Move reactive server instrumentation out of WebFilter

Prior to this commit, the Observation instrumentation for Reactive
server applications was implemented with a `WebFilter`. This allowed to
record observations and set up a tracing context for the controller
handlers.

The limitation of this approach is that all processing happening at a
lower level is not aware of any observation. Here, the
`HttpWebHandlerAdapter` handles several interesting aspects:

* logging of HTTP requests and responses at the TRACE level
* logging of client disconnect errors
* handling of unresolved errors

With the current instrumentation, these logging statements will miss the
tracing context information. As a result, this commit deprecates the
`ServerHttpObservationFilter` in favor of a more direct instrumentation
of the `HttpWebHandlerAdapter`. This enables a more precise
instrumentattion and allows to set up the current observation earlier in
the reactor context: log statements will now contain the relevant
information.

Fixes gh-30013

spring-web/src/test/java/org/springframework/web/filter/reactive/ServerHttpObservationFilterTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * 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.
@@ -40,6 +40,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  *
  * @author Brian Clozel
  */
+@SuppressWarnings("removal")
 class ServerHttpObservationFilterTests {
 
 	private final TestObservationRegistry observationRegistry = TestObservationRegistry.create();

spring-web/src/test/java/org/springframework/web/server/adapter/HttpWebHandlerAdapterObservabilityTests.java

@@ -0,0 +1,158 @@
+/*
+ * 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.adapter;
+
+
+import java.util.List;
+import java.util.Optional;
+
+import io.micrometer.observation.contextpropagation.ObservationThreadLocalAccessor;
+import io.micrometer.observation.tck.TestObservationRegistry;
+import io.micrometer.observation.tck.TestObservationRegistryAssert;
+import org.junit.jupiter.api.Test;
+import reactor.core.publisher.Mono;
+import reactor.test.StepVerifier;
+import reactor.util.context.ContextView;
+
+import org.springframework.http.HttpStatus;
+import org.springframework.http.server.reactive.observation.ServerRequestObservationContext;
+import org.springframework.web.server.ServerWebExchange;
+import org.springframework.web.server.WebExceptionHandler;
+import org.springframework.web.server.WebHandler;
+import org.springframework.web.server.handler.ExceptionHandlingWebHandler;
+import org.springframework.web.testfixture.http.server.reactive.MockServerHttpRequest;
+import org.springframework.web.testfixture.http.server.reactive.MockServerHttpResponse;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Observability-related tests for {@link HttpWebHandlerAdapter}.
+ * @author Brian Clozel
+ */
+class HttpWebHandlerAdapterObservabilityTests {
+
+	private final TestObservationRegistry observationRegistry = TestObservationRegistry.create();
+
+	private final MockServerHttpRequest request = MockServerHttpRequest.post("/test/resource").build();
+
+	private final MockServerHttpResponse response = new MockServerHttpResponse();
+
+	@Test
+	void handlerShouldSetObservationContextOnExchange() {
+		HttpStatusSuccessStubWebHandler targetHandler = new HttpStatusSuccessStubWebHandler(HttpStatus.OK);
+		createWebHandler(targetHandler).handle(this.request, this.response).block();
+		assertThat(targetHandler.observationContext).isPresent();
+		assertThat(targetHandler.observationContext.get().getCarrier()).isEqualTo(this.request);
+		assertThat(targetHandler.observationContext.get().getResponse()).isEqualTo(this.response);
+		assertThatHttpObservation().hasLowCardinalityKeyValue("outcome", "SUCCESS");
+	}
+
+	@Test
+	void handlerShouldSetCurrentObservationInReactorContext() {
+		ReactorContextWebHandler targetHandler = new ReactorContextWebHandler();
+		createWebHandler(targetHandler).handle(this.request, this.response).block();
+		assertThat(targetHandler.contextView.getOrEmpty(ObservationThreadLocalAccessor.KEY)).isPresent();
+		assertThatHttpObservation().hasLowCardinalityKeyValue("outcome", "SUCCESS");
+	}
+
+	@Test
+	void handlerShouldSeeHandledException() {
+		ThrowingExceptionWebHandler throwingHandler = new ThrowingExceptionWebHandler(new IllegalStateException("testing error"));
+		ExceptionHandlingWebHandler targetHandler = new ExceptionHandlingWebHandler(throwingHandler, List.of(new BadRequestExceptionHandler()));
+		createWebHandler(targetHandler).handle(this.request, this.response).block();
+		assertThat(throwingHandler.observationContext).isPresent();
+		assertThat(throwingHandler.observationContext.get().getError()).isInstanceOf(IllegalStateException.class);
+		assertThatHttpObservation().hasLowCardinalityKeyValue("outcome", "CLIENT_ERROR");
+	}
+
+	@Test
+	void handlerShouldRecordObservationWhenCancelled() {
+		HttpStatusSuccessStubWebHandler targetHandler = new HttpStatusSuccessStubWebHandler(HttpStatus.OK);
+		StepVerifier.create(createWebHandler(targetHandler).handle(this.request, this.response)).thenCancel().verify();
+		assertThatHttpObservation().hasLowCardinalityKeyValue("outcome", "UNKNOWN");
+	}
+
+	private HttpWebHandlerAdapter createWebHandler(WebHandler targetHandler) {
+		HttpWebHandlerAdapter handlerAdapter = new HttpWebHandlerAdapter(targetHandler);
+		handlerAdapter.setObservationRegistry(this.observationRegistry);
+		return handlerAdapter;
+	}
+
+	private TestObservationRegistryAssert.TestObservationRegistryAssertReturningObservationContextAssert assertThatHttpObservation() {
+		return TestObservationRegistryAssert.assertThat(this.observationRegistry)
+				.hasObservationWithNameEqualTo("http.server.requests").that();
+	}
+
+	private static class HttpStatusSuccessStubWebHandler implements WebHandler {
+
+		private final HttpStatus responseStatus;
+
+		private Optional<ServerRequestObservationContext> observationContext;
+
+		public HttpStatusSuccessStubWebHandler(HttpStatus responseStatus) {
+			this.responseStatus = responseStatus;
+		}
+
+		@Override
+		public Mono<Void> handle(ServerWebExchange exchange) {
+			this.observationContext = ServerRequestObservationContext.findCurrent(exchange);
+			exchange.getResponse().setStatusCode(this.responseStatus);
+			return Mono.empty();
+		}
+	}
+
+	private static class ReactorContextWebHandler implements WebHandler {
+
+		ContextView contextView;
+
+		@Override
+		public Mono<Void> handle(ServerWebExchange exchange) {
+			exchange.getResponse().setStatusCode(HttpStatus.OK);
+			return Mono.deferContextual(contextView -> {
+				this.contextView = contextView;
+				return Mono.empty();
+			});
+		}
+	}
+
+	private static class ThrowingExceptionWebHandler implements WebHandler {
+
+		private final Throwable exception;
+
+		private Optional<ServerRequestObservationContext> observationContext;
+
+		private ThrowingExceptionWebHandler(Throwable exception) {
+			this.exception = exception;
+		}
+
+		@Override
+		public Mono<Void> handle(ServerWebExchange exchange) {
+			this.observationContext = ServerRequestObservationContext.findCurrent(exchange);
+			return Mono.error(this.exception);
+		}
+	}
+
+	private static class BadRequestExceptionHandler implements WebExceptionHandler {
+
+		@Override
+		public Mono<Void> handle(ServerWebExchange exchange, Throwable ex) {
+			exchange.getResponse().setStatusCode(HttpStatus.BAD_REQUEST);
+			return exchange.getResponse().setComplete();
+		}
+	}
+
+}

