[JENKINS-58302] call refresh on the Extension providers again

In the case the ExtensionFinder is itself extensible we refresh it again
to catch extensions that would be discovered by a newly installed
extension.

This seems a little overzerlous, as there is only the variant plugin
that I know of, however calling `Jenkins.get().refreshExtensions() from
any point in the `variant` plugin would seem to be a little bit
dangerous as it would call back into a place where the reactor is
already calling back from.

core/src/main/java/hudson/ExtensionFinder.java

@@ -298,9 +298,11 @@ protected Injector resolve() {
         }
 
         private void refreshExtensionAnnotations() {
+            LOGGER.finer(() -> "refreshExtensionAnnotations()");
             for (ExtensionComponent<GuiceExtensionAnnotation> ec : moduleFinder.find(GuiceExtensionAnnotation.class, Hudson.getInstance())) {
                 GuiceExtensionAnnotation gea = ec.getInstance();
                 extensionAnnotations.put(gea.annotationType, gea);
+                LOGGER.finer(() -> "found " + gea.getClass());
             }
         }
 
@@ -328,6 +330,7 @@ public Injector getContainer() {
          */
         @Override
         public synchronized ExtensionComponentSet refresh() throws ExtensionRefreshException {
+            LOGGER.finer(() -> "refresh()");
             refreshExtensionAnnotations();
             // figure out newly discovered sezpoz components
             List<IndexItem<?, Object>> delta = new ArrayList<>();

core/src/main/java/jenkins/model/Jenkins.java

@@ -2897,13 +2897,16 @@ public ExtensionList getExtensionList(String extensionType) throws ClassNotFound
      */
     public void refreshExtensions() throws ExtensionRefreshException {
         ExtensionList<ExtensionFinder> finders = getExtensionList(ExtensionFinder.class);
+        LOGGER.finer(() -> "refreshExtensions " + finders);
         for (ExtensionFinder ef : finders) {
             if (!ef.isRefreshable())
                 throw new ExtensionRefreshException(ef + " doesn't support refresh");
         }
 
         List<ExtensionComponentSet> fragments = new ArrayList<>();
+
         for (ExtensionFinder ef : finders) {
+            LOGGER.finer(() -> "searching " + ef);
             fragments.add(ef.refresh());
         }
         ExtensionComponentSet delta = ExtensionComponentSet.union(fragments).filtered();
@@ -2912,12 +2915,21 @@ public void refreshExtensions() throws ExtensionRefreshException {
         List<ExtensionComponent<ExtensionFinder>> newFinders = new ArrayList<>(delta.find(ExtensionFinder.class));
         while (!newFinders.isEmpty()) {
             ExtensionFinder f = newFinders.remove(newFinders.size() - 1).getInstance();
+            LOGGER.finer(() -> "found new ExtensionFinder " + f);
 
             ExtensionComponentSet ecs = ExtensionComponentSet.allOf(f).filtered();
             newFinders.addAll(ecs.find(ExtensionFinder.class));
             delta = ExtensionComponentSet.union(delta, ecs);
         }
 
+        // we may not have found a new Extension finder but we may be using an extension finder that is extensible
+        // e.g. hudson.ExtensionFinder.GuiceFinder is extensible by GuiceExtensionAnnotation which is done by the variant plugin
+        // so lets give it one more chance.
+        for (ExtensionFinder ef : finders) {
+            LOGGER.finer(() -> "searching again in " + ef);
+            delta = ExtensionComponentSet.union(delta, ef.refresh().filtered());
+        }
+
         for (ExtensionList el : extensionLists.values()) {
             el.refresh(delta);
         }