Added a mechanism to filter extension points as they are discovered.

I couldn't quite find one place to do this, so it's spread apart
somewhat.

changelog.html

@@ -55,6 +55,8 @@ Upcoming changes</a>
 <!-- Record your changes in the trunk here. -->
 <div id="trunk" style="display:none"><!--=TRUNK-BEGIN=-->
 <ul class=image>
+  <li class=rfe>
+    Added a mechanism to filter extension points as they are discovered.
   <li class=rfe>
     Exposed the master's own node properties to be configured in <tt>/computer/(master)/configure</tt>
     (whereas <tt>/configure</tt> controls global node properties that apply to all nodes.)

core/src/main/java/hudson/ClassicPluginStrategy.java

@@ -23,6 +23,8 @@
  */
 package hudson;
 
+import com.google.common.collect.Iterables;
+import com.google.common.collect.Lists;
 import hudson.Plugin.DummyImpl;
 import hudson.PluginWrapper.Dependency;
 import hudson.model.Hudson;
@@ -33,6 +35,7 @@ import hudson.util.IOUtils;
 import hudson.util.MaskingClassLoader;
 import hudson.util.VersionNumber;
 import jenkins.ClassLoaderReflectionToolkit;
+import jenkins.ExtensionFilter;
 import org.apache.tools.ant.AntClassLoader;
 import org.apache.tools.ant.BuildException;
 import org.apache.tools.ant.Project;
@@ -54,6 +57,7 @@ import java.util.Collection;
 import java.util.Collections;
 import java.util.Enumeration;
 import java.util.HashSet;
+import java.util.Iterator;
 import java.util.List;
 import java.util.Vector;
 import java.util.jar.Attributes;
@@ -286,7 +290,7 @@ public class ClassicPluginStrategy implements PluginStrategy {
             finder.scout(type, hudson);
         }
 
-        List<ExtensionComponent<T>> r = new ArrayList<ExtensionComponent<T>>();
+        List<ExtensionComponent<T>> r = Lists.newArrayList();
         for (ExtensionFinder finder : finders) {
             try {
                 r.addAll(finder._find(type, hudson));
@@ -296,7 +300,14 @@ public class ClassicPluginStrategy implements PluginStrategy {
                     r.add(new ExtensionComponent<T>(t));
             }
         }
-        return r;
+
+        List<ExtensionComponent<T>> filtered = Lists.newArrayList();
+        for (ExtensionComponent<T> e : r) {
+            if (ExtensionFilter.isAllowed(type,e))
+                filtered.add(e);
+        }
+
+        return filtered;
     }
 
     public void load(PluginWrapper wrapper) throws IOException {

core/src/main/java/hudson/ExtensionComponent.java

@@ -24,7 +24,9 @@
 
 package hudson;
 
+import hudson.model.Describable;
 import hudson.model.Descriptor;
+import jenkins.ExtensionFilter;
 
 /**
  * Discovered {@link Extension} object with a bit of metadata for Hudson.
@@ -32,6 +34,8 @@ import hudson.model.Descriptor;
  *
  * @author Kohsuke Kawaguchi
  * @since 1.356
+ * @see ExtensionFinder
+ * @see ExtensionFilter
  */
 public class ExtensionComponent<T> implements Comparable<ExtensionComponent<T>> {
     private final T instance;
@@ -66,6 +70,15 @@ public class ExtensionComponent<T> implements Comparable<ExtensionComponent<T>>
         return instance;
     }
 
+    /**
+     * Checks if this component is a {@link Descriptor} describing the given type
+     *
+     * For example, {@code component.isDescriptorOf(Builder.class)}
+     */
+    public boolean isDescriptorOf(Class<? extends Describable> c) {
+        return instance instanceof Descriptor && ((Descriptor)instance).isSubTypeOf(c);
+    }
+
     /**
      * Sort {@link ExtensionComponent}s in the descending order of {@link #ordinal()}.
      */

