Add OpenTracing instrumentation for Action View Render events

This change adds three new instrumentations, driven through rails
notifications: render_template.action_view,
render_collection.action_view and render_partial.action_view.

These can help developers understand why renders are taking a long
time which may in turn help them to improve their performance.

3 files changed, 118 insertions, 8 deletions

diff --git a/lib/gitlab/tracing/rails/action_view_subscriber.rb b/lib/gitlab/tracing/rails/action_view_subscriber.rb
new file mode 100644
index 00000000000..88816e1fb32
--- /dev/null
+++ b/lib/gitlab/tracing/rails/action_view_subscriber.rb
@@ -0,0 +1,75 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Tracing
+    module Rails
+      class ActionViewSubscriber
+        include RailsCommon
+
+        COMPONENT_TAG = 'ActionView'
+        RENDER_TEMPLATE_NOTIFICATION_TOPIC = 'render_template.action_view'
+        RENDER_COLLECTION_NOTIFICATION_TOPIC = 'render_collection.action_view'
+        RENDER_PARTIAL_NOTIFICATION_TOPIC = 'render_partial.action_view'
+
+        # Instruments Rails ActionView events for opentracing.
+        # Returns a lambda, which, when called will unsubscribe from the notifications
+        def self.instrument
+          subscriber = new
+
+          subscriptions = [
+            ActiveSupport::Notifications.subscribe(RENDER_TEMPLATE_NOTIFICATION_TOPIC) do |_, start, finish, _, payload|
+              subscriber.notify_render_template(start, finish, payload)
+            end,
+            ActiveSupport::Notifications.subscribe(RENDER_COLLECTION_NOTIFICATION_TOPIC) do |_, start, finish, _, payload|
+              subscriber.notify_render_collection(start, finish, payload)
+            end,
+            ActiveSupport::Notifications.subscribe(RENDER_PARTIAL_NOTIFICATION_TOPIC) do |_, start, finish, _, payload|
+              subscriber.notify_render_partial(start, finish, payload)
+            end
+          ]
+
+          create_unsubscriber subscriptions
+        end
+
+        # For more information on the payloads: https://guides.rubyonrails.org/active_support_instrumentation.html
+        def notify_render_template(start, finish, payload)
+          generate_span_for_notification("render_template", start, finish, payload, tags_for_render_template(payload))
+        end
+
+        def notify_render_collection(start, finish, payload)
+          generate_span_for_notification("render_collection", start, finish, payload, tags_for_render_collection(payload))
+        end
+
+        def notify_render_partial(start, finish, payload)
+          generate_span_for_notification("render_partial", start, finish, payload, tags_for_render_partial(payload))
+        end
+
+        private
+
+        def tags_for_render_template(payload)
+          {
+            'component' =>       COMPONENT_TAG,
+            'template.id' =>     payload[:identifier],
+            'template.layout' => payload[:layout]
+          }
+        end
+
+        def tags_for_render_collection(payload)
+          {
+            'component' =>            COMPONENT_TAG,
+            'template.id' =>          payload[:identifier],
+            'template.count' =>       payload[:count] || 0,
+            'template.cache.hits' =>  payload[:cache_hits] || 0
+          }
+        end
+
+        def tags_for_render_partial(payload)
+          {
+            'component' =>            COMPONENT_TAG,
+            'template.id' =>          payload[:identifier]
+          }
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/tracing/rails/active_record_subscriber.rb b/lib/gitlab/tracing/rails/active_record_subscriber.rb
index 214eac47e14..32f5658e57e 100644
--- a/lib/gitlab/tracing/rails/active_record_subscriber.rb
+++ b/lib/gitlab/tracing/rails/active_record_subscriber.rb
@@ -4,24 +4,37 @@ module Gitlab
   module Tracing
     module Rails
       class ActiveRecordSubscriber
-        include Gitlab::Tracing::Common
+        include RailsCommon
 
         ACTIVE_RECORD_NOTIFICATION_TOPIC = 'sql.active_record'
-        DEFAULT_OPERATION_NAME = "sqlquery"
+        OPERATION_NAME_PREFIX = 'active_record:'
+        DEFAULT_OPERATION_NAME = 'sqlquery'
 
+        # Instruments Rails ActiveRecord events for opentracing.
+        # Returns a lambda, which, when called will unsubscribe from the notifications
         def self.instrument
           subscriber = new
 
-          ActiveSupport::Notifications.subscribe(ACTIVE_RECORD_NOTIFICATION_TOPIC) do |_, start, finish, _, payload|
+          subscription = ActiveSupport::Notifications.subscribe(ACTIVE_RECORD_NOTIFICATION_TOPIC) do |_, start, finish, _, payload|
             subscriber.notify(start, finish, payload)
           end
+
+          create_unsubscriber [subscription]
         end
 
         # For more information on the payloads: https://guides.rubyonrails.org/active_support_instrumentation.html
         def notify(start, finish, payload)
-          operation_name = payload[:name].presence || DEFAULT_OPERATION_NAME
-          exception = payload[:exception]
-          tags = {
+          generate_span_for_notification(notification_name(payload), start, finish, payload, tags_for_notification(payload))
+        end
+
+        private
+
+        def notification_name(payload)
+          OPERATION_NAME_PREFIX + (payload[:name].presence || DEFAULT_OPERATION_NAME)
+        end
+
+        def tags_for_notification(payload)
+          {
             'component' =>        'ActiveRecord',
             'span.kind' =>        'client',
             'db.type' =>          'sql',
@@ -29,8 +42,6 @@ module Gitlab
             'db.cached' =>        payload[:cached] || false,
             'db.statement' =>     payload[:sql]
           }
-
-          postnotify_span("active_record:#{operation_name}", start, finish, tags: tags, exception: exception)
         end
       end
     end
diff --git a/lib/gitlab/tracing/rails/rails_common.rb b/lib/gitlab/tracing/rails/rails_common.rb
new file mode 100644
index 00000000000..88e914f62f8
--- /dev/null
+++ b/lib/gitlab/tracing/rails/rails_common.rb
@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Tracing
+    module Rails
+      module RailsCommon
+        extend ActiveSupport::Concern
+        include Gitlab::Tracing::Common
+
+        class_methods do
+          def create_unsubscriber(subscriptions)
+            -> { subscriptions.each { |subscriber| ActiveSupport::Notifications.unsubscribe(subscriber) } }
+          end
+        end
+
+        def generate_span_for_notification(operation_name, start, finish, payload, tags)
+          exception = payload[:exception]
+
+          postnotify_span(operation_name, start, finish, tags: tags, exception: exception)
+        end
+      end
+    end
+  end
+end