Created attachment 1597001 [details]
Example service, edit exitN to return test exit codes exit0 should always return 0

Description of problem:
Unable to repeatedly "reload" a oneshot service.

Red Hat Enterprise Linux 8.0 (Ootpa) -- affected
Fedora 30 (Workstation Edition) -- affected
CentOS Linux 7 (Core) -- not affected


Version-Release number of selected component (if applicable):
systemd 239


How reproducible:
always


Steps to Reproduce:
1. Create a oneshot service
2. Start it
3. Modify ExecReload script to return non 0 exit code
4. Reload it
5. Reload it again


Actual results:
Reload 1 fails due to exit code != 0
> Unit state is now: Active: failed (Result: exit-code)
Reload 2 fails with: `test.service is not active, cannot reload.`


Expected results:
Reload 1 fails due to exit code != 0
> Unit state is now: Active: active (exited) (Result: exit-code)
Reload 2 fails due to exit code != 0
# Modify ExecReload script to return exit code 0
Reload 3 succeeds


Additional info:
This affects units such as nftables.service such that a syntax error during reload causes the unit to enter a failed state, and must either be restarted by hand or the reload command run manually.

Restarting units like nftables.service will flush the firewall and then attempt to start it again, leaving you without a firewall until it gets started again.

By utilizing reload on units such as nftables, I can fix the syntax error and reload it again, using the atomic rule replacement feature of nftables.

https://wiki.nftables.org/wiki-nftables/index.php/Atomic_rule_replacement

It is possible to change the ExecReload command and prefix it with `-` but that causes the reload command to not notify you of syntax errors or similar.