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https://github.com/mozilla/gecko-dev.git
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514 lines
24 KiB
C++
514 lines
24 KiB
C++
/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim:set ts=2 sw=2 sts=2 et cindent: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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#ifndef MediaCache_h_
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#define MediaCache_h_
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#include "nsTArray.h"
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#include "nsIPrincipal.h"
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#include "nsCOMPtr.h"
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namespace mozilla {
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// defined in MediaResource.h
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class ChannelMediaResource;
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class MediaByteRange;
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class MediaResource;
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class ReentrantMonitorAutoEnter;
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/**
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* Media applications want fast, "on demand" random access to media data,
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* for pausing, seeking, etc. But we are primarily interested
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* in transporting media data using HTTP over the Internet, which has
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* high latency to open a connection, requires a new connection for every
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* seek, may not even support seeking on some connections (especially
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* live streams), and uses a push model --- data comes from the server
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* and you don't have much control over the rate. Also, transferring data
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* over the Internet can be slow and/or unpredictable, so we want to read
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* ahead to buffer and cache as much data as possible.
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*
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* The job of the media cache is to resolve this impedance mismatch.
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* The media cache reads data from Necko channels into file-backed storage,
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* and offers a random-access file-like API to the stream data
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* (MediaCacheStream). Along the way it solves several problems:
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* -- The cache intelligently reads ahead to prefetch data that may be
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* needed in the future
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* -- The size of the cache is bounded so that we don't fill up
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* storage with read-ahead data
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* -- Cache replacement is managed globally so that the most valuable
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* data (across all streams) is retained
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* -- The cache can suspend Necko channels temporarily when their data is
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* not wanted (yet)
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* -- The cache translates file-like seek requests to HTTP seeks,
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* including optimizations like not triggering a new seek if it would
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* be faster to just keep reading until we reach the seek point. The
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* "seek to EOF" idiom to determine file size is also handled efficiently
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* (seeking to EOF and then seeking back to the previous offset does not
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* trigger any Necko activity)
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* -- The cache also handles the case where the server does not support
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* seeking
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* -- Necko can only send data to the main thread, but MediaCacheStream
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* can distribute data to any thread
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* -- The cache exposes APIs so clients can detect what data is
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* currently held
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*
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* Note that although HTTP is the most important transport and we only
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* support transport-level seeking via HTTP byte-ranges, the media cache
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* works with any kind of Necko channels and provides random access to
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* cached data even for, e.g., FTP streams.
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*
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* The media cache is not persistent. It does not currently allow
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* data from one load to be used by other loads, either within the same
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* browser session or across browser sessions. The media cache file
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* is marked "delete on close" so it will automatically disappear in the
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* event of a browser crash or shutdown.
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*
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* The media cache is block-based. Streams are divided into blocks of a
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* fixed size (currently 4K) and we cache blocks. A single cache contains
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* blocks for all streams.
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*
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* The cache size is controlled by the media.cache_size preference
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* (which is in KB). The default size is 500MB.
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*
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* The replacement policy predicts a "time of next use" for each block
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* in the cache. When we need to free a block, the block with the latest
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* "time of next use" will be evicted. Blocks are divided into
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* different classes, each class having its own predictor:
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* FREE_BLOCK: these blocks are effectively infinitely far in the future;
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* a free block will always be chosen for replacement before other classes
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* of blocks.
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* METADATA_BLOCK: these are blocks that contain data that has been read
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* by the decoder in "metadata mode", e.g. while the decoder is searching
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* the stream during a seek operation. These blocks are managed with an
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* LRU policy; the "time of next use" is predicted to be as far in the
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* future as the last use was in the past.
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* PLAYED_BLOCK: these are blocks that have not been read in "metadata
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* mode", and contain data behind the current decoder read point. (They
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* may not actually have been read by the decoder, if the decoder seeked
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* forward.) These blocks are managed with an LRU policy except that we add
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* REPLAY_DELAY seconds of penalty to their predicted "time of next use",
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* to reflect the uncertainty about whether replay will actually happen
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* or not.
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* READAHEAD_BLOCK: these are blocks that have not been read in
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* "metadata mode" and that are entirely ahead of the current decoder
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* read point. (They may actually have been read by the decoder in the
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* past if the decoder has since seeked backward.) We predict the
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* time of next use for these blocks by assuming steady playback and
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* dividing the number of bytes between the block and the current decoder
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* read point by the decoder's estimate of its playback rate in bytes
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* per second. This ensures that the blocks farthest ahead are considered
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* least valuable.
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* For efficient prediction of the "latest time of next use", we maintain
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* linked lists of blocks in each class, ordering blocks by time of
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* next use. READAHEAD_BLOCKS have one linked list per stream, since their
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* time of next use depends on stream parameters, but the other lists
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* are global.
