Add Coroutines support for WebClient and WebFlux.fn

This commit is the first part of a more complete Coroutines
support coming in Spring Framework 5.2. It introduces suspendable
Kotlin extensions for Mono based methods in WebFlux classes like
WebClient, ServerRequest, ServerResponse as well as a Coroutines
router usable via `coRouter { }`.

Coroutines extensions use `await` prefix or `AndAwait` suffix,
and most are using names close to their Reactive counterparts,
except `exchange` in `WebClient.RequestHeadersSpec`
which translates to `awaitResponse`.

Upcoming expected changes are:
 - Leverage `Dispatchers.Unconfined` (Kotlin/kotlinx.coroutines#972)
 - Expose extensions for `Flux` based API (Kotlin/kotlinx.coroutines#254)
 - Introduce interop with `CoroutineContext` (Kotlin/kotlinx.coroutines#284)
 - Support Coroutines in `ReactiveAdapterRegistry`
 - Support Coroutines for WebFlux annotated controllers
 - Fix return type of Kotlin suspending functions (gh-21058)

See gh-19975

spring-webflux/spring-webflux.gradle

@@ -40,6 +40,8 @@ dependencies {
 	optional("org.jetbrains.kotlin:kotlin-reflect:${kotlinVersion}")
 	optional("org.jetbrains.kotlin:kotlin-stdlib:${kotlinVersion}")
 	optional("com.google.protobuf:protobuf-java-util:3.6.1")
+	optional("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.1.1")
+	optional("org.jetbrains.kotlinx:kotlinx-coroutines-reactor:1.1.1")
 	testCompile("javax.xml.bind:jaxb-api:2.3.1")
 	testCompile("com.fasterxml:aalto-xml:1.1.1")
 	testCompile("org.hibernate:hibernate-validator:6.0.14.Final")

spring-webflux/src/main/kotlin/org/springframework/web/reactive/function/client/ClientResponseExtensions.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,6 +16,7 @@
 
 package org.springframework.web.reactive.function.client
 
+import kotlinx.coroutines.reactive.awaitSingle
 import org.springframework.core.ParameterizedTypeReference
 import org.springframework.http.ResponseEntity
 import reactor.core.publisher.Flux
@@ -64,3 +65,30 @@ inline fun <reified T : Any> ClientResponse.toEntity(): Mono<ResponseEntity<T>>
  */
 inline fun <reified T : Any> ClientResponse.toEntityList(): Mono<ResponseEntity<List<T>>> =
 		toEntityList(object : ParameterizedTypeReference<T>() {})
+
+/**
+ * Coroutines variant of [ClientResponse.bodyToMono].
+ *
+ * @author Sebastien Deleuze
+ * @since 5.2
+ */
+suspend inline fun <reified T : Any> ClientResponse.awaitBody(): T =
+		bodyToMono<T>().awaitSingle()
+
+/**
+ * Coroutines variant of [ClientResponse.toEntity].
+ *
+ * @author Sebastien Deleuze
+ * @since 5.2
+ */
+suspend inline fun <reified T : Any> ClientResponse.awaitEntity(): ResponseEntity<T> =
+		toEntity<T>().awaitSingle()
+
+/**
+ * Coroutines variant of [ClientResponse.toEntityList].
+ *
+ * @author Sebastien Deleuze
+ * @since 5.2
+ */
+suspend inline fun <reified T : Any> ClientResponse.awaitEntityList(): ResponseEntity<List<T>> =
+		toEntityList<T>().awaitSingle()

spring-webflux/src/main/kotlin/org/springframework/web/reactive/function/client/WebClientExtensions.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,6 +16,9 @@
 
 package org.springframework.web.reactive.function.client
 
+import kotlinx.coroutines.GlobalScope
+import kotlinx.coroutines.reactive.awaitSingle
+import kotlinx.coroutines.reactor.mono
 import org.reactivestreams.Publisher
 import org.springframework.core.ParameterizedTypeReference
 import org.springframework.web.reactive.function.client.WebClient.RequestBodySpec
@@ -57,3 +60,30 @@ inline fun <reified T : Any> WebClient.ResponseSpec.bodyToMono(): Mono<T> =
  */
 inline fun <reified T : Any> WebClient.ResponseSpec.bodyToFlux(): Flux<T> =
 		bodyToFlux(object : ParameterizedTypeReference<T>() {})
+
+/**
+ * Coroutines variant of [WebClient.RequestHeadersSpec.exchange].
+ *
+ * @author Sebastien Deleuze
+ * @since 5.2
+ */
+suspend fun WebClient.RequestHeadersSpec<out WebClient.RequestHeadersSpec<*>>.awaitResponse(): ClientResponse =
+		exchange().awaitSingle()
+
+/**
+ * Coroutines variant of [WebClient.RequestBodySpec.body].
+ *
+ * @author Sebastien Deleuze
+ * @since 5.2
+ */
+inline fun <reified T: Any> WebClient.RequestBodySpec.body(crossinline supplier: suspend () -> T)
+		= body(GlobalScope.mono { supplier.invoke() })
+
+/**
+ * Coroutines variant of [WebClient.ResponseSpec.bodyToMono].
+ *
+ * @author Sebastien Deleuze
+ * @since 5.2
+ */
+suspend inline fun <reified T : Any> WebClient.ResponseSpec.awaitBody() : T =
+		bodyToMono<T>().awaitSingle()