postinst is in neutron-common 2:8.0.0-0ubuntu1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 | #!/bin/sh
set -e 
if [ "$1" = "configure" ]
then
	if ! getent group neutron > /dev/null 2>&1
	then
		addgroup --system neutron >/dev/null
	fi
	if ! getent passwd neutron > /dev/null 2>&1
	then
		adduser --system --home /var/lib/neutron --ingroup neutron --no-create-home --shell /bin/false neutron
	fi
	chown -R neutron:neutron /var/lib/neutron/
	chown -R neutron:adm /var/log/neutron/ 
	chmod 0750 /var/log/neutron/
    chown -R root:neutron /etc/neutron/
    # NOTE(jamespage): In order to execute unit test in decomposed
    #                  vendor drivers, /etc/neutron must be readable
    #                  other than by root or neutron; limitation of
    #                  access to neutron.conf is sufficient as this
    #                  file contains access details for DB, keystone
    #                  and message bus.
    chmod 0755 /etc/neutron/
    chmod -f 0640 /etc/neutron/neutron.conf || :
	chown root:root /etc/neutron/rootwrap.conf
	chown -R root:root /etc/neutron/rootwrap.d
	chmod 0755 /etc/neutron/rootwrap.d
	if [ -f /etc/sudoers.d/neutron_sudoers ] ; then
		chmod 0440 /etc/sudoers.d/neutron_sudoers
	fi
fi
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