isothrum 2.1.3

Fixed

• A published port's preferred number that the computer already holds moves to a free port.
  With ISOLOOM_PUBLISH_FIXED (a lab's own ports), a published port took its preferred number
  whenever no other run of this runner had reserved it. A number some other process held (another
  lab's VM already forwarding 8080, any program) then failed the start port_in_use at its
  forwarded-port check, though Isoloom calls the number preferred, not fixed. The pick now passes
  over a number the host holds and gives the port a free one, as an ephemeral port gets; the run's
  outputs give that address. Compose and Vagrant pick the same way. Two starts in two processes
  that pick the same free number at the same moment can still collide; that's #74's second part.