Avoid holding client lock while scanning pieces in File.State

Some torrent data backends are expensive to query.

file.go

@@ -35,8 +35,6 @@ type FilePieceState struct {
 
 // Returns the state of pieces in this file.
 func (f *File) State() (ret []FilePieceState) {
-	f.t.cl.mu.Lock()
-	defer f.t.cl.mu.Unlock()
 	pieceSize := int64(f.t.usualPieceSize())
 	off := f.offset % pieceSize
 	remaining := f.length
@@ -48,7 +46,10 @@ func (f *File) State() (ret []FilePieceState) {
 		if len1 > remaining {
 			len1 = remaining
 		}
-		ret = append(ret, FilePieceState{len1, f.t.pieceState(i)})
+		f.t.cl.mu.RLock()
+		ps := f.t.pieceState(i)
+		f.t.cl.mu.RUnlock()
+		ret = append(ret, FilePieceState{len1, ps})
 		off = 0
 		remaining -= len1
 	}