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

@@ -41,6 +41,7 @@ import hudson.init.InitMilestone;
 import hudson.model.Descriptor;
 import hudson.model.Hudson;
 import jenkins.ExtensionComponentSet;
+import jenkins.ExtensionFilter;
 import jenkins.ExtensionRefreshException;
 import jenkins.ProxyInjector;
 import jenkins.model.Jenkins;
@@ -72,10 +73,11 @@ import java.lang.reflect.Method;
  *
  * <p>
  * {@link ExtensionFinder} itself is an extension point, but to avoid infinite recursion,
- * Hudson discovers {@link ExtensionFinder}s through {@link Sezpoz} and that alone.
+ * Jenkins discovers {@link ExtensionFinder}s through {@link Sezpoz} and that alone.
  *
  * @author Kohsuke Kawaguchi
  * @since 1.286
+ * @see ExtensionFilter
  */
 public abstract class ExtensionFinder implements ExtensionPoint {
     /**
@@ -126,7 +128,7 @@ public abstract class ExtensionFinder implements ExtensionPoint {
      * <p>
      * This method should return all the known components at the time of the call, including
      * those that are discovered later via {@link #refresh()}, even though those components
-     * are separately retruend in {@link ExtensionComponentSet}.
+     * are separately returned in {@link ExtensionComponentSet}.
      *
      * @param <T>
      *      The type of the extension points. This is not bound to {@link ExtensionPoint} because

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

@@ -3,6 +3,7 @@ package hudson.model;
 import hudson.ExtensionList;
 import hudson.ExtensionPoint;
 import hudson.scm.SCMDescriptor;
+import jenkins.ExtensionFilter;
 import jenkins.model.Jenkins;
 
 import java.util.ArrayList;
@@ -13,6 +14,7 @@ import java.util.List;
  *
  * @author Kohsuke Kawaguchi
  * @since 1.393
+ * @see ExtensionFilter
  */
 public abstract class DescriptorVisibilityFilter implements ExtensionPoint {
 

core/src/main/java/jenkins/ExtensionComponentSet.java

@@ -59,6 +59,24 @@ public abstract class ExtensionComponentSet {
      */
     public abstract <T> Collection<ExtensionComponent<T>> find(Class<T> type);
 
+    /**
+     * Apply {@link ExtensionFilter}s and returns a filtered set.
+     */
+    public final ExtensionComponentSet filtered() {
+        final ExtensionComponentSet base = this;
+        return new ExtensionComponentSet() {
+            @Override
+            public <T> Collection<ExtensionComponent<T>> find(Class<T> type) {
+                List<ExtensionComponent<T>> a = Lists.newArrayList();
+                for (ExtensionComponent<T> c : base.find(type)) {
+                    if (ExtensionFilter.isAllowed(type,c))
+                        a.add(c);
+                }
+                return a;
+            }
+        };
+    }
+
     /**
      * Constant that has zero component in it.
      */

