Restored world reload lifecycle

- Generation admission retires after complete runtime cleanup
- Fresh world loads can activate pending compatible updates
- Failed generator closes remain sealed and retryable

adapters/modded-common/src/main/java/art/arcane/iris/modded/ModdedWorldEngines.java

@@ -28,6 +28,7 @@ import art.arcane.iris.engine.history.GenerationActivation;
 import art.arcane.iris.engine.history.GenerationEpoch;
 import art.arcane.iris.engine.history.GenerationEpochContractFactory;
 import art.arcane.iris.engine.history.GenerationHistory;
+import art.arcane.iris.engine.history.GenerationAdmission;
 import art.arcane.iris.engine.history.GenerationHistoryRuntimeRouter;
 import art.arcane.iris.engine.history.IrisBoundarySignatureSampler;
 import art.arcane.iris.engine.history.TransitionGenerationPlan;
@@ -191,44 +192,46 @@ public final class ModdedWorldEngines {
             ModdedGenerationHistoryStorage.ActivePack opened
     ) throws IOException {
         GenerationHistory history = opened.history();
-        history.prepareCurrentGenerator(IrisSettings.get().getGenerator().getGenerationTransitionWidthBlocks());
-        opened = ModdedGenerationHistoryStorage.resolveActive(history);
-        long openedActivationId = history.activeActivation().activationId();
-        IrisEngine candidate = buildEngine(level, seed, opened);
-        boolean ready = false;
-        try {
-            GenerationHistoryRuntimeRouter router = candidate.attachGenerationHistory(
-                    history,
-                    IrisBoundarySignatureSampler.INSTANCE,
-                    IrisSettings.get().getGenerator().getGenerationTransitionWidthBlocks()
-            );
-            router.preloadActiveRuntimes();
-            if (history.activeActivation().activationId() != openedActivationId) {
-                close(candidate);
-                ModdedGenerationHistoryStorage.ActivePack promoted =
-                        ModdedGenerationHistoryStorage.resolveActive(history);
-                candidate = buildEngine(level, seed, promoted);
-                router = GenerationHistoryRuntimeRouter.attach(
-                        candidate,
+        try (GenerationAdmission.RuntimeLease startupAdmission = history.retainRuntime()) {
+            history.prepareCurrentGenerator(IrisSettings.get().getGenerator().getGenerationTransitionWidthBlocks());
+            opened = ModdedGenerationHistoryStorage.resolveActive(history);
+            long openedActivationId = history.activeActivation().activationId();
+            IrisEngine candidate = buildEngine(level, seed, opened);
+            boolean ready = false;
+            try {
+                GenerationHistoryRuntimeRouter router = candidate.attachGenerationHistory(
                         history,
-                        IrisBoundarySignatureSampler.INSTANCE
+                        IrisBoundarySignatureSampler.INSTANCE,
+                        IrisSettings.get().getGenerator().getGenerationTransitionWidthBlocks()
                 );
                 router.preloadActiveRuntimes();
-            }
-            requireReady(candidate, level, ModdedGenerationMode.PERSISTENT_RESTORE);
-            ready = true;
-            ModdedIrisLog.info("Iris engine up for {}: pack={} dim={} seed={} height={}..{} activation={}",
-                    level.dimension().identifier(),
-                    candidate.getData().getDataFolder().getAbsolutePath(),
-                    candidate.getDimension().getLoadKey(),
-                    seed,
-                    candidate.getDimension().getMinHeight(),
-                    candidate.getDimension().getMaxHeight(),
-                    history.activeActivation().activationId());
-            return candidate;
-        } finally {
-            if (!ready) {
-                close(candidate);
+                if (history.activeActivation().activationId() != openedActivationId) {
+                    close(candidate);
+                    ModdedGenerationHistoryStorage.ActivePack promoted =
+                            ModdedGenerationHistoryStorage.resolveActive(history);
+                    candidate = buildEngine(level, seed, promoted);
+                    router = GenerationHistoryRuntimeRouter.attach(
+                            candidate,
+                            history,
+                            IrisBoundarySignatureSampler.INSTANCE
+                    );
+                    router.preloadActiveRuntimes();
+                }
+                requireReady(candidate, level, ModdedGenerationMode.PERSISTENT_RESTORE);
+                ready = true;
+                ModdedIrisLog.info("Iris engine up for {}: pack={} dim={} seed={} height={}..{} activation={}",
+                        level.dimension().identifier(),
+                        candidate.getData().getDataFolder().getAbsolutePath(),
+                        candidate.getDimension().getLoadKey(),
+                        seed,
+                        candidate.getDimension().getMinHeight(),
+                        candidate.getDimension().getMaxHeight(),
+                        history.activeActivation().activationId());
+                return candidate;
+            } finally {
+                if (!ready) {
+                    close(candidate);
+                }
             }
         }
     }

core/src/main/java/art/arcane/iris/engine/EngineShutdownSequence.java

@@ -27,11 +27,16 @@ import art.arcane.iris.engine.framework.EngineTarget;
 import art.arcane.iris.engine.framework.NativeStructureOwnershipStore;
 import art.arcane.iris.engine.framework.PreservationRegistry;
 import art.arcane.iris.engine.mantle.EngineMantle;
+import art.arcane.iris.engine.history.GenerationAdmission;
+import art.arcane.iris.engine.history.GenerationHistory;
 import art.arcane.iris.spi.IrisLogging;
 import art.arcane.iris.spi.IrisServices;
 
 import java.util.Collections;
 import java.util.IdentityHashMap;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Iterator;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
 
