Fix missing contextPath when mutating ServerHttpRequest

This commit ensures that when mutating `ServerHttpRequest` instances,
the original contextPath information is copied to the request being
built.

Note that mutation on the `contextPath(String)` or `path(String)` should
be reflected to the other. (See their Javadoc for more information).

Fixes spring-projectsgh-25279

spring-web/src/main/java/org/springframework/http/server/reactive/DefaultServerHttpRequestBuilder.java

@@ -71,6 +71,7 @@ public DefaultServerHttpRequestBuilder(ServerHttpRequest original) {
 
 		this.uri = original.getURI();
 		this.httpMethodValue = original.getMethodValue();
+		this.contextPath = original.getPath().contextPath().value();
 		this.body = original.getBody();
 
 		this.httpHeaders = HttpHeaders.writableHttpHeaders(original.getHeaders());

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

@@ -37,6 +37,7 @@
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 import static org.mockito.Mockito.mock;
 
 /**
@@ -163,14 +164,42 @@ public void mutateHeaderBySettingHeaderValues() throws Exception {
 		assertThat(request.getHeaders().get(headerName)).containsExactly(headerValue3);
 	}
 
+	@Test
+	void mutateWithExistingContextPath() throws Exception {
+		ServerHttpRequest request = createHttpRequest("/context/path", "/context");
+
+		ServerHttpRequest mutated = request.mutate().build();
+		assertThat(mutated.getPath().contextPath().value()).isEqualTo("/context");
+		assertThat(mutated.getPath().pathWithinApplication().value()).isEqualTo("/path");
+		assertThat(mutated.getURI().getRawPath()).isEqualTo("/context/path");
+
+		mutated = request.mutate().contextPath("/other").path("/other/path").build();
+		assertThat(mutated.getPath().contextPath().value()).isEqualTo("/other");
+		assertThat(mutated.getPath().pathWithinApplication().value()).isEqualTo("/path");
+		assertThat(mutated.getURI().getRawPath()).isEqualTo("/other/path");
+	}
+
+	@Test
+	void mutateContextPathWithoutUpdatingPathShouldFail() throws Exception {
+		ServerHttpRequest request = createHttpRequest("/context/path", "/context");
+
+		assertThatThrownBy(() -> request.mutate().path("/fail").build())
+				.isInstanceOf(IllegalArgumentException.class)
+				.hasMessage("Invalid contextPath '/context': must match the start of requestPath: '/fail'");
+	}
+
 	private ServerHttpRequest createHttpRequest(String uriString) throws Exception {
+		return createHttpRequest(uriString, "");
+	}
+
+	private ServerHttpRequest createHttpRequest(String uriString, String contextPath) throws Exception {
 		URI uri = URI.create(uriString);
 		MockHttpServletRequest request = new TestHttpServletRequest(uri);
+		request.setContextPath(contextPath);
 		AsyncContext asyncContext = new MockAsyncContext(request, new MockHttpServletResponse());
 		return new ServletServerHttpRequest(request, asyncContext, "", new DefaultDataBufferFactory(), 1024);
 	}
 
-
 	private static class TestHttpServletRequest extends MockHttpServletRequest {
 
 		TestHttpServletRequest(URI uri) {