ConcurrentModificationException When IntegrationFlow uses gateway

[venkatg-v]

In what version(s) of Spring Integration are you seeing this issue?

6.5.1

Describe the bug

It appears that MockIntegrationContext.autoStartupCandidates is updated when SpringIntegrationTestExecutionListener#prepareTestInstance is called causing a ConcurrentModificationException. This happens because the method's iteration over AbstractEndpoints triggers GatewayMessageHandler#initialize for any IntegrationFlows with a gateway, which in turn calls MockIntegrationContext#postProcessBeforeInitialization to add a new candidate.

Note: in #10018 the GatewayMessageHandler#start method was updated which indirectly results in MockIntegrationContext#postProcessBeforeInitialization to be called so I did not see this issue in Spring Integration 6.4.x.

To Reproduce

1. Create an IntegrationFlow where it uses a gateway.
2. Create a Test with annotations @SpringBootTest and @SpringIntegrationTest.
3. Run the test and you will see java.util.ConcurrentModificationException from org.springframework.integration.test.context.SpringIntegrationTestExecutionListener#prepareTestInstance

Expected behavior

The test should not run into a ConcurrentModificationException. Prior to #10018 we did not call MockIntegrationContext#postProcessBeforeInitialization when a gateway was found on an IntegrationFlow.

Sample

Sample Project

[artembilan]

Thank you for great issue!
So, the problem appears here:

		this.autoStartupCandidates = mockIntegrationContext.getAutoStartupCandidates();
		this.autoStartupCandidates.stream()
				.filter(endpoint -> !match(endpoint.getBeanName(), patterns))
				.peek(endpoint -> endpoint.setAutoStartup(true))
				.forEach(AbstractEndpoint::start);

That AbstractEndpoint::start causes to the collection to be modified due to the mentioned postProcessBeforeInitialization called from the GatewayMessageHandler#initialize.

It doesn't look like we need to expose an internal GatewayProxyFactoryBean from that GatewayMessageHandler to the application context at all.

I've some change and it confirms that no ConcurrentModificationException any more.
Observing the rest of tests in the project...

[venkatg-v]

Thanks @artembilan for quickly taking a look at this issue!