spring-web/src/test/java/org/springframework/web/server/handler/ExceptionHandlingWebHandlerTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * 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.
@@ -45,13 +45,13 @@ public class ExceptionHandlingWebHandlerTests {
 
 
 	@Test
-	public void handleErrorSignal() throws Exception {
+	void handleErrorSignal() {
 		createWebHandler(new BadRequestExceptionHandler()).handle(this.exchange).block();
 		assertThat(this.exchange.getResponse().getStatusCode()).isEqualTo(HttpStatus.BAD_REQUEST);
 	}
 
 	@Test
-	public void handleErrorSignalWithMultipleHttpErrorHandlers() throws Exception {
+	void handleErrorSignalWithMultipleHttpErrorHandlers() {
 		createWebHandler(
 				new UnresolvedExceptionHandler(),
 				new UnresolvedExceptionHandler(),
@@ -62,17 +62,14 @@ public class ExceptionHandlingWebHandlerTests {
 	}
 
 	@Test
-	public void unresolvedException() throws Exception {
+	void unresolvedException() {
 		Mono<Void> mono = createWebHandler(new UnresolvedExceptionHandler()).handle(this.exchange);
 		StepVerifier.create(mono).expectErrorMessage("boo").verify();
 		assertThat(this.exchange.getResponse().getStatusCode()).isNull();
 	}
 
 	@Test
-	public void unresolvedExceptionWithWebHttpHandlerAdapter() throws Exception {
-
-		// HttpWebHandlerAdapter handles unresolved errors
-
+	void unresolvedExceptionWithWebHttpHandlerAdapter() {
 		new HttpWebHandlerAdapter(createWebHandler(new UnresolvedExceptionHandler()))
 				.handle(this.exchange.getRequest(), this.exchange.getResponse()).block();
 
@@ -80,11 +77,18 @@ public class ExceptionHandlingWebHandlerTests {
 	}
 
 	@Test
-	public void thrownExceptionBecomesErrorSignal() throws Exception {
+	void thrownExceptionBecomesErrorSignal() {
 		createWebHandler(new BadRequestExceptionHandler()).handle(this.exchange).block();
 		assertThat(this.exchange.getResponse().getStatusCode()).isEqualTo(HttpStatus.BAD_REQUEST);
 	}
 
+	@Test
+	void thrownExceptionIsStoredAsExchangeAttribute() {
+		createWebHandler(new BadRequestExceptionHandler()).handle(this.exchange).block();
+		Exception exceptionAttribute = this.exchange.getAttribute(ExceptionHandlingWebHandler.HANDLED_WEB_EXCEPTION);
+		assertThat(exceptionAttribute).isInstanceOf(IllegalStateException.class);
+	}
+
 	private WebHandler createWebHandler(WebExceptionHandler... handlers) {
 		return new ExceptionHandlingWebHandler(this.targetHandler, Arrays.asList(handlers));
 	}