Fix absolute paths when transforming resources

Prior to this commit, `ResourceTransformerSupport.toAbsolutePath`
would call `StringUtils.applyRelativePath` in all cases. But this
implementation is prepending the given path even if the relative path
starts with `"/"`.

This commit skips the entire operation if the given path is absolute,
i.e. it starts with `"/"`.

Issue: SPR-17432
(Cherry-picked from 2146e13787)

spring-webflux/src/main/java/org/springframework/web/reactive/resource/ResourceTransformerSupport.java

@@ -96,7 +96,7 @@ public abstract class ResourceTransformerSupport implements ResourceTransformer
 	 */
 	protected String toAbsolutePath(String path, ServerWebExchange exchange) {
 		String requestPath = exchange.getRequest().getURI().getPath();
-		String absolutePath = StringUtils.applyRelativePath(requestPath, path);
+		String absolutePath = path.startsWith("/") ? path : StringUtils.applyRelativePath(requestPath, path);
 		return StringUtils.cleanPath(absolutePath);
 	}
 

spring-webflux/src/test/java/org/springframework/web/reactive/resource/ResourceTransformerSupportTests.java

@@ -101,6 +101,16 @@ public class ResourceTransformerSupportTests {
 		assertEquals("../bar-11e16cf79faee7ac698c805cf28248d2.css", actual);
 	}
 
+	@Test
+	public void toAbsolutePath() {
+		MockServerWebExchange exchange = MockServerWebExchange.from(MockServerHttpRequest.get("/resources/main.css"));
+		String absolute = this.transformer.toAbsolutePath("img/image.png", exchange);
+		assertEquals("/resources/img/image.png", absolute);
+
+		absolute = this.transformer.toAbsolutePath("/img/image.png", exchange);
+		assertEquals("/img/image.png", absolute);
+	}
+
 
 	private static class TestResourceTransformerSupport extends ResourceTransformerSupport {
 

spring-webmvc/src/main/java/org/springframework/web/servlet/resource/ResourceTransformerSupport.java

@@ -94,10 +94,13 @@ public abstract class ResourceTransformerSupport implements ResourceTransformer
 	 * @return the absolute request path for the given resource path
 	 */
 	protected String toAbsolutePath(String path, HttpServletRequest request) {
-		ResourceUrlProvider urlProvider = findResourceUrlProvider(request);
-		Assert.state(urlProvider != null, "No ResourceUrlProvider");
-		String requestPath = urlProvider.getUrlPathHelper().getRequestUri(request);
-		String absolutePath = StringUtils.applyRelativePath(requestPath, path);
+		String absolutePath = path;
+		if(!path.startsWith("/")) {
+			ResourceUrlProvider urlProvider = findResourceUrlProvider(request);
+			Assert.state(urlProvider != null, "No ResourceUrlProvider");
+			String requestPath = urlProvider.getUrlPathHelper().getRequestUri(request);
+			absolutePath = StringUtils.applyRelativePath(requestPath, path);
+		}
 		return StringUtils.cleanPath(absolutePath);
 	}
 

spring-webmvc/src/test/java/org/springframework/web/servlet/resource/ResourceTransformerSupportTests.java

@@ -94,6 +94,17 @@ public class ResourceTransformerSupportTests {
 		assertEquals("../bar-11e16cf79faee7ac698c805cf28248d2.css", actual);
 	}
 
+	@Test
+	public void toAbsolutePath() {
+		String absolute = this.transformer.toAbsolutePath("img/image.png",
+				new MockHttpServletRequest("GET", "/resources/style.css"));
+		assertEquals("/resources/img/image.png", absolute);
+
+		absolute = this.transformer.toAbsolutePath("/img/image.png",
+				new MockHttpServletRequest("GET", "/resources/style.css"));
+		assertEquals("/img/image.png", absolute);
+	}
+
 
 	private static class TestResourceTransformerSupport extends ResourceTransformerSupport {