Bug#801065: Documenting how to not fail postinst on service fails to start

[Sam Hartman <hartmans@debian.org>]

>>>>> "Wouter" == Wouter Verhelst <wouter@debian.org> writes:

    Wouter> On Wed, Feb 15, 2023 at 02:38:10PM -0500, Marvin Renich wrote:
    >> > > > - the service fails to start in the postinst.
    >> 
    >> This implies that "the service is running" is part of "the
    >> service is configured", which is where I disagree.

    Wouter> What Steve said is that if

    Wouter> - The service fails to start, *AND* - The service was
    Wouter> previously running (or this is a new install)

    Wouter> *THEN*

I think I disagree with Steve that postinst should fail on a new
install.

I think that failing postinst on a failed restart during upgrade is more
commonly the correct answer than ignoring the issue.
I also agree that it should be RECOMMENDED that if the restart fails
that be flagged to the admin somehow.

But in the case of krb5 and I think a few other services, there is not a
good way to detect at install time *whether* the service is sufficiently
configured to run from a systemd unit.  I could for example include a
ConditionPathExists on the Kerberos database.  That's wrong though
because in an LDAP deployment there will be no such database.

--Sam