@@ -44,6 +49,7 @@ final class EngineShutdownSequence {
     private static final long CLOSE_RETRY_DRAIN_TIMEOUT_MILLIS = 5000L;
 
     private final IrisEngine engine;
+    private final List<GenerationAdmission.RuntimeLease> generationAdmissions = new ArrayList<>(1);
     private final Set<RuntimeAssembly> incompleteAssemblies = ConcurrentHashMap.newKeySet();
     private final Set<EngineRuntime> unpublishedRuntimes = Collections.synchronizedSet(
             Collections.newSetFromMap(new IdentityHashMap<>()));
@@ -58,6 +64,10 @@ final class EngineShutdownSequence {
         this.engine = engine;
     }
 
+    void retainGenerationHistory(GenerationHistory history) {
+        generationAdmissions.add(history.retainRuntime());
+    }
+
     void close() {
         if (!engine.beginShutdown()) {
             return;
@@ -161,9 +171,12 @@ final class EngineShutdownSequence {
                     && targetReleased
                     && engineDataReleased
                     && preservationReleased) {
-                engine.closed = true;
-                engine.lifecycleState = LifecycleState.CLOSED;
-                IrisLogging.debug("Engine Fully Shutdown!");
+                failure = releaseGenerationAdmissions(failure);
+                if (failure == null) {
+                    engine.closed = true;
+                    engine.lifecycleState = LifecycleState.CLOSED;
+                    IrisLogging.debug("Engine Fully Shutdown!");
+                }
             }
         }
         if (failure != null) {
@@ -171,6 +184,19 @@ final class EngineShutdownSequence {
         }
     }
 
+    private Throwable releaseGenerationAdmissions(Throwable failure) {
+        Iterator<GenerationAdmission.RuntimeLease> admissions = generationAdmissions.iterator();
+        while (admissions.hasNext()) {
+            GenerationAdmission.RuntimeLease admission = admissions.next();
+            Throwable admissionFailure = runCleanup(null, admission::close);
+            failure = appendFailure(failure, admissionFailure);
+            if (admissionFailure == null) {
+                admissions.remove();
+            }
+        }
+        return failure;
+    }
+
     private void reportIncompleteClose(Throwable failure) {
         IrisLogging.error("Iris engine shutdown remains incomplete after cleanup failures for " + engine.getWorld().name() + ".");
         IrisLogging.reportError(failure);
@@ -218,7 +244,10 @@ final class EngineShutdownSequence {
                 cleanupFailure = releasePreservation(cleanupFailure);
             }
             if (runtimeReleased && targetReleased && engineDataReleased && preservationReleased) {
-                engine.closed = true;
+                cleanupFailure = releaseGenerationAdmissions(cleanupFailure);
+                if (generationAdmissions.isEmpty()) {
+                    engine.closed = true;
+                }
             }
         }
         if (cleanupFailure != null && cleanupFailure != original) {

core/src/main/java/art/arcane/iris/engine/IrisEngine.java

@@ -1010,6 +1010,7 @@ public class IrisEngine implements Engine {
                 if (generationHistoryRuntimeRouter != null) {
                     throw new IllegalStateException("Iris engine already has a generation-history runtime router.");
                 }
+                shutdownSequence.retainGenerationHistory(required.history());
                 generationHistoryRuntimeRouter = required;
                 generationHistoryRoutingRequired = true;
             }

core/src/main/java/art/arcane/iris/engine/history/GenerationAdmission.java

@@ -16,7 +16,12 @@ public final class GenerationAdmission {
         Path root = Objects.requireNonNull(dimensionRoot, "dimensionRoot")
                 .toAbsolutePath()
                 .normalize();
-        this.state = STATES.computeIfAbsent(root, ignored -> new State());
+        this.state = STATES.computeIfAbsent(root, State::new);
+    }
+
+    public RuntimeLease retainRuntime() {
+        state.retainRuntime();
+        return new RuntimeLease(state);
     }
 
     public StageLease enterStage() {
@@ -34,6 +39,23 @@ public final class GenerationAdmission {
         return new CutoverLease(state);
     }
 
+    public static final class RuntimeLease implements AutoCloseable {
+        private final State state;
+        private boolean closed;
+
+        private RuntimeLease(State state) {
+            this.state = state;
+        }
+
+        @Override
+        public synchronized void close() {
+            if (!closed) {
+                state.releaseRuntime();
+                closed = true;
+            }
+        }
+    }
+
     public static final class StageLease implements AutoCloseable {
         private final State state;
         private final AtomicBoolean closed;
@@ -69,26 +91,71 @@ public final class GenerationAdmission {
     }
 
     private static final class State {
+        private final Path root;
         private final ReentrantLock lock;
         private final Condition changed;
+        private int runtimeOwners;
+        private int waitingStages;
         private int activeStages;
         private int waitingCutovers;
         private boolean cutoverActive;
         private boolean generationAdmissionOpened;
+        private boolean closed;
 
-        private State() {
+        private State(Path root) {
+            this.root = root;
             this.lock = new ReentrantLock(true);
             this.changed = lock.newCondition();
         }
 
