Fail-closed modded decoration

- A generator with a refused or stopping engine refuses imported feature settings
- Those settings no longer fall back to vanilla ones
- Decoration fails when a sealed runtime has no settled feature table
- Decoration fails when the feature table was retired before placement
- Stacked initial spawns fail when the surface spawn biome lookup is rejected
- Those spawns no longer fall back to the chunk's top biome
- Bind failures from a stopping runtime stay out of the refusal memo and banner
- The next bind after such a failure retries

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

@@ -73,6 +73,7 @@ import art.arcane.iris.pack.loading.IrisData;
 import art.arcane.iris.pack.PackValidationRegistry;
 import art.arcane.iris.generation.runtime.IrisEngine;
 import art.arcane.iris.generation.runtime.Engine;
+import art.arcane.iris.generation.runtime.GenerationFailures;
 import art.arcane.iris.generation.runtime.GenerationSessionException;
 import art.arcane.iris.generation.runtime.GenerationSessionLease;
 import art.arcane.iris.structure.nativegen.NativeFeatureGenerationPolicy;
@@ -517,6 +518,12 @@ public final class IrisModdedChunkGenerator implements NativeGeneratorOwner, Nat
     }
 
     private void recordBindFailure(NativeWorld level, Throwable error) {
+        // A runtime that is stopping says nothing about the world: fail this call, let the next bind try again.
+        if (GenerationFailures.isShutdownFailure(error)) {
+            ModdedIrisLog.warn("Iris bind of {} ({}) stopped with its runtime: {}", level.name(), dimensionKey,
+                    error.toString());
+            return;
+        }
         engineBinding.fail(error);
         for (String line : bindFailureNotice(level.name(), dimensionKey, error)) {
             ModdedIrisLog.error(line);
@@ -588,21 +595,11 @@ public final class IrisModdedChunkGenerator implements NativeGeneratorOwner, Nat
         }
     }
 
-    Engine engineOrNull() {
-        requireBindingAllowed();
-        Engine cached = engine;
-        requireCompletedShutdown(cached);
-        if (cached != null && !cached.isClosed()) {
-            return cached;
-        }
-        try {
-            return engine();
-        } catch (Throwable ignored) {
-            return null;
-        }
-    }
-
-    Engine structureEngineOrNull() {
+    /**
+     * Null only while no level uses this generator (world creation screens, codec validation). A bound level whose
+     * engine is refused, unloading or stopping throws, so no stage can mistake it for an unbound generator.
+     */
+    Engine boundEngineOrNull() {
         requireBindingAllowed();
         engineBinding.throwIfFailed(dimensionKey);
         Engine cached = engine;
@@ -1007,7 +1004,7 @@ public final class IrisModdedChunkGenerator implements NativeGeneratorOwner, Nat
     @Override
     public <H, S> NativeModdedBiomePolicy<H, S> biomePolicy(NativeBiomeSourceAccess<H, S> source) {
         ModdedBiomePolicy<H, S> policy = new ModdedBiomePolicy<>(source);
-        policy.bind(new ModdedBiomePolicy.RuntimeCallbacks(this::structureEngineOrNull, this::awaitStructureEngine,
+        policy.bind(new ModdedBiomePolicy.RuntimeCallbacks(this::boundEngineOrNull, this::awaitStructureEngine,
                 this::allowsGenerationHistoryBypass, this::configuredStructureBiomeKeys,
                 IrisModdedChunkGenerator::retainedBiomeKeys, this::visibleBiomeSeed));
         return policy;

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

@@ -266,7 +266,7 @@ final class ModdedBiomePolicy<H, S> implements NativeModdedBiomePolicy<H, S> {
 
     @Override
     public H getNoiseBiome(int quartX, int quartY, int quartZ, S sampler) {
-        Engine engine = engineOrNull();
+        Engine engine = boundEngineOrNull();
         if (engine == null) {
             return access.serializedNoise(quartX, quartY, quartZ, sampler);
         }
@@ -288,7 +288,7 @@ final class ModdedBiomePolicy<H, S> implements NativeModdedBiomePolicy<H, S> {
 
