How do I create/destroy instances dynamically when config files appear/disappear?

[systemd-devel@xxxxxxx (John Ioannidis)]

On Mon, Mar 19, 2018 at 5:52 PM, John Ioannidis <systemd-devel at tla.org>
wrote:

> I want to have a service with several instances, each of which has a
> configuration file; when configuration files appear and disappear, I want
> the corresponding instances to be created and started, and die,
> respectively, and in particular have the running processes corresponding to
> the removed units be killed.
>
> I can get them to start the first time, but they do not stop when
> configuration files change. If I systemctl daemon-reload, the links in
> /run/systemd/system-generators get properly adjusted, but I cannot figure
> out how to tell my service to kill the no-longer-needed processes. If I
> also systemctl reload the service, all existing processes are killed,
> which is absolutely not what I want.
>
> Here is what I've done: what am I doing wrong?
>
> I tried modelling this after what OpenVPN does:
>
> # *cat /etc/systemd/system/letters.service*
> [Unit]
> Description=Letters service
> After=network.target
>
> [Service]
> Type=oneshot
> RemainAfterExit=yes
> ExecStart=/bin/true
> ExecReload=/bin/true
> WorkingDirectory=/var/tmp/letters
>
> [Install]
> WantedBy=multi-user.target
>
> # *cat /etc/systemd/system/letters at .service*
> [Unit]
> Description=Letters for %i
> PartOf=letters.service
> ReloadPropagatedFrom=letters.service
> Before=systemd-user-sessions.service
> After=syslog.target network-online.target
> Wants=network-online.target
>
> [Service]
> Type=simple
> PrivateTmp=false
> WorkingDirectory=/var/tmp/letters
> ExecStart=/usr/local/bin/letter --daemon --pidfile /run/letters/%i.pid
> PIDFile=/run/letters/%i.pid
> ExecReload=/bin/kill -HUP $MAINPID
> DeviceAllow=/dev/null rw
> ProtectSystem=true
> ProtectHome=true
>
> [Install]
> WantedBy=multi-user.target
>
> # *cat /etc/systemd/system-generators/letters*
> #!/bin/sh
> exec 2>> /tmp/opg2
> set -eux
> GENDIR="$1"
> WANTDIR="$1/letters.service.wants"
> SERVICEFILE="/lib/systemd/system/letters at .service"
> CONFIG_DIR=/var/letters/
> mkdir -p "$WANTDIR"
> for CONFIG in `cd $CONFIG_DIR; ls *.conf 2> /dev/null`; do
>  NAME=${CONFIG%%.conf}
>  ln -s "$SERVICEFILE" "$WANTDIR/letters@$NAME.service"
> done
> exit 0
>
>
> Now for the fun:
>
> # *touch /var/letters/aleph.conf /var/letters/e.conf
> /var/letters/zeta.conf*
> # systemctl daemon-reload
> # ls -l /run/systemd/generator/letters.service.wants
> total 0
> lrwxrwxrwx 1 root root 36 Mar 19 17:37 letters at aleph.service ->
> /lib/systemd/system/letters at .service
> lrwxrwxrwx 1 root root 36 Mar 19 17:37 letters at e.service ->
> /lib/systemd/system/letters at .service
> lrwxrwxrwx 1 root root 36 Mar 19 17:37 letters at zeta.service ->
> /lib/systemd/system/letters at .service
> # *systemctl enable letters.service*
> Created symlink /etc/systemd/system/multi-user.target.wants/letters.service
> â?? /etc/systemd/system/letters.service.
> # *systemctl start letters.service*
> # *ps ax | grep lett*
> 22853 ?        S      0:00 /usr/local/bin/letter --daemon --pidfile /run/
> letters/e.pid
> 22927 ?        S      0:00 /usr/local/bin/letter --daemon --pidfile /run/
> letters/aleph.pid
> 22928 ?        S      0:00 /usr/local/bin/letter --daemon --pidfile /run/
> letters/zeta.pid
>
> So far so good. This is working as expected. Even better:
>
> # *touch /var/letters/tcha.conf*
> # *systemctl daemon-reload*
> # *systemctl start letters.service*
> # *ps ax | grep lett*
> 22853 ?        S      0:00 /usr/local/bin/letter --daemon --pidfile /run/
> letters/e.pid
> 22927 ?        S      0:00 /usr/local/bin/letter --daemon --pidfile /run/
> letters/aleph.pid
> 22928 ?        S      0:00 /usr/local/bin/letter --daemon --pidfile /run/
> letters/zeta.pid
> 23083 ?        S      0:00 /usr/local/bin/letter --daemon --pidfile /run/
> letters/tcha.pid
>
> But now the trouble begins:
>
> # *rm /var/letters/e.conf*
> # *systemctl daemon-reload *
> # *ls -l /run/systemd/generator/letters.service.wants*
> total 0
> lrwxrwxrwx 1 root root 36 Mar 19 17:44 letters at aleph.service ->
> /lib/systemd/system/letters at .service
> lrwxrwxrwx 1 root root 36 Mar 19 17:44 letters at tcha.service ->
> /lib/systemd/system/letters at .service
> lrwxrwxrwx 1 root root 36 Mar 19 17:44 letters at zeta.service ->
> /lib/systemd/system/letters at .service
>
> The link letters at e.service has been correctly removed,  but the
> corresponding process still exists:
> # *ps ax | grep e.pid*
> 22853 ?        S      0:00 /usr/local/bin/letter --daemon --pidfile
> /run/letters/e.pid
>
>
>
>
I also tried this with *requires *instead of *wants*, but the behavior is
the same.
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