Suggest in docs and daemon messages how command-signals vs. PID files vs. systemd units interact

[jimklimov]

Follow-up from #1721 discussions, mostly to clarify docs and messages to be more actionable with regard to changes in NUT 2.8.0 and systemd unit wrapping (e.g. that by default it starts upsd -F and so does not save PID files).

Also clarifies some example texts to be more consistent between files (e.g. NUT user for upsmon).

[jimklimov]

For builds with systemd:

$ ./server/upsd -c reload
Network UPS Tools upsd 2.8.0-175-g3d5537378
fopen /var/state/ups/upsd.pid: No such file or directory
Try 'systemctl reload nut-server.service' or add '-P $PID' argument

$ ./server/upsd -c reload -P 123123
Network UPS Tools upsd 2.8.0-175-g3d5537378
kill: No such process
Try 'systemctl reload nut-server.service'

For builds on other systems:

$ ./server/upsd -c reload
Network UPS Tools upsd 2.8.0-182-gdbe5e39e6
fopen /var/state/ups/upsd.pid: No such file or directory
Failed to signal the currently running daemon
Try to add '-P $PID' argument

$ ./server/upsd -c reload -P 123123
Network UPS Tools upsd 2.8.0-182-gdbe5e39e6
kill: No such process
Failed to signal the currently running daemon

While not a 100% exact science (we might delve into detailing different return codes from the sendsignal*() but do not do so here), this is hopefully actionable enough for troubleshooting.

[jimklimov]

Getting closer to "some % exact science" -- by refactoring the code for detection and reporting of probe/signal outcomes into one block:

$ ./server/upsd -c reload
Network UPS Tools upsd 2.8.0-182-gdbe5e39e6
fopen /var/state/ups/upsd.pid: No such file or directory
Could not find PID file '/var/state/ups/upsd.pid' to see if previous upsd instance is already running!
Failed to signal the currently running daemon (if any)
Try to add '-P $PID' argument

$ ./server/upsd -c reload -P1
Network UPS Tools upsd 2.8.0-182-gdbe5e39e6
kill: Operation not permitted
Failed to signal the currently running daemon (if any)

$ ./server/upsd -c reload -P123123
Network UPS Tools upsd 2.8.0-182-gdbe5e39e6
kill: No such process
Failed to signal the currently running daemon (if any)

As can be seen in make check-NIT the normal start-up is "reasonably noisy":

[INFO] Starting UPSD for sandbox
Network UPS Tools upsd 2.8.0-188-gc78e889c4
fopen /home/jim/nut/tests/NIT/tmp/run/upsd.pid: No such file or directory
Could not find PID file '/home/jim/nut/tests/NIT/tmp/run/upsd.pid' to see if previous upsd instance is already running!
listening on ::1 port 35064
listening on 127.0.0.1 port 35064

• It did not find a PID file from previous instance; only the future can tell what it means:
  • this may mean nobody to conflict with
  • this may mean poor tracking of someone to conflict with
• in this example there was no conflict (port available)

[AppVeyorBot]

✅ Build nut 2.8.0.198-master completed (commit 447ade2ae2 by @jimklimov)

[jimklimov]

See some more testing of commands vs. starts with and without a competing daemon in #1728 research trail.