WebTestClient.mutate() should not impact future .mutate() invocations

Issue: SPR-16059

spring-test/src/main/java/org/springframework/test/web/reactive/server/DefaultWebTestClient.java

@@ -66,13 +66,13 @@ class DefaultWebTestClient implements WebTestClient {
 
 	private final Duration timeout;
 
-	private final WebTestClient.Builder builder;
+	private final DefaultWebTestClientBuilder builder;
 
 	private final AtomicLong requestIndex = new AtomicLong();
 
 
 	DefaultWebTestClient(WebClient.Builder clientBuilder, ClientHttpConnector connector,
-			@Nullable Duration timeout, WebTestClient.Builder webTestClientBuilder) {
+			@Nullable Duration timeout, DefaultWebTestClientBuilder webTestClientBuilder) {
 
 		Assert.notNull(clientBuilder, "WebClient.Builder is required");
 		this.wiretapConnector = new WiretapConnector(connector);
@@ -133,7 +133,7 @@ class DefaultWebTestClient implements WebTestClient {
 
 	@Override
 	public Builder mutate() {
-		return this.builder;
+		return new DefaultWebTestClientBuilder(this.builder);
 	}
 
 	@Override

spring-test/src/main/java/org/springframework/test/web/reactive/server/DefaultWebTestClientBuilder.java

@@ -60,6 +60,11 @@ class DefaultWebTestClientBuilder implements WebTestClient.Builder {
 		this(null, httpHandlerBuilder, null, null);
 	}
 
+	DefaultWebTestClientBuilder(DefaultWebTestClientBuilder other) {
+		this(other.webClientBuilder.clone(), other.httpHandlerBuilder, other.connector,
+				other.responseTimeout);
+	}
+
 	DefaultWebTestClientBuilder(@Nullable WebClient.Builder webClientBuilder,
 			@Nullable WebHttpHandlerBuilder httpHandlerBuilder, @Nullable ClientHttpConnector connector,
 			@Nullable Duration responseTimeout) {
@@ -150,12 +155,8 @@ class DefaultWebTestClientBuilder implements WebTestClient.Builder {
 			connectorToUse = new HttpHandlerConnector(this.httpHandlerBuilder.build());
 		}
 
-		DefaultWebTestClientBuilder webTestClientBuilder = new DefaultWebTestClientBuilder(
-				this.webClientBuilder.clone(), this.httpHandlerBuilder,
-				this.connector, this.responseTimeout);
-
 		return new DefaultWebTestClient(this.webClientBuilder,
-				connectorToUse, this.responseTimeout, webTestClientBuilder);
+				connectorToUse, this.responseTimeout, new DefaultWebTestClientBuilder(this));
 	}
 
 }

spring-test/src/test/java/org/springframework/test/web/reactive/server/MockServerIntegrationTests.java

@@ -24,6 +24,7 @@ import org.springframework.http.ResponseCookie;
 import org.springframework.web.server.WebHandler;
 
 import static java.nio.charset.StandardCharsets.UTF_8;
+import static org.junit.Assert.*;
 
 /**
  * Mock server integration test scenarios.
@@ -68,4 +69,39 @@ public class MockServerIntegrationTests {
 				.expectBody(String.class).isEqualTo("bar");
 	}
 
+	@Test
+	public void mutateDoesCopy() throws Exception {
+
+		WebTestClient.Builder builder = WebTestClient.bindToWebHandler(exchange -> exchange.getResponse().setComplete()).configureClient();
+		builder.filter((request, next) -> next.exchange(request));
+		builder.defaultHeader("foo", "bar");
+		builder.defaultCookie("foo", "bar");
+		WebTestClient client1 = builder.build();
+
+		builder.filter((request, next) -> next.exchange(request));
+		builder.defaultHeader("baz", "qux");
+		builder.defaultCookie("baz", "qux");
+		WebTestClient client2 = builder.build();
+
+		WebTestClient.Builder mutatedBuilder = client1.mutate();
+
+		mutatedBuilder.filter((request, next) -> next.exchange(request));
+		mutatedBuilder.defaultHeader("baz", "qux");
+		mutatedBuilder.defaultCookie("baz", "qux");
+		WebTestClient clientFromMutatedBuilder = mutatedBuilder.build();
+
+		client1.mutate().filters(filters -> assertEquals(1, filters.size()));
+		client1.mutate().defaultHeaders(headers -> assertEquals(1, headers.size()));
+		client1.mutate().defaultCookies(cookies -> assertEquals(1, cookies.size()));
+
+		client2.mutate().filters(filters -> assertEquals(2, filters.size()));
+		client2.mutate().defaultHeaders(headers -> assertEquals(2, headers.size()));
+		client2.mutate().defaultCookies(cookies -> assertEquals(2, cookies.size()));
+
+		clientFromMutatedBuilder.mutate().filters(filters -> assertEquals(2, filters.size()));
+		clientFromMutatedBuilder.mutate().defaultHeaders(headers -> assertEquals(2, headers.size()));
+		clientFromMutatedBuilder.mutate().defaultCookies(cookies -> assertEquals(2, cookies.size()));
+	}
+
+
 }