Fix startup crash in Puma single mode

Changelog: fixed

config.ru

@@ -4,17 +4,7 @@
 
 require ::File.expand_path('../config/environment', __FILE__)
 
-def master_process?
-  Prometheus::PidProvider.worker_id == 'puma_master'
-end
-
 warmup do |app|
-  # The following is necessary to ensure stale Prometheus metrics don't accumulate over time.
-  # It needs to be done as early as here to ensure metrics files aren't deleted.
-  # After we hit our app in `warmup`, first metrics and corresponding files already being created,
-  # for example in `lib/gitlab/metrics/requests_rack_middleware.rb`.
-  Prometheus::CleanupMultiprocDirService.new.execute if master_process?
-
   client = Rack::MockRequest.new(app)
   client.get('/')
 end

config/initializers/7_prometheus_metrics.rb

@@ -13,6 +13,24 @@ def prometheus_default_multiproc_dir
   end
 end
 
+def puma_metrics_server_process?
+  Prometheus::PidProvider.worker_id == 'puma_master'
+end
+
+def sidekiq_metrics_server_process?
+  Gitlab::Runtime.sidekiq? && (!ENV['SIDEKIQ_WORKER_ID'] || ENV['SIDEKIQ_WORKER_ID'] == '0')
+end
+
+if puma_metrics_server_process? || sidekiq_metrics_server_process?
+  # The following is necessary to ensure stale Prometheus metrics don't accumulate over time.
+  # It needs to be done as early as here to ensure metrics files aren't deleted.
+  # After we hit our app in `warmup`, first metrics and corresponding files already being created,
+  # for example in `lib/gitlab/metrics/requests_rack_middleware.rb`.
+  Prometheus::CleanupMultiprocDirService.new.execute
+
+  ::Prometheus::Client.reinitialize_on_pid_change(force: true)
+end
+
 ::Prometheus::Client.configure do |config|
   config.logger = Gitlab::AppLogger
 
@@ -49,15 +67,9 @@ if Gitlab::Runtime.sidekiq? && (!ENV['SIDEKIQ_WORKER_ID'] || ENV['SIDEKIQ_WORKER
 end
 
 Gitlab::Cluster::LifecycleEvents.on_master_start do
-  # When running Puma in a Single mode, `on_master_start` and `on_worker_start` are the same.
-  # Thus, we order these events to run `reinitialize_on_pid_change` with `force: true` first.
-  ::Prometheus::Client.reinitialize_on_pid_change(force: true)
-
   Gitlab::Metrics.gauge(:deployments, 'GitLab Version', {}, :max).set({ version: Gitlab::VERSION, revision: Gitlab.revision }, 1)
 
   if Gitlab::Runtime.puma?
-    Gitlab::Metrics::RequestsRackMiddleware.initialize_metrics
-
     Gitlab::Metrics::Samplers::PumaSampler.instance.start
 
     # Starts a metrics server to export metrics from the Puma primary.

config/initializers/zz_metrics.rb

@@ -36,6 +36,10 @@ if Gitlab::Metrics.enabled? && !Rails.env.test? && !(Rails.env.development? && d
     config.middleware.use(Gitlab::Metrics::ElasticsearchRackMiddleware)
   end
 
+  if Gitlab::Runtime.puma?
+    Gitlab::Metrics::RequestsRackMiddleware.initialize_metrics
+  end
+
   GC::Profiler.enable
 
   module TrackNewRedisConnections

lib/gitlab/cluster/lifecycle_events.rb

@@ -70,7 +70,7 @@ module Gitlab
         # Hook registration methods (called from initializers)
         #
         def on_worker_start(&block)
-          if in_clustered_environment?
+          if in_clustered_puma?
             # Defer block execution
             (@worker_start_hooks ||= []) << block
           else
@@ -101,7 +101,7 @@ module Gitlab
         end
 
         def on_master_start(&block)
-          if in_clustered_environment?
+          if in_clustered_puma?
             on_before_fork(&block)
           else
             on_worker_start(&block)
@@ -158,21 +158,8 @@ module Gitlab
           end
         end
 
-        def in_clustered_environment?
-          # Sidekiq doesn't fork
-          return false if Gitlab::Runtime.sidekiq?
-
-          # Puma sometimes forks
-          return true if in_clustered_puma?
-
-          # Default assumption is that we don't fork
-          false
-        end
-
         def in_clustered_puma?
-          return false unless Gitlab::Runtime.puma?
-
-          @puma_options && @puma_options[:workers] && @puma_options[:workers] > 0
+          Gitlab::Runtime.puma? && @puma_options && @puma_options[:workers] && @puma_options[:workers] > 0
         end
       end
     end