Previously, when reaping child processes, if there were no remaining
reapable children, the parent strand would only nap 1, which puts
unnecessary load on respirate unless at least one child strand
exits in the next second.
Change this approach by having the exiting child strands, after they
release the lease, schedule their parent immediately if the parent
has no non-exited child strands.
When doing this, you need to be careful to make sure there are not
race conditions that would delay the scheduling of the parent.
There are two potential situations you need to handle:
1. Multiple children exiting at the same time
2. Parent currently running while child is exiting
By waiting until after the child strand leases are released, you
still have a race condition with 1, but the race condition is that
multiple child strands exiting concurrently could both reschedule
the parent strand. However, that isn't a problem. You want to avoid
the case where neither child strand schedules the parent, which
rescheduling after releasing the lease should do.
To handle 2, inside reap use Model#lock! to lock the parent strand.
This will make exiting child strands block if they UPDATE the parent
strand with a new schedule, until the parent strand's transaction
commits. However, it's possible that a child strand already UPDATED
the parent. To handle this situation, before calling lock!, store
the cached schedule value in a local variable. lock! implicitly does
a reload, so compare the schedule value after reload. If the schedule
has changed, likely a child scheduled the parent for immediate
execution, so nap 0 in that case.
Just in case there are unforeseen race conditions that are not handled,
only nap for 120 seconds if there are active children. Worst case
scenario, this results in a 2 minute delay for running the parent.
However, this can potentially result in 120x less load from parent
strands polling children.
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