Apparently bash (via POSIX) only guarantees a small (32ish) number of
children can be started and their statuses retrieved at any given
point. On larger jobs with lots of plugins and additional work, we
may go over that limit, especially for long-lived children, such
as the install_tempest task.
This works around that issue by creating a fifo for each child at
spawn time. When the child is complete, it will block on a read
against that fifo (and thus not exit). When the parent goes to wait
on the child, it first writes to that fifo, unblocking the child so
that it can exit near the time we go to wait.
Closes-Bug: #1923728
Change-Id: Id755bdb1e7f1664ec08742d034c174e87a3d2902
aa5c38727b