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

[systemd-devel@xxxxxxx (John Ioannidis)]

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
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