+        private void retainRuntime() {
+            lock.lock();
+            try {
+                requireOpen();
+                runtimeOwners++;
+            } finally {
+                lock.unlock();
+            }
+        }
+
+        private void releaseRuntime() {
+            lock.lock();
+            try {
+                requireOpen();
+                if (runtimeOwners == 1) {
+                    if (activeStages > 0 || waitingStages > 0 || waitingCutovers > 0 || cutoverActive) {
+                        throw new IllegalStateException("Generation admission still has active stages or cutovers.");
+                    }
+                    closed = true;
+                    STATES.remove(root, this);
+                }
+                runtimeOwners--;
+            } finally {
+                lock.unlock();
+            }
+        }
+
+        private void requireOpen() {
+            if (closed) {
+                throw new IllegalStateException("Generation admission belongs to a closed runtime.");
+            }
+        }
+
         private void enterStage() {
             lock.lock();
             try {
+                requireOpen();
                 generationAdmissionOpened = true;
-                while (cutoverActive || waitingCutovers > 0) {
-                    changed.awaitUninterruptibly();
+                waitingStages++;
+                try {
+                    while (cutoverActive || waitingCutovers > 0) {
+                        changed.awaitUninterruptibly();
+                        requireOpen();
+                    }
+                    activeStages++;
+                } finally {
+                    waitingStages--;
                 }
-                activeStages++;
             } finally {
                 lock.unlock();
             }
@@ -112,6 +179,7 @@ public final class GenerationAdmission {
         private void beginCutover(boolean startupOnly) {
             lock.lock();
             try {
+                requireOpen();
                 if (startupOnly && generationAdmissionOpened) {
                     throw new IllegalStateException(
                             "Generation activation promotion is only allowed before generation admission opens."
@@ -121,6 +189,7 @@ public final class GenerationAdmission {
                 try {
                     while (cutoverActive || activeStages > 0) {
                         changed.awaitUninterruptibly();
+                        requireOpen();
                     }
                     cutoverActive = true;
                 } finally {

core/src/main/java/art/arcane/iris/engine/history/GenerationHistory.java

@@ -64,6 +64,10 @@ public final class GenerationHistory {
         validateReferencedState();
     }
 
+    public GenerationAdmission.RuntimeLease retainRuntime() {
+        return admission.retainRuntime();
+    }
+
     public SavedBiomeStore savedBiomes() {
         return savedBiomes;
     }

core/src/main/java/art/arcane/iris/engine/platform/BukkitChunkGenerator.java

@@ -48,6 +48,7 @@ import art.arcane.iris.engine.framework.WrongEngineBroException;
 import art.arcane.iris.engine.history.GenerationActivation;
 import art.arcane.iris.engine.history.GenerationEpoch;
 import art.arcane.iris.engine.history.GenerationHistory;
+import art.arcane.iris.engine.history.GenerationAdmission;
 import art.arcane.iris.engine.history.GenerationHistoryRuntimeRouter;
 import art.arcane.iris.engine.history.IrisBoundarySignatureSampler;
 import art.arcane.iris.engine.history.TransitionGenerationPlan;
@@ -396,66 +397,68 @@ public class BukkitChunkGenerator extends ChunkGenerator implements PlatformChun
     }
 
     private void setupEngine() {
-        lastMode = StudioMode.NORMAL;
-        lastJigsawStudioRequestId = null;
-        if (generationHistory != null) {
-            try {
-                generationHistory.prepareCurrentGenerator(IrisSettings.get().getGenerator().getGenerationTransitionWidthBlocks());
-                targetCache.reset();
-            } catch (IOException failure) {
-                throw new IllegalStateException("Unable to prepare saved Iris terrain for the current generator.", failure);
-            }
-        }
-        EngineTarget engineTarget = getTarget();
-        String packKey = engineTarget.getDimension().getLoadKey();
-        JigsawStudioActivation.Request request = studio
-                ? JigsawStudioActivation.getGeneratorRequest(packKey)
-                : null;
-        JigsawStudioSession session = request == null
-                ? null
-                : JigsawStudioActivation.getGeneratorSession(packKey);
-        jigsawStudioActive = request != null
-                && session != null
-                && session.sessionId().equals(request.requestId());
-        objectStudioActive = studio && ObjectStudioActivation.isActive(packKey);
-        IrisEngine createdEngine = createEngine(engineTarget);
-        if (generationHistory != null) {
-            long initialActivationId = generationHistory.activeActivation().activationId();
-            int transitionWidthBlocks = IrisSettings.get()
-                    .getGenerator()
-                    .getGenerationTransitionWidthBlocks();
-            try {
-                createdEngine.attachGenerationHistory(
-                        generationHistory,
-                        IrisBoundarySignatureSampler.INSTANCE,
-                        transitionWidthBlocks);
-                if (generationHistory.activeActivation().activationId() != initialActivationId) {
-                    createdEngine.close();
+        try (GenerationAdmission.RuntimeLease startupAdmission = generationHistory == null ? null : generationHistory.retainRuntime()) {
+            lastMode = StudioMode.NORMAL;
+            lastJigsawStudioRequestId = null;
+            if (generationHistory != null) {
+                try {
+                    generationHistory.prepareCurrentGenerator(IrisSettings.get().getGenerator().getGenerationTransitionWidthBlocks());
                     targetCache.reset();
-                    createdEngine = createEngine(loadActiveGenerationHistoryTarget());
+                } catch (IOException failure) {
+                    throw new IllegalStateException("Unable to prepare saved Iris terrain for the current generator.", failure);
+                }
+            }
+            EngineTarget engineTarget = getTarget();
+            String packKey = engineTarget.getDimension().getLoadKey();
+            JigsawStudioActivation.Request request = studio
+                    ? JigsawStudioActivation.getGeneratorRequest(packKey)
+                    : null;
+            JigsawStudioSession session = request == null
+                    ? null
+                    : JigsawStudioActivation.getGeneratorSession(packKey);
+            jigsawStudioActive = request != null
+                    && session != null
+                    && session.sessionId().equals(request.requestId());
+            objectStudioActive = studio && ObjectStudioActivation.isActive(packKey);
+            IrisEngine createdEngine = createEngine(engineTarget);
+            if (generationHistory != null) {
+                long initialActivationId = generationHistory.activeActivation().activationId();
+                int transitionWidthBlocks = IrisSettings.get()
+                        .getGenerator()
+                        .getGenerationTransitionWidthBlocks();
+                try {
                     createdEngine.attachGenerationHistory(
                             generationHistory,
                             IrisBoundarySignatureSampler.INSTANCE,
                             transitionWidthBlocks);
-                }
-            } catch (Throwable failure) {
-                try {
-                    createdEngine.close();
-                } catch (Throwable closeFailure) {
-                    if (failure != closeFailure) {
-                        failure.addSuppressed(closeFailure);
+                    if (generationHistory.activeActivation().activationId() != initialActivationId) {
+                        createdEngine.close();
+                        targetCache.reset();
+                        createdEngine = createEngine(loadActiveGenerationHistoryTarget());
+                        createdEngine.attachGenerationHistory(
+                                generationHistory,
+                                IrisBoundarySignatureSampler.INSTANCE,
+                                transitionWidthBlocks);
                     }
+                } catch (Throwable failure) {
+                    try {
+                        createdEngine.close();
+                    } catch (Throwable closeFailure) {
+                        if (failure != closeFailure) {
+                            failure.addSuppressed(closeFailure);
+                        }
+                    }
+                    throw propagateEngineSetupFailure(failure);
                 }
-                throw propagateEngineSetupFailure(failure);
             }
+            createdEngine.setNativeStructureVolumeQueriesEnabled(shouldGenerateNativeStructures(
+                    isAuthoringStudio(),
+                    studioEntryBootstrapActive.get(),
+                    initializationFailure != null));
+            engine = createdEngine;
+            populators.clear();
+            targetCache.reset();
         }
-        createdEngine.setNativeStructureVolumeQueriesEnabled(shouldGenerateNativeStructures(
-                isAuthoringStudio(),
-                studioEntryBootstrapActive.get(),
-                initializationFailure != null));
-        engine = createdEngine;
-        populators.clear();
-        targetCache.reset();
     }
 
