NoMethodError: undefined method `stats' for nil:NilClass

[paneq]

Hey, thanks for this very useful plugin.

We are experiencing a bug during deployments. It appears that metrics endpoint is served earlier than the rest of the application. This leads to:

NoMethodError: undefined method `stats' for nil:NilClass` exception.

Software versions: puma (6.3.1), puma-metrics (1.3.0)

Stacktrace:

NoMethodError: undefined method `stats' for nil:NilClass (NoMethodError)

      }.merge(@server.stats).merge(super)
                     ^^^^^^
  from puma/single.rb:20:in `stats'
  from puma/launcher.rb:115:in `stats'
  from puma/metrics/app.rb:26:in `retrieve_and_parse_stats!'
  from puma/metrics/app.rb:17:in `call'
  from puma/request.rb:100:in `block in handle_request'
  from puma/thread_pool.rb:344:in `with_force_shutdown'
  from puma/request.rb:99:in `handle_request'
  from puma/server.rb:443:in `process_client'
  from puma/server.rb:245:in `block in run'
  from puma/thread_pool.rb:151:in `block in spawn_thread'

I am not exactly sure how this can be fixed, should Puma check if @server is nil before invoking merge:

  class Single < Runner
    # @!attribute [r] stats
    def stats
      {
        started_at: utc_iso8601(@started_at)
      }.merge(@server.stats).merge(super)
    end

or should the metrics plugin check some status on @launcher before invoking stats.

      def retrieve_and_parse_stats!
        puma_stats = @launcher.stats

Related code:

• https://github.com/puma/puma/blob/c2f73304655711219fed45fd083963174735a734/lib/puma/launcher.rb#L97-L109
• https://github.com/puma/puma/blob/c2f73304655711219fed45fd083963174735a734/lib/puma/single.rb#L41-L53

[harmjanblok]

As the method retrieve_and_parse_stats! is called every scrape (like every 30 seconds, depending on your prometheus configuration), I think adding a rescue or additional condition to the if/else clause should be sufficient?
Assuming the described NoMethodError applies to boottime only?