[JENKINS-73760] Updates fail due to invalid JSON from HTTP Update Center

core/src/main/java/hudson/model/UpdateSite.java

@@ -538,14 +538,17 @@ public String getDownloadUrl() {
 
     /**
      * Is this the legacy default update center site?
-     * @deprecated
-     *      Will be removed, currently returns always false.
-     * @since 2.343
+     * @since 1.357
      */
-    @Deprecated
     @Restricted(NoExternalUse.class)
     public boolean isLegacyDefault() {
-        return false;
+        return isJenkinsCI();
+    }
+
+    private boolean isJenkinsCI() {
+        return url != null
+                && UpdateCenter.PREDEFINED_UPDATE_SITE_ID.equals(id)
+                && url.startsWith("http://updates.jenkins-ci.org/");
     }
 
     /**

test/src/test/java/hudson/model/UpdateCenterMigrationTest.java

@@ -0,0 +1,35 @@
+package hudson.model;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+
+import org.junit.Rule;
+import org.junit.Test;
+import org.jvnet.hudson.test.Issue;
+import org.jvnet.hudson.test.JenkinsRule;
+import org.jvnet.hudson.test.recipes.LocalData;
+
+public class UpdateCenterMigrationTest {
+
+    @Rule
+    public JenkinsRule j = new JenkinsRule() {
+        @Override
+        protected void configureUpdateCenter() {
+            // Avoid reverse proxy
+            DownloadService.neverUpdate = true;
+            UpdateSite.neverUpdate = true;
+        }
+    };
+
+    @Issue("JENKINS-73760")
+    @LocalData
+    @Test
+    public void updateCenterMigration() {
+        UpdateSite site = j.jenkins.getUpdateCenter().getSites().stream()
+                .filter(s -> UpdateCenter.PREDEFINED_UPDATE_SITE_ID.equals(s.getId()))
+                .findFirst()
+                .orElseThrow();
+        assertFalse(site.isLegacyDefault());
+        assertEquals(j.jenkins.getUpdateCenter().getDefaultBaseUrl() + "update-center.json", site.getUrl());
+    }
+}

test/src/test/java/hudson/model/UpdateSiteTest.java

@@ -72,6 +72,7 @@
 import org.junit.Before;
 import org.junit.Rule;
 import org.junit.Test;
+import org.jvnet.hudson.test.Issue;
 import org.jvnet.hudson.test.JenkinsRule;
 
 public class UpdateSiteTest {
@@ -205,6 +206,19 @@ public void shutdownWebserver() throws Exception {
         assertNotEquals("plugin data is present", Collections.emptyMap(), site.getData().plugins);
     }
 
+    @Issue("JENKINS-73760")
+    @Test
+    public void isLegacyDefault() {
+        assertFalse("isLegacyDefault should be false with null id", new UpdateSite(null, "url").isLegacyDefault());
+        assertFalse(
+                "isLegacyDefault should be false when id is not default and url is http://updates.jenkins-ci.org/",
+                new UpdateSite("dummy", "http://updates.jenkins-ci.org/").isLegacyDefault());
+        assertTrue(
+                "isLegacyDefault should be true when id is default and url is http://updates.jenkins-ci.org/",
+                new UpdateSite(UpdateCenter.PREDEFINED_UPDATE_SITE_ID, "http://updates.jenkins-ci.org/").isLegacyDefault());
+        assertFalse("isLegacyDefault should be false with null url", new UpdateSite(null, null).isLegacyDefault());
+    }
+
     @Test public void getAvailables() throws Exception {
         UpdateSite site = getUpdateSite("/plugins/available-update-center.json");
         List<UpdateSite.Plugin> available = site.getAvailables();

test/src/test/resources/hudson/model/UpdateCenterMigrationTest/updateCenterMigration/hudson.model.UpdateCenter.xml

@@ -0,0 +1,7 @@
+<?xml version='1.0' encoding='UTF-8'?>
+<sites>
+  <site>
+    <id>default</id>
+    <url>http://updates.jenkins-ci.org/update-center.json</url>
+  </site>
+</sites>