config: Add hostname and worker_id short-cut. fix #1473

[repeatedly]

No description provided.

[repeatedly]

This approach shows warnings with verbose mode but I'm not sure how to avoid it...

[mururu]

I can't see any warnings on my environment. Could you share the detail?

[repeatedly]

See https://travis-ci.org/fluent/fluentd/jobs/327180965#L541

[mururu]

Ah, I see. How about using Binding#local_variable_set?

[repeatedly]

With instance_eval, local_variable_set doesn't work. Try this:

context = Kernel.binding
context.local_variable_set(:worker_id, 1)
p context.instance_eval('"foo#{worker_id}"')

[mururu]

Can't we use #eval instead of #instance_eval here?

[repeatedly]

I'm not sure it doesn't break existing behaviour or not.

@nurse Do you see any problem for using eval here?

[nurse]

Just add to code seems simple.

code = <<eom + code
hostname = Socket.gethostname
worker_id = ENV['SERVERENGINE_WORKER_ID'] || ''
eom
         @eval_context.instance_eval(code)

[mururu]

With this conf

<source>
  @type dummy
  @id "dummy#{worker_id}"
  tag test
</source>
<match test>
  @type null
</match>

dumped config is like this:

2018-01-11 01:04:34 +0900 [info]: using configuration file: <ROOT>
  <source>
    @type dummy
    @id dummy
    tag "test"
  </source>
  <match test>
    @type null
  </match>
</ROOT>

but I think @id "dummy#{worker_id}" is better.

[repeatedly]

Yeah. The problem is "#{}" is processed during configuration parsing and it doesn't store raw parameter.
This is the Element problem so we need other issue/pr for it.

[mururu]

Makes sense.

[repeatedly]

Applied reviews. Will merge after test passed.