Introduce CoWebFilter

This commit introduces the CoWebFilter, an abstract WebFilter
implementation that allows for coroutine usage.

Closes gh-29672

spring-web/src/main/kotlin/org/springframework/web/server/CoWebFilter.kt

@@ -0,0 +1,66 @@
+/*
+ * Copyright 2002-2023 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.web.server
+
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.reactor.awaitSingleOrNull
+import kotlinx.coroutines.reactor.mono
+import reactor.core.publisher.Mono
+
+/**
+ * Kotlin-specific implementation of the [WebFilter] interface that allows for
+ * using co-routines.
+ *
+ * @author Arjen Poutsma
+ * @since 6.0.5
+ */
+abstract class CoWebFilter : WebFilter {
+
+	final override fun filter(exchange: ServerWebExchange, chain: WebFilterChain): Mono<Void> {
+		return mono(Dispatchers.Unconfined) {
+			filter(exchange, object : CoWebFilterChain {
+				override suspend fun filter(exchange: ServerWebExchange) {
+					return chain.filter(exchange).cast(Unit.javaClass).awaitSingleOrNull() ?: Unit
+				}
+			})}.then()
+	}
+
+	/**
+	 * Process the Web request and (optionally) delegate to the next
+	 * `WebFilter` through the given [WebFilterChain].
+	 * @param exchange the current server exchange
+	 * @param chain provides a way to delegate to the next filter
+	 */
+	protected abstract suspend fun filter(exchange: ServerWebExchange, chain: CoWebFilterChain)
+
+}
+
+/**
+ * Kotlin-specific adaption of [WebFilterChain] that allows for co-routines.
+ *
+ * @author Arjen Poutsma
+ * @since 6.0.5
+ */
+interface CoWebFilterChain {
+
+	/**
+	 * Delegate to the next [WebFilter] in the chain.
+	 * @param exchange the current server exchange
+	 */
+	suspend fun filter(exchange: ServerWebExchange)
+
+}