implicit linkage

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

Andrei Popescu:
> I recently needed something to run imapfilter and restart it in case
> it might exit, so I had a look at daemontools. I gave up quickly [...]

And here's how one can do it with the nosh package 
(http://homepage.ntlworld.com./jonathan.deboynepollard/Softwares/nosh.html).

I took a template service unit for imapfilter from the WWW and tweaked 
it a little bit:

    root /var/sv #cat imapfilter@.service
    [Unit]
    Description=%p daemon service for %I

    [Service]
    Type=simple
    ExecStart=%p -c /home/%I/.%p/config.lua
    Restart=always
    User=%I

    [Install]
    WantedBy=workstation.target
    root /var/sv #

The nosh package's native service configuration is of course the service 
bundle, a compatible superset of the daemontools-family service/ and 
supervise/ directories; and the package has a tool for converting 
systemd service, socket, and target units (within some fairly broad 
limits, as explained in depth in the chapter on converting systemd units 
in the nosh Guide).  So I ran it, instatiating the template:

    root /var/sv #system-control convert-systemd-units 
./imapfilter@popescu.service
    root /var/sv #

It produced an entire service bundle.  Since you've been talking about 
how, supposedly, hard run scripts are to write, the interesting part is 
the auto-generated run script that the import tool produced.  Even this 
isn't particularly complex:

    root /var/sv #cat imapfilter@popescu/service/run
    #!/bin/nosh
    #Run file generated from ./imapfilter@.service
    #imapfilter daemon service for popescu
    chdir /
    setuidgid popescu
    imapfilter -c /home/popescu/.imapfilter/config.lua
    root /var/sv #

It uses one of those minimum-functionality script interpreters that I 
mentioned before.  One could of course write a similarly short shell 
script by hand:

    root /var/sv #cat imapfilter@popescu/service/run.sh
    #!/bin/sh -e
    exec setuidgid popescu imapfilter -c 
/home/popescu/.imapfilter/config.lua
    root /var/sv #

But I'm illustrating starting from a systemd unit here, and letting the 
import tool write the scripts.

For kicks, I decided to log the output of this service to a dedicated 
logging service.  I started with the cyclog@.service template that comes 
in the nosh source archive and instantiated it with the same systemd 
unit import tool as before:

    root /var/sv #system-control convert-systemd-units 
./cyclog@imapfilter@popescu.service
    root /var/sv #

This made me another service bundle.  Again, the interesting part for 
this discussion is the run script that the import tool made:

    root /var/sv #cat cyclog@imapfilter@popescu/service/run
    #!/bin/nosh
    #Run file generated from ./cyclog@.service
    #Standard format cyclog logging service for imapfilter@popescu
    chdir /var/log/sv/
    setuidgid "imapfilter@popescu-log"
    cyclog imapfilter@popescu/
    root /var/sv #

Setting up /var/log/sv/imapfilter@popescu and its dedicated logging user 
(or editing the run script to use, say, "popescu" as the logging user) 
is a simple exercise in making a directory and adding a user to the 
system's user database (and editing a script file), of course.  I could 
have made one big logging service for /var/log/sv/popescu instead:

    root /var/sv #system-control convert-systemd-units 
./cyclog@popescu.service
    root /var/sv #

This is because I can plumb "main" services to "log" services however I 
like, including many-to-one fan-in and 1:1 dedicated logging services.  
I chose the latter:

    root /var/sv #ln -s ../cyclog@imapfilter@popescu imapfilter@popescu/log
    root /var/sv #

The rest of the process goes beyond this discussion of how simple it is 
to write daemontools-family run scripts and is an exercise in enabling 
the two services and starting them:

    root /var/sv #systemctl enable imapfilter@popescu.service 
cyclog@imapfilter@popescu.service
    root /var/sv #systemctl start imapfilter@popescu.service 
cyclog@imapfilter@popescu.service
    root /var/sv #

Were this machine running the daemontools compatibility old-style 
service scanner, enabling the two services and starting them would have 
been:

    root /var/sv #ln -s /var/sv/imapfilter@popescu/ /var/service/
    root /var/sv #

But this machine is running the nosh package's system manager, has the 
"standard targets" system in place, and is always starting things with 
system-control (a.k.a. systemctl).  It was the same machine as used at 
http://homepage.ntlworld.com./jonathan.deboynepollard/Softwares/nosh-example.html 
in fact (which will come as a surprise to those who remember that we're 
starting from a systemd unit file here).