Return empty feed instead of 204 from the DiscoJuice feeds endpoint (#1422)

* Return empty feed instead of 204 from the DiscoJuice feeds endpoint

When no discofeed content is cached — the default, since
`shibboleth.discofeed.allowed` is off — `/api/discojuice/feeds`
responded with `204 No Content`. The DiscoJuice IdP-discovery widget on
the login and register pages loads this endpoint via JSONP and passes
the body straight to `jQuery.merge()`. An empty body leaves the callback
argument undefined, so the widget throws
`TypeError: Cannot read properties of undefined (reading 'length')`
on every default deployment.

Respond with an empty but valid feed instead: `callback([])` when a
callback is given, `[]` otherwise. The JSONP callback then always fires
with a well-formed array and the widget no longer crashes; deployments
with feeds enabled are unaffected.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* Shorten the empty-feed comment

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Matus Kasak <matus.kasak@dataquest.sk>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: 3 people

dspace-server-webapp/src/main/java/org/dspace/app/rest/repository/ClarinDiscoJuiceFeedsController.java

@@ -9,9 +9,6 @@
 
 import static org.springframework.web.bind.annotation.RequestMethod.GET;
 
-import java.io.IOException;
-
-import jakarta.servlet.http.HttpServletResponse;
 import org.apache.commons.lang3.StringUtils;
 import org.apache.logging.log4j.Logger;
 import org.dspace.app.rest.ClarinDiscoJuiceFeedsDownloadService;
@@ -45,13 +42,13 @@ public class ClarinDiscoJuiceFeedsController {
 
     @RequestMapping(method = GET, produces = APPLICATION_JAVASCRIPT_UTF8)
     @PreAuthorize("permitAll()")
-    public ResponseEntity getDiscojuiceFeeds(@RequestParam(value = "callback", required = false) String callback,
-                                             HttpServletResponse response) throws IOException {
+    public ResponseEntity getDiscojuiceFeeds(@RequestParam(value = "callback", required = false) String callback) {
         // Download feeds
         String feedsContent = clarinDiscoJuiceFeedsUpdateScheduler.getFeedsContent();
         if (StringUtils.isBlank(feedsContent)) {
-            response.sendError(HttpServletResponse.SC_NO_CONTENT);
-            return null;
+            // Return an empty but valid feed, not a 204: the JSONP client passes the body to
+            // jQuery.merge() and an empty response throws on `undefined.length`.
+            feedsContent = "[]";
         }
 
         // If callback is not null wrap the feedsContent to the callback string.

dspace-server-webapp/src/test/java/org/dspace/app/rest/ClarinDiscoJuiceFeedsControllerIT.java

@@ -16,6 +16,7 @@
 
 import java.io.IOException;
 import java.io.InputStreamReader;
+import java.lang.reflect.Field;
 import java.net.URL;
 import java.net.URLConnection;
 import javax.net.ssl.HttpsURLConnection;
@@ -108,4 +109,30 @@ public void getDiscoFeeds() throws Exception {
 
         configurationService.setProperty(configKey, origVal);
     }
+
+    @Test
+    public void getDiscoFeedsWithoutContentReturnsEmptyFeed() throws Exception {
+        // Simulate the default state where no feeds have been downloaded (discofeed disabled).
+        Field feedsContentField = ClarinDiscoJuiceFeedsUpdateScheduler.class.getDeclaredField("feedsContent");
+        feedsContentField.setAccessible(true);
+        Object origFeedsContent = feedsContentField.get(null);
+        feedsContentField.set(null, null);
+
+        try {
+            // With a callback the JSONP wrapper must still be valid so the widget callback fires
+            // with an (empty) array instead of `undefined`.
+            getClient().perform(get("/api/discojuice/feeds?callback=dj_md_1"))
+                    .andExpect(status().isOk())
+                    .andExpect(content().contentType(APPLICATION_JAVASCRIPT_UTF8))
+                    .andExpect(content().string("dj_md_1([])"));
+
+            // Without a callback an empty JSON array is returned instead of a 204 No Content.
+            getClient().perform(get("/api/discojuice/feeds"))
+                    .andExpect(status().isOk())
+                    .andExpect(content().contentType(APPLICATION_JAVASCRIPT_UTF8))
+                    .andExpect(content().string("[]"));
+        } finally {
+            feedsContentField.set(null, origFeedsContent);
+        }
+    }
 }