Fix race condition when mods create tags on different threads

Author: embeddedt

common/src/main/java/org/embeddedt/modernfix/common/mixin/bugfix/concurrency/MappedRegistryMixin.java

@@ -0,0 +1,40 @@
+package org.embeddedt.modernfix.common.mixin.bugfix.concurrency;
+
+import net.minecraft.core.HolderSet;
+import net.minecraft.core.MappedRegistry;
+import net.minecraft.tags.TagKey;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Overwrite;
+import org.spongepowered.asm.mixin.Shadow;
+
+import java.util.IdentityHashMap;
+import java.util.Map;
+
+@Mixin(value = MappedRegistry.class, priority = 500)
+public abstract class MappedRegistryMixin<T> {
+    @Shadow private volatile Map<TagKey<T>, HolderSet.Named<T>> tags;
+
+    @Shadow protected abstract HolderSet.Named<T> createTag(TagKey<T> key);
+
+    /**
+     * @author embeddedt (issue found by Uncandango)
+     * @reason vanilla does not use the correct double-checked locking paradigm, which leads to race conditions
+     */
+    @Overwrite
+    public HolderSet.Named<T> getOrCreateTag(TagKey<T> key) {
+        HolderSet.Named<T> named = this.tags.get(key);
+        if (named == null) {
+            // synchronize and check again - this is the bugfix
+            synchronized (this) {
+                named = this.tags.get(key);
+                if (named == null) {
+                    named = this.createTag(key);
+                    Map<TagKey<T>, HolderSet.Named<T>> map = new IdentityHashMap<>(this.tags);
+                    map.put(key, named);
+                    this.tags = map;
+                }
+            }
+        }
+        return named;
+    }
+}

forge/src/main/java/org/embeddedt/modernfix/forge/mixin/bugfix/concurrency/NamespacedWrapperMixin.java

@@ -0,0 +1,39 @@
+package org.embeddedt.modernfix.forge.mixin.bugfix.concurrency;
+
+import net.minecraft.core.HolderSet;
+import net.minecraft.tags.TagKey;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Overwrite;
+import org.spongepowered.asm.mixin.Shadow;
+
+import java.util.IdentityHashMap;
+import java.util.Map;
+
+@Mixin(targets = {"net/minecraftforge/registries/NamespacedWrapper"}, priority = 500)
+public abstract class NamespacedWrapperMixin<T> {
+    @Shadow private volatile Map<TagKey<T>, HolderSet.Named<T>> tags;
+
+    @Shadow(aliases = {"createTag"}) protected abstract HolderSet.Named<T> m_211067_(TagKey<T> key);
+
+    /**
+     * @author embeddedt (issue found by Uncandango)
+     * @reason vanilla does not use the correct double-checked locking paradigm, which leads to race conditions
+     */
+    @Overwrite
+    public HolderSet.Named<T> getOrCreateTag(TagKey<T> key) {
+        HolderSet.Named<T> named = this.tags.get(key);
+        if (named == null) {
+            // synchronize and check again - this is the bugfix
+            synchronized (this) {
+                named = this.tags.get(key);
+                if (named == null) {
+                    named = this.m_211067_(key);
+                    Map<TagKey<T>, HolderSet.Named<T>> map = new IdentityHashMap<>(this.tags);
+                    map.put(key, named);
+                    this.tags = map;
+                }
+            }
+        }
+        return named;
+    }
+}