We need to do this because when e.g. a thread in client process dies and
wants to abort the whole process, kernel sends FUSE INTERRUPT request to
interrup READ syscall on-client pinner is waiting on for /head/watch.
And if this cancellation is not handled, the client process is left hang
forever and even kill -9 does not stop it.

xio.Pipe to be committed/pushed yet to go123.

test_wcfs_watch_robust becomes broken for now.