The Tor Browser: content/media/FileBlockCache.h@44a2da4a2ab2 (annotated)

content/media/FileBlockCache.h@44a2da4a2ab2 (annotated)

content/media/FileBlockCache.h

Fri, 16 Jan 2015 04:50:19 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Fri, 16 Jan 2015 04:50:19 +0100
branch: TOR_BUG_9701
changeset 13: 44a2da4a2ab2
permissions: -rw-r--r--

Replace accessor implementation with direct member state manipulation, by
request https://trac.torproject.org/projects/tor/ticket/9701#comment:32

 /* -*- 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 http://mozilla.org/MPL/2.0/. */
 #ifndef FILE_BLOCK_CACHE_H_
 #define FILE_BLOCK_CACHE_H_
 #include "mozilla/Attributes.h"
 #include "mozilla/Monitor.h"
 #include "nsTArray.h"
 #include "MediaCache.h"
 #include "nsDeque.h"
 #include "nsThreadUtils.h"
 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 nsRunnable {
 public:
   enum {
     BLOCK_SIZE = MediaCacheStream::BLOCK_SIZE
   };
   FileBlockCache();
   ~FileBlockCache();
   // Assumes ownership of aFD.
   nsresult Open(PRFileDesc* aFD);
   // Closes writer, shuts down thread.
   void Close();
   // Can be called on any thread. This defers to a non-main thread.
   nsresult WriteBlock(uint32_t aBlockIndex, const uint8_t* aData);
   // Performs block writes and block moves on its own thread.
   NS_IMETHOD Run() MOZ_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);
   // 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);
   // 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 MOZ_FINAL {
     NS_INLINE_DECL_THREADSAFE_REFCOUNTING(BlockChange)
     // This block is waiting in memory to be written.
     // Stores a copy of the block, so we can write it asynchronously.
     BlockChange(const uint8_t* aData)
       : mSourceBlockIndex(-1)
     {
       mData = new uint8_t[BLOCK_SIZE];
       memcpy(mData.get(), aData, BLOCK_SIZE);
     }
     // This block's contents are located in another file
     // block, i.e. this block has been moved.
     BlockChange(int32_t aSourceBlockIndex)
       : mSourceBlockIndex(aSourceBlockIndex) {}
     nsAutoArrayPtr<uint8_t> mData;
     const int32_t mSourceBlockIndex;
     bool IsMove() const {
       return mSourceBlockIndex != -1;
     }
     bool IsWrite() const {
       return mSourceBlockIndex == -1 &&
              mData.get() != nullptr;
     }
   private:
     // Private destructor, to discourage deletion outside of Release():
     ~BlockChange()
     {
     }
   };
   class Int32Queue : private nsDeque {
   public:
     int32_t PopFront() {
       int32_t front = ObjectAt(0);
       nsDeque::PopFront();
       return front;
     }
     void PushBack(int32_t aValue) {
       nsDeque::Push(reinterpret_cast<void*>(aValue));
     }
     bool Contains(int32_t aValue) {
       for (int32_t i = 0; i < GetSize(); ++i) {
         if (ObjectAt(i) == aValue) {
           return true;
         }
       }
       return false;
     }
     bool IsEmpty() {
       return nsDeque::GetSize() == 0;
     }
   private:
     int32_t ObjectAt(int32_t aIndex) {
       void* v = nsDeque::ObjectAt(aIndex);
       return reinterpret_cast<uintptr_t>(v);
     }
   };
 private:
   int64_t BlockIndexToOffset(int32_t aBlockIndex) {
     return static_cast<int64_t>(aBlockIndex) * BLOCK_SIZE;
   }
   // Monitor which controls access to mFD and mFDCurrentPos. Don't hold
   // mDataMonitor while holding mFileMonitor! mFileMonitor must be owned
   // while accessing any of the following data fields or methods.
   Monitor mFileMonitor;
   // 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;
   // Monitor which controls access to all data in this class, except mFD
   // and mFDCurrentPos. Don't hold mDataMonitor while holding mFileMonitor!
   // mDataMonitor must be owned while accessing any of the following data
   // fields or methods.
   Monitor mDataMonitor;
   // Ensures we either are running the event to preform IO, or an event
   // has been dispatched to preform the IO.
   // mDataMonitor 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< nsRefPtr<BlockChange> > mBlockChanges;
   // Thread upon which block writes and block moves are performed. This is
   // created upon open, and shutdown (asynchronously) upon close (on the
   // main thread).
   nsCOMPtr<nsIThread> mThread;
   // Queue of pending block indexes that need to be written or moved.
   //nsAutoTArray<int32_t, 8> mChangeIndexList;
   Int32Queue mChangeIndexList;
   // True if we've dispatched an event to commit all pending block changes
   // to file on mThread.
   bool mIsWriteScheduled;
   // True if the writer is ready to write data to file.
   bool mIsOpen;
 };
 } // End namespace mozilla.
 #endif /* FILE_BLOCK_CACHE_H_ */

The Tor Browser / annotate

content/media/FileBlockCache.h@44a2da4a2ab2 (annotated)

content/media/FileBlockCache.h