JRuby fix for concurrency issue with manager cache

lib/new_relic/agent/configuration/manager.rb

@@ -382,6 +382,14 @@ def reset_to_defaults
         def reset_cache
           return new_cache unless defined?(@cache) && @cache
 
+          # Modifying the @cache hash under JRuby - even with a `synchronize do`
+          # block and a `Hash#dup` operation - has been known to cause issues
+          # with JRuby for concurrent access of the hash while it is being
+          # modified. The hash really only needs to be modified for the benefit
+          # of the security agent, so if JRuby is in play and the security agent
+          # is not, don't attempt to modify the hash at all and return early.
+          return @cache if NewRelic::LanguageSupport.jruby? && !Agent.config[:'security.agent.enabled']
+
           @lock.synchronize do
             preserved = @cache.dup.select { |_k, v| DEPENDENCY_DETECTION_VALUES.include?(v) }
             new_cache

test/new_relic/agent/configuration/manager_test.rb

@@ -537,6 +537,19 @@ def test_logger_does_not_receive_excluded_settings
       refute_includes(log, ':license_key')
     end
 
+    def test_reset_cache_return_early_for_jruby
+      phony_cache = {dup_called: false}
+      def phony_cache.dup; self[:dup_called] = true; self; end
+      @manager.instance_variable_set(:@cache, phony_cache)
+      NewRelic::LanguageSupport.stub :jruby?, true do
+        @manager.reset_cache
+      end
+
+      refute phony_cache[:dup_called], 'Expected the use of JRuby to prevent the Hash#dup call!'
+    ensure
+      @manager.new_cache
+    end
+
     private
 
     def assert_parsed_labels(expected)