class DEBUGGER__::UI_ServerBase

def sock skip: false
  if s = @sock         # already connection
    # ok
  elsif skip == true   # skip process
    no_sock = true
    r = @accept_m.synchronize do
      if @sock
        no_sock = false
      else
        yield nil
      end
    end
    return r if no_sock
  else                 # wait for connection
    until s = @sock
      @accept_m.synchronize{
        unless @sock
          DEBUGGER__.warn "wait for debugger connection..."
          @accept_cv.wait(@accept_m)
        end
      }
    end
  end
  yield s
rescue Errno::EPIPE
  # ignore
end