ * Finally, in the COMPLETE state the consumer can call CompletionStatus() to
+ * get the status passed to SourceBuffer::Complete().
+ */
+class SourceBufferIterator MOZ_FINAL
+{
+public:
+  enum State {
+    START,    // The iterator is at the beginning of the buffer.
+    READY,    // The iterator is pointing to new data.
+    WAITING,  // The iterator is blocked and the caller must yield.
+    COMPLETE  // The iterator is pointing to the end of the buffer.
+  };
+
+  explicit SourceBufferIterator(SourceBuffer* aOwner)
+    : mOwner(aOwner)
+    , mState(START)
+  {
+    MOZ_ASSERT(aOwner);
+    mData.mIterating.mChunk = 0;
+    mData.mIterating.mData = nullptr;
+    mData.mIterating.mOffset = 0;
+    mData.mIterating.mLength = 0;
+  }
+
+  SourceBufferIterator(SourceBufferIterator&& aOther)
+    : mOwner(Move(aOther.mOwner))
+    , mState(aOther.mState)
+    , mData(aOther.mData)
+  { }
+
+  SourceBufferIterator& operator=(SourceBufferIterator&& aOther)
+  {
+    mOwner = Move(aOther.mOwner);
+    mState = aOther.mState;
+    mData = aOther.mData;
+    return *this;
+  }
+
+  /**
+   * Returns true if there are no more than @aBytes remaining in the
+   * SourceBuffer. If the SourceBuffer is not yet complete, returns false.
+   */
+  bool RemainingBytesIsNoMoreThan(size_t aBytes) const;
+
+  /**
+   * Advances the iterator through the SourceBuffer if possible. If not,
+   * arranges to call the @aConsumer's Resume() method when more data is
+   * available.
+   */
+  State AdvanceOrScheduleResume(IResumable* aConsumer);
+
+  /// If at the end, returns the status passed to SourceBuffer::Complete().
+  nsresult CompletionStatus() const
+  {
+    MOZ_ASSERT(mState == COMPLETE, "Calling CompletionStatus() in the wrong state");
+    return mState == COMPLETE ? mData.mAtEnd.mStatus : NS_OK;
+  }
+
+  /// If we're ready to read, returns a pointer to the new data.
+  const char* Data() const
+  {
+    MOZ_ASSERT(mState == READY, "Calling Data() in the wrong state");
+    return mState == READY ? mData.mIterating.mData + mData.mIterating.mOffset
+                           : nullptr;
+  }
+
+  /// If we're ready to read, returns the length of the new data.
+  size_t Length() const
+  {
+    MOZ_ASSERT(mState == READY, "Calling Length() in the wrong state");
+    return mState == READY ? mData.mIterating.mLength : 0;
+  }
+
+private:
+  friend class SourceBuffer;
+
+  SourceBufferIterator(const SourceBufferIterator&) = delete;
+  SourceBufferIterator& operator=(const SourceBufferIterator&) = delete;
+
+  bool HasMore() const { return mState != COMPLETE; }
+
+  State SetReady(uint32_t aChunk, const char* aData,
+                size_t aOffset, size_t aLength)
+  {
+    MOZ_ASSERT(mState != COMPLETE);
+    mData.mIterating.mChunk = aChunk;
+    mData.mIterating.mData = aData;
+    mData.mIterating.mOffset = aOffset;
+    mData.mIterating.mLength = aLength;
+    return mState = READY;
+  }
+
+  State SetWaiting()
+  {
+    MOZ_ASSERT(mState != COMPLETE);
+    MOZ_ASSERT(mState != WAITING, "Did we get a spurious wakeup somehow?");
+    return mState = WAITING;
+  }
+
+  State SetComplete(nsresult aStatus)
+  {
+    mData.mAtEnd.mStatus = aStatus;
+    return mState = COMPLETE;
+  }
+
+  nsRefPtr<SourceBuffer> mOwner;
+
+  State mState;
+
+  /**
+   * This union contains our iteration state if we're still iterating (for
+   * states START, READY, and WAITING) and the status the SourceBuffer was
+   * completed with if we're in state COMPLETE.
+   */
+  union {
+    struct {
+      uint32_t mChunk;
