fix: restore CopyOnWriteArrayList for unique and text index value lists

The #1260 index rework (4.4.0) switched the per-key `List<NitriteId>`
value used by unique and full-text indexes from CopyOnWriteArrayList to a
plain ArrayList mutated in place. MVStore serializes dirty page values on a
background thread, so mutating that list after it is written to the map races
with the serializer, throwing ConcurrentModificationException (and could
corrupt the id list, later surfacing as a spurious UniqueConstraintException)
even under single-threaded use.

Restore CopyOnWriteArrayList for these classic list-valued index paths so
each mutation swaps the backing array atomically and the background
serializer always sees a stable snapshot. The composite-key layout for
non-unique indexes (the actual #1260 optimization) is unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: anidotnet

CHANGELOG.md

@@ -6,6 +6,12 @@
   - Opening a file-based store runs `MVStoreUtils.testForMigration()`, which for a legacy v1-format file deserialized stored values through `ObjectInputStream.readObject()` with no class restriction. Any `Serializable` class on the embedding application's classpath could be instantiated, so a suitable gadget chain (e.g. from commons-collections) made a malicious `.db` file a remote-code-execution vector.
   - The v1-compat deserializer now enforces a JEP 290 allowlist filter that only permits Nitrite's own types and standard JDK types; any other class is rejected before its `readObject`/`readResolve` callbacks can run. Applications that open Nitrite database files from untrusted sources (e.g. "import"/"restore backup" features) should upgrade.
 
+### Issue Fixes
+
+- Fix intermittent `ConcurrentModificationException` and spurious `UniqueConstraintException` from unique and full-text indexes on the MVStore backend (regression introduced by the #1260 index rework in 4.4.0)
+  - Unique and full-text indexes still store a `List<NitriteId>` value per key. 4.4.0 switched that list from `CopyOnWriteArrayList` to a plain `ArrayList`, which is mutated in place after being written to the map. MVStore serializes dirty page values on a background thread, so that in-place mutation races with the serializer and threw `ConcurrentModificationException` (and could corrupt the id list, surfacing later as a false unique-key violation) — even under single-threaded use.
+  - These index value lists are `CopyOnWriteArrayList` again, so each mutation swaps the backing array atomically and the background serializer always sees a stable snapshot. The composite-key layout for non-unique indexes (the actual #1260 optimization) is unchanged.
+
 ## Release 4.4.0 - Jul 2, 2026
 
 ### Upgrade Notes

nitrite/src/main/java/org/dizitart/no2/index/NitriteIndex.java

@@ -23,9 +23,9 @@
 import org.dizitart.no2.exceptions.UniqueConstraintException;
 import org.dizitart.no2.exceptions.ValidationException;
 
-import java.util.ArrayList;
 import java.util.LinkedHashSet;
 import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
 
 import static org.dizitart.no2.common.util.ValidationUtils.validateArrayIndexField;
 import static org.dizitart.no2.common.util.ValidationUtils.validateIterableIndexField;
@@ -109,11 +109,15 @@ default void validateIndexField(Object value, String field) {
      */
     default List<NitriteId> addNitriteIds(List<NitriteId> nitriteIds, FieldValues fieldValues) {
         if (nitriteIds == null) {
-            // a plain ArrayList gives amortized O(1) appends; index reads and
-            // writes are guarded by the collection's read-write lock, so the
-            // copy-on-write semantics previously used here were unnecessary and
-            // made every insert O(n) for keys with many values (issue #1260)
-            nitriteIds = new ArrayList<>();
+            // this list is stored as a value in the backing NitriteMap. MVStore serializes
+            // dirty page values on a background thread, so a plain ArrayList mutated in place
+            // after being put races with that serialization and throws
+            // ConcurrentModificationException. CopyOnWriteArrayList swaps its backing array
+            // atomically on each mutation, so the background serializer always sees a stable
+            // snapshot. Non-unique indexes avoid list values entirely via the composite layout
+            // (issue #1260); only unique indexes and the text index reach this path, where the
+            // per-key list is small enough that copy-on-write cost is negligible.
+            nitriteIds = new CopyOnWriteArrayList<>();
         }
 
         if (isUnique() && nitriteIds.size() == 1) {

nitrite/src/main/java/org/dizitart/no2/index/TextIndex.java

@@ -30,11 +30,11 @@
 import org.dizitart.no2.store.NitriteMap;
 import org.dizitart.no2.store.NitriteStore;
 
-import java.util.ArrayList;
 import java.util.HashSet;
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Set;
+import java.util.concurrent.CopyOnWriteArrayList;
 
 import static org.dizitart.no2.common.util.IndexUtils.deriveIndexMapName;
 import static org.dizitart.no2.common.util.ObjectUtils.convertToObjectArray;
@@ -157,7 +157,7 @@ public LinkedHashSet<NitriteId> findNitriteIds(FindPlan findPlan) {
 
     private NitriteMap<String, List<?>> findIndexMap() {
         String mapName = deriveIndexMapName(indexDescriptor);
-        return nitriteStore.openMap(mapName, String.class, ArrayList.class);
+        return nitriteStore.openMap(mapName, String.class, CopyOnWriteArrayList.class);
     }
 
     @SuppressWarnings("unchecked")
@@ -168,7 +168,9 @@ private void addIndexElement(NitriteMap<String, List<?>> indexMap, FieldValues f
             List<NitriteId> nitriteIds = (List<NitriteId>) indexMap.get(word);
 
             if (nitriteIds == null) {
-                nitriteIds = new ArrayList<>();
+                // CopyOnWriteArrayList so in-place mutation is safe against MVStore's
+                // background page serializer (see NitriteIndex.addNitriteIds)
+                nitriteIds = new CopyOnWriteArrayList<>();
             }
 
             nitriteIds = addNitriteIds(nitriteIds, fieldValues);