     private EngineTarget loadActiveGenerationHistoryTarget() throws IOException {
@@ -672,7 +675,6 @@ public class BukkitChunkGenerator extends ChunkGenerator implements PlatformChun
 
     @Override
     public CompletableFuture<Void> closeAsync() {
-        closing = true;
         // Outside the exclusive-control block so every close path detaches the WorldInit
         // listener, including a rollback where the world never materialized. Guarded: with no
         // hosted plugin (unit tests, teardown) volmitPlugin() throws, and that must not stop
@@ -689,6 +691,8 @@ public class BukkitChunkGenerator extends ChunkGenerator implements PlatformChun
             }
         }
 
+        boolean alreadyClosing = closing;
+        closing = true;
         CompletableFuture<Void> operation;
         try {
             operation = withExclusiveControlFuture(() -> {
@@ -712,11 +716,13 @@ public class BukkitChunkGenerator extends ChunkGenerator implements PlatformChun
                 populators.clear();
             });
         } catch (Throwable throwable) {
-            future.completeExceptionally(throwable);
+            if (!alreadyClosing) {
+                // A failed close must stay retryable; leaving closing latched would permanently
+                // reject generation for a world that may still be loaded.
+                closing = false;
+            }
             closeFuture.compareAndSet(future, null);
-            // A failed close must stay retryable; leaving closing latched would permanently
-            // reject generation for a world that may still be loaded.
-            closing = false;
+            future.completeExceptionally(throwable);
             return future;
         }
 
@@ -727,6 +733,7 @@ public class BukkitChunkGenerator extends ChunkGenerator implements PlatformChun
                 // The close body already ran and tore the engine down; unlike the pre-dispatch
                 // failure above, resetting the gate here would advertise a healthy generator
                 // over a CLOSED or FAILED engine. Stay latched and surface the failure.
+                closeFuture.compareAndSet(future, null);
                 future.completeExceptionally(throwable);
             }
         });

core/src/test/java/art/arcane/iris/engine/EngineShutdownDrainTest.java

@@ -12,6 +12,13 @@ import art.arcane.iris.engine.framework.GenerationSessionException;
 import art.arcane.iris.engine.framework.GenerationSessionManager;
 import art.arcane.iris.engine.framework.NativeStructureOwnershipStore;
 import art.arcane.iris.engine.history.GenerationKernelRegistry;
+import art.arcane.iris.engine.history.GenerationHistory;
+import art.arcane.iris.engine.history.GenerationAdmission;
+import art.arcane.iris.engine.history.GenerationHistoryRuntimeRouter;
+import art.arcane.iris.engine.history.GenerationEpoch;
+import art.arcane.iris.engine.history.GenerationEpochContractFactory;
+import art.arcane.iris.engine.history.GenerationPackFingerprint;
+import art.arcane.iris.engine.history.GenerationRegistryContract;
 import art.arcane.iris.engine.object.IrisWorld;
 import art.arcane.iris.engine.framework.EngineMode;
 import art.arcane.iris.engine.mantle.EngineMantle;
@@ -22,16 +29,21 @@ import art.arcane.iris.spi.PlatformRegistries;
 import org.junit.AfterClass;
 import org.junit.BeforeClass;
 import org.junit.Test;
+import org.junit.Rule;
+import org.junit.rules.TemporaryFolder;
 import org.mockito.InOrder;
 import org.mockito.MockedStatic;
 
