package torrent import ( "context" "errors" "fmt" "io" "sync" "github.com/anacrolix/log" "github.com/anacrolix/missinggo/v2" ) // Accesses Torrent data via a Client. Reads block until the data is available. Seeks and readahead // also drive Client behaviour. Not safe for concurrent use. type Reader interface { io.ReadSeekCloser missinggo.ReadContexter // Configure the number of bytes ahead of a read that should also be prioritized in preparation // for further reads. Overridden by non-nil readahead func, see SetReadaheadFunc. SetReadahead(int64) // If non-nil, the provided function is called when the implementation needs to know the // readahead for the current reader. Calls occur during Reads and Seeks, and while the Client is // locked. SetReadaheadFunc(ReadaheadFunc) // Don't wait for pieces to complete and be verified. Read calls return as soon as they can when // the underlying chunks become available. SetResponsive() } // Piece range by piece index, [begin, end). type pieceRange struct { begin, end pieceIndex } type ReadaheadContext struct { ContiguousReadStartPos int64 CurrentPos int64 } // Returns the desired readahead for a Reader. type ReadaheadFunc func(ReadaheadContext) int64 type reader struct { t *Torrent // Adjust the read/seek window to handle Readers locked to File extents and the like. offset, length int64 // Function to dynamically calculate readahead. If nil, readahead is static. readaheadFunc ReadaheadFunc // Required when modifying pos and readahead. mu sync.Locker readahead, pos int64 // Position that reads have continued contiguously from. contiguousReadStartPos int64 // The cached piece range this reader wants downloaded. The zero value corresponds to nothing. // We cache this so that changes can be detected, and bubbled up to the Torrent only as // required. pieces pieceRange // Reads have been initiated since the last seek. This is used to prevent readaheads occurring // after a seek or with a new reader at the starting position. reading bool responsive bool } var _ io.ReadSeekCloser = (*reader)(nil) func (r *reader) SetResponsive() { r.responsive = true r.t.cl.event.Broadcast() } // Disable responsive mode. TODO: Remove? func (r *reader) SetNonResponsive() { r.responsive = false r.t.cl.event.Broadcast() } func (r *reader) SetReadahead(readahead int64) { r.mu.Lock() r.readahead = readahead r.readaheadFunc = nil r.posChanged() r.mu.Unlock() } func (r *reader) SetReadaheadFunc(f ReadaheadFunc) { r.mu.Lock() r.readaheadFunc = f r.posChanged() r.mu.Unlock() } // How many bytes are available to read. Max is the most we could require. func (r *reader) available(off, max int64) (ret int64) { off += r.offset for max > 0 { req, ok := r.t.offsetRequest(off) if !ok { break } if !r.responsive && !r.t.pieceComplete(pieceIndex(req.Index)) { break } if !r.t.haveChunk(req) { break } len1 := int64(req.Length) - (off - r.t.requestOffset(req)) max -= len1 ret += len1 off += len1 } // Ensure that ret hasn't exceeded our original max. if max < 0 { ret += max } return } // Calculates the pieces this reader wants downloaded, ignoring the cached value at r.pieces. func (r *reader) piecesUncached() (ret pieceRange) { ra := r.readahead if r.readaheadFunc != nil { ra = r.readaheadFunc(ReadaheadContext{ ContiguousReadStartPos: r.contiguousReadStartPos, CurrentPos: r.pos, }) } if ra < 1 { // Needs to be at least 1, because [x, x) means we don't want // anything. ra = 1 } if !r.reading { ra = 0 } if ra > r.length-r.pos { ra = r.length - r.pos } ret.begin, ret.end = r.t.byteRegionPieces(r.torrentOffset(r.pos), ra) return } func (r *reader) Read(b []byte) (n int, err error) { return r.ReadContext(context.Background(), b) } func (r *reader) ReadContext(ctx context.Context, b []byte) (n int, err error) { if len(b) > 0 { r.reading = true // TODO: Rework reader piece priorities so we don't have to push updates in to the Client // and take the lock here. r.mu.Lock() r.posChanged() r.mu.Unlock() } n, err = r.readOnceAt(ctx, b, r.pos) if n == 0 { if err == nil && len(b) > 0 { panic("expected error") } else { return } } r.mu.Lock() r.pos += int64(n) r.posChanged() r.mu.Unlock() if r.pos >= r.length { err = io.EOF } else if err == io.EOF { err = io.ErrUnexpectedEOF } return } var closedChan = make(chan struct{}) func init() { close(closedChan) } // Wait until some data should be available to read. Tickles the client if it isn't. Returns how // much should be readable without blocking. func (r *reader) waitAvailable(ctx context.Context, pos, wanted int64, wait bool) (avail int64, err error) { t := r.t for { r.t.cl.rLock() avail = r.available(pos, wanted) readerCond := t.piece(int((r.offset + pos) / t.info.PieceLength)).readerCond.Signaled() r.t.cl.rUnlock() if avail != 0 { return } var dontWait <-chan struct{} if !wait || wanted == 0 { dontWait = closedChan } select { case <-r.t.closed.Done(): err = errors.New("torrent closed") return case <-ctx.Done(): err = ctx.Err() return case <-r.t.dataDownloadDisallowed.On(): err = errors.New("torrent data downloading disabled") case <-r.t.networkingEnabled.Off(): err = errors.New("torrent networking disabled") return case <-dontWait: return case <-readerCond: } } } // Adds the reader's torrent offset to the reader object offset (for example the reader might be // constrainted to a particular file within the torrent). func (r *reader) torrentOffset(readerPos int64) int64 { return r.offset + readerPos } // Performs at most one successful read to torrent storage. func (r *reader) readOnceAt(ctx context.Context, b []byte, pos int64) (n int, err error) { if pos >= r.length { err = io.EOF return } for { var avail int64 avail, err = r.waitAvailable(ctx, pos, int64(len(b)), n == 0) if avail == 0 { return } firstPieceIndex := pieceIndex(r.torrentOffset(pos) / r.t.info.PieceLength) firstPieceOffset := r.torrentOffset(pos) % r.t.info.PieceLength b1 := missinggo.LimitLen(b, avail) n, err = r.t.readAt(b1, r.torrentOffset(pos)) if n != 0 { err = nil return } if r.t.closed.IsSet() { err = fmt.Errorf("reading from closed torrent: %w", err) return } r.t.cl.lock() // I think there's a panic here caused by the Client being closed before obtaining this // lock. TestDropTorrentWithMmapStorageWhileHashing seems to tickle occasionally in CI. func() { // Just add exceptions already. defer r.t.cl.unlock() if r.t.closed.IsSet() { // Can't update because Torrent's piece order is removed from Client. return } // TODO: Just reset pieces in the readahead window. This might help // prevent thrashing with small caches and file and piece priorities. r.log(log.Fstr("error reading torrent %s piece %d offset %d, %d bytes: %v", r.t.infoHash.HexString(), firstPieceIndex, firstPieceOffset, len(b1), err)) if !r.t.updatePieceCompletion(firstPieceIndex) { r.log(log.Fstr("piece %d completion unchanged", firstPieceIndex)) } // Update the rest of the piece completions in the readahead window, without alerting to // changes (since only the first piece, the one above, could have generated the read error // we're currently handling). if r.pieces.begin != firstPieceIndex { panic(fmt.Sprint(r.pieces.begin, firstPieceIndex)) } for index := r.pieces.begin + 1; index < r.pieces.end; index++ { r.t.updatePieceCompletion(index) } }() } } // Hodor func (r *reader) Close() error { r.t.cl.lock() r.t.deleteReader(r) r.t.cl.unlock() return nil } func (r *reader) posChanged() { to := r.piecesUncached() from := r.pieces if to == from { return } r.pieces = to // log.Printf("reader pos changed %v->%v", from, to) r.t.readerPosChanged(from, to) } func (r *reader) Seek(off int64, whence int) (newPos int64, err error) { switch whence { case io.SeekStart: newPos = off r.mu.Lock() case io.SeekCurrent: r.mu.Lock() newPos = r.pos + off case io.SeekEnd: newPos = r.length + off r.mu.Lock() default: return 0, errors.New("bad whence") } if newPos != r.pos { r.reading = false r.pos = newPos r.contiguousReadStartPos = newPos r.posChanged() } r.mu.Unlock() return } func (r *reader) log(m log.Msg) { r.t.logger.LogLevel(log.Debug, m.Skip(1)) } // Implementation inspired by https://news.ycombinator.com/item?id=27019613. func defaultReadaheadFunc(r ReadaheadContext) int64 { return r.CurrentPos - r.ContiguousReadStartPos }