     @Override
     public NativeBiomeSourcePolicy.BiomeLocation<H> findClosestBiome3d(ClosestQuery<H, S> query) {
-        Engine engine = engineOrNull();
+        Engine engine = boundEngineOrNull();
         if (engine == null) {
             return query.unboundFallback().get();
         }
@@ -355,7 +355,7 @@ final class ModdedBiomePolicy<H, S> implements NativeModdedBiomePolicy<H, S> {
     }
 
     public H getVisibleNoiseBiome(int quartX, int quartY, int quartZ, S sampler) {
-        Engine engine = engineOrNull();
+        Engine engine = boundEngineOrNull();
         if (engine == null) {
             return access.serializedNoise(quartX, quartY, quartZ, sampler);
         }
@@ -411,7 +411,7 @@ final class ModdedBiomePolicy<H, S> implements NativeModdedBiomePolicy<H, S> {
     }
 
     public H getVisibleSurfaceBiome(int blockX, int blockZ) {
-        Engine engine = engineOrNull();
+        Engine engine = boundEngineOrNull();
         if (engine == null) {
             return null;
         }
@@ -420,15 +420,15 @@ final class ModdedBiomePolicy<H, S> implements NativeModdedBiomePolicy<H, S> {
         try (historyScope) {
             GenerationSessionLease lease = tryAcquireGenerationLease(engine, "modded_surface_spawn_biome");
             if (lease == null) {
-                return null;
+                throw new IllegalStateException("Iris surface spawn biome lookup was rejected during an engine transition");
             }
             try (lease; IrisContext.Scope ignored = IrisContext.open(engine, lease.sessionId(), null)) {
                 if (!isReady(engine)) {
-                    return null;
+                    throw new IllegalStateException("Iris surface spawn biome lookup has no active engine runtime");
                 }
                 NativeBiomeSourceAccess.RegistryView<H> registry = biomeRegistry();
                 if (registry == null) {
-                    return null;
+                    throw new IllegalStateException("Iris surface spawn biome lookup has no biome registry");
                 }
                 int quartX = ((blockX) >> 2);
                 int quartZ = ((blockZ) >> 2);
@@ -463,7 +463,7 @@ final class ModdedBiomePolicy<H, S> implements NativeModdedBiomePolicy<H, S> {
     }
 
     public boolean isStructureReachable(Iterable<H> biomes) {
-        Engine engine = engineOrNull();
+        Engine engine = boundEngineOrNull();
         if (engine == null) {
             return isStructureReachable(biomes, possibleStructureBiomeKeys());
         }
@@ -494,7 +494,7 @@ final class ModdedBiomePolicy<H, S> implements NativeModdedBiomePolicy<H, S> {
         int radius = query.radius();
         S sampler = query.sampler();
         int minQuartY = ((y - radius) >> 2);
-        Engine engine = engineOrNull();
+        Engine engine = boundEngineOrNull();
         if (engine == null) {
             return query.fallback().get();
         }
@@ -874,7 +874,7 @@ final class ModdedBiomePolicy<H, S> implements NativeModdedBiomePolicy<H, S> {
         return engine != null && !engine.isClosed() && engine.getComplex() != null;
     }
 
-    private Engine engineOrNull() {
+    private Engine boundEngineOrNull() {
         RuntimeCallbacks current = runtime;
         return current == null ? null : current.currentEngine().get();
     }