 import java.lang.reflect.Field;
+import java.nio.file.Files;
+import java.nio.file.Path;
 import java.util.Optional;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.atomic.AtomicBoolean;
 
 import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertSame;
 import static org.junit.Assert.assertTrue;
@@ -42,6 +54,7 @@ import static org.mockito.ArgumentMatchers.anyInt;
 import static org.mockito.ArgumentMatchers.same;
 import static org.mockito.ArgumentMatchers.isNull;
 import static org.mockito.Mockito.doThrow;
+import static org.mockito.Mockito.doCallRealMethod;
 import static org.mockito.Mockito.doNothing;
 import static org.mockito.Mockito.doReturn;
 import static org.mockito.Mockito.doAnswer;
@@ -58,6 +71,9 @@ import static org.mockito.Mockito.when;
 public class EngineShutdownDrainTest {
     private static IrisPlatform previousPlatform;
 
+    @Rule
+    public TemporaryFolder temporaryFolder = new TemporaryFolder();
+
     @BeforeClass
     public static void bindPlatform() {
         previousPlatform = IrisPlatforms.isBound() ? IrisPlatforms.get() : null;
@@ -77,6 +93,100 @@ public class EngineShutdownDrainTest {
         }
     }
 
+    @Test
+    public void successfulEngineCloseAllowsTheSameWorldToPrepareGenerationAgain() throws Exception {
+        GenerationHistory history = history();
+        ShutdownFixture fixture = new ShutdownFixture();
+        fixture.attachHistory(history);
+        try (GenerationHistory.GenerationStage ignored = history.openStage(2, 3)) {
+        }
+        assertThrows(IllegalStateException.class, () -> history.prepareCurrentGenerator(32));
+        try (MockedStatic<NativeStructureOwnershipStore> ownership = mockStatic(NativeStructureOwnershipStore.class)) {
+            fixture.shutdown.close();
+        }
+        assertTrue(fixture.engine.closed);
+        GenerationHistory reopened = GenerationHistory.open(history.paths().dimensionRoot());
+        reopened.prepareCurrentGenerator(32);
+        try (GenerationHistory.GenerationStage ignored = reopened.openStage(3, 4)) {
+        }
+    }
+
+    @Test
+    public void failedEngineCloseKeepsAdmissionUntilSuccessfulRetry() throws Exception {
+        GenerationHistory history = history();
+        ShutdownFixture fixture = new ShutdownFixture();
+        fixture.attachHistory(history);
+        try (GenerationHistory.GenerationStage ignored = history.openStage(2, 3)) {
+        }
+        doThrow(new IllegalStateException("Planner still active")).when(fixture.complex).close();
+        try (MockedStatic<NativeStructureOwnershipStore> ownership = mockStatic(NativeStructureOwnershipStore.class)) {
+            assertThrows(IllegalStateException.class, fixture.shutdown::close);
+            GenerationHistory premature = GenerationHistory.open(history.paths().dimensionRoot());
+            assertThrows(IllegalStateException.class, () -> premature.prepareCurrentGenerator(32));
+            doNothing().when(fixture.complex).close();
+            fixture.shutdown.close();
+        }
+        GenerationHistory reopened = GenerationHistory.open(history.paths().dimensionRoot());
+        reopened.prepareCurrentGenerator(32);
+    }
+
+    @Test
+    public void startupOwnershipSpansReplacementEnginesUsingTheSameHistory() throws Exception {
+        GenerationHistory history = history();
+        ShutdownFixture first = new ShutdownFixture();
+        ShutdownFixture replacement = new ShutdownFixture();
+        try (GenerationAdmission.RuntimeLease startup = history.retainRuntime();
+             MockedStatic<NativeStructureOwnershipStore> ownership = mockStatic(NativeStructureOwnershipStore.class)) {
+            first.attachHistory(history);
+            first.shutdown.close();
+            history.prepareCurrentGenerator(32);
+            replacement.attachHistory(history);
+        }
+        try (GenerationHistory.GenerationStage ignored = history.openStage(2, 3)) {
+        }
+        try (MockedStatic<NativeStructureOwnershipStore> ownership = mockStatic(NativeStructureOwnershipStore.class)) {
+            replacement.shutdown.close();
+        }
+        GenerationHistory.open(history.paths().dimensionRoot()).prepareCurrentGenerator(32);
+    }
+
+    @Test
+    public void reopenedWorldPromotesPendingGenerationAfterTheOldEngineCloses() throws Exception {
+        GenerationHistory history = history();
+        ShutdownFixture fixture = new ShutdownFixture();
+        fixture.attachHistory(history);
+        try (GenerationHistory.GenerationStage ignored = history.openStage(2, 3)) {
+        }
+        Path replacementPack = temporaryFolder.newFolder("replacement-pack").toPath();
+        Files.createDirectories(replacementPack.resolve("dimensions"));
+        Files.writeString(replacementPack.resolve("dimensions/main.json"), "{\"name\":\"replacement\"}");
+        history.stageUpdate(replacementPack,
+                GenerationPackFingerprint.compute(replacementPack, GenerationPackFingerprint.CURRENT_VERSION),
+                history.activeEpoch().dimensionContract(), GenerationRegistryContract.empty(), 32);
+        assertThrows(IllegalStateException.class, () -> history.prepareCurrentGenerator(32));
+        try (MockedStatic<NativeStructureOwnershipStore> ownership = mockStatic(NativeStructureOwnershipStore.class)) {
+            fixture.shutdown.close();
+        }
+        GenerationHistory reopened = GenerationHistory.open(history.paths().dimensionRoot());
+        reopened.prepareCurrentGenerator(32);
+        assertEquals(2L, reopened.activeActivation().activationId());
+        assertFalse(reopened.pendingActivation().isPresent());
+    }
+
+    private GenerationHistory history() throws Exception {
+        Path world = temporaryFolder.newFolder("history-world").toPath();
+        Path pack = temporaryFolder.newFolder("history-pack").toPath();
+        Files.createDirectories(pack.resolve("dimensions"));
+        Files.writeString(pack.resolve("dimensions/main.json"), "{}");
+        GenerationEpoch.DimensionContract contract = new GenerationEpoch.DimensionContract(
+                "overworld", "iris:overworld_type", "NORMAL", "OVERWORLD", 127,
+                -64, 384, 384, 1D, false, "none", 0, "0".repeat(64),
+                GenerationEpochContractFactory.CURRENT_DIMENSION_TYPE_FINGERPRINT_SCHEMA, "c".repeat(64));
+        return GenerationHistory.create(world, pack,
+                GenerationPackFingerprint.compute(pack, GenerationPackFingerprint.CURRENT_VERSION),
+                42L, contract, GenerationRegistryContract.empty());
+    }
+
     @Test
     public void failedPlannerDrainRetainsRuntimeStorage() {
         GenerationRuntime runtime = mock(GenerationRuntime.class);
@@ -491,6 +601,15 @@ public class EngineShutdownDrainTest {
             return kernel;
         }
 