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*
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* A block containing a current decoder read point can contain data
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* both behind and ahead of the read point. It will be classified as a
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* PLAYED_BLOCK but we will give it special treatment so it is never
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* evicted --- it actually contains the highest-priority readahead data
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* as well as played data.
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*
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* "Time of next use" estimates are also used for flow control. When
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* reading ahead we can predict the time of next use for the data that
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* will be read. If the predicted time of next use is later then the
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* prediction for all currently cached blocks, and the cache is full, then
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* we should suspend reading from the Necko channel.
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*
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* Unfortunately suspending the Necko channel can't immediately stop the
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* flow of data from the server. First our desire to suspend has to be
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* transmitted to the server (in practice, Necko stops reading from the
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* socket, which causes the kernel to shrink its advertised TCP receive
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* window size to zero). Then the server can stop sending the data, but
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* we will receive data roughly corresponding to the product of the link
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* bandwidth multiplied by the round-trip latency. We deal with this by
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* letting the cache overflow temporarily and then trimming it back by
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* moving overflowing blocks back into the body of the cache, replacing
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* less valuable blocks as they become available. We try to avoid simply
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* discarding overflowing readahead data.
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*
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* All changes to the actual contents of the cache happen on the main
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* thread, since that's where Necko's notifications happen.
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*
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* The media cache maintains at most one Necko channel for each stream.
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* (In the future it might be advantageous to relax this, e.g. so that a
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* seek to near the end of the file can happen without disturbing
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* the loading of data from the beginning of the file.) The Necko channel
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* is managed through ChannelMediaResource; MediaCache does not
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* depend on Necko directly.
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*
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* Every time something changes that might affect whether we want to
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* read from a Necko channel, or whether we want to seek on the Necko
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* channel --- such as data arriving or data being consumed by the
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* decoder --- we asynchronously trigger MediaCache::Update on the main
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* thread. That method implements most cache policy. It evaluates for
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* each stream whether we want to suspend or resume the stream and what
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* offset we should seek to, if any. It is also responsible for trimming
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* back the cache size to its desired limit by moving overflowing blocks
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* into the main part of the cache.
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*
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* Streams can be opened in non-seekable mode. In non-seekable mode,
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* the cache will only call ChannelMediaResource::CacheClientSeek with
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* a 0 offset. The cache tries hard not to discard readahead data
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* for non-seekable streams, since that could trigger a potentially
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* disastrous re-read of the entire stream. It's up to cache clients
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* to try to avoid requesting seeks on such streams.
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*
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* MediaCache has a single internal monitor for all synchronization.
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* This is treated as the lowest level monitor in the media code. So,
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* we must not acquire any MediaDecoder locks or MediaResource locks
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* while holding the MediaCache lock. But it's OK to hold those locks
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* and then get the MediaCache lock.
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*
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* MediaCache associates a principal with each stream. CacheClientSeek
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* can trigger new HTTP requests; due to redirects to other domains,
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* each HTTP load can return data with a different principal. This
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* principal must be passed to NotifyDataReceived, and MediaCache
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* will detect when different principals are associated with data in the
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* same stream, and replace them with a null principal.
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*/
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class MediaCache;
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/**
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* If the cache fails to initialize then Init will fail, so nonstatic
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* methods of this class can assume gMediaCache is non-null.
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*
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* This class can be directly embedded as a value.
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*/
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class MediaCacheStream {
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public:
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enum {
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// This needs to be a power of two
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BLOCK_SIZE = 32768
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};
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enum ReadMode {
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MODE_METADATA,
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MODE_PLAYBACK
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};
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// aClient provides the underlying transport that cache will use to read
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// data for this stream.
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MediaCacheStream(ChannelMediaResource* aClient)
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: mClient(aClient), mInitialized(false),
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mHasHadUpdate(false),
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mClosed(false),
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mDidNotifyDataEnded(false), mResourceID(0),
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mIsTransportSeekable(false), mCacheSuspended(false),
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mChannelEnded(false),
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mChannelOffset(0), mStreamLength(-1),
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mStreamOffset(0), mPlaybackBytesPerSecond(10000),
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mPinCount(0), mCurrentMode(MODE_PLAYBACK),
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mMetadataInPartialBlockBuffer(false) {}
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~MediaCacheStream();
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// Set up this stream with the cache. Can fail on OOM. One
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// of InitAsClone or Init must be called before any other method on
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// this class. Does nothing if already initialized.
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nsresult Init();
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// Set up this stream with the cache, assuming it's for the same data
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// as the aOriginal stream. Can fail on OOM. Exactly one
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// of InitAsClone or Init must be called before any other method on
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// this class. Does nothing if already initialized.
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nsresult InitAsClone(MediaCacheStream* aOriginal);
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// These are called on the main thread.
