Handed in via ProcessParallelOptions.stallPhaseHandle (#4902) and filled
in synchronously by the pool with a live getStallPhase() reader. Absent
getStallPhase (before the pool runs, or on a code path that doesn't wire
one) is the caller's signal that no phase is known yet.
Handed in via
ProcessParallelOptions.stallPhaseHandle(#4902) and filled in synchronously by the pool with a livegetStallPhase()reader. AbsentgetStallPhase(before the pool runs, or on a code path that doesn't wire one) is the caller's signal that no phase is known yet.