core/src/main/java/jenkins/ExtensionFilter.java

@@ -0,0 +1,91 @@
+/*
+ * The MIT License
+ *
+ * Copyright (c) 2012, CloudBees, Inc.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package jenkins;
+
+import hudson.ExtensionComponent;
+import hudson.ExtensionFinder;
+import hudson.ExtensionList;
+import hudson.ExtensionPoint;
+import hudson.model.AdministrativeMonitor;
+import hudson.model.Describable;
+import hudson.model.Descriptor;
+import hudson.model.DescriptorVisibilityFilter;
+import jenkins.model.Jenkins;
+
+/**
+ * Filters out {@link ExtensionComponent}s discovered by {@link ExtensionFinder}s,
+ * as if they were never discovered.
+ *
+ * <p>
+ * This is useful for those who are deploying restricted/simplified version of Jenkins
+ * by reducing the functionality.
+ *
+ * <p>
+ * Because of the way {@link ExtensionFinder} works, even when an extension component
+ * is rejected by a filter, its instance still gets created first.
+ *
+ * @author Kohsuke Kawaguchi
+ * @since 1.472
+ * @see DescriptorVisibilityFilter
+ * @see ExtensionComponentSet#filtered()
+ */
+public abstract class ExtensionFilter implements ExtensionPoint {
+    /**
+     * Checks if a newly discovered extension is allowed to participate into Jenkins.
+     *
+     * <p>
+     * To filter {@link Descriptor}s based on the {@link Describable} subtypes, do as follows:
+     *
+     * <pre>
+     * return !component.isDescriptorOf(Builder.class);
+     * </pre>
+     *
+     * @param type
+     *      The type of the extension that we are discovering. This is not the actual instance
+     *      type, but the contract type, such as {@link Descriptor}, {@link AdministrativeMonitor}, etc.
+     * @return
+     *      true to let the component into Jenkins. false to drop it and pretend
+     *      as if it didn't exist. When any one of {@link ExtensionFilter}s veto
+     *      a component, it gets dropped.
+     */
+    public abstract <T> boolean allows(Class<T> type, ExtensionComponent<T> component);
+
+    public static <T> boolean isAllowed(Class<T> type, ExtensionComponent<T> component) {
+        // to avoid infinite recursion, those extension points are handled differently.
+        if (type==ExtensionFilter.class || type==ExtensionFinder.class)
+            return true;
+
+        for (ExtensionFilter f : all())
+            if (!f.allows(type, component))
+                return false;
+        return true;
+    }
+
+    /**
+     * All registered {@link ExtensionFilter} instances.
+     */
+    public static ExtensionList<ExtensionFilter> all() {
+        return Jenkins.getInstance().getExtensionList(ExtensionFilter.class);
+    }
+}

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

@@ -2055,14 +2055,14 @@ public class Jenkins extends AbstractCIBase implements ModifiableItemGroup<TopLe
         for (ExtensionFinder ef : finders) {
             fragments.add(ef.refresh());
         }
-        ExtensionComponentSet delta = ExtensionComponentSet.union(fragments);
+        ExtensionComponentSet delta = ExtensionComponentSet.union(fragments).filtered();
 
         // if we find a new ExtensionFinder, we need it to list up all the extension points as well
         List<ExtensionComponent<ExtensionFinder>> newFinders = Lists.newArrayList(delta.find(ExtensionFinder.class));
         while (!newFinders.isEmpty()) {
             ExtensionFinder f = newFinders.remove(newFinders.size()-1).getInstance();
 
-            ExtensionComponentSet ecs = ExtensionComponentSet.allOf(f);
+            ExtensionComponentSet ecs = ExtensionComponentSet.allOf(f).filtered();
             newFinders.addAll(ecs.find(ExtensionFinder.class));
             delta = ExtensionComponentSet.union(delta, ecs);
         }

test/src/test/java/jenkins/ExtensionFilterTest.java

@@ -0,0 +1,29 @@
+package jenkins;
+
+import hudson.ExtensionComponent;
+import hudson.console.ConsoleAnnotatorFactory;
+import hudson.model.PageDecorator;
+import org.jvnet.hudson.test.HudsonTestCase;
+import org.jvnet.hudson.test.TestExtension;
+
+/**
+ * @author Kohsuke Kawaguchi
+ */
+public class ExtensionFilterTest extends HudsonTestCase {
+    public void testFilter() {
+        assertTrue(PageDecorator.all().isEmpty());
+        assertTrue(ConsoleAnnotatorFactory.all().isEmpty());
+    }
+
+    @TestExtension("testFilter")
+    public static class Impl extends ExtensionFilter {
+        @Override
+        public <T> boolean allows(Class<T> type, ExtensionComponent<T> component) {
+            if (type==ConsoleAnnotatorFactory.class)
+                return false;
+            if (component.isDescriptorOf(PageDecorator.class))
+                return false;
+            return true;
+        }
+    }
+}