XNU stops signalling a process as soon as it starts exiting but posts
NOTE_EXIT later in the same exit. A KILL that lands in that window gets
EPERM while the exit probe still reads "alive", so stopping a child that
TERM had just ended could fail with "Operation not permitted". This
flaked the macOS run of contained_exit_probe_preserves_a_live_child.
The EPERM branch now re-probes for up to 250 ms before treating the error
as a live, unsignalable root. Live roots, reaped roots and probe failures
are still errors.
Palash Debnath
8cf7bebb61