feat: Only send index.html when the browser requests a HTML page (#12571) (#12969)

Fixes #12569

Co-authored-by: Artur <artur@vaadin.com>

Author: vaadin-bot

flow-server/src/main/java/com/vaadin/flow/server/communication/IndexHtmlRequestHandler.java

@@ -19,7 +19,10 @@
 import java.io.IOException;
 import java.io.Serializable;
 import java.io.UncheckedIOException;
+import java.util.Collections;
+import java.util.HashSet;
 import java.util.Optional;
+import java.util.Set;
 
 import org.jsoup.Jsoup;
 import org.jsoup.nodes.DataNode;
@@ -66,6 +69,30 @@ public class IndexHtmlRequestHandler extends JavaScriptBootstrapHandler {
 
     private static final String SCRIPT = "script";
     private static final String SCRIPT_INITIAL = "initial";
+    private static final Set<String> nonHtmlFetchDests;
+    static {
+        // Full list at
+        // https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Sec-Fetch-Dest
+        Set<String> dests = new HashSet<>();
+        dests.add("audio");
+        dests.add("audioworklet");
+        dests.add("font");
+        dests.add("image");
+        dests.add("manifest");
+        dests.add("paintworklet");
+        dests.add("script"); // NOSONAR
+        dests.add("serviceworker");
+        dests.add("sharedworker");
+        dests.add("style");
+        dests.add("track");
+        dests.add("video");
+        dests.add("worker");
+        dests.add("xslt");
+
+        // "empty" requests are used when service worker caches / so they need
+        // to be allowed
+        nonHtmlFetchDests = Collections.unmodifiableSet(dests);
+    }
 
     @Override
     public boolean synchronizedHandleRequest(VaadinSession session,
@@ -227,8 +254,31 @@ private void includeInitialUidl(JsonObject initialJson,
 
     @Override
     protected boolean canHandleRequest(VaadinRequest request) {
-        return !BootstrapHandler.isFrameworkInternalRequest(request) && request
-                .getService().getBootstrapUrlPredicate().isValidUrl(request);
+        return isRequestForHtml(request)
+                && !BootstrapHandler.isFrameworkInternalRequest(request)
+                && request.getService().getBootstrapUrlPredicate()
+                        .isValidUrl(request);
+    }
+
+    /**
+     * Checks if the request is potentially a request for a HTML page.
+     * 
+     * @param request
+     *            the request to check
+     * @return {@code true} if the request is potentially for HTML,
+     *         {@code false} if it is certain that it is a request for a script,
+     *         image or something else
+     */
+    protected boolean isRequestForHtml(VaadinRequest request) {
+        String fetchDest = request.getHeader("Sec-Fetch-Dest");
+        if (fetchDest == null) {
+            // Old browsers do not send the header at all
+            return true;
+        }
+        if (nonHtmlFetchDests.contains(fetchDest)) {
+            return false;
+        }
+        return true;
     }
 
     @Override

flow-server/src/test/java/com/vaadin/flow/server/communication/IndexHtmlRequestHandlerTest.java

@@ -256,6 +256,36 @@ public void canHandleRequest_withBootstrapUrlPredicate() {
                         .canHandleRequest(createVaadinRequest("/.htaccess")));
     }
 
+    @Test
+    public void canHandleRequest_allow_oldBrowser() {
+        Assert.assertTrue(indexHtmlRequestHandler.canHandleRequest(
+                createRequestWithDestination("/", null, null)));
+    }
+
+    @Test
+    public void canHandleRequest_handle_indexHtmlRequest() {
+        Assert.assertTrue(indexHtmlRequestHandler.canHandleRequest(
+                createRequestWithDestination("/", "document", "navigate")));
+    }
+
+    @Test
+    public void canHandleRequest_doNotHandle_scriptRequest() {
+        Assert.assertFalse(indexHtmlRequestHandler.canHandleRequest(
+                createRequestWithDestination("/", "script", "no-cors")));
+    }
+
+    @Test
+    public void canHandleRequest_doNotHandle_imageRequest() {
+        Assert.assertFalse(indexHtmlRequestHandler.canHandleRequest(
+                createRequestWithDestination("/", "image", "no-cors")));
+    }
+
+    @Test
+    public void canHandleRequest_handle_serviceWorkerDocumentRequest() {
+        Assert.assertTrue(indexHtmlRequestHandler.canHandleRequest(
+                createRequestWithDestination("/", "empty", "same-origin")));
+    }
+
     @Test
     public void bootstrapListener_addListener_responseIsModified()
             throws IOException {
@@ -756,6 +786,21 @@ public void tearDown() throws Exception {
         mocks.cleanup();
     }
 
+    private VaadinServletRequest createRequestWithDestination(String pathInfo,
+            String fetchDest, String fetchMode) {
+        VaadinServletRequest req = createVaadinRequest(pathInfo);
+        Mockito.when(req.getHeader(Mockito.anyString())).thenAnswer(arg -> {
+            if ("Sec-Fetch-Dest".equals(arg.getArgument(0))) {
+                return fetchDest;
+            } else if ("Sec-Fetch-Mode".equals(arg.getArgument(0))) {
+                return fetchMode;
+            }
+            return null;
+        });
+
+        return req;
+    }
+
     private VaadinServletRequest createVaadinRequest(String pathInfo) {
         HttpServletRequest request = createRequest(pathInfo);
         return new VaadinServletRequest(request, Mockito.spy(service));