Add coroutine variant of WebExceptionHandler

See gh-32931

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

@@ -0,0 +1,15 @@
+package org.springframework.web.server
+
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.reactor.mono
+import reactor.core.publisher.Mono
+import kotlin.coroutines.CoroutineContext
+
+abstract class CoWebExceptionHandler : WebExceptionHandler {
+	final override fun handle(exchange: ServerWebExchange, ex: Throwable): Mono<Void> {
+		val context = exchange.attributes[CoWebFilter.COROUTINE_CONTEXT_ATTRIBUTE] as CoroutineContext?
+		return mono(context ?: Dispatchers.Unconfined) { coHandle(exchange, ex) }.then()
+	}
+
+	protected abstract suspend fun coHandle(exchange: ServerWebExchange, ex: Throwable)
+}

spring-web/src/test/kotlin/org/springframework/web/server/CoWebExceptionHandlerTests.kt

@@ -0,0 +1,28 @@
+package org.springframework.web.server
+
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+import org.springframework.web.testfixture.http.server.reactive.MockServerHttpRequest
+import org.springframework.web.testfixture.server.MockServerWebExchange
+import reactor.test.StepVerifier
+
+class CoWebExceptionHandlerTest {
+	@Test
+	fun handle() {
+		val exchange = MockServerWebExchange.from(MockServerHttpRequest.get("https://example.com"))
+		val ex = RuntimeException()
+
+		val handler = MyCoWebExceptionHandler()
+		val result = handler.handle(exchange, ex)
+
+		StepVerifier.create(result).verifyComplete()
+
+		assertThat(exchange.attributes["foo"]).isEqualTo("bar")
+	}
+}
+
+private class MyCoWebExceptionHandler : CoWebExceptionHandler() {
+	override suspend fun coHandle(exchange: ServerWebExchange, ex: Throwable) {
+		exchange.attributes["foo"] = "bar"
+	}
+}