Commit 4518c798 intended to block signals in regular backends that
allocate DSM segments, but dsm_impl_resize() is also reached by
dsm_postmaster_startup(). It's not OK to clobber the postmaster's
signal mask, so only manipulate the signal mask when under the
postmaster.
Back-patch to all releases, like 4518c798.
Discussion: https://postgr.es/m/CA%2BhUKGKNpK%3D2OMeea_AZwpLg7Bm4%3DgYWk7eDjZ5F6YbozfOf8w%40mail.gmail.com
80845b7c0b