Use set once for systemd extender (#55497)

When Elasticsearch is starting up, we schedule a thread to repeatedly
let systemd know that we are still in the process of starting up. Today
we use a non-final field for this. This commit changes this to be a set
once so we can mark the field as final, and get stronger guarantees when
reasoning about the state of execution here.

modules/systemd/src/main/java/org/elasticsearch/systemd/SystemdPlugin.java

@@ -21,6 +21,7 @@ package org.elasticsearch.systemd;
 
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
+import org.apache.lucene.util.SetOnce;
 import org.elasticsearch.Build;
 import org.elasticsearch.client.Client;
 import org.elasticsearch.cluster.metadata.IndexNameExpressionResolver;
@@ -79,7 +80,7 @@ public class SystemdPlugin extends Plugin implements ClusterPlugin {
         enabled = Boolean.TRUE.toString().equals(esSDNotify);
     }
 
-    Scheduler.Cancellable extender;
+    final SetOnce<Scheduler.Cancellable> extender = new SetOnce<>();
 
     @Override
     public Collection<Object> createComponents(
@@ -94,15 +95,18 @@ public class SystemdPlugin extends Plugin implements ClusterPlugin {
         final NamedWriteableRegistry namedWriteableRegistry,
         final IndexNameExpressionResolver expressionResolver,
         final Supplier<RepositoriesService> repositoriesServiceSupplier) {
-        if (enabled) {
+        if (enabled == false) {
+            extender.set(null);
+            return Collections.emptyList();
+        }
         /*
          * Since we have set the service type to notify, by default systemd will wait up to sixty seconds for the process to send the
-             * READY=1 status via sd_notify. Since our startup can take longer than that (e.g., if we are upgrading on-disk metadata) then
+         * READY=1 status via sd_notify. Since our startup can take longer than that (e.g., if we are upgrading on-disk metadata) then we
-             * we need to repeatedly notify systemd that we are still starting up by sending EXTEND_TIMEOUT_USEC with an extension to the
+         * need to repeatedly notify systemd that we are still starting up by sending EXTEND_TIMEOUT_USEC with an extension to the timeout.
-             * timeout. Therefore, every fifteen seconds we send systemd a message via sd_notify to extend the timeout by thirty seconds.
+         * Therefore, every fifteen seconds we send systemd a message via sd_notify to extend the timeout by thirty seconds. We will cancel
-             * We will cancel this scheduled task after we successfully notify systemd that we are ready.
+         * this scheduled task after we successfully notify systemd that we are ready.
          */
-            extender = threadPool.scheduleWithFixedDelay(
+        extender.set(threadPool.scheduleWithFixedDelay(
             () -> {
                 final int rc = sd_notify(0, "EXTEND_TIMEOUT_USEC=30000000");
                 if (rc < 0) {
@@ -110,8 +114,7 @@ public class SystemdPlugin extends Plugin implements ClusterPlugin {
                 }
             },
             TimeValue.timeValueSeconds(15),
-                ThreadPool.Names.SAME);
+            ThreadPool.Names.SAME));
-        }
         return Collections.emptyList();
     }
 
@@ -124,6 +127,7 @@ public class SystemdPlugin extends Plugin implements ClusterPlugin {
     @Override
     public void onNodeStarted() {
         if (enabled == false) {
+            assert extender.get() == null;
             return;
         }
         final int rc = sd_notify(0, "READY=1");
@@ -131,8 +135,8 @@ public class SystemdPlugin extends Plugin implements ClusterPlugin {
             // treat failure to notify systemd of readiness as a startup failure
             throw new RuntimeException("sd_notify returned error [" + rc + "]");
         }
-        assert extender != null;
+        assert extender.get() != null;
-        final boolean cancelled = extender.cancel();
+        final boolean cancelled = extender.get().cancel();
         assert cancelled;
     }
 

modules/systemd/src/test/java/org/elasticsearch/systemd/SystemdPluginTests.java

@@ -63,28 +63,28 @@ public class SystemdPluginTests extends ESTestCase {
         final SystemdPlugin plugin = new SystemdPlugin(false, randomPackageBuildType, Boolean.TRUE.toString());
         plugin.createComponents(null, null, threadPool, null, null, null, null, null, null, null, null);
         assertTrue(plugin.isEnabled());
-        assertNotNull(plugin.extender);
+        assertNotNull(plugin.extender.get());
     }
 
     public void testIsNotPackageDistribution() {
         final SystemdPlugin plugin = new SystemdPlugin(false, randomNonPackageBuildType, Boolean.TRUE.toString());
         plugin.createComponents(null, null, threadPool, null, null, null, null, null, null, null, null);
         assertFalse(plugin.isEnabled());
-        assertNull(plugin.extender);
+        assertNull(plugin.extender.get());
     }
 
     public void testIsImplicitlyNotEnabled() {
         final SystemdPlugin plugin = new SystemdPlugin(false, randomPackageBuildType, null);
         plugin.createComponents(null, null, threadPool, null, null, null, null, null, null, null, null);
         assertFalse(plugin.isEnabled());
-        assertNull(plugin.extender);
+        assertNull(plugin.extender.get());
     }
 
     public void testIsExplicitlyNotEnabled() {
         final SystemdPlugin plugin = new SystemdPlugin(false, randomPackageBuildType, Boolean.FALSE.toString());
         plugin.createComponents(null, null, threadPool, null, null, null, null, null, null, null, null);
         assertFalse(plugin.isEnabled());
-        assertNull(plugin.extender);
+        assertNull(plugin.extender.get());
     }
 
     public void testInvalid() {
@@ -102,7 +102,7 @@ public class SystemdPluginTests extends ESTestCase {
             randomIntBetween(0, Integer.MAX_VALUE),
             (maybe, plugin) -> {
                 assertThat(maybe, OptionalMatchers.isEmpty());
-                verify(plugin.extender).cancel();
+                verify(plugin.extender.get()).cancel();
             });
     }
 
@@ -185,7 +185,7 @@ public class SystemdPluginTests extends ESTestCase {
         if (Boolean.TRUE.toString().equals(esSDNotify)) {
             assertNotNull(plugin.extender);
         } else {
-            assertNull(plugin.extender);
+            assertNull(plugin.extender.get());
         }
 
         boolean success = false;