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/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim:set ts=2 sw=2 sts=2 et cindent: */
/* 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 */
#include "mozilla/Attributes.h"
#include "mozilla/MozPromise.h"
#include "mozilla/Mutex.h"
#include "mozilla/UniquePtr.h"
#include "mozilla/AbstractThread.h"
#include "nsTArray.h"
#include "MediaBlockCacheBase.h"
#include "nsDeque.h"
#include "nsThreadUtils.h"
#include <deque>
struct PRFileDesc;
namespace mozilla {
// Manages file I/O for the media cache. Data comes in over the network
// via callbacks on the main thread, however we don't want to write the
// incoming data to the media cache on the main thread, as this could block
// causing UI jank.
// So FileBlockCache provides an abstraction for a temporary file accessible
// as an array of blocks, which supports a block move operation, and
// allows synchronous reading and writing from any thread, with writes being
// buffered so as not to block.
// Writes and cache block moves (which require reading) are deferred to
// their own non-main thread. This object also ensures that data which has
// been scheduled to be written, but hasn't actually *been* written, is read
// as if it had, i.e. pending writes are cached in readable memory until
// they're flushed to file.
// To improve efficiency, writes can only be done at block granularity,
// whereas reads can be done with byte granularity.
// Note it's also recommended not to read from the media cache from the main
// thread to prevent jank.
// When WriteBlock() or MoveBlock() are called, data about how to complete
// the block change is added to mBlockChanges, indexed by block index, and
// the block index is appended to the mChangeIndexList. This enables
// us to quickly tell if a block has been changed, and ensures we can perform
// the changes in the correct order. An event is dispatched to perform the
// changes listed in mBlockChanges to file. Read() checks mBlockChanges and
// determines the current data to return, reading from file or from
// mBlockChanges as necessary.
class FileBlockCache : public MediaBlockCacheBase {
virtual ~FileBlockCache();
// Launch thread and open temporary file.
nsresult Init() override;
// Will discard pending changes if any.
void Flush() override;
// Maximum number of blocks allowed in this block cache.
// Calculated from "media.cache_size" pref.
size_t GetMaxBlocks(size_t aCacheSizeInKB) const override;
// Can be called on any thread. This defers to a non-main thread.
nsresult WriteBlock(uint32_t aBlockIndex, Span<const uint8_t> aData1,
Span<const uint8_t> aData2) override;
// Synchronously reads data from file. May read from file or memory
// depending on whether written blocks have been flushed to file yet.
// Not recommended to be called from the main thread, as can cause jank.
nsresult Read(int64_t aOffset, uint8_t* aData, int32_t aLength,
int32_t* aBytes) override;
// Moves a block asynchronously. Can be called on any thread.
// This defers file I/O to a non-main thread.
nsresult MoveBlock(int32_t aSourceBlockIndex,
int32_t aDestBlockIndex) override;
// Represents a change yet to be made to a block in the file. The change
// is either a write (and the data to be written is stored in this struct)
// or a move (and the index of the source block is stored instead).
struct BlockChange final {
// This block is waiting in memory to be written.
// Stores a copy of the block, so we can write it asynchronously.
explicit BlockChange(const uint8_t* aData) : mSourceBlockIndex(-1) {
mData = MakeUnique<uint8_t[]>(BLOCK_SIZE);
memcpy(mData.get(), aData, BLOCK_SIZE);
BlockChange(Span<const uint8_t> aData1, Span<const uint8_t> aData2)
: mSourceBlockIndex(-1) {
MOZ_ASSERT(aData1.Length() + aData2.Length() == BLOCK_SIZE);
mData = MakeUnique<uint8_t[]>(BLOCK_SIZE);
memcpy(mData.get(), aData1.Elements(), aData1.Length());
memcpy(mData.get() + aData1.Length(), aData2.Elements(), aData2.Length());
// This block's contents are located in another file
// block, i.e. this block has been moved.
explicit BlockChange(int32_t aSourceBlockIndex)
: mSourceBlockIndex(aSourceBlockIndex) {}
UniquePtr<uint8_t[]> mData;
const int32_t mSourceBlockIndex;
bool IsMove() const { return mSourceBlockIndex != -1; }
bool IsWrite() const {
return mSourceBlockIndex == -1 && mData.get() != nullptr;
// Private destructor, to discourage deletion outside of Release():
~BlockChange() = default;
int64_t BlockIndexToOffset(int32_t aBlockIndex) {
return static_cast<int64_t>(aBlockIndex) * BLOCK_SIZE;
void SetCacheFile(PRFileDesc* aFD);
// Close file in thread and terminate thread.
void Close();
// Performs block writes and block moves on its own thread.
void PerformBlockIOs();
// Mutex which controls access to mFD and mFDCurrentPos. Don't hold
// mDataMutex while holding mFileMutex! mFileMutex must be owned
// while accessing any of the following data fields or methods.
Mutex mFileMutex;
// Moves a block already committed to file.
nsresult MoveBlockInFile(int32_t aSourceBlockIndex, int32_t aDestBlockIndex);
// Seeks file pointer.
nsresult Seek(int64_t aOffset);
// Reads data from file offset.
nsresult ReadFromFile(int64_t aOffset, uint8_t* aDest, int32_t aBytesToRead,
int32_t& aBytesRead);
nsresult WriteBlockToFile(int32_t aBlockIndex, const uint8_t* aBlockData);
// File descriptor we're writing to. This is created externally, but
// shutdown by us.
PRFileDesc* mFD;
// The current file offset in the file.
int64_t mFDCurrentPos;
// Mutex which controls access to all data in this class, except mFD
// and mFDCurrentPos. Don't hold mDataMutex while holding mFileMutex!
// mDataMutex must be owned while accessing any of the following data
// fields or methods.
Mutex mDataMutex;
// Ensures we either are running the event to preform IO, or an event
// has been dispatched to preform the IO.
// mDataMutex must be owned while calling this.
void EnsureWriteScheduled();
// Array of block changes to made. If mBlockChanges[offset/BLOCK_SIZE] ==
// nullptr, then the block has no pending changes to be written, but if
// mBlockChanges[offset/BLOCK_SIZE] != nullptr, then either there's a block
// cached in memory waiting to be written, or this block is the target of a
// block move.
nsTArray<RefPtr<BlockChange> > mBlockChanges;
// Event target upon which block writes and block moves are performed. This is
// created upon open, and dropped on close.
nsCOMPtr<nsISerialEventTarget> mBackgroundET;
// Queue of pending block indexes that need to be written or moved.
std::deque<int32_t> mChangeIndexList;
// True if we've dispatched an event to commit all pending block changes
// to file on mBackgroundET.
bool mIsWriteScheduled;
// True when a read is happening. Pending writes may be postponed, to give
// higher priority to reads (which may be blocking the caller).
bool mIsReading;
// True if we've got a temporary file descriptor. Note: we don't use mFD
// directly as that's synchronized via mFileMutex and we need to make
// decisions about whether we can write while holding mDataMutex.
bool mInitialized = false;
} // End namespace mozilla.
#endif /* FILE_BLOCK_CACHE_H_ */