The Tor Browser: content/media/MediaCache.h@6474c204b198 (annotated)

content/media/MediaCache.h@6474c204b198 (annotated)

content/media/MediaCache.h

Wed, 31 Dec 2014 06:09:35 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Wed, 31 Dec 2014 06:09:35 +0100
changeset 0: 6474c204b198
permissions: -rw-r--r--

Cloned upstream origin tor-browser at tor-browser-31.3.0esr-4.5-1-build1
revision ID fc1c9ff7c1b2defdbc039f12214767608f46423f for hacking purpose.

 /* -*- 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 MediaCache_h_
 #define MediaCache_h_
 #include "nsTArray.h"
 #include "nsCOMPtr.h"
 #include "nsHashKeys.h"
 #include "nsTHashtable.h"
 class nsIPrincipal;
 namespace mozilla {
 // defined in MediaResource.h
 class ChannelMediaResource;
 class MediaByteRange;
 class MediaResource;
 class ReentrantMonitorAutoEnter;
 /**
  * Media applications want fast, "on demand" random access to media data,
  * for pausing, seeking, etc. But we are primarily interested
  * in transporting media data using HTTP over the Internet, which has
  * high latency to open a connection, requires a new connection for every
  * seek, may not even support seeking on some connections (especially
  * live streams), and uses a push model --- data comes from the server
  * and you don't have much control over the rate. Also, transferring data
  * over the Internet can be slow and/or unpredictable, so we want to read
  * ahead to buffer and cache as much data as possible.
  *
  * The job of the media cache is to resolve this impedance mismatch.
  * The media cache reads data from Necko channels into file-backed storage,
  * and offers a random-access file-like API to the stream data
  * (MediaCacheStream). Along the way it solves several problems:
  * -- The cache intelligently reads ahead to prefetch data that may be
  * needed in the future
  * -- The size of the cache is bounded so that we don't fill up
  * storage with read-ahead data
  * -- Cache replacement is managed globally so that the most valuable
  * data (across all streams) is retained
  * -- The cache can suspend Necko channels temporarily when their data is
  * not wanted (yet)
  * -- The cache translates file-like seek requests to HTTP seeks,
  * including optimizations like not triggering a new seek if it would
  * be faster to just keep reading until we reach the seek point. The
  * "seek to EOF" idiom to determine file size is also handled efficiently
  * (seeking to EOF and then seeking back to the previous offset does not
  * trigger any Necko activity)
  * -- The cache also handles the case where the server does not support
  * seeking
  * -- Necko can only send data to the main thread, but MediaCacheStream
  * can distribute data to any thread
  * -- The cache exposes APIs so clients can detect what data is
  * currently held
  *
  * Note that although HTTP is the most important transport and we only
  * support transport-level seeking via HTTP byte-ranges, the media cache
  * works with any kind of Necko channels and provides random access to
  * cached data even for, e.g., FTP streams.
  *
  * The media cache is not persistent. It does not currently allow
  * data from one load to be used by other loads, either within the same
  * browser session or across browser sessions. The media cache file
  * is marked "delete on close" so it will automatically disappear in the
  * event of a browser crash or shutdown.
  *
  * The media cache is block-based. Streams are divided into blocks of a
  * fixed size (currently 4K) and we cache blocks. A single cache contains
  * blocks for all streams.
  *
  * The cache size is controlled by the media.cache_size preference
  * (which is in KB). The default size is 500MB.
  *
  * The replacement policy predicts a "time of next use" for each block
  * in the cache. When we need to free a block, the block with the latest
  * "time of next use" will be evicted. Blocks are divided into
  * different classes, each class having its own predictor:
  * FREE_BLOCK: these blocks are effectively infinitely far in the future;
  * a free block will always be chosen for replacement before other classes
  * of blocks.
  * METADATA_BLOCK: these are blocks that contain data that has been read
  * by the decoder in "metadata mode", e.g. while the decoder is searching
  * the stream during a seek operation. These blocks are managed with an
  * LRU policy; the "time of next use" is predicted to be as far in the
  * future as the last use was in the past.
  * PLAYED_BLOCK: these are blocks that have not been read in "metadata
  * mode", and contain data behind the current decoder read point. (They
  * may not actually have been read by the decoder, if the decoder seeked
  * forward.) These blocks are managed with an LRU policy except that we add
  * REPLAY_DELAY seconds of penalty to their predicted "time of next use",
  * to reflect the uncertainty about whether replay will actually happen
  * or not.
