Re: Systemd unit parser and s6 generator

[Jonathan de Boyne Pollard <J.deBoynePollard-newsgroups@NTLWorld.COM>]

> [...]  We've discussed internally if we change that process and try to 
> write a systemd unit parser, because all units are there in Ubuntu. If 
> we could catch 90% of all cases, we need, we would be happy. If it 
> would take 2 weeks of work, that would be fine. Did somebody of you 
> try to implement something? What are your thoughts?
>
So it seems that I am, years later, still the only person in the world 
to have done this (except not targetting skarnet toolsets). (-:

As the voice of experience:

Yes, you'll not get total coverage.  You'll get "good enough for most 
cases", though I couldn't put an exact percentage on that.

No, with the s6 tooling as it stands, you'll not be able to do 
everything.  One of the things that one has to add is support for 
systemd's idiosyncratic non-UCSPI TCP/UDP service handling. Contrast the 
--systemd-compatibility flag to the tcp-socket-listen program in the 
nosh toolset to what s6-tcpserver4-socketbinder in s6-networking does.

* https://jdebp.uk/Softwares/nosh/guide/commands/tcp-socket-listen.xml

* https://skarnet.org/software/s6-networking/s6-tcpserver4-socketbinder.html

That flag is used by convert-systemd-units to handle the conversion of 
socket units.

* https://jdebp.uk/Softwares/nosh/guide/commands/convert-systemd-units.xml

Most of the issues for s6 will be things like that, minor tweaks for 
(sometimes reasonable, sometimes downright absurd) systemd quirks and 
the odd missing tool.  I wrote a set-control-group-knob command, for 
example, used by convert-systemd-units to translate the various unit 
file settings that end up twiddling knobs on control groups.  I haven't 
seen a skarnet tool for doing that, yet; so that will be one necessary 
addition, for example.

* https://jdebp.uk/Softwares/nosh/guide/commands/set-control-group-knob.xml

s6-networking also does not appear to have UDP listening tools, which 
socket units for UDP sockets need.  Timer units need tools like 
time-pause-until and time-env-add, similarly.  Notice the 
--systemd-compatibility flag for that latter, too, and the notions like 
systemd month lengths.

* https://jdebp.uk/Softwares/nosh/guide/commands/time-pause-until.xml

* https://jdebp.uk/Softwares/nosh/guide/commands/time-env-add.xml

There is, furthermore, a lot of stuff that you'll find that s6/nosh/&c. 
service run scripts do a lot better than systemd can.  As you'll see 
from the convert-systemd-units(1) manual page, I added quite a number of 
things to the systemd unit file syntax for things that one can do in the 
aforementioned, such as UCSPI access control mechanisms (UCSPI being 
entirely alien to systemd) for starters.