Replacing pendingWrites WaitGroup with Mutex/Cond/int

Apparently I've been using WaitGroups wrong all along.

client.go

@@ -1440,7 +1440,12 @@ another:
 
 func (me *Client) sendChunk(t *torrent, c *connection, r request) error {
 	b := make([]byte, r.Length)
-	t.Pieces[r.Index].pendingWrites.Wait()
+	tp := t.Pieces[r.Index]
+	tp.pendingWritesMutex.Lock()
+	for tp.pendingWrites != 0 {
+		tp.noPendingWrites.Wait()
+	}
+	tp.pendingWritesMutex.Unlock()
 	p := t.Info.Piece(int(r.Index))
 	n, err := dataReadAt(t.data, b, p.Offset()+int64(r.Begin))
 	if err != nil {
@@ -2536,9 +2541,18 @@ func (me *Client) downloadedChunk(t *torrent, c *connection, msg *pp.Message) er
 
 	me.upload(t, c)
 
-	piece.pendingWrites.Add(1)
+	piece.pendingWritesMutex.Lock()
+	piece.pendingWrites++
+	piece.pendingWritesMutex.Unlock()
 	go func() {
-		defer piece.pendingWrites.Done()
+		defer func() {
+			piece.pendingWritesMutex.Lock()
+			piece.pendingWrites--
+			if piece.pendingWrites == 0 {
+				piece.noPendingWrites.Broadcast()
+			}
+			piece.pendingWritesMutex.Unlock()
+		}()
 		// Write the chunk out.
 		tr := perf.NewTimer()
 		err := t.writeChunk(int(msg.Index), int64(msg.Begin), msg.Piece)

piece.go

@@ -30,7 +30,10 @@ type piece struct {
 	EverHashed        bool
 	Event             sync.Cond
 	Priority          piecePriority
-	pendingWrites     sync.WaitGroup
+
+	pendingWritesMutex sync.Mutex
+	pendingWrites      int
+	noPendingWrites    sync.Cond
 }
 
 func (p *piece) pendingChunk(cs chunkSpec, chunkSize pp.Integer) bool {

reader.go

@@ -128,7 +128,11 @@ again:
 	if int64(len(b1)) > ip.Length()-po {
 		b1 = b1[:ip.Length()-po]
 	}
-	tp.pendingWrites.Wait()
+	tp.pendingWritesMutex.Lock()
+	for tp.pendingWrites != 0 {
+		tp.noPendingWrites.Wait()
+	}
+	tp.pendingWritesMutex.Unlock()
 	n, err = dataReadAt(r.t.data, b1, pos)
 	if n != 0 {
 		err = nil

torrent.go

@@ -225,6 +225,7 @@ func (t *torrent) setMetadata(md *metainfo.Info, infoBytes []byte, eventLocker s
 	for _, hash := range infoPieceHashes(md) {
 		piece := &piece{}
 		piece.Event.L = eventLocker
+		piece.noPendingWrites.L = &piece.pendingWritesMutex
 		missinggo.CopyExact(piece.Hash[:], hash)
 		t.Pieces = append(t.Pieces, piece)
 	}
@@ -638,7 +639,11 @@ func (t *torrent) pieceLength(piece int) (len_ pp.Integer) {
 func (t *torrent) hashPiece(piece pp.Integer) (ps pieceSum) {
 	hash := pieceHash.New()
 	p := t.Pieces[piece]
-	p.pendingWrites.Wait()
+	p.pendingWritesMutex.Lock()
+	for p.pendingWrites != 0 {
+		p.noPendingWrites.Wait()
+	}
+	p.pendingWritesMutex.Unlock()
 	t.data.WriteSectionTo(hash, int64(piece)*t.Info.PieceLength, t.Info.PieceLength)
 	missinggo.CopyExact(ps[:], hash.Sum(nil))
 	return