  * READAHEAD_BLOCK: these are blocks that have not been read in
  * "metadata mode" and that are entirely ahead of the current decoder
  * read point. (They may actually have been read by the decoder in the
  * past if the decoder has since seeked backward.) We predict the
  * time of next use for these blocks by assuming steady playback and
  * dividing the number of bytes between the block and the current decoder
  * read point by the decoder's estimate of its playback rate in bytes
  * per second. This ensures that the blocks farthest ahead are considered
  * least valuable.
  * For efficient prediction of the "latest time of next use", we maintain
  * linked lists of blocks in each class, ordering blocks by time of
  * next use. READAHEAD_BLOCKS have one linked list per stream, since their
  * time of next use depends on stream parameters, but the other lists
  * are global.
  *
  * A block containing a current decoder read point can contain data
  * both behind and ahead of the read point. It will be classified as a
  * PLAYED_BLOCK but we will give it special treatment so it is never
  * evicted --- it actually contains the highest-priority readahead data
  * as well as played data.
  *
  * "Time of next use" estimates are also used for flow control. When
  * reading ahead we can predict the time of next use for the data that
  * will be read. If the predicted time of next use is later then the
  * prediction for all currently cached blocks, and the cache is full, then
  * we should suspend reading from the Necko channel.
  *
  * Unfortunately suspending the Necko channel can't immediately stop the
  * flow of data from the server. First our desire to suspend has to be
  * transmitted to the server (in practice, Necko stops reading from the
  * socket, which causes the kernel to shrink its advertised TCP receive
  * window size to zero). Then the server can stop sending the data, but
  * we will receive data roughly corresponding to the product of the link
  * bandwidth multiplied by the round-trip latency. We deal with this by
  * letting the cache overflow temporarily and then trimming it back by
  * moving overflowing blocks back into the body of the cache, replacing
  * less valuable blocks as they become available. We try to avoid simply
  * discarding overflowing readahead data.
  *
  * All changes to the actual contents of the cache happen on the main
  * thread, since that's where Necko's notifications happen.
  *
  * The media cache maintains at most one Necko channel for each stream.
  * (In the future it might be advantageous to relax this, e.g. so that a
  * seek to near the end of the file can happen without disturbing
  * the loading of data from the beginning of the file.) The Necko channel
  * is managed through ChannelMediaResource; MediaCache does not
  * depend on Necko directly.
  *
  * Every time something changes that might affect whether we want to
  * read from a Necko channel, or whether we want to seek on the Necko
  * channel --- such as data arriving or data being consumed by the
  * decoder --- we asynchronously trigger MediaCache::Update on the main
  * thread. That method implements most cache policy. It evaluates for
  * each stream whether we want to suspend or resume the stream and what
  * offset we should seek to, if any. It is also responsible for trimming
  * back the cache size to its desired limit by moving overflowing blocks
  * into the main part of the cache.
  *
  * Streams can be opened in non-seekable mode. In non-seekable mode,
  * the cache will only call ChannelMediaResource::CacheClientSeek with
  * a 0 offset. The cache tries hard not to discard readahead data
  * for non-seekable streams, since that could trigger a potentially
  * disastrous re-read of the entire stream. It's up to cache clients
  * to try to avoid requesting seeks on such streams.
  *
  * MediaCache has a single internal monitor for all synchronization.
  * This is treated as the lowest level monitor in the media code. So,
  * we must not acquire any MediaDecoder locks or MediaResource locks
  * while holding the MediaCache lock. But it's OK to hold those locks
  * and then get the MediaCache lock.
  *
  * MediaCache associates a principal with each stream. CacheClientSeek
  * can trigger new HTTP requests; due to redirects to other domains,
  * each HTTP load can return data with a different principal. This
  * principal must be passed to NotifyDataReceived, and MediaCache
  * will detect when different principals are associated with data in the
  * same stream, and replace them with a null principal.
  */
 class MediaCache;
 /**
  * If the cache fails to initialize then Init will fail, so nonstatic
  * methods of this class can assume gMediaCache is non-null.
  *
  * This class can be directly embedded as a value.
  */
 class MediaCacheStream {
 public:
   enum {
     // This needs to be a power of two
     BLOCK_SIZE = 32768
   };
   enum ReadMode {
     MODE_METADATA,
     MODE_PLAYBACK
   };
   // aClient provides the underlying transport that cache will use to read
   // data for this stream.
