Coroutine context propagation for Reactor to coroutines API migration

[SokolovaMaria]

We've supported the following cases of the coroutine context propagation for Reactor to coroutines API migration:

• kotlinx.coroutines.reactive contains extension functions for (Mono/Flux) to suspending function: suspend fun <T> Publisher<T>.await*(): T?. Coroutine context of these await functions is propagated into Publisher's context. Consider the use case:

suspend fun foo() {
    ...
    bar().awaitFirst()
}

fun bar(): Mono<T> { ... } 

• The same idea introduced for migration from Reactor's Publisher to Flow: suspending collect call propagates it's coroutine context to the source Publisher context.

// function that uses new coroutines Flow API
suspend fun bar(): Flow<T> {
     return oldBar().asFlow()
}
// legacy function that returns Publisher
fun oldBar(): Flux<T> {...}

suspend fun foo() {
     bar().collect() 
}

This propagation is implemented the following way:

• We provide ContextInjector service API that contains the only function to inject the given coroutine context into the Publisher:

public interface ContextInjector {
    public fun <T> injectCoroutineContext(publisher: Publisher<T>, coroutineContext: CoroutineContext): Publisher<T>
}

• reactor module contains the implementation of this interface (ReactorContextInjector) that enriches Reactor's Mono/Flux contexts with values from coroutineContext[ReactorContext], instances of other Publisher types are returned unchanged.

• Publisher<T>.await*() functions search for the available ContextInjector services (if reactor module is included - ReactorContextInjector is available, otherwise no services found) and enrich Publisher's context if possible.
  PublisherAsFlow injects the coroutine context of collect call the same way.

Available services are searched once, if reactor module is not included - the list of services is empty, so no overhead added.

[elizarov]

I've resolved some of the issues, squashed commits, dropped .flow package from reactive modules, remove @JvmName("from") from conversion methods (as it is not used in other conversion methods of reactive module), but moved them to a more future-proof FlowKt JVM class.

Force-pushed

[qwwdfsad]

Integration in produceImpl is missing

[qwwdfsad]

LGTM

[elizarov]

Look great now.