GH-500: Workaround for non-serializable header (#503)

Fixes #500

When `listeners` are provided for `DefaultKafkaConsumerFactory`,
the target `KafkaConsumer` instance is proxied.
The `java.lang.reflect.Proxy` is `Serializable`,
but the value it is wrapping is not.
When the `MessageHeaders` is serialized (e.g. into persistent `MessageStore`),
it checks for `Serializable` type only on top-level object of the header.
Therefore, the `Proxy` is passing condition, but eventually we fail
with `NotSerializableException`, since the proxied object is not like that

* Remove `kafka_consumer` from a message before it reaches an aggregator
with its logic to serialize message into the store

This is a workaround until Spring for Apache Kafka is released
with the fix: spring-projects/spring-kafka#2822

functions/function/aggregator-function/src/main/java/org/springframework/cloud/fn/aggregator/AggregatorFunctionConfiguration.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2020-2022 the original author or authors.
+ * Copyright 2020-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -43,6 +43,7 @@
 import org.springframework.integration.channel.FluxMessageChannel;
 import org.springframework.integration.config.AggregatorFactoryBean;
 import org.springframework.integration.store.MessageGroupStore;
+import org.springframework.integration.support.MessageBuilder;
 import org.springframework.lang.Nullable;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageChannel;
@@ -67,7 +68,12 @@ public Function<Flux<Message<?>>, Flux<Message<?>>> aggregatorFunction(
 		FluxMessageChannel outputChannel
 	) {
 		return input -> Flux.from(outputChannel)
-			.doOnRequest((request) -> inputChannel.subscribeTo(input));
+			.doOnRequest((request) ->
+					inputChannel.subscribeTo(
+							input.map((inputMessage) ->
+									MessageBuilder.fromMessage(inputMessage)
+											.removeHeader("kafka_consumer")
+											.build())));
 	}
 
 	@Bean

functions/function/aggregator-function/src/test/java/org/springframework/cloud/fn/aggregator/RedisMessageStoreAggregatorTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2020-2020 the original author or authors.
+ * Copyright 2020-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,13 +16,16 @@
 
 package org.springframework.cloud.fn.aggregator;
 
+import java.io.ByteArrayInputStream;
+import java.io.InputStream;
 import java.time.Duration;
 import java.util.List;
 
 import org.junit.jupiter.api.Test;
 import reactor.core.publisher.Flux;
 import reactor.test.StepVerifier;
 
+import org.springframework.aop.framework.ProxyFactory;
 import org.springframework.cloud.fn.consumer.redis.RedisTestContainerSupport;
 import org.springframework.integration.IntegrationMessageHeaderAccessor;
 import org.springframework.integration.redis.store.RedisMessageStore;
@@ -46,11 +49,14 @@ static void redisProperties(DynamicPropertyRegistry registry) {
 
 	@Test
 	public void test() {
+		InputStream fakeNonSerializableKafkaConsumer = new ByteArrayInputStream(new byte[0]);
+
 		Flux<Message<?>> input =
 			Flux.just(MessageBuilder.withPayload("2")
 					.setHeader(IntegrationMessageHeaderAccessor.CORRELATION_ID, "my_correlation")
 					.setHeader(IntegrationMessageHeaderAccessor.SEQUENCE_NUMBER, 2)
 					.setHeader(IntegrationMessageHeaderAccessor.SEQUENCE_SIZE, 2)
+					.setHeader("kafka_consumer", new ProxyFactory(fakeNonSerializableKafkaConsumer).getProxy())
 					.build(),
 				MessageBuilder.withPayload("1")
 					.setHeader(IntegrationMessageHeaderAccessor.CORRELATION_ID, "my_correlation")