   MediaCacheStream(ChannelMediaResource* aClient);
   ~MediaCacheStream();
   // Set up this stream with the cache. Can fail on OOM. One
   // of InitAsClone or Init must be called before any other method on
   // this class. Does nothing if already initialized.
   nsresult Init();
   // Set up this stream with the cache, assuming it's for the same data
   // as the aOriginal stream. Can fail on OOM. Exactly one
   // of InitAsClone or Init must be called before any other method on
   // this class. Does nothing if already initialized.
   nsresult InitAsClone(MediaCacheStream* aOriginal);
   // These are called on the main thread.
   // Tell us whether the stream is seekable or not. Non-seekable streams
   // will always pass 0 for aOffset to CacheClientSeek. This should only
   // be called while the stream is at channel offset 0. Seekability can
   // change during the lifetime of the MediaCacheStream --- every time
   // we do an HTTP load the seekability may be different (and sometimes
   // is, in practice, due to the effects of caching proxies).
   void SetTransportSeekable(bool aIsTransportSeekable);
   // This must be called (and return) before the ChannelMediaResource
   // used to create this MediaCacheStream is deleted.
   void Close();
   // This returns true when the stream has been closed
   bool IsClosed() const { return mClosed; }
   // Returns true when this stream is can be shared by a new resource load
   bool IsAvailableForSharing() const
   {
     return !mClosed &&
       (!mDidNotifyDataEnded || NS_SUCCEEDED(mNotifyDataEndedStatus));
   }
   // Get the principal for this stream. Anything accessing the contents of
   // this stream must have a principal that subsumes this principal.
   nsIPrincipal* GetCurrentPrincipal() { return mPrincipal; }
   // Ensure a global media cache update has run with this stream present.
   // This ensures the cache has had a chance to suspend or unsuspend this stream.
   // Called only on main thread. This can change the state of streams, fire
   // notifications, etc.
   void EnsureCacheUpdate();
   // These callbacks are called on the main thread by the client
   // when data has been received via the channel.
   // Tells the cache what the server said the data length is going to be.
   // The actual data length may be greater (we receive more data than
   // specified) or smaller (the stream ends before we reach the given
   // length), because servers can lie. The server's reported data length
   // *and* the actual data length can even vary over time because a
   // misbehaving server may feed us a different stream after each seek
   // operation. So this is really just a hint. The cache may however
   // stop reading (suspend the channel) when it thinks we've read all the
   // data available based on an incorrect reported length. Seeks relative
   // EOF also depend on the reported length if we haven't managed to
   // read the whole stream yet.
   void NotifyDataLength(int64_t aLength);
   // Notifies the cache that a load has begun. We pass the offset
   // because in some cases the offset might not be what the cache
   // requested. In particular we might unexpectedly start providing
   // data at offset 0. This need not be called if the offset is the
   // offset that the cache requested in
   // ChannelMediaResource::CacheClientSeek. This can be called at any
   // time by the client, not just after a CacheClientSeek.
   void NotifyDataStarted(int64_t aOffset);
   // Notifies the cache that data has been received. The stream already
   // knows the offset because data is received in sequence and
   // the starting offset is known via NotifyDataStarted or because
   // the cache requested the offset in
   // ChannelMediaResource::CacheClientSeek, or because it defaulted to 0.
   // We pass in the principal that was used to load this data.
   void NotifyDataReceived(int64_t aSize, const char* aData,
                           nsIPrincipal* aPrincipal);
   // Notifies the cache that the current bytes should be written to disk.
   // Called on the main thread.
   void FlushPartialBlock();
   // Notifies the cache that the channel has closed with the given status.
   void NotifyDataEnded(nsresult aStatus);
   // These methods can be called on any thread.
   // Cached blocks associated with this stream will not be evicted
   // while the stream is pinned.
   void Pin();
   void Unpin();
   // See comments above for NotifyDataLength about how the length
   // can vary over time. Returns -1 if no length is known. Returns the
   // reported length if we haven't got any better information. If
   // the stream ended normally we return the length we actually got.
   // If we've successfully read data beyond the originally reported length,
   // we return the end of the data we've read.
   int64_t GetLength();
   // Returns the unique resource ID. Call only on the main thread or while
   // holding the media cache lock.
   int64_t GetResourceID() { return mResourceID; }
   // Returns the end of the bytes starting at the given offset
   // which are in cache.
   int64_t GetCachedDataEnd(int64_t aOffset);
   // Returns the offset of the first byte of cached data at or after aOffset,
   // or -1 if there is no such cached data.
   int64_t GetNextCachedData(int64_t aOffset);
   // Fills aRanges with the ByteRanges representing the data which is currently
   // cached. Locks the media cache while running, to prevent any ranges
   // growing. The stream should be pinned while this runs and while its results
   // are used, to ensure no data is evicted.
