Audit plugin exit states to determine if another state (e.g., UNKNOWN) would be more applicable

[atc0005]

Overview

While implementing performance data handling as part of atc0005/check-cert#445 I found myself choosing to do one of:

• emitting an error and continuing
• exiting with an UNKNOWN status

Either seemed more appropriate than exiting with either of CRITICAL or WARNING state.

To keep mostly current logic intact I opted to just emit the error as a log message and continue:

	pd, perfDataErr := getPerfData(certChain, cfg.AgeCritical, cfg.AgeWarning)
	if perfDataErr != nil {
		log.Error().
			Err(perfDataErr).
			Msg("failed to generate performance data")

		// Surface the error in plugin output.
		plugin.AddError(perfDataErr)

		// TODO: Abort plugin execution with UNKNOWN status?
	}

	if err := plugin.AddPerfData(false, pd...); err != nil {
		log.Error().
			Err(err).
			Msg("failed to add performance data")

		// Surface the error in plugin output.
		plugin.AddError(err)

		// TODO: Abort plugin execution with UNKNOWN status?
	}

I'm filing this GH issue as a reminder to revisit that decision in the larger context of current plugin logic as it pertains to the Nagios Plugin Guidelines (see refs):

Numeric Value	Service Status	Status Description
0	OK	The plugin was able to check the service and it appeared to be functioning properly
1	Warning	The plugin was able to check the service, but it appeared to be above some "warning" threshold or did not appear to be working properly
2	Critical	The plugin detected that either the service was not running or it was above some "critical" threshold
3	Unknown	Invalid command line arguments were supplied to the plugin or low-level failures internal to the plugin (such as unable to fork, or open a tcp socket) that prevent it from performing the specified operation. Higher-level errors (such as name resolution errors, socket timeouts, etc) are outside of the control of plugins and should generally NOT be reported as UNKNOWN states.

References

• https://nagios-plugins.org/doc/guidelines.html#AEN78
• Surface plugin.AddPerfData() error, exit with UNKNOWN exit state todo#53
• Use UNKNOWN state for invalid command-line args todo#55

[atc0005]

#187 resolved one issue, but others remain.