+        private void attachHistory(GenerationHistory history) throws Exception {
+            GenerationHistoryRuntimeRouter router = mock(GenerationHistoryRuntimeRouter.class);
+            when(router.engine()).thenReturn(engine);
+            when(router.history()).thenReturn(history);
+            setField(engine, "generationHistoryRuntimeRouterLock", new Object());
+            doCallRealMethod().when(engine).attachGenerationHistoryRuntimeRouter(router);
+            engine.attachGenerationHistoryRuntimeRouter(router);
+        }
+
         private ShutdownFixture() throws Exception {
             EngineBackgroundTasks background = mock(EngineBackgroundTasks.class);
             when(background.drainBackgroundTasks(anyString()))

core/src/test/java/art/arcane/iris/engine/IrisEngineGenerationRuntimeScopeTest.java

@@ -18,6 +18,8 @@ import art.arcane.iris.engine.framework.SeedManager;
 import art.arcane.iris.engine.framework.IrisEngineMode;
 import art.arcane.iris.engine.framework.NativeStructureVolumeMemo;
 import art.arcane.iris.engine.history.GenerationHistoryRuntimeRouter;
+import art.arcane.iris.engine.history.GenerationHistory;
+import art.arcane.iris.engine.history.GenerationAdmission;
 import art.arcane.iris.engine.mantle.EngineMantle;
 import art.arcane.iris.engine.object.IrisDimension;
 import art.arcane.iris.engine.object.IrisBiome;
@@ -515,9 +517,7 @@ public class IrisEngineGenerationRuntimeScopeTest {
     public void attachedHistoryRouterIsClearedWhenEngineShutdownBegins() throws Exception {
         RuntimeFixture active = runtime(1, 1D, 1D, 1D);
         IrisEngine engine = engine(active.runtime, mock(EngineEffects.class), mock(EngineWorldManager.class));
-        GenerationHistoryRuntimeRouter router = mock(GenerationHistoryRuntimeRouter.class);
-        when(router.biomes()).thenReturn(mock(SavedBiomeRuntime.class));
-        when(router.engine()).thenReturn(engine);
+        GenerationHistoryRuntimeRouter router = historyRouter(engine);
         engine.attachGenerationHistoryRuntimeRouter(router);
 
         assertTrue(engine.beginShutdown());
@@ -531,9 +531,7 @@ public class IrisEngineGenerationRuntimeScopeTest {
     public void attachedHistoryRejectsGenerationRuntimeHotload() throws Exception {
         RuntimeFixture active = runtime(1, 1D, 1D, 1D);
         IrisEngine engine = engine(active.runtime, mock(EngineEffects.class), mock(EngineWorldManager.class));
-        GenerationHistoryRuntimeRouter router = mock(GenerationHistoryRuntimeRouter.class);
-        when(router.biomes()).thenReturn(mock(SavedBiomeRuntime.class));
-        when(router.engine()).thenReturn(engine);
+        GenerationHistoryRuntimeRouter router = historyRouter(engine);
         engine.attachGenerationHistoryRuntimeRouter(router);
 
         IllegalStateException failure = assertThrows(IllegalStateException.class, engine::hotloadComplex);
@@ -548,9 +546,7 @@ public class IrisEngineGenerationRuntimeScopeTest {
         RuntimeFixture active = runtime(1, 1D, 1D, 1D);
         IrisEngine engine = engine(active.runtime, mock(EngineEffects.class), mock(EngineWorldManager.class));
         assertNull(engine.openGenerationHistoryCoordinateScope(0, 0));
-        GenerationHistoryRuntimeRouter router = mock(GenerationHistoryRuntimeRouter.class);
-        when(router.biomes()).thenReturn(mock(SavedBiomeRuntime.class));
-        when(router.engine()).thenReturn(engine);
+        GenerationHistoryRuntimeRouter router = historyRouter(engine);
         engine.attachGenerationHistoryRuntimeRouter(router);
         engine.detachGenerationHistoryRuntimeRouter(router);
 
