Re-enable async dispatches in Observation Filter

Prior to this commit, the fix for gh-32730 disabled the involvment of
the osbervation filter for async dispatches. Instead of relying on ASYNC
dispatches to close the observation for async requests, this is now
using an async listener instead: async dispatches are not guaranteed to
happen once the async request is handled.

This change caused another side-effect: because async dispatches are not
considered anymore by this filter, the observation scope is not
reinstated for async dispatches. For example, `ResponseBodyAdvice`
implementations do not have the observation scope opened during their
execution.

This commit re-enables async dispatches for this filter, but ensures
that observations are not closed during such dispatches as this will be
done by the async listener.

Fixes gh-33128

spring-web/src/main/java/org/springframework/web/filter/ServerHttpObservationFilter.java

@@ -23,6 +23,7 @@ import io.micrometer.observation.Observation;
 import io.micrometer.observation.ObservationRegistry;
 import jakarta.servlet.AsyncEvent;
 import jakarta.servlet.AsyncListener;
+import jakarta.servlet.DispatcherType;
 import jakarta.servlet.FilterChain;
 import jakarta.servlet.RequestDispatcher;
 import jakarta.servlet.ServletException;
@@ -97,6 +98,11 @@ public class ServerHttpObservationFilter extends OncePerRequestFilter {
 		return Optional.ofNullable((ServerRequestObservationContext) request.getAttribute(CURRENT_OBSERVATION_CONTEXT_ATTRIBUTE));
 	}
 
+	@Override
+	protected boolean shouldNotFilterAsyncDispatch() {
+		return false;
+	}
+
 	@Override
 	@SuppressWarnings("try")
 	protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
@@ -116,8 +122,9 @@ public class ServerHttpObservationFilter extends OncePerRequestFilter {
 			if (request.isAsyncStarted()) {
 				request.getAsyncContext().addListener(new ObservationAsyncListener(observation));
 			}
-			// Stop Observation right now if async processing has not been started.
-			else {
+			// scope is opened for ASYNC dispatches, but the observation will be closed
+			// by the async listener.
+			else if (request.getDispatcherType() != DispatcherType.ASYNC){
 				Throwable error = fetchException(request);
 				if (error != null) {
 					observation.error(error);
@@ -176,7 +183,6 @@ public class ServerHttpObservationFilter extends OncePerRequestFilter {
 		@Override
 		public void onError(AsyncEvent event) {
 			this.currentObservation.error(unwrapServletException(event.getThrowable()));
-			this.currentObservation.stop();
 		}
 
 	}

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

@@ -16,15 +16,24 @@
 
 package org.springframework.web.filter;
 
+import java.io.IOException;
+
 import io.micrometer.observation.ObservationRegistry;
 import io.micrometer.observation.tck.TestObservationRegistry;
 import io.micrometer.observation.tck.TestObservationRegistryAssert;
+import jakarta.servlet.AsyncEvent;
+import jakarta.servlet.AsyncListener;
+import jakarta.servlet.DispatcherType;
 import jakarta.servlet.RequestDispatcher;
 import jakarta.servlet.ServletException;
+import jakarta.servlet.http.HttpServlet;
+import jakarta.servlet.http.HttpServletRequest;
+import jakarta.servlet.http.HttpServletResponse;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.http.HttpMethod;
 import org.springframework.http.server.observation.ServerRequestObservationContext;
+import org.springframework.web.testfixture.servlet.MockAsyncContext;
 import org.springframework.web.testfixture.servlet.MockFilterChain;
 import org.springframework.web.testfixture.servlet.MockHttpServletRequest;
 import org.springframework.web.testfixture.servlet.MockHttpServletResponse;
@@ -41,18 +50,18 @@ class ServerHttpObservationFilterTests {
 
 	private final TestObservationRegistry observationRegistry = TestObservationRegistry.create();
 
