silent exit on pipelines reloading

[colinsurprenant]

under these conditions:

• pipeline reloading disabled
• using a system pipeline like the monitoring pipeline
• very long pipeline startup time due to the pipeline configuration complexity

logstash may silently exit when issued a SIGHUP to signal pipelines reloading.

This can be trivially reproduced by setting the sleep to 0.1 in:

logstash/logstash-core/lib/logstash/agent.rb

Lines 119 to 124 in e6d59de

	while !Stud.stop?
	if clean_state? || running_user_defined_pipelines?
	sleep(0.5)
	else
	break
	end

And simulating a slow reload time by forcing a 5 seconds sleep on pipeline reloading in just before the Create.new:

logstash/logstash-core/lib/logstash/pipeline_action/reload.rb

Lines 54 to 56 in e6d59de

	if stop_result.successful?
	Create.new(@pipeline_config, @metric).execute(agent, pipelines)
	else

Using any dummy pipeline configured in pipelines.yml, disabling pipeline reloading and enabling monitoring when issuing a SIGHUP, logstash will silently terminate.