@@ -710,6 +706,16 @@ public class IrisEngineGenerationRuntimeScopeTest {
         assertSame(active.dimension, engine.getDimension());
     }
 
+    private static GenerationHistoryRuntimeRouter historyRouter(IrisEngine engine) {
+        GenerationHistory history = mock(GenerationHistory.class);
+        when(history.retainRuntime()).thenReturn(mock(GenerationAdmission.RuntimeLease.class));
+        GenerationHistoryRuntimeRouter router = mock(GenerationHistoryRuntimeRouter.class);
+        when(router.biomes()).thenReturn(mock(SavedBiomeRuntime.class));
+        when(router.engine()).thenReturn(engine);
+        when(router.history()).thenReturn(history);
+        return router;
+    }
+
     private static IrisEngine engine(
             GenerationRuntime generationRuntime,
             EngineEffects effects,

core/src/test/java/art/arcane/iris/engine/history/GenerationAdmissionTest.java

@@ -4,6 +4,7 @@ import org.junit.Rule;
 import org.junit.Test;
 import org.junit.rules.TemporaryFolder;
 
+import java.nio.file.Path;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.FutureTask;
@@ -103,6 +104,99 @@ public class GenerationAdmissionTest {
         assertThrows(IllegalStateException.class, admission::beginStartupCutover);
     }
 
+    @Test
+    public void lastRuntimeReleaseAllowsStartupOnAFreshAdmissionOnly() {
+        Path root = temporaryFolder.getRoot().toPath();
+        GenerationAdmission admission = new GenerationAdmission(root);
+        GenerationAdmission stale = new GenerationAdmission(root);
+        GenerationAdmission.RuntimeLease runtime = admission.retainRuntime();
+        try (GenerationAdmission.StageLease ignored = admission.enterStage()) {
+        }
+        runtime.close();
+        GenerationAdmission reopened = new GenerationAdmission(root);
+        try (GenerationAdmission.CutoverLease ignored = reopened.beginStartupCutover()) {
+        }
+        assertThrows(IllegalStateException.class, stale::enterStage);
+        assertThrows(IllegalStateException.class, stale::beginCutover);
+        assertThrows(IllegalStateException.class, stale::retainRuntime);
+        runtime.close();
+        try (GenerationAdmission.StageLease ignored = reopened.enterStage()) {
+        }
+    }
+
+    @Test
+    public void everyRuntimeMustReleaseBeforeAdmissionCanRetire() {
+        Path root = temporaryFolder.getRoot().toPath();
+        GenerationAdmission first = new GenerationAdmission(root);
+        GenerationAdmission second = new GenerationAdmission(root);
+        GenerationAdmission.RuntimeLease firstRuntime = first.retainRuntime();
+        GenerationAdmission.RuntimeLease secondRuntime = second.retainRuntime();
+        try (GenerationAdmission.StageLease ignored = first.enterStage()) {
+        }
+        firstRuntime.close();
+        assertThrows(IllegalStateException.class, new GenerationAdmission(root)::beginStartupCutover);
+        try (GenerationAdmission.StageLease ignored = second.enterStage()) {
+        }
+        secondRuntime.close();
+        try (GenerationAdmission.CutoverLease ignored = new GenerationAdmission(root).beginStartupCutover()) {
+        }
+    }
+
+    @Test
+    public void failedRetirementRetainsTheLeaseUntilActiveStagesFinish() {
+        GenerationAdmission admission = new GenerationAdmission(temporaryFolder.getRoot().toPath());
+        GenerationAdmission.RuntimeLease runtime = admission.retainRuntime();
+        GenerationAdmission.StageLease stage = admission.enterStage();
+        assertThrows(IllegalStateException.class, runtime::close);
+        assertThrows(IllegalStateException.class, admission::beginStartupCutover);
+        stage.close();
+        runtime.close();
+        assertThrows(IllegalStateException.class, admission::enterStage);
+    }
+
+    @Test
+    public void failedRetirementRetainsTheLeaseUntilActiveCutoversFinish() {
+        GenerationAdmission admission = new GenerationAdmission(temporaryFolder.getRoot().toPath());
+        GenerationAdmission.RuntimeLease runtime = admission.retainRuntime();
+        GenerationAdmission.CutoverLease cutover = admission.beginCutover();
+        assertThrows(IllegalStateException.class, runtime::close);
+        cutover.close();
+        runtime.close();
+        assertThrows(IllegalStateException.class, admission::beginCutover);
+    }
+
+    @Test
+    public void waitingCutoversKeepAdmissionOwnedUntilTheirWorkCompletes() throws Exception {
+        GenerationAdmission admission = new GenerationAdmission(temporaryFolder.getRoot().toPath());
+        GenerationAdmission.RuntimeLease runtime = admission.retainRuntime();
+        GenerationAdmission.StageLease stage = admission.enterStage();
+        CountDownLatch entered = new CountDownLatch(1);
+        CountDownLatch release = new CountDownLatch(1);
+        FutureTask<Void> cutover = new FutureTask<>(() -> {
+            try (GenerationAdmission.CutoverLease ignored = admission.beginCutover()) {
+                entered.countDown();
+                await(release);
+            }
+        }, null);
+        Thread thread = Thread.ofPlatform().daemon().unstarted(cutover);
+        try {
+            thread.start();
+            awaitWaiting(thread);
+            assertThrows(IllegalStateException.class, runtime::close);
+            stage.close();
+            assertTrue(entered.await(5L, TimeUnit.SECONDS));
+            assertThrows(IllegalStateException.class, runtime::close);
+            release.countDown();
+            cutover.get(5L, TimeUnit.SECONDS);
+            runtime.close();
+            assertThrows(IllegalStateException.class, admission::beginCutover);
+        } finally {
+            stage.close();
+            release.countDown();
+            thread.join(5_000L);
+        }
+    }
+
     private static void awaitWaiting(Thread thread) throws InterruptedException {
         long deadline = System.nanoTime() + TimeUnit.SECONDS.toNanos(5L);
         while (thread.getState() != Thread.State.WAITING && System.nanoTime() < deadline) {

core/src/test/java/art/arcane/iris/engine/platform/BukkitChunkGeneratorCloseTest.java

