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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 */
#ifndef MediaCache_h_
#define MediaCache_h_
#include "DecoderDoctorLogger.h"
#include "Intervals.h"
#include "mozilla/Result.h"
#include "mozilla/UniquePtr.h"
#include "mozilla/dom/MediaDebugInfoBinding.h"
#include "nsCOMPtr.h"
#include "nsHashKeys.h"
#include "nsTArray.h"
#include "nsTHashtable.h"
#include "MediaChannelStatistics.h"
class nsIEventTarget;
class nsIPrincipal;
namespace mozilla {
// defined in MediaResource.h
class ChannelMediaResource;
typedef media::IntervalSet<int64_t> MediaByteRangeSet;
class MediaResource;
class MonitorAutoLock;
* 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 DecoderDoctorLifeLogger<MediaCacheStream> {
using AutoLock = MonitorAutoLock;
// This needs to be a power of two
static const int64_t BLOCK_SIZE = 32768;
// aClient provides the underlying transport that cache will use to read
// data for this stream.
MediaCacheStream(ChannelMediaResource* aClient, bool aIsPrivateBrowsing);
// Set up this stream with the cache. Can fail on OOM.
// aContentLength is the content length if known, otherwise -1.
// Exactly one of InitAsClone or Init must be called before any other method
// on this class. Does nothing if already initialized.
nsresult Init(int64_t aContentLength);
// Set up this stream with the cache, assuming it's for the same data
// as the aOriginal stream.
// Exactly one of InitAsClone or Init must be called before any other method
// on this class.
void InitAsClone(MediaCacheStream* aOriginal);
nsISerialEventTarget* OwnerThread() const;
// These are called on the main thread.
// This must be called (and resolve) before the ChannelMediaResource
// used to create this MediaCacheStream is deleted.
RefPtr<GenericPromise> Close();
// This returns true when the stream has been closed.
bool IsClosed(AutoLock&) const { return mClosed; }
// Returns true when this stream is can be shared by a new resource load.
// Called on the main thread only.
bool IsAvailableForSharing() const { return !mIsPrivateBrowsing; }
// These callbacks are called on the main thread by the client
// when data has been received via the channel.
// 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.
// aSeekable tells 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).
// aLength 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 NotifyDataStarted(uint32_t aLoadID, int64_t aOffset, bool aSeekable,
int64_t aLength);
// 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.
void NotifyDataReceived(uint32_t aLoadID, uint32_t aCount,
const uint8_t* aData);
// Set the load ID so the following NotifyDataEnded() call can work properly.
// Used in some rare cases where NotifyDataEnded() is called without the
// preceding NotifyDataStarted().
void NotifyLoadID(uint32_t aLoadID);
// Notifies the cache that the channel has closed with the given status.
void NotifyDataEnded(uint32_t aLoadID, nsresult aStatus);
// Notifies the stream that the suspend status of the client has changed.
// Main thread only.
void NotifyClientSuspended(bool aSuspended);
// Notifies the stream to resume download at the current offset.
void NotifyResume();
// 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 NotifyDataStarted 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() const;
// Return the length and offset where next channel data will write to. Main
// thread only.
// This method should be removed as part of bug 1464045.
struct LengthAndOffset {
int64_t mLength;
int64_t mOffset;
LengthAndOffset GetLengthAndOffset() const;
// 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(MediaByteRangeSet& aRanges);
double GetDownloadRate(bool* aIsReliable);
// 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, uint32_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 true when all streams for this resource are suspended or their
// channel has ended.
bool AreAllStreamsForResourceSuspended(AutoLock&);
// 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.
// *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(AutoLock&, 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);
void ThrottleReadahead(bool bThrottle);
size_t SizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const;
void GetDebugInfo(dom::MediaCacheStreamDebugInfo& aInfo);
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 {
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();
void Verify() {}
size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
struct Entry : public nsUint32HashKey {
explicit Entry(KeyTypePointer aKey)
: nsUint32HashKey(aKey), mNextBlock(0), mPrevBlock(0) {}
Entry(const Entry& toCopy)
: nsUint32HashKey(&toCopy.GetKey()),
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;
// Read data from the partial block and return the number of bytes read
// successfully. 0 if aOffset is not an offset in the partial block or there
// is nothing to read.
uint32_t ReadPartialBlock(AutoLock&, int64_t aOffset, Span<char> aBuffer);
// Read data from the cache block specified by aOffset. Return the number of
// bytes read successfully or an error code if any failure.
Result<uint32_t, nsresult> ReadBlockFromCache(AutoLock&, int64_t aOffset,
Span<char> aBuffer,
bool aNoteBlockUsage = false);
// Non-main thread only.
nsresult Seek(AutoLock&, int64_t aOffset);
// 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(AutoLock&, 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(AutoLock&, int64_t aOffset);
// Used by |NotifyDataEnded| to write |mPartialBlock| to disk.
// 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(AutoLock&, bool aNotify);
void NotifyDataStartedInternal(uint32_t aLoadID, int64_t aOffset,
bool aSeekable, int64_t aLength);
void NotifyDataEndedInternal(uint32_t aLoadID, nsresult aStatus);
void UpdateDownloadStatistics(AutoLock&);
void CloseInternal(AutoLock&);
void InitAsCloneInternal(MediaCacheStream* aOriginal);
// Instance of MediaCache to use with this MediaCacheStream.
RefPtr<MediaCache> mMediaCache;
ChannelMediaResource* const mClient;
// 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.
// Set to true when the stream has been closed either explicitly or
// due to an internal cache error
bool mClosed = false;
// 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.
// Initialized to 0 as invalid. Will be allocated a valid ID (always positive)
// from the cache.
int64_t mResourceID = 0;
// 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 following fields are protected by the cache's monitor and can be
// written by any thread.
// The reported or discovered length of the data, or -1 if nothing is known
int64_t mStreamLength = -1;
// The offset where the next data from the channel will arrive
int64_t mChannelOffset = 0;
// 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;
// True if CacheClientNotifyDataEnded has been called for this stream.
bool mDidNotifyDataEnded = false;
// The status used when we did CacheClientNotifyDataEnded. Only valid
// when mDidNotifyDataEnded is true.
nsresult mNotifyDataEndedStatus;
// The last reported read mode
ReadMode mCurrentMode = MODE_METADATA;
// True if some data in mPartialBlockBuffer has been read as metadata
bool mMetadataInPartialBlockBuffer;
// The load ID of the current channel. Used to check whether the data is
// coming from an old channel and should be discarded.
uint32_t mLoadID = 0;
// The seek target initiated by MediaCache. -1 if no seek is going on.
int64_t mSeekTarget = -1;
bool mThrottleReadahead = false;
// 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.
// Heap allocate this buffer since the exact power-of-2 will cause allocation
// slop when combined with the rest of the object members.
// This partial buffer should always be read/write within the cache's monitor.
const UniquePtr<uint8_t[]> mPartialBlockBuffer =
// True if associated with a private browsing window.
const bool mIsPrivateBrowsing;
// True if the client is suspended. Accessed on the owner thread only.
bool mClientSuspended = false;
MediaChannelStatistics mDownloadStatistics;
} // namespace mozilla