+      const char* mData;
+      size_t mOffset;
+      size_t mLength;
+    } mIterating;
+    struct {
+      nsresult mStatus;
+    } mAtEnd;
+  } mData;
+};
+
+/**
+ * SourceBuffer is a parallel data structure used for storing image source
+ * (compressed) data.
+ *
+ * SourceBuffer is a single producer, multiple consumer data structure. The
+ * single producer calls Append() to append data to the buffer. In parallel,
+ * multiple consumers can call Iterator(), which returns a SourceBufferIterator
+ * that they can use to iterate through the buffer. The SourceBufferIterator
+ * returns a series of pointers which remain stable for lifetime of the
+ * SourceBuffer, and the data they point to is immutable, ensuring that the
+ * producer never interferes with the consumers.
+ *
+ * In order to avoid blocking, SourceBuffer works with SourceBufferIterator to
+ * keep a list of consumers which are waiting for new data, and to resume them
+ * when the producer appends more. All consumers must implement the IResumable
+ * interface to make this possible.
+ *
+ * XXX(seth): We should add support for compacting a SourceBuffer. To do this,
+ * we need to have SourceBuffer keep track of how many live
+ * SourceBufferIterator's point to it. When the SourceBuffer is complete and no
+ * live SourceBufferIterator's for it remain, we can compact its contents into a
+ * single chunk.
+ */
+class SourceBuffer MOZ_FINAL
+{
+public:
+  MOZ_DECLARE_REFCOUNTED_TYPENAME(image::SourceBuffer)
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(image::SourceBuffer)
+
+  SourceBuffer();
+
+  //////////////////////////////////////////////////////////////////////////////
+  // Producer methods.
+  //////////////////////////////////////////////////////////////////////////////
+
+  /**
+   * If the producer knows how long the source data will be, it should call
+   * ExpectLength, which enables SourceBuffer to preallocate its buffer.
+   */
+  nsresult ExpectLength(size_t aExpectedLength);
+
+  /// Append the provided data to the buffer.
+  nsresult Append(const char* aData, size_t aLength);
+
+  /**
+   * Mark the buffer complete, with a status that will be available to
+   * consumers. Further calls to Append() are forbidden after Complete().
+   */
+  void Complete(nsresult aStatus);
+
+  /// Returns true if the buffer is complete.
+  bool IsComplete();
+
+  /// Memory reporting.
+  size_t SizeOfIncludingThisWithComputedFallback(MallocSizeOf) const;
+
+
+  //////////////////////////////////////////////////////////////////////////////
+  // Consumer methods.
+  //////////////////////////////////////////////////////////////////////////////
+
+  /// Returns an iterator to this SourceBuffer.
+  SourceBufferIterator Iterator() { return SourceBufferIterator(this); }
+
+
+private:
+  friend class SourceBufferIterator;
+
+  virtual ~SourceBuffer();
+
+
+  //////////////////////////////////////////////////////////////////////////////
+  // Chunk type and chunk-related methods.
+  //////////////////////////////////////////////////////////////////////////////
+
+  class Chunk
+  {
+  public:
+    explicit Chunk(size_t aCapacity)
+      : mCapacity(aCapacity)
+      , mLength(0)
+      , mData(MakeUnique<char[]>(mCapacity))
+    {
+      MOZ_ASSERT(aCapacity > 0, "Creating zero-capacity chunk");
+    }
+
+    Chunk(Chunk&& aOther)
+      : mCapacity(aOther.mCapacity)
+      , mLength(aOther.mLength)
+      , mData(Move(aOther.mData))
+    {
+      aOther.mCapacity = aOther.mLength = 0;
+    }
+
+    Chunk& operator=(Chunk&& aOther)
+    {