@@ -0,0 +1,107 @@
+package art.arcane.iris.engine.platform;
+
+import art.arcane.iris.engine.framework.Engine;
+import art.arcane.volmlib.util.collection.KList;
+import art.arcane.volmlib.util.io.ReactiveFolder;
+import org.bukkit.generator.BlockPopulator;
+import org.junit.Test;
+
+import java.lang.reflect.Field;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ConcurrentLinkedQueue;
+import java.util.concurrent.atomic.AtomicReference;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotSame;
+import static org.junit.Assert.assertSame;
+import static org.junit.Assert.assertThrows;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.CALLS_REAL_METHODS;
+import static org.mockito.Mockito.doAnswer;
+import static org.mockito.Mockito.doThrow;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+
+public class BukkitChunkGeneratorCloseTest {
+    @Test
+    public void asynchronousCloseFailureCanRetryWithoutReopeningGeneration() throws Exception {
+        CloseFixture fixture = new CloseFixture();
+        IllegalStateException failure = new IllegalStateException("Planner remains active");
+        doThrow(failure).doNothing().when(fixture.engine).close();
+        CompletableFuture<Void> firstOperation = fixture.queueOperation();
+        CompletableFuture<Void> firstClose = fixture.generator.closeAsync();
+        assertSame(firstClose, fixture.generator.closeAsync());
+        firstOperation.completeExceptionally(assertThrows(IllegalStateException.class, fixture.operation.get()::run));
+        assertTrue(firstClose.isCompletedExceptionally());
+        assertTrue(fixture.generator.isClosing());
+
+        CompletableFuture<Void> secondOperation = fixture.queueOperation();
+        CompletableFuture<Void> secondClose = fixture.generator.closeAsync();
+        assertNotSame(firstClose, secondClose);
+        fixture.operation.get().run();
+        secondOperation.complete(null);
+        secondClose.join();
+        assertTrue(fixture.generator.isClosing());
+        assertSame(secondClose, fixture.generator.closeAsync());
+        verify(fixture.engine, times(2)).close();
+    }
+
+    @Test
+    public void failedRetryDispatchKeepsPartiallyClosedGenerationSealed() throws Exception {
+        CloseFixture fixture = new CloseFixture();
+        CompletableFuture<Void> operation = fixture.queueOperation();
+        CompletableFuture<Void> firstClose = fixture.generator.closeAsync();
+        operation.completeExceptionally(new IllegalStateException("Planner remains active"));
+        assertTrue(firstClose.isCompletedExceptionally());
+        doThrow(new IllegalStateException("Scheduler unavailable")).when(fixture.generator)
+                .withExclusiveControlFuture(any(Runnable.class));
+
+        CompletableFuture<Void> retry = fixture.generator.closeAsync();
+        assertNotSame(firstClose, retry);
+        assertTrue(retry.isCompletedExceptionally());
+        assertTrue(fixture.generator.isClosing());
+    }
+
+    @Test
+    public void initialDispatchFailureLeavesTheUnchangedGeneratorAvailable() throws Exception {
+        CloseFixture fixture = new CloseFixture();
+        doThrow(new IllegalStateException("Scheduler unavailable")).when(fixture.generator)
+                .withExclusiveControlFuture(any(Runnable.class));
+
+        CompletableFuture<Void> failedClose = fixture.generator.closeAsync();
+        assertTrue(failedClose.isCompletedExceptionally());
+        assertFalse(fixture.generator.isClosing());
+    }
+
+    private static void setField(BukkitChunkGenerator generator, String name, Object value) throws Exception {
+        Field field = BukkitChunkGenerator.class.getDeclaredField(name);
+        field.setAccessible(true);
+        field.set(generator, value);
+    }
+
+    private static final class CloseFixture {
+        private final BukkitChunkGenerator generator = mock(BukkitChunkGenerator.class, CALLS_REAL_METHODS);
+        private final Engine engine = mock(Engine.class);
+        private final AtomicReference<Runnable> operation = new AtomicReference<>();
+        private final ConcurrentLinkedQueue<CompletableFuture<Void>> operations = new ConcurrentLinkedQueue<>();
+
+        private CloseFixture() throws Exception {
+            setField(generator, "closeFuture", new AtomicReference<CompletableFuture<Void>>());
+            setField(generator, "engine", engine);
+            setField(generator, "folder", mock(ReactiveFolder.class));
+            setField(generator, "populators", new KList<BlockPopulator>());
+            doAnswer(invocation -> {
+                operation.set(invocation.getArgument(0, Runnable.class));
+                return operations.remove();
+            }).when(generator).withExclusiveControlFuture(any(Runnable.class));
+        }
+
+        private CompletableFuture<Void> queueOperation() {
+            CompletableFuture<Void> future = new CompletableFuture<>();
+            operations.add(future);
+            return future;
+        }
+    }
+}