post-start commands are not supported

[Dmitrii Shcherbakov]

I have the following scenario:

1) tgtd starts up and notifies systemd via sd_notify before entering its event loop;
https://git.launchpad.net/ubuntu/+source/tgt/tree/usr/tgtd.c?h=ubuntu/focal#n658

  tgtd:
    command: usr/sbin/tgtd -f
    stop-command: bin/tgtd-stop
    daemon: notify
----
   Status: "Starting event loop..."
    Tasks: 1 (limit: 7372)
   CGroup: /system.slice/snap.microstack.tgtd.service
           └─22652 /snap/microstack/x2/usr/sbin/tgtd -f

2) tgtd does not automatically read config files - there is no option to do this upstream;
http://manpages.ubuntu.com/manpages/focal/man8/tgtd.8.html

3) after tgtd starts, `tgt-admin -e -c $SNAP_COMMON/etc/tgt/targets.conf` needs to be executed in order to load the iscsi target config into tgtd.

For this to work, I need to execute a post-start command similar to how it is done in the Ubuntu distribution packaging:

https://git.launchpad.net/ubuntu/+source/tgt/tree/debian/tgt.service#n11
ExecStartPost=/usr/sbin/tgt-admin -e -c /etc/tgt/targets.conf

A systemd doc reference: https://www.freedesktop.org/software/systemd/man/systemd.service.html#ExecStartPre=

Alternatively, a separate service like this could be used with the PartOf stanza (https://www.freedesktop.org/software/systemd/man/systemd.unit.html#PartOf=):

  tgtd-load-targets:
    command: usr/sbin/tgt-admin -e -c $SNAP_COMMON/etc/tgt/targets.conf
    daemon: oneshot
    after:
      - tgtd

However, it is not supported in snapd either.