With sufficiently bad luck, it was possible for a parallel worker to
attempt attach to a DSA area after all other backends have detached
from it, which is not legal.  If the worker had waited a little longer
to get started, the DSM itself would have been destroyed, which is why
this wasn't noticed before.

Thomas Munro, per a report from Andreas Seltenreich

Discussion: http://postgr.es/m/87h92g83t3.fsf@credativ.de