+      mCapacity = aOther.mCapacity;
+      mLength = aOther.mLength;
+      mData = Move(aOther.mData);
+      aOther.mCapacity = aOther.mLength = 0;
+      return *this;
+    }
+
+    bool AllocationFailed() const { return !mData; }
+    size_t Capacity() const { return mCapacity; }
+    size_t Length() const { return mLength; }
+    char* Data() const { return mData.get(); }
+
+    void AddLength(size_t aAdditionalLength)
+    {
+      MOZ_ASSERT(mLength + aAdditionalLength <= mCapacity);
+      mLength += aAdditionalLength;
+    }
+
+    void Release()
+    {
+      mCapacity = mLength = 0;
+      mData.reset();
+    }
+
+  private:
+    Chunk(const Chunk&) = delete;
+    Chunk& operator=(const Chunk&) = delete;
+
+    size_t mCapacity;
+    size_t mLength;
+    UniquePtr<char[]> mData;
+  };
+
+  nsresult AppendChunk(Maybe<Chunk>&& aChunk);
+  Maybe<Chunk> CreateChunk(size_t aCapacity);
+  size_t FibonacciCapacityWithMinimum(size_t aMinCapacity);
+
+
+  //////////////////////////////////////////////////////////////////////////////
+  // Iterator / consumer methods.
+  //////////////////////////////////////////////////////////////////////////////
+
+  void AddWaitingConsumer(IResumable* aConsumer);
+  void ResumeWaitingConsumers();
+
+  typedef SourceBufferIterator::State State;
+
+  State AdvanceIteratorOrScheduleResume(SourceBufferIterator& aIterator,
+                                        IResumable* aConsumer);
+  bool RemainingBytesIsNoMoreThan(const SourceBufferIterator& aIterator,
+                                  size_t aBytes) const;
+
+
+  //////////////////////////////////////////////////////////////////////////////
+  // Helper methods.
+  //////////////////////////////////////////////////////////////////////////////
+
+  nsresult HandleError(nsresult aError);
+  bool IsEmpty();
+  bool IsLastChunk(uint32_t aChunk);
+
+
+  //////////////////////////////////////////////////////////////////////////////
+  // Member variables.
+  //////////////////////////////////////////////////////////////////////////////
+
+  static const size_t MIN_CHUNK_CAPACITY = 4096;
+
+  /// All private members are protected by mMutex.
+  mutable Mutex mMutex;
+
+  /// The data in this SourceBuffer, stored as a series of Chunks.
+  FallibleTArray<Chunk> mChunks;
+
+  /// Consumers which are waiting to be notified when new data is available.
+  nsTArray<nsRefPtr<IResumable>> mWaitingConsumers;
+
+  /// If present, marks this SourceBuffer complete with the given final status.
+  Maybe<nsresult> mStatus;
+};
+
+} // namespace image
+} // namespace mozilla
+
+#endif // MOZILLA_IMAGELIB_SOURCEBUFFER_H_

image/src/SurfaceCache.cpp

@@ -824,6 +824,16 @@ SurfaceCache::CanHold(const IntSize& aSize)
   return sInstance->CanHold(cost);
 }
 
+/* static */ bool
+SurfaceCache::CanHold(size_t aSize)
+{
+  if (!sInstance) {
+    return false;
+  }
+
+  return sInstance->CanHold(aSize);
+}
+
 /* static */ void
 SurfaceCache::LockImage(Image* aImageKey)
 {

image/src/SurfaceCache.h

@@ -236,6 +236,7 @@ struct SurfaceCache
    * @return false if the surface cache can't hold a surface of that size.
    */
   static bool CanHold(const IntSize& aSize);
+  static bool CanHold(size_t aSize);
 
   /**
    * Locks an image, preventing any of that image's surfaces from expiring

image/src/moz.build

@@ -33,6 +33,7 @@ UNIFIED_SOURCES += [
     'OrientedImage.cpp',
     'ScriptedNotificationObserver.cpp',
     'ShutdownTracker.cpp',
+    'SourceBuffer.cpp',
     'SurfaceCache.cpp',
     'SVGDocumentWrapper.cpp',
     'VectorImage.cpp',