fix: Drop Rails dependency for ActiveSupport Instrumentation

This makes it possible to use this instrumentation in non-Rails applications that leverage AS::Notifications.

In addition to that it mitigates a race condition where a subscription may be registered by a separate thread before obtaining a lock on the notifications object.

See open-telemetry#218

instrumentation/active_support/Appraisals

@@ -4,23 +4,16 @@
 #
 # SPDX-License-Identifier: Apache-2.0
 
-# Rails 5.2 is incompatible with Ruby 3.
-if RUBY_VERSION < '3'
-  appraise 'rails-5.2' do
-    gem 'rails', '~> 5.2.0'
-  end
-end
-
-appraise 'rails-6.0' do
-  gem 'rails', '~> 6.0.0'
+appraise 'activesupport-6.0' do
+  gem 'activesupport', '~> 6.0.0'
 end
 
-appraise 'rails-6.1' do
-  gem 'rails', '~> 6.1.0'
+appraise 'activesupport-6.1' do
+  gem 'activesupport', '~> 6.1.0'
 end
 
 if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('2.7.0')
-  appraise 'rails-7.0' do
-    gem 'rails', '~> 7.0.0'
+  appraise 'activesupport-7.0' do
+    gem 'activesupport', '~> 7.0.0'
   end
 end

instrumentation/active_support/lib/opentelemetry/instrumentation/active_support/span_subscriber.rb

@@ -8,12 +8,15 @@ module OpenTelemetry
   module Instrumentation
     # rubocop:disable Style/Documentation
     module ActiveSupport
+      NOTIFIER_MAJOR_VERSION_6 = 6
+
       # The SpanSubscriber is a special ActiveSupport::Notification subscription
       # handler which turns notifications into generic spans, taking care to handle
       # context appropriately.
 
       # A very hacky way to make sure that OpenTelemetry::Instrumentation::ActiveSupport::SpanSubscriber
       # gets invoked first
+      # Rails 6+ https://github.com/rails/rails/blob/0f0ec9908e25af36df2d937dc431f626a4102b3d/activesupport/lib/active_support/notifications/fanout.rb#L51
       #
       def self.subscribe(
         tracer,
@@ -29,13 +32,25 @@ def self.subscribe(
         )
 
         subscriber_object = ::ActiveSupport::Notifications.subscribe(pattern, subscriber)
+        active_support_major_version = ::ActiveSupport.version.canonical_segments.first
+
         ::ActiveSupport::Notifications.notifier.synchronize do
-          if ::Rails::VERSION::MAJOR >= 6
-            s = ::ActiveSupport::Notifications.notifier.instance_variable_get(:@string_subscribers)[pattern].pop
-            ::ActiveSupport::Notifications.notifier.instance_variable_get(:@string_subscribers)[pattern].unshift(s)
+          if active_support_major_version >= NOTIFIER_MAJOR_VERSION_6
+            subscribers = ::ActiveSupport::Notifications.notifier.instance_variable_get(:@string_subscribers)[pattern]
+          else
+            subscribers = ::ActiveSupport::Notifications.notifier.instance_variable_get(:@subscribers)
+          end
+
+          if subscribers.nil?
+            OpenTelemetry.handle_error(
+              message: "Unable to move OTEL ActiveSupport Notifications subscriber to the front of the notifications list which may cause incomplete traces." +
+                        "Please report an issue here: " +
+                        "https://github.com/open-telemetry/opentelemetry-ruby-contrib/issues/new?labels=bug&template=bug_report.md&title=ActiveSupport%20Notifications%20subscribers%20list%20is%20nil"
+            )
           else
-            s = ::ActiveSupport::Notifications.notifier.instance_variable_get(:@subscribers).pop
-            ::ActiveSupport::Notifications.notifier.instance_variable_get(:@subscribers).unshift(s)
+            subscribers.unshift(
+              subscribers.delete(subscriber_object)
+            )
           end
         end
         subscriber_object

instrumentation/active_support/opentelemetry-instrumentation-active_support.gemspec

@@ -28,6 +28,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency 'opentelemetry-api', '~> 1.0'
   spec.add_dependency 'opentelemetry-instrumentation-base', '~> 0.21.0'
 
+  spec.add_development_dependency 'activesupport'
   spec.add_development_dependency 'appraisal', '~> 2.2.0'
   spec.add_development_dependency 'bundler', '~> 2.4'
   spec.add_development_dependency 'minitest', '~> 5.0'

instrumentation/active_support/test/opentelemetry/instrumentation/active_support/span_subscriber_test.rb

@@ -6,7 +6,7 @@
 
 require 'test_helper'
 
-describe OpenTelemetry::Instrumentation::ActiveSupport::SpanSubscriber do
+describe 'OpenTelemetry::Instrumentation::ActiveSupport::SpanSubscriber' do
   let(:instrumentation) { OpenTelemetry::Instrumentation::ActiveSupport::Instrumentation.instance }
   let(:tracer) { instrumentation.tracer }
   let(:exporter) { EXPORTER }

instrumentation/active_support/test/test_helper.rb

@@ -7,11 +7,14 @@
 require 'bundler/setup'
 Bundler.require(:default, :development, :test)
 
+require 'active_support'
+require 'active_support/core_ext'
 require 'opentelemetry-instrumentation-active_support'
 
 require 'minitest/autorun'
 require 'webmock/minitest'
 
+
 # global opentelemetry-sdk setup:
 EXPORTER = OpenTelemetry::SDK::Trace::Export::InMemorySpanExporter.new
 span_processor = OpenTelemetry::SDK::Trace::Export::SimpleSpanProcessor.new(EXPORTER)