Use Dispatchers.Unconfined for Coroutines

As of Coroutines 1.2.0-alpha, Dispatchers.Unconfined
is a stable API so we can leverage it in order to get
better performances in our Reactive to Coroutines
bridge.

See gh-19975

Author: sdeleuze

Diff for: spring-core-coroutines/src/main/kotlin/org/springframework/core/CoroutinesUtils.kt

@@ -18,6 +18,7 @@
 package org.springframework.core
 
 import kotlinx.coroutines.Deferred
+import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.GlobalScope
 import kotlinx.coroutines.async
 import kotlinx.coroutines.reactive.awaitFirstOrNull
@@ -36,15 +37,17 @@ import kotlin.reflect.jvm.kotlinFunction
  * @author Sebastien Deleuze
  * @since 5.2
  */
-internal fun <T: Any> deferredToMono(source: Deferred<T>) = GlobalScope.mono { source.await() }
+internal fun <T: Any> deferredToMono(source: Deferred<T>) =
+		GlobalScope.mono(Dispatchers.Unconfined) { source.await() }
 
 /**
  * Convert a [Mono] instance to a [Deferred] one.
  *
  * @author Sebastien Deleuze
  * @since 5.2
  */
-internal fun <T: Any> monoToDeferred(source: Mono<T>) = GlobalScope.async { source.awaitFirstOrNull() }
+internal fun <T: Any> monoToDeferred(source: Mono<T>) =
+		GlobalScope.async(Dispatchers.Unconfined) { source.awaitFirstOrNull() }
 
 /**
  * Invoke an handler method converting suspending method to [Mono] if necessary.
@@ -55,7 +58,8 @@ internal fun <T: Any> monoToDeferred(source: Mono<T>) = GlobalScope.async { sour
 internal fun invokeHandlerMethod(method: Method, bean: Any, vararg args: Any?): Any? {
 	val function = method.kotlinFunction!!
 	return if (function.isSuspend) {
-		GlobalScope.mono { function.callSuspend(bean, *args.sliceArray(0..(args.size-2)))
+		GlobalScope.mono(Dispatchers.Unconfined) {
+			function.callSuspend(bean, *args.sliceArray(0..(args.size-2)))
 				.let { if (it == Unit) null else it} }
 				.onErrorMap(InvocationTargetException::class) { it.targetException }
 	}

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

@@ -16,6 +16,7 @@
 
 package org.springframework.web.reactive.function.client
 
+import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.GlobalScope
 import kotlinx.coroutines.reactive.awaitSingle
 import kotlinx.coroutines.reactor.mono
@@ -77,7 +78,7 @@ suspend fun WebClient.RequestHeadersSpec<out WebClient.RequestHeadersSpec<*>>.aw
  * @since 5.2
  */
 inline fun <reified T: Any> WebClient.RequestBodySpec.body(crossinline supplier: suspend () -> T)
-		= body(GlobalScope.mono { supplier.invoke() })
+		= body(GlobalScope.mono(Dispatchers.Unconfined) { supplier.invoke() })
 
 /**
  * Coroutines variant of [WebClient.ResponseSpec.bodyToMono].

Diff for: spring-webflux/src/main/kotlin/org/springframework/web/reactive/function/server/CoRouterFunctionDsl.kt

@@ -16,6 +16,7 @@
 
 package org.springframework.web.reactive.function.server
 
+import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.GlobalScope
 import kotlinx.coroutines.reactor.mono
 import org.springframework.core.io.Resource
@@ -389,7 +390,7 @@ open class CoRouterFunctionDsl(private val init: (CoRouterFunctionDsl.() -> Unit
      */
     fun resources(lookupFunction: suspend (ServerRequest) -> Resource?) {
         builder.resources {
-            GlobalScope.mono {
+            GlobalScope.mono(Dispatchers.Unconfined) {
                 lookupFunction.invoke(it)
             }
         }
@@ -404,7 +405,7 @@ open class CoRouterFunctionDsl(private val init: (CoRouterFunctionDsl.() -> Unit
     }
 
     private fun asHandlerFunction(init: suspend (ServerRequest) -> ServerResponse) = HandlerFunction {
-        GlobalScope.mono {
+        GlobalScope.mono(Dispatchers.Unconfined) {
             init(it)
         }
     }

Diff for: spring-webflux/src/main/kotlin/org/springframework/web/reactive/server/ServerWebExchangeExtensions.kt

@@ -16,6 +16,7 @@
 
 package org.springframework.web.reactive.server
 
+import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.GlobalScope
 import kotlinx.coroutines.reactive.awaitSingle
 import kotlinx.coroutines.reactor.mono
@@ -68,4 +69,4 @@ suspend fun ServerWebExchange.awaitSession(): WebSession =
  * @since 5.2
  */
 fun ServerWebExchange.Builder.principal(supplier: suspend () -> Principal): ServerWebExchange.Builder
-        = principal(GlobalScope.mono { supplier.invoke() })
+        = principal(GlobalScope.mono(Dispatchers.Unconfined) { supplier.invoke() })