[JENKINS-72111] Allow Lifecycle to load implementations from plugins

core/src/main/java/hudson/PluginManager.java

@@ -506,8 +506,10 @@ protected void reactOnCycle(PluginWrapper q, List<PluginWrapper> cycle) {
 
                                         // obtain topologically sorted list and overwrite the list
                                         for (PluginWrapper p : cgd.getSorted()) {
-                                            if (p.isActive())
+                                            if (p.isActive()) {
                                                 activePlugins.add(p);
+                                                ((UberClassLoader) uberClassLoader).clearCacheMisses();
+                                            }
                                         }
                                     } catch (CycleDetectedException e) { // TODO this should be impossible, since we override reactOnCycle to not throw the exception
                                         stop(); // disable all plugins since classloading from them can lead to StackOverflow
@@ -932,9 +934,10 @@ public void dynamicLoad(File arc, boolean removeExisting, @CheckForNull List<Plu
             // so existing plugins can't be depending on this newly deployed one.
 
             plugins.add(p);
-            if (p.isActive())
+            if (p.isActive()) {
                 activePlugins.add(p);
-            ((UberClassLoader) uberClassLoader).loaded.clear();
+                ((UberClassLoader) uberClassLoader).clearCacheMisses();
+            }
 
             // TODO antimodular; perhaps should have a PluginListener to complement ExtensionListListener?
             CustomClassFilter.Contributed.load();
@@ -2385,6 +2388,10 @@ protected Enumeration<URL> findResources(String name) throws IOException {
             return Collections.enumeration(resources);
         }
 
+        void clearCacheMisses() {
+            loaded.values().removeIf(Optional::isEmpty);
+        }
+
         @Override
         public String toString() {
             // only for debugging purpose

core/src/main/java/hudson/lifecycle/Lifecycle.java

@@ -29,6 +29,8 @@
 import hudson.ExtensionPoint;
 import hudson.Functions;
 import hudson.Util;
+import hudson.init.InitMilestone;
+import hudson.init.Initializer;
 import java.io.File;
 import java.io.IOException;
 import java.io.UncheckedIOException;
@@ -40,6 +42,8 @@
 import jenkins.model.Jenkins;
 import jenkins.util.SystemProperties;
 import org.apache.commons.io.FileUtils;
+import org.kohsuke.accmod.Restricted;
+import org.kohsuke.accmod.restrictions.NoExternalUse;
 
 /**
  * Provides the capability for starting/stopping/restarting/uninstalling Hudson.
@@ -48,7 +52,8 @@
  * The steps to perform these operations depend on how Hudson is launched,
  * so the concrete instance of this method (which is VM-wide singleton) is discovered
  * by looking up a FQCN from the system property "hudson.lifecycle".
- *
+ * (This may be set to a class defined in a plugin,
+ * in which case the singleton switches during startup.)
  * @author Kohsuke Kawaguchi
  * @since 1.254
  */
@@ -57,9 +62,8 @@ public abstract class Lifecycle implements ExtensionPoint {
 
     /**
      * Gets the singleton instance.
-     *
-     * @return never null
      */
+    @NonNull
     public static synchronized Lifecycle get() {
         if (INSTANCE == null) {
             Lifecycle instance;
@@ -81,9 +85,8 @@ public static synchronized Lifecycle get() {
                     x.initCause(e);
                     throw x;
                 } catch (ClassNotFoundException e) {
-                    NoClassDefFoundError x = new NoClassDefFoundError(e.getMessage());
-                    x.initCause(e);
-                    throw x;
+                    LOGGER.log(Level.FINE, e, () -> "Failed to load " + p + " so will try again later");
+                    instance = new PlaceholderLifecycle();
                 } catch (InvocationTargetException e) {
                     Throwable t = e.getCause();
                     if (t instanceof RuntimeException) {
@@ -307,5 +310,23 @@ public void onStatusUpdate(String status) {
         LOGGER.log(Level.INFO, status);
     }
 
+    @Restricted(NoExternalUse.class)
+    public static final class PlaceholderLifecycle extends ExitLifecycle {
+
+        @Initializer(after = InitMilestone.PLUGINS_STARTED, before = InitMilestone.EXTENSIONS_AUGMENTED)
+        public static synchronized void replacePlaceholder() {
+            if (get() instanceof PlaceholderLifecycle) {
+                String p = SystemProperties.getString("hudson.lifecycle");
+                try {
+                    INSTANCE = (Lifecycle) Jenkins.get().getPluginManager().uberClassLoader.loadClass(p).getConstructor().newInstance();
+                    LOGGER.fine(() -> "Updated to " + INSTANCE);
+                } catch (Exception | LinkageError x) {
+                    LOGGER.log(Level.WARNING, x, () -> "Failed to load " + p + "; using fallback exit lifecycle");
+                }
+            }
+        }
+
+    }
+
     private static final Logger LOGGER = Logger.getLogger(Lifecycle.class.getName());
 }

test/src/test/java/hudson/lifecycle/LifecycleTest.java

@@ -0,0 +1,62 @@
+/*
+ * The MIT License
+ *
+ * Copyright 2023 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 hudson.lifecycle;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.is;
+
+import java.lang.reflect.Field;
+import java.util.logging.Level;
+import jenkins.model.Jenkins;
+import org.junit.Rule;
+import org.junit.Test;
+import org.jvnet.hudson.test.JenkinsRule;
+import org.jvnet.hudson.test.RealJenkinsRule;
+
+public final class LifecycleTest {
+
+    @Rule
+    public RealJenkinsRule rr = new RealJenkinsRule()
+        .addPlugins("plugins/custom-lifecycle.hpi")
+        .javaOptions("-Dhudson.lifecycle=test.custom_lifecycle.CustomLifecycle")
+        .withLogger(Lifecycle.class, Level.FINE);
+
+    @Test
+    public void definedInPlugin() throws Throwable {
+        rr.then(LifecycleTest::_definedInPlugin);
+    }
+
+    private static void _definedInPlugin(JenkinsRule r) throws Throwable {
+        Class<? extends Lifecycle> type = Jenkins.get().getPluginManager().uberClassLoader
+            .loadClass("test.custom_lifecycle.CustomLifecycle").asSubclass(Lifecycle.class);
+        Lifecycle l = Lifecycle.get();
+        assertThat(l.getClass(), is(type));
+        Field count = type.getField("count");
+        assertThat(count.get(l), is(0));
+        Lifecycle.get().restart();
+        assertThat(count.get(l), is(1));
+    }
+
+}