Restrict the number of concurrent piece hashes

This fixes bad behaviour where running out of file descriptors, and overloading the system with goroutines and concurrent I/O may cause newly started torrents to fail to hash data.

torrent.go

@@ -131,6 +131,7 @@ type Torrent struct {
 	completedPieces bitmap.Bitmap
 	// Pieces that need to be hashed.
 	piecesQueuedForHash bitmap.Bitmap
+	activePieceHashes   int
 
 	// A pool of piece priorities []int for assignment to new connections.
 	// These "inclinations" are used to give connections preference for
@@ -382,6 +383,7 @@ func (t *Torrent) onSetInfo() {
 	t.updateWantPeersEvent()
 	t.pendingRequests = make(map[request]int)
 	t.lastRequested = make(map[request]*time.Timer)
+	t.tryCreateMorePieceHashers()
 }
 
 // Called when metadata for a torrent becomes available.
@@ -1503,11 +1505,13 @@ func (t *Torrent) SetMaxEstablishedConns(max int) (oldMax int) {
 }
 
 func (t *Torrent) pieceHashed(piece pieceIndex, correct bool) {
-	log.Fmsg("hashed piece %d", piece).Add("piece", piece).Add("passed", correct).AddValue(debugLogValue).Log(t.logger)
+	t.logger.Log(log.Fstr("hashed piece %d (passed=%t)", piece, correct).WithValues(debugLogValue))
+	p := t.piece(piece)
+	p.numVerifies++
+	t.cl.event.Broadcast()
 	if t.closed.IsSet() {
 		return
 	}
-	p := &t.pieces[piece]
 	touchers := t.reapPieceTouchers(piece)
 	if p.storageCompletionOk {
 		// Don't score the first time a piece is hashed, it could be an
@@ -1602,37 +1606,54 @@ func (t *Torrent) onIncompletePiece(piece pieceIndex) {
 	}
 }
 
-func (t *Torrent) verifyPiece(piece pieceIndex) {
-	cl := t.cl
-	cl.lock()
-	defer cl.unlock()
-	p := &t.pieces[piece]
-	defer func() {
-		p.numVerifies++
-		cl.event.Broadcast()
-	}()
-	for p.hashing || t.storage == nil {
-		cl.event.Wait()
+func (t *Torrent) tryCreateMorePieceHashers() {
+	for t.activePieceHashes < 2 && t.tryCreatePieceHasher() {
 	}
-	if !p.t.piecesQueuedForHash.Remove(bitmap.BitIndex(piece)) {
-		panic("piece was not queued")
+}
+
+func (t *Torrent) tryCreatePieceHasher() bool {
+	if t.storage == nil {
+		return false
 	}
-	t.updatePiecePriority(piece)
-	if t.closed.IsSet() {
-		return
+	pi, ok := t.getPieceToHash()
+	if !ok {
+		return false
 	}
+	p := t.piece(pi)
+	t.piecesQueuedForHash.Remove(pi)
 	p.hashing = true
-	t.publishPieceChange(piece)
-	t.updatePiecePriority(piece)
+	t.publishPieceChange(pi)
+	t.updatePiecePriority(pi)
 	t.storageLock.RLock()
-	cl.unlock()
-	sum := t.hashPiece(piece)
+	t.activePieceHashes++
+	go t.pieceHasher(pi)
+	return true
+}
+
+func (t *Torrent) getPieceToHash() (ret pieceIndex, ok bool) {
+	t.piecesQueuedForHash.IterTyped(func(i pieceIndex) bool {
+		if t.piece(i).hashing {
+			return true
+		}
+		ret = i
+		ok = true
+		return false
+	})
+	return
+}
+
+func (t *Torrent) pieceHasher(index pieceIndex) {
+	p := t.piece(index)
+	sum := t.hashPiece(index)
 	t.storageLock.RUnlock()
-	cl.lock()
+	t.cl.lock()
+	defer t.cl.unlock()
 	p.hashing = false
-	t.updatePiecePriority(piece)
-	t.pieceHashed(piece, sum == *p.hash)
-	t.publishPieceChange(piece)
+	t.updatePiecePriority(index)
+	t.pieceHashed(index, sum == *p.hash)
+	t.publishPieceChange(index)
+	t.activePieceHashes--
+	t.tryCreateMorePieceHashers()
 }
 
 // Return the connections that touched a piece, and clear the entries while
@@ -1655,14 +1676,14 @@ func (t *Torrent) connsAsSlice() (ret []*connection) {
 
 // Currently doesn't really queue, but should in the future.
 func (t *Torrent) queuePieceCheck(pieceIndex pieceIndex) {
-	piece := &t.pieces[pieceIndex]
+	piece := t.piece(pieceIndex)
 	if piece.queuedForHash() {
 		return
 	}
 	t.piecesQueuedForHash.Add(bitmap.BitIndex(pieceIndex))
 	t.publishPieceChange(pieceIndex)
 	t.updatePiecePriority(pieceIndex)
-	go t.verifyPiece(pieceIndex)
+	t.tryCreateMorePieceHashers()
 }
 
 // Forces all the pieces to be re-hashed. See also Piece.VerifyData.