Merge pull request #543 from basil/cacheExecutor2

[JENKINS-72325] Define an executor and scheduler for `SandboxResolvingClassLoader`

src/main/java/org/jenkinsci/plugins/scriptsecurity/sandbox/groovy/SandboxResolvingClassLoader.java

@@ -3,10 +3,22 @@
 import com.github.benmanes.caffeine.cache.Cache;
 import com.github.benmanes.caffeine.cache.Caffeine;
 import com.github.benmanes.caffeine.cache.LoadingCache;
+import com.github.benmanes.caffeine.cache.Scheduler;
+import groovy.lang.GroovyClassLoader;
 import groovy.lang.GroovyShell;
+import hudson.util.ClassLoaderSanityThreadFactory;
+import hudson.util.DaemonThreadFactory;
+import hudson.util.NamingThreadFactory;
 import java.net.URL;
+import java.security.AccessControlContext;
+import java.security.ProtectionDomain;
 import java.time.Duration;
 import java.util.Optional;
+import java.util.concurrent.Executor;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ForkJoinPool;
+import java.util.concurrent.ForkJoinWorkerThread;
+import java.util.concurrent.ThreadFactory;
 import java.util.function.Supplier;
 import java.util.logging.Level;
 import java.util.logging.Logger;
@@ -23,6 +35,51 @@ class SandboxResolvingClassLoader extends ClassLoader {
 
     private static final Logger LOGGER = Logger.getLogger(SandboxResolvingClassLoader.class.getName());
 
+    /**
+     * Care must be taken to avoid leaking instances of {@link GroovyClassLoader} when computing the cached value.
+     * This can happen in several ways, depending on the Caffeine configuration:
+     *
+     * <ul>
+     *   <li>In its default configuration, Caffeine uses {@link ForkJoinPool#commonPool} as its {@link Executor}.
+     *       As of recent Java versions, {@link ForkJoinPool} can capture a reference to {@link GroovyClassLoader} by
+     *       creating a {@link ForkJoinWorkerThread} whose {@link Thread#inheritedAccessControlContext} refers to an
+     *       {@link AccessControlContext} whose {@link ProtectionDomain} refers to {@link GroovyClassLoader}.
+     *   <li>When Caffeine is configured with an {@link Executor} returned by {@link Executors#newCachedThreadPool},
+     *       that {@link Executor} can capture a reference to {@link GroovyClassLoader} by creating a {@link Thread}
+     *       whose {@link Thread#inheritedAccessControlContext} refers to an {@link AccessControlContext} whose {@link
+     *       ProtectionDomain} refers to {@link GroovyClassLoader}. Additionally, when the thread pool's {@link
+     *       ThreadFactory} is not wrapped by {@link ClassLoaderSanityThreadFactory}, the {@link Executor} can sometimes
+     *       create {@link Thread} instances whose {@link Thread#contextClassLoader} refers to {@link
+     *       GroovyClassLoader}.
+     * </ul>
+     *
+     * As of <a href="https://openjdk.org/jeps/411">JEP-411</a>, {@link Thread#inheritedAccessControlContext} is
+     * deprecated for removal, but in the meantime we must contend with this issue. We therefore create a dedicated
+     * {@link Executors#newSingleThreadExecutor}, which is safe for use with Caffeine from a memory perspective because:
+     *
+     * <ul>
+     *   <li>In contrast to {@link ForkJoinPool#commonPool}, the thread is eagerly created and avoids references to
+     *       {@link GroovyClassLoader} in {@link Thread#inheritedAccessControlContext}.
+     *   <li>In contrast to {@link Executors#newCachedThreadPool}, the thread is eagerly created and avoids references
+     *       to {@link GroovyClassLoader} in {@link Thread#inheritedAccessControlContext}.
+     *   <li>In contrast to {@link Executors#newCachedThreadPool}, the thread is eagerly created and avoids references
+     *       to {@link GroovyClassLoader} in {@link Thread#contextClassLoader}, thereby avoiding the need for {@link
+     *       ClassLoaderSanityThreadFactory}.
+     * </ul>
+     *
+     * A single-threaded {@link Executor} is safe for use with Caffeine from a CPU perspective because <a
+     * href="https://stackoverflow.com/a/68105121">the cache's work is implemented with cheap O(1) algorithms</a>.
+     *
+     * <p>In the medium term, once {@link Thread#inheritedAccessControlContext} is removed upstream, we could possibly
+     * switch to a combination of {@link Executors#newCachedThreadPool} and {@link ClassLoaderSanityThreadFactory}.
+     *
+     * <p>In the long term, a listener should be added to inform this class when dynamically installed plugins become
+     * available, as described in the comments to {@link #makeParentCache(boolean)}, in which case the use of Caffeine
+     * could possibly be removed entirely.
+     */
+    private static final Executor cacheExecutor = Executors.newSingleThreadExecutor(new NamingThreadFactory(
+            new DaemonThreadFactory(), SandboxResolvingClassLoader.class.getName() + ".cacheExecutor"));
+
     static final LoadingCache<ClassLoader, Cache<String, Class<?>>> parentClassCache = makeParentCache(true);
 
