Regression: Function returning List<Message<>> does not produce multiple messages in 4.2.2, but did in 4.2.0

[lbilger]

I have the following simple reproducer application:

package com.example.demo

import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.runApplication
import org.springframework.context.annotation.Bean
import org.springframework.messaging.Message
import org.springframework.messaging.MessageHeaders
import org.springframework.messaging.support.MessageBuilder
import org.springframework.stereotype.Component

@SpringBootApplication
class DemoApplication {
    @Bean
    fun kafkaProcessorBean(): (String) -> List<Message<Char>> = { it.map { MessageBuilder.createMessage(it, MessageHeaders(emptyMap())) } }
}

@Component
class KafkaProcessorComponent : (String) -> List<Message<Char>> {
    override fun invoke(input: String): List<Message<Char>> {
        return input.map { MessageBuilder.createMessage(it, MessageHeaders(emptyMap())) }
    }
}

fun main(args: Array<String>) {
    runApplication<DemoApplication>(*args)
}

with spring-cloud-stream-kafka on the classpath and this configuration:

spring.cloud.function.definition=kafkaProcessorBean;kafkaProcessorComponent

When I run this application with Spring Cloud 2024.0.1 and write a message into kafkaProcessorComponent-in-0, I get only one output message in kafkaProcessorComponent-out-0:

kcat -b localhost -t kafkaProcessorComponent-in-0 -P <<< bla

# kafkaProcessorComponent-out-0 now contains:
[{"payload":"b","headers":{"id":"70a871ab-556e-b618-fd62-72e790cc0c62","timestamp":1744104346397}},{"payload":"l","headers":{"id":"bb009a60-0f8f-15ac-bb7d-d14ef372f8b1","timestamp":1744104346397}},{"payload":"a","headers":{"id":"4064ed6a-21f0-308f-a7d7-5a6a545e7d7e","timestamp":1744104346397}}]

With 2024.0.0, I get (as expected) three output messages:

"b"                                                                                                                                                                 
"l"
"a"

I played around with the versions and found out that Spring Cloud 2024.0.0 with spring-cloud-function 4.2.2 also has this problem, while Spring Cloud 2024.0.1 with spring-cloud-function 4.2.0 does not. That's why I'm reporting this here and not in spring-cloud-stream.

The kafkaProcessorBean function works as expected in all versions:

kcat -b localhost -t kafkaProcessorBean-in-0 -P <<< bla

# kafkaProcessorBean-out-0 now contains:
"b"                                                                                                                                                                 
"l"
"a"

[olegz]

So basically you are saying that @Component style bean is the issue, correct?

[lbilger]

Yes, exactly. But only since 4.2.1 (just tried it out: works in 4.2.0, broken in 4.2.1)

[olegz]

It appears to be Kotlin related issue as it works well with java regardless of the version.

Looking. . .

[olegz]

The problem is indeed related to Kotlin's type discovery and yes it is a regression as we made some major refactoring in type discovery logic and there were few Kotlin regressions reported/fixed already. Hoping this is the last one.