Fix MC-149777 - java.util.ConcurrentModificationException when using Java 11 or above (#1176)

* Fix MC-149777

* Fix license

fabric-structure-api-v1/src/main/java/net/fabricmc/fabric/mixin/structure/StructureManagerMixin.java

@@ -0,0 +1,51 @@
+/*
+ * Copyright (c) 2016, 2017, 2018, 2019 FabricMC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package net.fabricmc.fabric.mixin.structure;
+
+import java.util.Collections;
+import java.util.Map;
+
+import org.spongepowered.asm.mixin.Final;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Mutable;
+import org.spongepowered.asm.mixin.Shadow;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Inject;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
+
+import net.minecraft.structure.Structure;
+import net.minecraft.structure.StructureManager;
+import net.minecraft.util.Identifier;
+
+/**
+ * This fixes a rare CME in the StructureManager when using Java 11 or newer.
+ *
+ * <p>See: https://bugs.mojang.com/browse/MC-149777
+ *
+ */
+@Mixin(StructureManager.class)
+public abstract class StructureManagerMixin {
+	@Shadow
+	@Final
+	@Mutable
+	private Map<Identifier, Structure> structures;
+
+	@Inject(method = "<init>", at = @At(value = "RETURN"))
+	private void init(CallbackInfo info) {
+		structures = Collections.synchronizedMap(structures);
+	}
+}

fabric-structure-api-v1/src/main/resources/fabric-structure-api-v1.mixins.json

@@ -9,6 +9,7 @@
     "MixinChunkGeneratorSettings",
     "MixinStructuresConfig",
     "StructureFeatureAccessor",
+    "StructureManagerMixin",
     "StructuresConfigAccessor"
   ],
   "client": [