     static final LoadingCache<ClassLoader, Cache<String, Optional<URL>>> parentResourceCache = makeParentCache(false);
@@ -98,16 +155,35 @@ private static <T> T load(LoadingCache<ClassLoader, Cache<String, T>> cache, Str
     private static <T> LoadingCache<ClassLoader, Cache<String, T>> makeParentCache(boolean weakValuesInnerCache) {
         // The outer cache has weak keys, so that we do not leak class loaders, but strong values, because the
         // inner caches are only referenced by the outer cache internally.
-        Caffeine<Object, Object> outerBuilder = Caffeine.newBuilder().recordStats().weakKeys();
+        Caffeine<Object, Object> outerBuilder = Caffeine.newBuilder()
+                .executor(cacheExecutor)
+                .scheduler(Scheduler.systemScheduler())
+                .recordStats()
+                .weakKeys();
         // The inner cache has strong keys, since they are just strings, and expires entries 15 minutes after they are
         // added to the cache, so that classes defined by dynamically installed plugins become available even if there
-        // were negative cache hits prior to the installation (ideally this would be done with a listener). The values
-        // for the inner cache may be weak if needed, for example parentClassCache uses weak values to avoid leaking
-        // classes and their loaders.
-        Caffeine<Object, Object> innerBuilder = Caffeine.newBuilder().recordStats().expireAfterWrite(Duration.ofMinutes(15));
+        // were negative cache hits prior to the installation (ideally this would be done with a listener, in which case
+        // this two-level Caffeine cache could possibly be replaced by something based on ClassValue, like
+        // org.kohsuke.stapler.ClassLoaderValue). The values for the inner cache may be weak if needed; for example,
+        // parentClassCache uses weak values to avoid leaking classes and their loaders.
+        Caffeine<Object, Object> innerBuilder = Caffeine.newBuilder()
+                .executor(cacheExecutor)
+                .scheduler(Scheduler.systemScheduler())
+                .recordStats()
+                .expireAfterWrite(Duration.ofMinutes(15));
         if (weakValuesInnerCache) {
             innerBuilder.weakValues();
         }
+        // In both cases above, note that by default Caffeine does not perform cleanup and evict values "automatically"
+        // or instantly after a value expires. Instead, it performs small amounts of maintenance work after write
+        // operations (or occasionally after read operations if writes are rare). When Caffeine is configured with its
+        // default Executor of ForkJoinPool#commonPool, it immediately schedules an asynchronous eviction event after
+        // such write operations; however, when using a custom executor, a scheduler is required in order to run the
+        // maintenance activity in the near future rather than deferring it to a subsequent cache operation. Since
+        // Caffeine does not define a default scheduler, we explicitly configure its scheduler to the recommended
+        // dedicated system-wide Scheduler#systemScheduler. This preserves, as closely as possible, Caffeine's behavior
+        // when using ForkJoinPool#commonPool. See
+        // com.github.benmanes.caffeine.cache.BoundedLocalCache#rescheduleCleanUpIfIncomplete for details.
 
         return outerBuilder.build(parentLoader -> innerBuilder.build());
     }