   nsresult GetCachedRanges(nsTArray<MediaByteRange>& aRanges);
   // Reads from buffered data only. Will fail if not all data to be read is
   // in the cache. Will not mark blocks as read. Can be called from the main
   // thread. It's the caller's responsibility to wrap the call in a pin/unpin,
   // and also to check that the range they want is cached before calling this.
   nsresult ReadFromCache(char* aBuffer,
                          int64_t aOffset,
                          int64_t aCount);
   // IsDataCachedToEndOfStream returns true if all the data from
   // aOffset to the end of the stream (the server-reported end, if the
   // real end is not known) is in cache. If we know nothing about the
   // end of the stream, this returns false.
   bool IsDataCachedToEndOfStream(int64_t aOffset);
   // The mode is initially MODE_PLAYBACK.
   void SetReadMode(ReadMode aMode);
   // This is the client's estimate of the playback rate assuming
   // the media plays continuously. The cache can't guess this itself
   // because it doesn't know when the decoder was paused, buffering, etc.
   // Do not pass zero.
   void SetPlaybackRate(uint32_t aBytesPerSecond);
   // Returns the last set value of SetTransportSeekable.
   bool IsTransportSeekable();
   // Returns true when all streams for this resource are suspended or their
   // channel has ended.
   bool AreAllStreamsForResourceSuspended();
   // These methods must be called on a different thread from the main
   // thread. They should always be called on the same thread for a given
   // stream.
   // This can fail when aWhence is NS_SEEK_END and no stream length
   // is known.
   nsresult Seek(int32_t aWhence, int64_t aOffset);
   int64_t Tell();
   // *aBytes gets the number of bytes that were actually read. This can
   // be less than aCount. If the first byte of data is not in the cache,
   // this will block until the data is available or the stream is
   // closed, otherwise it won't block.
   nsresult Read(char* aBuffer, uint32_t aCount, uint32_t* aBytes);
   // Seeks to aOffset in the stream then performs a Read operation. See
   // 'Read' for argument and return details.
   nsresult ReadAt(int64_t aOffset, char* aBuffer,
                   uint32_t aCount, uint32_t* aBytes);
   size_t SizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const;
 private:
   friend class MediaCache;
   /**
    * A doubly-linked list of blocks. Add/Remove/Get methods are all
    * constant time. We declare this here so that a stream can contain a
    * BlockList of its read-ahead blocks. Blocks are referred to by index
    * into the MediaCache::mIndex array.
    *
    * Blocks can belong to more than one list at the same time, because
    * the next/prev pointers are not stored in the block.
    */
   class BlockList {
   public:
     BlockList() : mFirstBlock(-1), mCount(0) {}
     ~BlockList() {
       NS_ASSERTION(mFirstBlock == -1 && mCount == 0,
                    "Destroying non-empty block list");
     }
     void AddFirstBlock(int32_t aBlock);
     void AddAfter(int32_t aBlock, int32_t aBefore);
     void RemoveBlock(int32_t aBlock);
     // Returns the first block in the list, or -1 if empty
     int32_t GetFirstBlock() const { return mFirstBlock; }
     // Returns the last block in the list, or -1 if empty
     int32_t GetLastBlock() const;
     // Returns the next block in the list after aBlock or -1 if
     // aBlock is the last block
     int32_t GetNextBlock(int32_t aBlock) const;
     // Returns the previous block in the list before aBlock or -1 if
     // aBlock is the first block
     int32_t GetPrevBlock(int32_t aBlock) const;
     bool IsEmpty() const { return mFirstBlock < 0; }
     int32_t GetCount() const { return mCount; }
     // The contents of aBlockIndex1 and aBlockIndex2 have been swapped
     void NotifyBlockSwapped(int32_t aBlockIndex1, int32_t aBlockIndex2);
 #ifdef DEBUG
     // Verify linked-list invariants
     void Verify();
 #else
     void Verify() {}
 #endif
     size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
   private:
     struct Entry : public nsUint32HashKey {
       Entry(KeyTypePointer aKey) : nsUint32HashKey(aKey) { }
       Entry(const Entry& toCopy) : nsUint32HashKey(&toCopy.GetKey()),
         mNextBlock(toCopy.mNextBlock), mPrevBlock(toCopy.mPrevBlock) {}
       int32_t mNextBlock;
       int32_t mPrevBlock;
     };
     nsTHashtable<Entry> mEntries;
     // The index of the first block in the list, or -1 if the list is empty.
     int32_t mFirstBlock;
     // The number of blocks in the list.
     int32_t mCount;
   };
   // Returns the end of the bytes starting at the given offset
   // which are in cache.
