dwa

adapters/bukkit/nms/v26_2_R1/src/main/java/art/arcane/iris/core/nms/v26_2_R1/VanillaStructureBiomes.java

@@ -53,15 +53,19 @@ final class VanillaStructureBiomes {
         if (structure == null) {
             throw new IllegalArgumentException("Registered structure does not exist: " + structureKey);
         }
+        boolean hasFilterEntries = false;
         for (Holder<Biome> holder : structure.biomes()) {
+            hasFilterEntries = true;
             Optional<ResourceKey<Biome>> key = holder.unwrapKey();
             if (key.isPresent()) {
                 keys.add(key.get().identifier().toString());
             }
         }
-        if (keys.isEmpty()) {
+        // An empty biome filter is legal datapack content (opt-in structures whose tags only
+        // reference absent modded biomes); the structure is unreachable, not an error state.
+        if (keys.isEmpty() && hasFilterEntries) {
             throw new IllegalStateException("Registered structure '" + structureKey
-                    + "' exposes no registered biome keys");
+                    + "' has biome filter entries but none resolve to registered biome keys");
         }
         return keys;
     }

adapters/bukkit/nms/v26_2_R1/src/test/java/art/arcane/iris/core/nms/v26_2_R1/VanillaStructureBiomesEmptyFilterContractTest.java

@@ -0,0 +1,38 @@
+package art.arcane.iris.core.nms.v26_2_R1;
+
+import org.junit.Test;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+/**
+ * A registered structure with an EMPTY biome filter is legal datapack content: opt-in structures
+ * (e.g. Towns and Towers' towns_and_towers:exclusives/*) ship biome tags that only reference
+ * optional modded biomes, so the filter resolves to zero entries on a vanilla server. Resolving
+ * biome keys for such a structure must return an empty set (the structure is simply unreachable),
+ * never throw — a throw here aborts the whole /iris structure verify run and degrades /iris find.
+ * The defensive throw is only kept for a NON-empty filter whose holders all fail to resolve keys.
+ */
+public class VanillaStructureBiomesEmptyFilterContractTest {
+    @Test
+    public void emptyBiomeFilterReturnsEmptyKeysInsteadOfThrowing() throws IOException {
+        String source = Files.readString(Path.of(System.getProperty("iris.vanillaStructureBiomesSource")));
+
+        assertTrue(source.contains("if (keys.isEmpty() && hasFilterEntries)"));
+        assertTrue(source.contains("has biome filter entries but none resolve to registered biome keys"));
+        assertFalse(source.contains("exposes no registered biome keys"));
+    }
+
+    @Test
+    public void moddedHookMatchesEmptyFilterContract() throws IOException {
+        String source = Files.readString(Path.of(System.getProperty("iris.moddedStructureHooksSource")));
+
+        assertTrue(source.contains("if (keys.isEmpty() && hasFilterEntries)"));
+        assertTrue(source.contains("has biome filter entries but none resolve to registered biome keys"));
+        assertFalse(source.contains("exposes no registered biome keys"));
+    }
+}