Wrap NBTTagCompound#setTag in static helper with safeguards against circular references, fixes #299

src/main/java/electroblob/wizardry/util/MagicDamage.java

@@ -8,7 +8,10 @@ import net.minecraft.entity.monster.*;
 import net.minecraft.util.DamageSource;
 import net.minecraft.util.EntityDamageSource;
 
-import java.util.*;
+import java.util.Collections;
+import java.util.EnumSet;
+import java.util.HashMap;
+import java.util.Map;
 
 // A note on the use of the vanilla damagesources:
 // When using indirect damage sources, the SECOND argument is the original entity (i.e. the caster), and the

src/main/java/electroblob/wizardry/util/NBTExtras.java

@@ -51,8 +51,8 @@ public final class NBTExtras {
 
 		for(Map.Entry<K, V> entry : map.entrySet()){
 			NBTTagCompound mapping = new NBTTagCompound();
-			mapping.setTag(keyTagName, keyFunction.apply(entry.getKey()));
-			mapping.setTag(valueTagName, valueFunction.apply(entry.getValue()));
+			NBTExtras.storeTagSafely(mapping, keyTagName, keyFunction.apply(entry.getKey()));
+			NBTExtras.storeTagSafely(mapping, valueTagName, valueFunction.apply(entry.getValue()));
 			tagList.appendTag(mapping);
 		}
 
@@ -205,6 +205,51 @@ public final class NBTExtras {
 		tag.removeTag(key + "Least");
 	}
 
+	/**
+	 * Stores the given NBT tag inside the given NBT tag compound using the given key. Under normal circumstances, this
+	 * is equivalent to {@link NBTTagCompound#setTag(String, NBTBase)}, but this method performs safety checks to
+	 * prevent circular references. If storing the given tag would cause a circular reference, the tag is not stored
+	 * and an error is printed to the console.
+	 * @param compound The {@link NBTTagCompound} in which to store the tag.
+	 * @param key The key to store the tag under.
+	 * @param tag The tag to store.
+	 */
+	// This is a catch-all fix for issue #299.
+	public static void storeTagSafely(NBTTagCompound compound, String key, NBTBase tag){
+
+		if(compound == tag || deepContains(tag, compound)){
+			Wizardry.logger.error("Cannot store tag of type {} under key '{}' as it would result in a circular reference!",
+					NBTBase.getTypeName(tag.getId()), key);
+		}else{
+			compound.setTag(key, tag);
+		}
+	}
+
+	/**
+	 * Recursively searches within the first NBT tag for the second NBT tag. This handles both compound and list tags.
+	 * @param toSearch The NBT tag to search inside. If this is not a compound or list tag, this method will always
+	 *                 return false.
+	 * @param searchFor The NBT tag to search for.
+	 * @return True if the second tag appears anywhere within the NBT tree contained within the first tag, false if not.
+	 */
+	public static boolean deepContains(NBTBase toSearch, NBTBase searchFor){
+
+		if(toSearch instanceof NBTTagCompound){
+
+			for(String subKey : ((NBTTagCompound)toSearch).getKeySet()){
+				NBTBase subTag = ((NBTTagCompound)toSearch).getTag(subKey);
+				if(subTag == searchFor || deepContains(subTag, searchFor)) return true;
+			}
+
+		}else if(toSearch instanceof NBTTagList){
+			for(NBTBase subTag : (NBTTagList)toSearch){
+				if(subTag == searchFor || deepContains(subTag, searchFor)) return true;
+			}
+		}
+
+		return false;
+	}
+
 	/**
 	 * Returns an NBTTagCompound which contains only the given UUID, stored using
 	 * {@link NBTTagCompound#setUniqueId(String, UUID)}. Allows for neater storage to NBTTagLists.

src/main/java/electroblob/wizardry/util/WandHelper.java

@@ -398,7 +398,7 @@ public final class WandHelper {
 		if(wand.getTagCompound() == null) wand.setTagCompound((new NBTTagCompound()));
 
 		if(!wand.getTagCompound().hasKey(UPGRADES_KEY))
-			wand.getTagCompound().setTag(UPGRADES_KEY, new NBTTagCompound());
+			NBTExtras.storeTagSafely(wand.getTagCompound(), UPGRADES_KEY, new NBTTagCompound());
 
 		NBTTagCompound upgrades = wand.getTagCompound().getCompoundTag(UPGRADES_KEY);
 
@@ -406,7 +406,7 @@ public final class WandHelper {
 
 		if(key != null) upgrades.setInteger(key, upgrades.getInteger(key) + 1);
 
-		wand.getTagCompound().setTag(UPGRADES_KEY, upgrades);
+		NBTExtras.storeTagSafely(wand.getTagCompound(), UPGRADES_KEY, upgrades);
 	}
 
 	/** Returns true if the given item is a valid special wand upgrade. */