diff --git a/fabric-structure-api-v1/src/main/java/net/fabricmc/fabric/mixin/structure/ChunkSerializerMixin.java b/fabric-structure-api-v1/src/main/java/net/fabricmc/fabric/mixin/structure/ChunkSerializerMixin.java
new file mode 100644
index 000000000..e6db50314
--- /dev/null
+++ b/fabric-structure-api-v1/src/main/java/net/fabricmc/fabric/mixin/structure/ChunkSerializerMixin.java
@@ -0,0 +1,71 @@
+/*
+ * 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.Map;
+
+import it.unimi.dsi.fastutil.longs.LongSet;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Unique;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Inject;
+import org.spongepowered.asm.mixin.injection.Redirect;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfoReturnable;
+
+import net.minecraft.nbt.CompoundTag;
+import net.minecraft.util.math.ChunkPos;
+import net.minecraft.world.ChunkSerializer;
+import net.minecraft.world.chunk.Chunk;
+import net.minecraft.world.gen.feature.StructureFeature;
+
+// This is a bug fix, tracking issue: MC-194811
+@Mixin(ChunkSerializer.class)
+abstract class ChunkSerializerMixin {
+ @Unique
+ private static final ThreadLocal<Boolean> CHUNK_NEEDS_SAVING = ThreadLocal.withInitial(() -> false);
+
+ /**
+ * Remove objects keyed by `null` in the map.
+ * This data is likely bad since multiple missing structures will cause value mapped by `null` to change at least once.
+ *
+ * <p>If a null value is stored in this map, the chunk will fail to save, so we remove the value stored using null key.
+ *
+ * <p>Note that the chunk may continue to emit errors after being (un)loaded again.
+ * This is because of the way minecraft handles chunk saving.
+ * If the chunk is not modified, the game will keep the currently saved chunk on the disk.
+ * In order to affect this change, we must mark the chunk to be save in order force the game to save the chunk without the errors.
+ */
+ @Inject(method = "readStructureReferences", at = @At("TAIL"))
+ private static void removeNullKeys(ChunkPos pos, CompoundTag tag, CallbackInfoReturnable<Map<StructureFeature<?>, LongSet>> cir) {
+ if (cir.getReturnValue().remove(null) != null) {
+ ChunkSerializerMixin.CHUNK_NEEDS_SAVING.set(true);
+ }
+ }
+
+ @Redirect(method = "deserialize", at = @At(value = "INVOKE", target = "Lnet/minecraft/world/chunk/Chunk;setStructureReferences(Ljava/util/Map;)V"))
+ private static void forceChunkSavingIfNullKeysExist(Chunk chunk, Map<StructureFeature<?>, LongSet> structureReferences) {
+ // Redirect is much cleaner than local capture. The local capture would be very long
+ if (ChunkSerializerMixin.CHUNK_NEEDS_SAVING.get()) {
+ ChunkSerializerMixin.CHUNK_NEEDS_SAVING.set(false);
+ // Make the chunk save as soon as possible
+ chunk.setShouldSave(true);
+ }
+
+ // Replicate vanilla logic
+ chunk.setStructureReferences(structureReferences);
+ }
+}
diff --git a/fabric-structure-api-v1/src/main/resources/fabric-structure-api-v1.mixins.json b/fabric-structure-api-v1/src/main/resources/fabric-structure-api-v1.mixins.json
index f92380d76..ccb792a25 100644
--- a/fabric-structure-api-v1/src/main/resources/fabric-structure-api-v1.mixins.json
+++ b/fabric-structure-api-v1/src/main/resources/fabric-structure-api-v1.mixins.json
@@ -4,6 +4,7 @@
"compatibilityLevel": "JAVA_8",
"mixins": [
"BiomeAccessor",
+ "ChunkSerializerMixin",
"FlatChunkGeneratorConfigAccessor",
"MixinChunkGeneratorSettings",
"MixinStructuresConfig",
diff --git a/fabric-structure-api-v1/src/testmod/java/net/fabricmc/fabric/test/structure/mixin/ChunkSerializerMixin.java b/fabric-structure-api-v1/src/testmod/java/net/fabricmc/fabric/test/structure/mixin/ChunkSerializerMixin.java
new file mode 100644
index 000000000..5bcb30374
--- /dev/null
+++ b/fabric-structure-api-v1/src/testmod/java/net/fabricmc/fabric/test/structure/mixin/ChunkSerializerMixin.java
@@ -0,0 +1,40 @@
+/*
+ * 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.test.structure.mixin;
+
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.injection.Constant;
+import org.spongepowered.asm.mixin.injection.ModifyConstant;
+
+import net.minecraft.nbt.CompoundTag;
+import net.minecraft.structure.StructureManager;
+import net.minecraft.world.ChunkSerializer;
+
+@Mixin(ChunkSerializer.class)
+abstract class ChunkSerializerMixin {
+ /**
+ * @reason Changes the logging message for the `unknown structure start` to describe which chunk the missing structure is located in for debugging purposes.
+ */
+ @ModifyConstant(method = "readStructureStarts", constant = @Constant(stringValue = "Unknown structure start: {}"))
+ private static String modifyErrorMessage(String original, StructureManager structureManager, CompoundTag tag, long worldSeed) {
+ // Use coordinates in tag to determine the position of the chunk
+ final int xPos = tag.getInt("xPos");
+ final int zPos = tag.getInt("zPos");
+
+ return String.format("Unknown structure start: {} at chunk [%s, %s]", xPos, zPos);
+ }
+}
diff --git a/fabric-structure-api-v1/src/testmod/resources/fabric-structure-api-v1-testmod.mixins.json b/fabric-structure-api-v1/src/testmod/resources/fabric-structure-api-v1-testmod.mixins.json
index 22c796648..526fa071c 100644
--- a/fabric-structure-api-v1/src/testmod/resources/fabric-structure-api-v1-testmod.mixins.json
+++ b/fabric-structure-api-v1/src/testmod/resources/fabric-structure-api-v1-testmod.mixins.json
@@ -3,6 +3,7 @@
"package": "net.fabricmc.fabric.test.structure.mixin",
"compatibilityLevel": "JAVA_8",
"mixins": [
+ "ChunkSerializerMixin",
"MixinDefaultBiomeCreator"
],
"client": [