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

@@ -25,6 +25,7 @@ import art.arcane.volmlib.nativelib.terrain.feature.NativeFeatureTable;
 import art.arcane.volmlib.nativelib.minecraft26_2.modded.NativeModdedImportedFeatures;
 
 import art.arcane.iris.generation.runtime.Engine;
+import art.arcane.iris.generation.runtime.GenerationClosedException;
 import art.arcane.iris.generation.runtime.GenerationFailures;
 import art.arcane.iris.generation.runtime.GenerationSessionLease;
 import art.arcane.iris.structure.nativegen.NativeFeatureGenerationPolicy;
@@ -96,10 +97,12 @@ final class ModdedImportedFeatureStage implements NativeModdedGeneratorPolicy.Fe
      * the per-step feature lists and {@code BiomeFilter}'s hasFeature gate both see real features for a biome
      * whose datapack JSON declares none by design. Real registry biomes pass straight through.
      *
-     * <p>With {@code importedFeatures} disabled there is no table and this is vanilla's default getter.
+     * <p>With {@code importedFeatures} disabled there is no table and this is vanilla's default getter. So is an
+     * unbound generator, which only world creation and codec validation ask. A bound generator whose engine is
+     * refused or stopping throws: BiomeFilter would otherwise drop every imported feature of a custom biome.
      */
     public NativeFeatureTable currentTable() {
-        Engine engine = generator == null ? null : generator.engineOrNull();
+        Engine engine = generator == null ? null : generator.boundEngineOrNull();
         return engine == null ? null : featureTables.get(engine.getCacheID());
     }
 
@@ -112,8 +115,15 @@ final class ModdedImportedFeatureStage implements NativeModdedGeneratorPolicy.Fe
      * the build itself is serialized because {@code applyBiomeDecoration} calls this from every worldgen
      * thread, and two threads that both found the stage unprepared would each run {@code FeatureSorter}, whose
      * cycle detection is the expensive part. Waiting here is safe: this caller holds no generator monitor.
+     * A sealed runtime keeps the table it already settled but cannot build one, and decorating without it
+     * would persist a chunk with no imported features, so it fails the chunk instead.
      */
     public void prepare(Engine engine) {
+        if ((engine.isClosed() || engine.isClosing())
+                && !settled(engine.getCacheID(), biomeSource.packGeneration())) {
+            throw new GenerationClosedException("Iris imported features cannot prepare runtime "
+                    + engine.getCacheID() + " while its engine is closing.");
+        }
         prepare(engine, true);
     }
 
@@ -128,7 +138,7 @@ final class ModdedImportedFeatureStage implements NativeModdedGeneratorPolicy.Fe
     }
 
     private void prepare(Engine engine, boolean waitForBuild) {
-        if (engine == null || engine.isClosed() || engine.isClosing()) {
+        if (engine.isClosed() || engine.isClosing()) {
             return;
         }
         int runtimeIdentity = engine.getCacheID();
@@ -204,17 +214,24 @@ final class ModdedImportedFeatureStage implements NativeModdedGeneratorPolicy.Fe
         inertGenerations.put(runtimeIdentity, generation);
     }
 
+    /**
+     * Null only for a runtime whose features are settled off. A table retired or superseded since this chunk's
+     * prepare fails the chunk: skipping the pass would persist it without imported features.
+     */
     public NativeFeatureTable placementTable(Engine engine) {
         int runtimeIdentity = engine.getCacheID();
+        long generation = biomeSource.packGeneration();
         NativeFeatureTable table = featureTables.get(runtimeIdentity);
-        if (table == null) {
+        if (table == null && Long.valueOf(generation).equals(inertGenerations.get(runtimeIdentity))) {
             WorldCheckFeaturePlacement.recordFeaturesOff();
             return null;
         }
-        if (table.generation() != biomeSource.packGeneration()) {
-            featureTables.remove(runtimeIdentity, table);
-            inertGenerations.remove(runtimeIdentity);
-            return null;
+        if (table == null || table.generation() != generation) {
+            if (table != null) {
+                featureTables.remove(runtimeIdentity, table);
+            }
+            throw new GenerationClosedException("Iris imported feature table for runtime " + runtimeIdentity
+                    + " was retired before decoration.");
         }
         return generator == null ? null : table;
     }