Enhance Kotlin Support for Suppliers, Functions, and Consumers

[apoupard]

Description:

This issue tracks the need to improve and solidify Kotlin support within Spring Cloud Function, aiming for seamless integration with various Kotlin features like suspend functions, Flow types, and different functional arities/signatures.

Recent compatibility testing, including tests such as KotlinConsumerArityBeanTest, KotlinFunctionArityBeanTest, and KotlinSupplierArityBeanTest (commit related to these tests: "Test: Add arity tests for Kotlin functions, consumers, and suppliers"), revealed several areas where Kotlin function signatures are not handled as expected. This has led to runtime exceptions and test failures. The recent test execution for these arity bean tests showed 43 failures and 3 errors out of 118 total tests, highlighting significant areas for improvement.

The goal is to address these failures and ensure robust support for the wide range of functional styles available in Kotlin.

Tested Signature Compatibility Summary:

The following tables summarize the pass/fail status for the tested Kotlin signatures based on the recent test execution. "FAIL" indicates that at least one test case for that signature (or its variants, like Java-style/Kotlin-style implementations where applicable) resulted in a failure or error.

Suppliers
(Based on tests like KotlinSupplierArityBeanTest)

#	Signature	Bean Name Suffix	Status
1	() -> R	Plain	PASS
2	() -> Flow<R>	Flow	PASS
3	suspend () -> R	SuspendPlain	FAIL
4	suspend () -> Flow<R>	SuspendFlow	PASS
5	() -> Mono<R>	Mono	PASS
6	() -> Flux<R>	Flux	PASS
7	() -> Message<R>	Message	PASS
8	() -> Mono<Message<R>>	MonoMessage	PASS
9	suspend () -> Message<R>	SuspendMessage	FAIL
10	() -> Flux<Message<R>>	FluxMessage	PASS
11	() -> Flow<Message<R>>	FlowMessage	FAIL
12	suspend () -> Flow<Message<R>>	SuspendFlowMessage	PASS

Functions
(Based on tests like KotlinFunctionArityBeanTest)

#	Signature	Bean Name Suffix	Status
1	(T) -> R	PlainToPlain	PASS
2	(T) -> Flow<R>	PlainToFlow	PASS
3	(Flow<T>) -> R	FlowToPlain	FAIL
4	(Flow<T>) -> Flow<R>	FlowToFlow	FAIL
5	suspend (T) -> R	SuspendPlainToPlain	FAIL
6	suspend (T) -> Flow<R>	SuspendPlainToFlow	FAIL
7	suspend (Flow<T>) -> R	SuspendFlowToPlain	FAIL
8	suspend (Flow<T>) -> Flow<R>	SuspendFlowToFlow	FAIL
9	(T) -> Mono<R>	PlainToMono	PASS
10	(T) -> Flux<R>	PlainToFlux	PASS
11	(Mono<T>) -> Mono<R>	MonoToMono	PASS
12	(Flux<T>) -> Flux<R>	FluxToFlux	PASS
13	(Flux<T>) -> Mono<R>	FluxToMono	PASS
14	(Message<T>) -> Message<R>	MessageToMessage	PASS
15	suspend (Message<T>) -> Message<R>	SuspendMessageToMessage	FAIL
16	(Mono<Message<T>>) -> Mono<Message<R>>	MonoMessageToMonoMessage	PASS
17	(Flux<Message<T>>) -> Flux<Message<R>>	FluxMessageToFluxMessage	PASS
18	(Flow<Message<T>>) -> Flow<Message<R>>	FlowMessageToFlowMessage	FAIL
19	suspend (Flow<Message<T>>) -> Flow<Message<R>>	SuspendFlowMessageToFlowMessage	FAIL

Consumers
(Based on tests like KotlinConsumerArityBeanTest)

#	Signature	Bean Name Suffix	Status
1	(T) -> Unit	Plain	PASS
2	(Flow<T>) -> Unit	Flow	FAIL
3	suspend (T) -> Unit	SuspendPlain	FAIL
4	suspend (Flow<T>) -> Unit	SuspendFlow	PASS
5	(T) -> Mono<Void>	MonoInput	PASS
6	(Mono<T>) -> Mono<Void>	Mono	PASS
7	(Flux<T>) -> Mono<Void>	Flux	PASS
8	(Message<T>) -> Unit	Message	PASS
9	(Mono<Message<T>>) -> Mono<Void>	MonoMessage	PASS
10	suspend (Message<T>) -> Unit	SuspendMessage	FAIL
11	(Flux<Message<T>>) -> Unit	FluxMessage	PASS
12	(Flow<Message<T>>) -> Unit	FlowMessage	FAIL
13	suspend (Flow<Message<T>>) -> Unit	SuspendFlowMessage	PASS

Addressing these failures is crucial for providing a first-class Kotlin experience in Spring Cloud Function.

[apoupard]

Just wanted to update that a Merge Request has been created to address the Kotlin arity issues detailed in this issue: #1278

This MR includes the necessary changes to fix all the described arity issues for Kotlin functions, consumers, and suppliers.