Instead of copying over a mostly-unchanged `postgresql.conf`, we
transition to deploying a `conf.d/zulip.conf` which contains the
only material changes we made to the file, which were previously
appended to the end.
While shipping separate while `postgresql.conf` files for each
supported version is useful if there is large variety in supported
options between versions, there is not no such variation at current,
and the burden of overriding the entire default configuration is that
it must be keep up to date wit the package's version.
These hooks are run immediately around the critical section of the
upgrade. If the upgrade fails for preparatory reasons, the pre-deploy
hook may not be run; if it fails during the upgrade, the post-deploy
hook will not be run. Hooks are called from the CWD of the new
deploy, with arguments of the old version and the new version. If
they exit with non-0 exit code, the deploy aborts.
Anders Kaseorg
Alex Vandiver
David Rosa