Add Kotlin extension for registering suspend functions in BatchLoaderRegistry

[tomirio619]

Hey!

TLDR: add kotlin extension function to BatchLoaderRegistry to simplify registration of a suspend function.

Request

If you want to register a "mapped batch loader" in Spring GraphQL, you must/can provide a function that returns a Mono(source).

To register a suspend function, you can provide this function as: mono() { /*suspend/* } .
There is a problem with this approach: it fails to properly propagate/restore ThreadLocalAccessor values (and also tracing information I think).

You can reproduce this if you access SecurityContextHolder.getContext().authentication before and after the mono call.
If authentication used to be non-null before the mono call (e.g. TestingAuthenticationToken), it will suddenly be null after it.

In Spring Framework 7, a coroutine element called PropagationContextElement() was introduced to solve this (docs).

It would be nice to have kotlin extensions to register a suspend function in the correct way:

fun <K, V> BatchLoaderRegistry.RegistrationSpec<K, V>.registerSuspendMappedBatchLoader(
    dispatcher: CoroutineDispatcher = Executors.newVirtualThreadPerTaskExecutor().asCoroutineDispatcher(),
    batchLoader: suspend (keys: Set<K>, env: BatchLoaderEnvironment) -> Map<K, V>,
) {
    this.registerMappedBatchLoader { keys: Set<K>, env: BatchLoaderEnvironment ->
        mono(dispatcher + PropagationContextElement()) {
            batchLoader(keys, env)
        }
    }
}

fun <K, V> BatchLoaderRegistry.RegistrationSpec<K, V>.registerSuspendMappedBatchLoader(
    dispatcher: CoroutineDispatcher = Dispatchers.Default,
    batchLoader: suspend (keys: Set<K>) -> Map<K, V>,
) {
    this.registerMappedBatchLoader { keys: Set<K>, _: BatchLoaderEnvironment ->
        mono(dispatcher + PropagationContextElement()) {
            batchLoader(keys)
        }
    }
}

NOTE: I'm not sure about the default value of the dispatcher though.

Could we add such an extension function to Spring GraphQL?