   // This method assumes that the cache monitor is held and can be called on
   // any thread.
   int64_t GetCachedDataEndInternal(int64_t aOffset);
   // Returns the offset of the first byte of cached data at or after aOffset,
   // or -1 if there is no such cached data.
   // This method assumes that the cache monitor is held and can be called on
   // any thread.
   int64_t GetNextCachedDataInternal(int64_t aOffset);
   // Writes |mPartialBlock| to disk.
   // Used by |NotifyDataEnded| and |FlushPartialBlock|.
   // If |aNotifyAll| is true, this function will wake up readers who may be
   // waiting on the media cache monitor. Called on the main thread only.
   void FlushPartialBlockInternal(bool aNotify);
   // A helper function to do the work of closing the stream. Assumes
   // that the cache monitor is held. Main thread only.
   // aReentrantMonitor is the nsAutoReentrantMonitor wrapper holding the cache monitor.
   // This is used to NotifyAll to wake up threads that might be
   // blocked on reading from this stream.
   void CloseInternal(ReentrantMonitorAutoEnter& aReentrantMonitor);
   // Update mPrincipal given that data has been received from aPrincipal
   bool UpdatePrincipal(nsIPrincipal* aPrincipal);
   // These fields are main-thread-only.
   ChannelMediaResource*  mClient;
   nsCOMPtr<nsIPrincipal> mPrincipal;
   // Set to true when Init or InitAsClone has been called
   bool                   mInitialized;
   // Set to true when MediaCache::Update() has finished while this stream
   // was present.
   bool                   mHasHadUpdate;
   // Set to true when the stream has been closed either explicitly or
   // due to an internal cache error
   bool                   mClosed;
   // True if CacheClientNotifyDataEnded has been called for this stream.
   bool                   mDidNotifyDataEnded;
   // The following fields must be written holding the cache's monitor and
   // only on the main thread, thus can be read either on the main thread
   // or while holding the cache's monitor.
   // This is a unique ID representing the resource we're loading.
   // All streams with the same mResourceID are loading the same
   // underlying resource and should share data.
   int64_t mResourceID;
   // The last reported seekability state for the underlying channel
   bool mIsTransportSeekable;
   // True if the cache has suspended our channel because the cache is
   // full and the priority of the data that would be received is lower
   // than the priority of the data already in the cache
   bool mCacheSuspended;
   // True if the channel ended and we haven't seeked it again.
   bool mChannelEnded;
   // The offset where the next data from the channel will arrive
   int64_t      mChannelOffset;
   // The reported or discovered length of the data, or -1 if nothing is
   // known
   int64_t      mStreamLength;
   // The following fields are protected by the cache's monitor can can be written
   // by any thread.
   // The offset where the reader is positioned in the stream
   int64_t           mStreamOffset;
   // For each block in the stream data, maps to the cache entry for the
   // block, or -1 if the block is not cached.
   nsTArray<int32_t> mBlocks;
   // The list of read-ahead blocks, ordered by stream offset; the first
   // block is the earliest in the stream (so the last block will be the
   // least valuable).
   BlockList         mReadaheadBlocks;
   // The list of metadata blocks; the first block is the most recently used
   BlockList         mMetadataBlocks;
   // The list of played-back blocks; the first block is the most recently used
   BlockList         mPlayedBlocks;
   // The last reported estimate of the decoder's playback rate
   uint32_t          mPlaybackBytesPerSecond;
   // The number of times this stream has been Pinned without a
   // corresponding Unpin
   uint32_t          mPinCount;
   // The status used when we did CacheClientNotifyDataEnded. Only valid
   // when mDidNotifyDataEnded is true.
   nsresult          mNotifyDataEndedStatus;
   // The last reported read mode
   ReadMode          mCurrentMode;
   // True if some data in mPartialBlockBuffer has been read as metadata
   bool              mMetadataInPartialBlockBuffer;
   // The following field is protected by the cache's monitor but are
   // only written on the main thread.
   // Data received for the block containing mChannelOffset. Data needs
   // to wait here so we can write back a complete block. The first
   // mChannelOffset%BLOCK_SIZE bytes have been filled in with good data,
   // the rest are garbage.
   // Use int64_t so that the data is well-aligned.
   // Heap allocate this buffer since the exact power-of-2 will cause allocation
   // slop when combined with the rest of the object members.
   nsAutoArrayPtr<int64_t> mPartialBlockBuffer;
 };
 } // namespace mozilla
 #endif

The Tor Browser / annotate

content/media/MediaCache.h@6474c204b198 (annotated)

content/media/MediaCache.h