Bug#1091519: deb-systemd-helper: Fails when non-existing unit is listed in Also= line

[MichaIng]

I think false conclusions are drawn here:

Also= lines for non-existing units are not a problem, but skipped 
silently. The problem here is, there multiple units of libvirt packages 
have an Also= loop:

libvirtd-admin.socket:
```
Also=libvirtd.socket
Also=libvirtd-ro.socket
```

libvirtd-ro.socket:
```
Also=libvirtd.socket
Also=libvirtd-admin.socket
```

libvirtd.socket:
```
Also=libvirtd-ro.socket
Also=libvirtd-admin.socket
```

libvirt-daemon.postrm or on distro upgrades the previous 
libvirt-daemon-system.postrm call "deb-systemd-helper purge" for each of 
those, and perl even warns about the resulting recursion:
```
recursion on subroutine "main::remove_links" at 
/usr/bin/deb-systemd-helper line 488, <$fh> line 14.
```

The function contains this code block:
```
sub update_state {
...
     # Read $service_path, recurse for all Also= units.
     # This might not work when $service_path was already deleted,
     # i.e. after apt-get remove. In this case we just return
     # silently in order to not confuse the user about whether
     # disabling actually worked or not — the case is handled by
     # dh_installsystemd generating an appropriate disable
     # command by parsing the service file at debhelper-time.
     open(my $fh, '<', "$dpkg_root$service_path") or return;
     while (my $line = <$fh>) {
         chomp($line);
         my $service_link;

         if ($line =~ /^\s*Also=(.+)$/i) {
             remove_links(find_unit($1));
         }
     }
     close($fh);
```
So using deb-systemd-helper on libvirtd.socket repeats the function on 
libvirtd-ro.socket which repeats it on libvirtd.socket again and so on 
and so forth.

As can be seen and as the comment explains, missing units are gracefully 
handled, and they would even break the endless loop in this case.

Generally, for all such kind of dependencies and ordering, there must 
never be loops in systemd units, but they must have a clear 
single-direction parent/child or before/after structure. Choose one of 
those units as the primary one which is enabled on install and disabled 
on uninstall, and add Also= lines only to this one, but remove them from 
the other two. That should fix the hanging issue.

Best regards,
Micha
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