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// Tell us whether the stream is seekable or not. Non-seekable streams
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// will always pass 0 for aOffset to CacheClientSeek. This should only
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// be called while the stream is at channel offset 0. Seekability can
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// change during the lifetime of the MediaCacheStream --- every time
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// we do an HTTP load the seekability may be different (and sometimes
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// is, in practice, due to the effects of caching proxies).
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void SetTransportSeekable(bool aIsTransportSeekable);
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// This must be called (and return) before the ChannelMediaResource
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// used to create this MediaCacheStream is deleted.
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void Close();
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// This returns true when the stream has been closed
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bool IsClosed() const { return mClosed; }
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// Returns true when this stream is can be shared by a new resource load
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bool IsAvailableForSharing() const
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{
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return !mClosed &&
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(!mDidNotifyDataEnded || NS_SUCCEEDED(mNotifyDataEndedStatus));
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}
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// Get the principal for this stream. Anything accessing the contents of
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// this stream must have a principal that subsumes this principal.
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nsIPrincipal* GetCurrentPrincipal() { return mPrincipal; }
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// Ensure a global media cache update has run with this stream present.
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// This ensures the cache has had a chance to suspend or unsuspend this stream.
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// Called only on main thread. This can change the state of streams, fire
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// notifications, etc.
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void EnsureCacheUpdate();
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// These callbacks are called on the main thread by the client
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// when data has been received via the channel.
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// Tells the cache what the server said the data length is going to be.
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// The actual data length may be greater (we receive more data than
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// specified) or smaller (the stream ends before we reach the given
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// length), because servers can lie. The server's reported data length
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// *and* the actual data length can even vary over time because a
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// misbehaving server may feed us a different stream after each seek
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// operation. So this is really just a hint. The cache may however
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// stop reading (suspend the channel) when it thinks we've read all the
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// data available based on an incorrect reported length. Seeks relative
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// EOF also depend on the reported length if we haven't managed to
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// read the whole stream yet.
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void NotifyDataLength(int64_t aLength);
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// Notifies the cache that a load has begun. We pass the offset
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// because in some cases the offset might not be what the cache
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// requested. In particular we might unexpectedly start providing
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// data at offset 0. This need not be called if the offset is the
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// offset that the cache requested in
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// ChannelMediaResource::CacheClientSeek. This can be called at any
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// time by the client, not just after a CacheClientSeek.
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void NotifyDataStarted(int64_t aOffset);
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// Notifies the cache that data has been received. The stream already
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// knows the offset because data is received in sequence and
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// the starting offset is known via NotifyDataStarted or because
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// the cache requested the offset in
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// ChannelMediaResource::CacheClientSeek, or because it defaulted to 0.
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// We pass in the principal that was used to load this data.
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void NotifyDataReceived(int64_t aSize, const char* aData,
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nsIPrincipal* aPrincipal);
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// Notifies the cache that the current bytes should be written to disk.
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// Called on the main thread.
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void FlushPartialBlock();
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// Notifies the cache that the channel has closed with the given status.
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void NotifyDataEnded(nsresult aStatus);
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// These methods can be called on any thread.
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// Cached blocks associated with this stream will not be evicted
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// while the stream is pinned.
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void Pin();
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void Unpin();
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// See comments above for NotifyDataLength about how the length
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// can vary over time. Returns -1 if no length is known. Returns the
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// reported length if we haven't got any better information. If
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// the stream ended normally we return the length we actually got.
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// If we've successfully read data beyond the originally reported length,
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// we return the end of the data we've read.
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int64_t GetLength();
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// Returns the unique resource ID. Call only on the main thread or while
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// holding the media cache lock.
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int64_t GetResourceID() { return mResourceID; }
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// Returns the end of the bytes starting at the given offset
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// which are in cache.
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int64_t GetCachedDataEnd(int64_t aOffset);
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// Returns the offset of the first byte of cached data at or after aOffset,
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// or -1 if there is no such cached data.
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int64_t GetNextCachedData(int64_t aOffset);
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// Fills aRanges with the ByteRanges representing the data which is currently
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// cached. Locks the media cache while running, to prevent any ranges
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// growing. The stream should be pinned while this runs and while its results
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// are used, to ensure no data is evicted.
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nsresult GetCachedRanges(nsTArray<MediaByteRange>& aRanges);
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// Reads from buffered data only. Will fail if not all data to be read is
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// in the cache. Will not mark blocks as read. Can be called from the main
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// thread. It's the caller's responsibility to wrap the call in a pin/unpin,
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// and also to check that the range they want is cached before calling this.
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nsresult ReadFromCache(char* aBuffer,
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int64_t aOffset,
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int64_t aCount);
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// IsDataCachedToEndOfStream returns true if all the data from
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// aOffset to the end of the stream (the server-reported end, if the
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// real end is not known) is in cache. If we know nothing about the
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// end of the stream, this returns false.
