Fixed shutdown teardown and shutdown staging wait

- Server stop defers teardown only while the plugin class loader close is held
- Shutdown with no Iris world loaded tears down during plugin disable
- Pre-unload hook drains generators inline when the class loader close cannot be held
- INMSBinding reports whether the class loader close is held
- Shutdown tests cover the held and unheld pre-unload paths
- Shutdown waits for an in-progress world replacement to finish staging
- Replacements started after shutdown begins are refused
- Admission tests cover the shutdown wait and the refusal

adapters/bukkit/plugin/src/test/java/art/arcane/iris/IrisShutdownMaintenanceTest.java

@@ -16,6 +16,7 @@ import art.arcane.iris.world.IrisWorld;
 import art.arcane.iris.world.WorldMaintenance;
 import art.arcane.iris.world.runtime.BukkitEnginePlatformHooks;
 import art.arcane.iris.world.task.J;
+import art.arcane.volmlib.integration.ReloadAware;
 import art.arcane.volmlib.nativelib.terrain.ServerShutdownBoundary;
 import org.bukkit.Bukkit;
 import org.bukkit.NamespacedKey;
@@ -37,9 +38,11 @@ import java.util.concurrent.atomic.AtomicReference;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
+import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.anyLong;
 import static org.mockito.ArgumentMatchers.eq;
 import static org.mockito.Mockito.CALLS_REAL_METHODS;
+import static org.mockito.Mockito.doCallRealMethod;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.mockStatic;
 import static org.mockito.Mockito.never;
@@ -109,8 +112,6 @@ public class IrisShutdownMaintenanceTest {
                 }
                 verify(fixture.generator).quiesceForServerShutdown();
                 verify(fixture.generator, never()).closeAsync();
-                fixture.nms.verify(INMS::get);
-                verify(fixture.binding).deferPluginClassLoaderClose();
                 verify(fixture.binding, never()).releasePluginClassLoaderClose();
             }
             assertEquals(0, sessions.activeLeases());
@@ -151,6 +152,37 @@ public class IrisShutdownMaintenanceTest {
         }
     }
 
+    @Test
+    public void serverStopPreUnloadDefersTeardownWhileTheClassLoaderIsHeld() throws Exception {
+        try (Fixture fixture = new Fixture()) {
+            when(fixture.binding.deferPluginClassLoaderClose()).thenReturn(true);
+            setField(fixture.plugin, "postStopFinisherStarted", new AtomicBoolean(true));
+            try {
+                fixture.commitReloadDuringServerStop();
+                verify(fixture.generator).quiesceForServerShutdown();
+                verify(fixture.generator, never()).closeAsync();
+                assertTrue(fixture.serverStopTeardownDeferred());
+            } finally {
+                WorldMaintenance.endWorldMaintenance(fixture.identity, "pre-unload:HOT_UNLOAD");
+            }
+        }
+    }
+
+    @Test
+    public void serverStopPreUnloadDrainsInlineWhenTheClassLoaderCannotBeHeld() throws Exception {
+        try (Fixture fixture = new Fixture()) {
+            when(fixture.binding.deferPluginClassLoaderClose()).thenReturn(false);
+            try {
+                fixture.commitReloadDuringServerStop();
+                verify(fixture.generator).closeAsync();
+                verify(fixture.generator, never()).quiesceForServerShutdown();
+                assertFalse(fixture.serverStopTeardownDeferred());
+            } finally {
+                WorldMaintenance.endWorldMaintenance(fixture.identity, "pre-unload:HOT_UNLOAD", true);
+            }
+        }
+    }
+
     private static void setField(Iris plugin, String name, Object value) throws ReflectiveOperationException {
         Field field = Iris.class.getDeclaredField(name);
         field.setAccessible(true);
@@ -206,6 +238,19 @@ public class IrisShutdownMaintenanceTest {
             method.invoke(plugin, "manual_disable", 5L);
         }
 
