pgsql: Kill dead-end children when there's nothing else left

Kill dead-end children when there's nothing else left

Previously, the postmaster would never try to kill dead-end child
processes, even if there were no other processes left. A dead-end
backend will eventually exit, when authentication_timeout expires, but
if a dead-end backend is the only thing that's preventing the server
from shutting down, it seems better to kill it immediately. It's
particularly important, if there was a bug in the early startup code
that prevented a dead-end child from timing out and exiting normally.

Includes a test for that case where a dead-end backend previously
prevented the server from shutting down.

Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://www.postgresql.org/message-id/a102f15f-eac4-4ff2-af02-f9ff209ec66f@iki.fi

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/bb861414fea31073f27aaab75a0ceaf3638d7985

Modified Files
--------------
src/backend/postmaster/postmaster.c | 17 +++---
src/test/perl/PostgreSQL/Test/Cluster.pm | 10 +++-
src/test/postmaster/meson.build | 1 +
src/test/postmaster/t/002_start_stop.pl | 98 ++++++++++++++++++++++++++++++++
4 files changed, 116 insertions(+), 10 deletions(-)