debian: instances are stopped but not started after package upgrade

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The latest tor upgrade revealed a problem:
when upgrading, tor instances are properly stopped, but are not started again.

This is my current understanding of the root-cause:

Tor instances are stopped by prerm:

if [ -x "/etc/init.d/tor" ]; then
        invoke-rc.d tor stop || tor_error_init
fi

and started by postinst:

if [ -x "/etc/init.d/tor" ]; then
        update-rc.d tor defaults >/dev/null
        invoke-rc.d tor start || tor_error_init
fi

Starting tor.service does not start multi-instance services due to the semantics of PartOf= (which is only relevant in the context of stop and restart actions, but not for start):

PartOf=
           Configures dependencies similar to Requires=, but limited to stopping and restarting of units. When systemd stops or restarts the units listed here, the action is propagated to this unit. Note
           that this is a one-way dependency  changes to this unit do not affect the listed units.

Another relevant point here is that our instances are named after IP addresses and ORports:
IP_port
and therefore include non-alphanumeric characters (i.e. "_").

This is relevant because the generator, that solves the "PartOf= does not cover the start case" problem, filters non-alphanumeric instance names.
https://gitweb.torproject.org/debian/tor.git/tree/debian/systemd/tor-generator

Adding "." and "_" to the allowed characters should be fine since tor-instance-create invokes adduser with "--force-badname" already.

I'll ask the maintainer whether this change is acceptable or has any other unwanted side effects, but we havn't had any problems with these service names in a long time.

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One can override the maintainers generator starting with systemd 219 - so the fix is trivial on Debian Testing and Ubuntu 16.04.
Systemd in Debian Jessie (215) is to old for that feature, so we use Martin Pitt's (Debian Developer) suggestion to solve it on Debian Jessie via dpkg-statoverride:
https://lists.freedesktop.org/archives/systemd-devel/2016-August/037367.html

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https://lists.torproject.org/pipermail/tor-relays/2016-August/009875.html

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Our modification to the generator script:

23c23
<     if echo "x$name" | grep -q '[^a-zA-Z0-9]' ||
---
>     if echo "x$name" | grep -q '[^a-zA-Z0-9_.]' ||
31d30
< 

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Known side effect, error messages on debian 8:

systemd: /lib/systemd/system-generators/tor-generator failed with error
code 1.
systemd: Failed to execute /lib/systemd/system-generators/tor-generator:
Permission denied

Currently we run dpkg-statoverride every time, at some point we might want to change this to run as a handler (triggered by package installation).