fix: RestTestClient.mutate() should not have side effects

Before this change, DefaultRestTestClient.mutate() was reusing the
underlying builder all calls.
Building a new builder for each call clones the RestTestClientBuilder,
protecting from side effects.

Signed-off-by: Thomas Recloux <[email protected]>

Author: trecloux

spring-test/src/main/java/org/springframework/test/web/servlet/client/DefaultRestTestClient.java

@@ -129,7 +129,7 @@ private RequestBodyUriSpec methodInternal(HttpMethod httpMethod) {
 	@SuppressWarnings("unchecked")
 	@Override
 	public <B extends Builder<B>> Builder<B> mutate() {
-		return (Builder<B>) this.restTestClientBuilder;
+		return new DefaultRestTestClientBuilder<B>((DefaultRestTestClientBuilder<B>) this.restTestClientBuilder);
 	}
 
 

spring-test/src/test/java/org/springframework/test/web/servlet/client/DefaultRestTestClientBuilderTest.java

@@ -0,0 +1,17 @@
+package org.springframework.test.web.servlet.client;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+
+class DefaultRestTestClientBuilderTest {
+
+	@Test
+	void testMutateHasNoSideEffects() {
+		RestTestClient baseTestClient = new DefaultRestTestClientBuilder().baseUrl("http://localhost").build();
+		baseTestClient.mutate().defaultHeader("foo", "bar").build();
+		baseTestClient.mutate().defaultHeaders(headers -> assertThat(headers.containsHeader("foo")).isFalse());
+	}
+
+}