-	private final ServerHttpObservationFilter filter = new ServerHttpObservationFilter(this.observationRegistry);
-
-	private final MockFilterChain mockFilterChain = new MockFilterChain();
-
 	private final MockHttpServletRequest request = new MockHttpServletRequest(HttpMethod.GET.name(), "/resource/test");
 
 	private final MockHttpServletResponse response = new MockHttpServletResponse();
 
+	private MockFilterChain mockFilterChain = new MockFilterChain();
+
+	private ServerHttpObservationFilter filter = new ServerHttpObservationFilter(this.observationRegistry);
+
 
 	@Test
-	void filterShouldNotProcessAsyncDispatch() {
-		assertThat(this.filter.shouldNotFilterAsyncDispatch()).isTrue();
+	void filterShouldProcessAsyncDispatch() {
+		assertThat(this.filter.shouldNotFilterAsyncDispatch()).isFalse();
 	}
 
 	@Test
@@ -68,6 +77,12 @@ class ServerHttpObservationFilterTests {
 		assertThatHttpObservation().hasLowCardinalityKeyValue("outcome", "SUCCESS").hasBeenStopped();
 	}
 
+	@Test
+	void filterShouldOpenScope() throws Exception {
+		this.mockFilterChain = new MockFilterChain(new ScopeCheckingServlet(this.observationRegistry));
+		filter.doFilter(this.request, this.response, this.mockFilterChain);
+	}
+
 	@Test
 	void filterShouldAcceptNoOpObservationContext() throws Exception {
 		ServerHttpObservationFilter filter = new ServerHttpObservationFilter(ObservationRegistry.NOOP);
@@ -126,9 +141,52 @@ class ServerHttpObservationFilterTests {
 		assertThatHttpObservation().hasLowCardinalityKeyValue("outcome", "SUCCESS").hasBeenStopped();
 	}
 
+	@Test
+	void shouldCloseObservationAfterAsyncError() throws Exception {
+		this.request.setAsyncSupported(true);
+		this.request.startAsync();
+		this.filter.doFilter(this.request, this.response, this.mockFilterChain);
+		MockAsyncContext asyncContext = (MockAsyncContext) this.request.getAsyncContext();
+		for (AsyncListener listener : asyncContext.getListeners()) {
+			listener.onError(new AsyncEvent(this.request.getAsyncContext(), new IllegalStateException("test error")));
+		}
+		asyncContext.complete();
+		assertThatHttpObservation().hasLowCardinalityKeyValue("exception", "IllegalStateException").hasBeenStopped();
+	}
+
+	@Test
+	void shouldNotCloseObservationDuringAsyncDispatch() throws Exception {
+		this.mockFilterChain = new MockFilterChain(new ScopeCheckingServlet(this.observationRegistry));
+		this.request.setDispatcherType(DispatcherType.ASYNC);
+		this.filter.doFilter(this.request, this.response, this.mockFilterChain);
+		TestObservationRegistryAssert.assertThat(this.observationRegistry)
+				.hasObservationWithNameEqualTo("http.server.requests")
+				.that().isNotStopped();
+	}
+
 	private TestObservationRegistryAssert.TestObservationRegistryAssertReturningObservationContextAssert assertThatHttpObservation() {
+		TestObservationRegistryAssert.assertThat(this.observationRegistry)
+				.hasNumberOfObservationsWithNameEqualTo("http.server.requests", 1);
+
 		return TestObservationRegistryAssert.assertThat(this.observationRegistry)
-				.hasObservationWithNameEqualTo("http.server.requests").that();
+				.hasObservationWithNameEqualTo("http.server.requests")
+				.that()
+				.hasBeenStopped();
+	}
+
+	@SuppressWarnings("serial")
+	static class ScopeCheckingServlet extends HttpServlet {
+
+		private final ObservationRegistry observationRegistry;
+
+		public ScopeCheckingServlet(ObservationRegistry observationRegistry) {
+			this.observationRegistry = observationRegistry;
+		}
+
+		@Override
+		protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
+			assertThat(this.observationRegistry.getCurrentObservation()).isNotNull();
+		}
 	}
 
 }