but fids can't be `dangling'.  they're allocated by one request or another (eg, Tattach, Tauth, Twalk),
and deallocated by Tclunk or Tremove.  they can't `dangle'.
if the client has allocated a fid, it is valid until it explicitly deallocates it.
if the client hasn't allocated a fid but uses it in a request,
it's a protocol error by the client (eg, `fid in use', or `unknown fid').