Add support for Kotlin Coroutines

[jamesward]

I have a basic Kotlin coroutine controller method:

	@GetMapping("/")
	suspend fun index() = run {
		"aasdf"
	}

Works fine in the JVM but fails at runtime when using a native image (org.springframework.experimental:spring-graalvm-native:0.8.4):

java.lang.IllegalStateException: No primary or single public constructor found for interface kotlin.coroutines.Continuation - and no default constructor found either
	at org.springframework.beans.BeanUtils.getResolvableConstructor(BeanUtils.java:250) ~[na:na]

Without the suspend it works as expected:

	@GetMapping("/")
	fun index(): String {
		return "asdf"
	}

[jamesward]

Here is a project that reproduces the error (graalvm branch):
https://github.com/jamesward/springboot-kotlin-reactive-demo/tree/graalvm

[sdeleuze]

If have switch KotlinDetector initialization to build time and order to fix that, then added Continuation related reflection entries but that seems not sufficient, see https://github.com/spring-projects-experimental/spring-graalvm-native/tree/webflux-kotlin.

The error I get, maybe resolvable with deeper work, is:

java.lang.IllegalArgumentException: Callable expects 2 arguments, but 1 were provided.
	at kotlin.reflect.jvm.internal.calls.Caller$DefaultImpls.checkArguments(Caller.kt:20) ~[na:na]

I suspect kotlin-reflect to be too reflection based and big to be really a good fit for native, so in parrallel on working on that I have created spring-projects/spring-framework#21546 which would be an ambitious but IMO promising way to solve that.

[sdeleuze]

For the time being, I have documented that Coroutines are not supported yet.

[sdeleuze]

We should be able to use Coroutines with proper documentation (add required entries with @NativeHint) until KT-44594 fix is available in Kotlin 1.5-M2. To be check with a modified version of webmvc-kotlin that uses WebFlux and Coroutines. Depends on #453.