fix wstat panic on session rename

OnRename callback already rewrites fid paths, so by the time
wstat's fid update runs, f.path has changed. Use relPath for
the new path computation instead of slicing f.path.

server.go

@@ -948,15 +948,17 @@ func (s *Server) wstat(cs *connState, fc *plan9.Fcall) *plan9.Fcall {
 	if err := st.Rename(relPath, newDir.Name); err != nil {
 		return errFcall(fc, err.Error())
 	}
-	// Update fid path.
+	// Update fid path (for non-session renames; session renames are
+	// handled by the OnRename callback which rewrites all affected fids).
 	for _, r := range s.storeRoutes() {
 		if strings.HasPrefix(f.path, r.prefix) {
-			// Replace the last path component with the new name.
-			parent := f.path[:len(f.path)-len(oldName)]
-			cs.mu.Lock()
-			f.path = parent + newDir.Name
-			f.qid.Path = qidPath(f.path)
-			cs.mu.Unlock()
+			newPath := r.prefix + relPath[:len(relPath)-len(oldName)] + newDir.Name
+			if newPath != f.path {
+				cs.mu.Lock()
+				f.path = newPath
+				f.qid.Path = qidPath(f.path)
+				cs.mu.Unlock()
+			}
 			break
 		}
 	}