JRuby fix for concurrency issue with manager cache

The configuration manager's alteration of its internal cache hash to
preserve previously dynamically defined values has been known to cause
problems with JRuby given its concurrent access to the hash object. We
have used `synchronize` and `Hash#dup` fixes previously, but still have
reports of issues.

When it comes down to it, the alteration of the hash was only added and
still only needed for use by the security agent. So let's just not even
attempt to alter the hash if JRuby is in play but the security agent is
not.

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)