[NO-TICKET] Fix breaking application boot due to concurrency issue in tracing

lib/datadog/tracing/contrib/extensions.rb

@@ -110,6 +110,13 @@ def configure(&block)
           module Settings
             InvalidIntegrationError = Class.new(StandardError)
 
+            # Used to avoid concurrency issues between registering integrations (e.g. mutation) and reporting the
+            # current integrations for logging/debugging/telemetry purposes (e.g. iteration) in the
+            # `@instrumented_integrations` hash.
+            #
+            # See https://github.com/DataDog/dd-trace-rb/issues/2851 for details on the original issue.
+            INSTRUMENTED_INTEGRATIONS_LOCK = Mutex.new
+
             def self.included(base)
               base.class_eval do
                 settings :contrib do
@@ -161,7 +168,10 @@ def instrument(integration_name, options = {}, &block)
                 configuration_name = options[:describes] || :default
                 filtered_options = options.reject { |k, _v| k == :describes }
                 integration.configure(configuration_name, filtered_options, &block)
-                instrumented_integrations[integration_name] = integration
+                INSTRUMENTED_INTEGRATIONS_LOCK.synchronize do
+                  @instrumented_integrations ||= {}
+                  @instrumented_integrations[integration_name] = integration
+                end
 
                 # Add to activation list
                 integrations_pending_activation << integration
@@ -192,14 +202,16 @@ def integrations_pending_activation
               @integrations_pending_activation ||= Set.new
             end
 
+            # This method is only for logging/debugging/telemetry purposes (e.g. iteration) in the
+            # `@instrumented_integrations` hash.
             # @!visibility private
             def instrumented_integrations
-              @instrumented_integrations ||= {}
+              INSTRUMENTED_INTEGRATIONS_LOCK.synchronize { (@instrumented_integrations&.dup || {}).freeze }
             end
 
             # @!visibility private
             def reset!
-              instrumented_integrations.clear
+              INSTRUMENTED_INTEGRATIONS_LOCK.synchronize { @instrumented_integrations&.clear }
               super
             end
 

spec/datadog/tracing/contrib/extensions_spec.rb

@@ -26,6 +26,10 @@
     before { registry.add(integration_name, integration) }
   end
 
+  before do
+    allow(Datadog.logger).to receive(:warn)
+  end
+
   context 'for' do
     describe Datadog do
       describe '#configure' do
@@ -244,6 +248,12 @@ def self.patch
             end
           end
         end
+
+        describe '#instrumented_integrations' do
+          subject(:instrumented_integrations) { settings.instrumented_integrations }
+
+          it { is_expected.to be_frozen }
+        end
       end
     end
   end