Patch concurrency issue in ForgeRegistryTagManager

Author: embeddedt

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

@@ -0,0 +1,31 @@
+package org.embeddedt.modernfix.forge.mixin.bugfix.concurrency;
+
+import com.llamalad7.mixinextras.injector.wrapmethod.WrapMethod;
+import com.llamalad7.mixinextras.injector.wrapoperation.Operation;
+import net.minecraft.tags.TagKey;
+import net.minecraftforge.registries.tags.ITag;
+import org.jetbrains.annotations.NotNull;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Shadow;
+
+import java.util.Map;
+
+@Mixin(targets = {"net/minecraftforge/registries/ForgeRegistryTagManager"})
+public class ForgeRegistryTagManagerMixin<V> {
+    @Shadow private volatile Map<TagKey<V>, ITag<V>> tags;
+
+    /**
+     * @author embeddedt (issue found by Uncandango)
+     * @reason vanilla does not use the correct double-checked locking paradigm, which leads to race conditions
+     */
+    @WrapMethod(method = "getTag", remap = false)
+    private ITag<V> getTagSafe(@NotNull TagKey<V> name, Operation<ITag<V>> original) {
+        ITag<V> tag = this.tags.get(name);
+        if (tag == null) {
+            synchronized (this) {
+                tag = original.call(name);
+            }
+        }
+        return tag;
+    }
+}