First of all, we should not send the unix time, but the monotonic time.
Second of all, RELOADING= and MONOTONIC_USEC fields should be sent in
*single* message not two separate messages.
From the man page of [systemd.service](https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html#Type=)
> notification message via sd_notify(3) that contains the "RELOADING=1" field in
> combination with "MONOTONIC_USEC=" set to the current monotonic time (i.e.
> CLOCK_MONOTONIC in clock_gettime(2)) in μs, formatted as decimal string.
[sd_notify](https://www.freedesktop.org/software/systemd/man/latest/sd_notify.html)
now has code samples of the protocol to clarify.
Without these changes, if you'd set
Type=notify-reload on the agen'ts systemd unit, systemd
would kill the service due to the service not responding to reload
correctly.
Nomad agents expect to receive `SIGHUP` to reload their configuration. The
signal handler for this is installed fairly late in agent startup, after the
client or server components are up and running. This means that configuration
management tools can potentially reload the configuration before the agent can
handle it, causing the agent to crash.
We don't want to allow configuration reload during client or server component
startup, because it would significantly complicate initialization. Instead,
we'll implement the systemd notify protocol. This causes systemd to block
sending configuration reload signals until the agent is actually ready. Users
can still bypass this by sending signals directly.
Note that there are several Go libraries that implement the sdnotify protocol,
but most are part of much larger projects which would create a lot of dependabot
burden. The bits of the protocol we need are extremely simple to implement in a
just a couple of functions.
For non-Linux or non-systemd Linux systems, this feature is a no-op. In future
work we could potentially implement service notification for Windows as well.
Fixes: https://github.com/hashicorp/nomad/issues/3885