+        private void commitReloadDuringServerStop() throws ReflectiveOperationException {
+            setField(plugin, "alreadyDrained", new AtomicBoolean());
+            toolbelt.when(IrisToolbelt::isServerStopping).thenReturn(true);
+            doCallRealMethod().when(plugin).commitReload(any());
+            plugin.commitReload(ReloadAware.PreUnloadReason.HOT_UNLOAD);
+        }
+
+        private boolean serverStopTeardownDeferred() throws ReflectiveOperationException {
+            Field field = Iris.class.getDeclaredField("serverStopTeardownDeferred");
+            field.setAccessible(true);
+            return ((AtomicBoolean) field.get(plugin)).get();
+        }
+
         private void startFinisher() throws ReflectiveOperationException {
             Method method = Iris.class.getDeclaredMethod("startPostStopFinisher");
             method.setAccessible(true);

adapters/bukkit/plugin/src/test/java/art/arcane/iris/world/PendingWorldReplacementAdmissionTest.java

@@ -11,12 +11,16 @@ import art.arcane.iris.platform.bukkit.plugin.VolmitSender;
 import org.bukkit.NamespacedKey;
 import org.junit.Test;
 
+import java.io.IOException;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 import java.util.concurrent.Future;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicReference;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.assertThrows;
 import static org.mockito.Mockito.mock;
 
@@ -42,4 +46,55 @@ public class PendingWorldReplacementAdmissionTest {
             executor.awaitTermination(5L, TimeUnit.SECONDS);
         }
     }
+
+    @Test
+    public void shutdownRejectsReplacementsThatStartAfterIt() {
+        PendingWorldReplacementManager manager = new PendingWorldReplacementManager(mock(Iris.class));
+        manager.awaitStagingForShutdown();
+
+        IOException rejected = assertThrows(IOException.class, () -> manager.stageReplacement(
+                mock(VolmitSender.class), NamespacedKey.minecraft("overworld"), mock(IrisDimension.class), null));
+
+        assertTrue(rejected.getMessage().contains("shutting down"));
+        assertTrue(LifecycleOperationCoordinator.get().isIdle());
+    }
+
+    @Test
+    public void shutdownWaitsForTheReplacementBeingStaged() throws Exception {
+        PendingWorldReplacementManager manager = new PendingWorldReplacementManager(mock(Iris.class));
+        AtomicReference<Throwable> stagingFailure = new AtomicReference<>();
+        Thread staging = new Thread(() -> {
+            try {
+                manager.stageReplacement(mock(VolmitSender.class), NamespacedKey.minecraft("overworld"),
+                        mock(IrisDimension.class), null);
+            } catch (Throwable failure) {
+                stagingFailure.set(failure);
+            }
+        });
+        Thread shutdown = new Thread(manager::awaitStagingForShutdown);
+        synchronized (manager) {
+            staging.start();
+            awaitState(staging, Thread.State.BLOCKED);
+            shutdown.start();
+            awaitState(shutdown, Thread.State.WAITING);
+            assertTrue(staging.isAlive());
+        }
+        shutdown.join(TimeUnit.SECONDS.toMillis(10L));
+        staging.join(TimeUnit.SECONDS.toMillis(10L));
+
+        assertFalse(shutdown.isAlive());
+        assertFalse(staging.isAlive());
+        assertTrue(stagingFailure.get() instanceof Exception);
+        assertTrue(LifecycleOperationCoordinator.get().isIdle());
+    }
+
+    private static void awaitState(Thread thread, Thread.State state) throws InterruptedException {
+        long deadline = System.nanoTime() + TimeUnit.SECONDS.toNanos(5L);
+        while (thread.getState() != state) {
+            if (System.nanoTime() > deadline) {
+                throw new AssertionError(thread.getName() + " never reached " + state + "; it is " + thread.getState());
+            }
+            Thread.sleep(5L);
+        }
+    }
 }