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bool IsDataCachedToEndOfStream(int64_t aOffset);
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// The mode is initially MODE_PLAYBACK.
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void SetReadMode(ReadMode aMode);
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// This is the client's estimate of the playback rate assuming
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// the media plays continuously. The cache can't guess this itself
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// because it doesn't know when the decoder was paused, buffering, etc.
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// Do not pass zero.
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void SetPlaybackRate(uint32_t aBytesPerSecond);
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// Returns the last set value of SetTransportSeekable.
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bool IsTransportSeekable();
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// Returns true when all streams for this resource are suspended or their
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// channel has ended.
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// If aActiveResource is non-null, fills it with a pointer to a stream
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// for this resource that is not suspended or ended.
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bool AreAllStreamsForResourceSuspended(MediaResource** aActiveResource);
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// These methods must be called on a different thread from the main
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// thread. They should always be called on the same thread for a given
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// stream.
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// This can fail when aWhence is NS_SEEK_END and no stream length
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// is known.
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nsresult Seek(int32_t aWhence, int64_t aOffset);
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int64_t Tell();
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// *aBytes gets the number of bytes that were actually read. This can
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// be less than aCount. If the first byte of data is not in the cache,
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// this will block until the data is available or the stream is
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// closed, otherwise it won't block.
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nsresult Read(char* aBuffer, uint32_t aCount, uint32_t* aBytes);
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private:
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friend class MediaCache;
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/**
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* A doubly-linked list of blocks. Add/Remove/Get methods are all
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* constant time. We declare this here so that a stream can contain a
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* BlockList of its read-ahead blocks. Blocks are referred to by index
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* into the MediaCache::mIndex array.
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*
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* Blocks can belong to more than one list at the same time, because
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* the next/prev pointers are not stored in the block.
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*/
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class BlockList {
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public:
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BlockList() : mFirstBlock(-1), mCount(0) { mEntries.Init(); }
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~BlockList() {
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NS_ASSERTION(mFirstBlock == -1 && mCount == 0,
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"Destroying non-empty block list");
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}
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void AddFirstBlock(int32_t aBlock);
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void AddAfter(int32_t aBlock, int32_t aBefore);
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void RemoveBlock(int32_t aBlock);
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// Returns the first block in the list, or -1 if empty
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int32_t GetFirstBlock() const { return mFirstBlock; }
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// Returns the last block in the list, or -1 if empty
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int32_t GetLastBlock() const;
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// Returns the next block in the list after aBlock or -1 if
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// aBlock is the last block
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int32_t GetNextBlock(int32_t aBlock) const;
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// Returns the previous block in the list before aBlock or -1 if
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// aBlock is the first block
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int32_t GetPrevBlock(int32_t aBlock) const;
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bool IsEmpty() const { return mFirstBlock < 0; }
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int32_t GetCount() const { return mCount; }
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// The contents of aBlockIndex1 and aBlockIndex2 have been swapped
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void NotifyBlockSwapped(int32_t aBlockIndex1, int32_t aBlockIndex2);
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#ifdef DEBUG
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// Verify linked-list invariants
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void Verify();
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#else
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void Verify() {}
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#endif
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private:
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struct Entry : public nsUint32HashKey {
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Entry(KeyTypePointer aKey) : nsUint32HashKey(aKey) { }
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Entry(const Entry& toCopy) : nsUint32HashKey(&toCopy.GetKey()),
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mNextBlock(toCopy.mNextBlock), mPrevBlock(toCopy.mPrevBlock) {}
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int32_t mNextBlock;
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int32_t mPrevBlock;
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};
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nsTHashtable<Entry> mEntries;
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// The index of the first block in the list, or -1 if the list is empty.
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int32_t mFirstBlock;
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// The number of blocks in the list.
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int32_t mCount;
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};
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// Returns the end of the bytes starting at the given offset
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// which are in cache.
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// This method assumes that the cache monitor is held and can be called on
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// any thread.
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int64_t GetCachedDataEndInternal(int64_t aOffset);
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// Returns the offset of the first byte of cached data at or after aOffset,
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// or -1 if there is no such cached data.
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// This method assumes that the cache monitor is held and can be called on
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// any thread.
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int64_t GetNextCachedDataInternal(int64_t aOffset);
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// Writes |mPartialBlock| to disk.
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// Used by |NotifyDataEnded| and |FlushPartialBlock|.
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// If |aNotifyAll| is true, this function will wake up readers who may be
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// waiting on the media cache monitor. Called on the main thread only.
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void FlushPartialBlockInternal(bool aNotify);
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// A helper function to do the work of closing the stream. Assumes
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|
// 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.
|
|
int64_t mPartialBlockBuffer[BLOCK_SIZE/sizeof(int64_t)];
|
|
};
|
|
|
|
} // namespace mozilla
|
|
|
|
#endif
|