Defensive checks in WebClient and Reactor connector

Since there is no reason for an exchange to ever complete without a
ClientResponse I've added a switchIfEmpty check at the WebClient level.

Also, temporarily a second check closer to the problem in the
ReactorClientHttpConnector suggesting a workaround and providing a
reference to the Reactor Netty issue #138.

Issue: SPR-15784

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultWebClient.java

@@ -63,6 +63,10 @@ import org.springframework.web.util.UriBuilderFactory;
  */
 class DefaultWebClient implements WebClient {
 
+	private static final Mono<ClientResponse> NO_HTTP_CLIENT_RESPONSE_ERROR = Mono.error(
+			new IllegalStateException("The underlying HTTP client completed without emitting a response."));
+
+
 	private final ExchangeFunction exchangeFunction;
 
 	private final UriBuilderFactory uriBuilderFactory;
@@ -309,7 +313,7 @@ class DefaultWebClient implements WebClient {
 			ClientRequest request = (this.inserter != null ?
 					initRequestBuilder().body(this.inserter).build() :
 					initRequestBuilder().build());
-			return exchangeFunction.exchange(request);
+			return exchangeFunction.exchange(request).switchIfEmpty(NO_HTTP_CLIENT_RESPONSE_ERROR);
 		}
 
 		private ClientRequest.Builder initRequestBuilder() {

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/DefaultWebClientTests.java

@@ -16,6 +16,7 @@
 
 package org.springframework.web.reactive.function.client;
 
+import java.time.Duration;
 import java.util.Collections;
 
 import org.junit.Before;
@@ -25,12 +26,15 @@ import org.mockito.Captor;
 import org.mockito.Mockito;
 import org.mockito.MockitoAnnotations;
 import reactor.core.publisher.Mono;
+import reactor.test.StepVerifier;
 
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.MediaType;
 
-import static org.junit.Assert.*;
-import static org.mockito.Mockito.*;
+import static org.junit.Assert.assertEquals;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verifyNoMoreInteractions;
+import static org.mockito.Mockito.when;
 
 /**
  * Unit tests for {@link DefaultWebClient}.
@@ -160,7 +164,7 @@ public class DefaultWebClientTests {
 	@Test
 	public void apply() {
 		WebClient client = builder()
-					.apply(builder -> builder.defaultHeader("Accept", "application/json").defaultCookie("id", "123"))
+				.apply(builder -> builder.defaultHeader("Accept", "application/json").defaultCookie("id", "123"))
 				.build();
 		client.get().uri("/path").exchange();
 
@@ -170,6 +174,12 @@ public class DefaultWebClientTests {
 		verifyNoMoreInteractions(this.exchangeFunction);
 	}
 
+	@Test
+	public void switchToErrorOnEmptyClientResponseMono() throws Exception {
+		StepVerifier.create(builder().build().get().uri("/path").exchange())
+				.expectErrorMessage("The underlying HTTP client completed without emitting a response.")
+				.verify(Duration.ofSeconds(5));
+	}
 
 
 	private WebClient.Builder builder() {