Use copy-on-write list for TransactionOperations synchronizations

Expected Behavior

A beforeCommit transaction synchronization can register an afterCommit one.

Actual Behaviour

java.util.ConcurrentModificationException
        at java.base/java.util.ArrayList$Itr.checkForComodification(ArrayList.java:1096)
        at java.base/java.util.ArrayList$Itr.next(ArrayList.java:1050)
        at io.micronaut.transaction.impl.AbstractInternalTransaction.triggerBeforeCommit(AbstractInternalTransaction.java:78)
        at io.micronaut.transaction.support.AbstractTransactionOperations.commitInternal(AbstractTransactionOperations.java:358)
        at io.micronaut.transaction.support.AbstractTransactionOperations.executeTransactional(AbstractTransactionOperations.java:327)
        at io.micronaut.transaction.support.AbstractTransactionOperations.doExecute(AbstractTransactionOperations.java:126)
        at io.micronaut.transaction.support.AbstractPropagatedStatusTransactionOperations.execute(AbstractPropagatedStatusTransactionOperations.java:64)
        at io.micronaut.transaction.interceptor.TransactionalInterceptor.intercept(TransactionalInterceptor.java:142)
        at io.micronaut.aop.chain.MethodInterceptorChain.proceed(MethodInterceptorChain.java:142)
        at com.example.$HomeController$Definition$Intercepted.getUserCount(Unknown Source)

Small demo application: demo.zip

Steps To Reproduce

1. Have a transaction synchronization to register another in any of the overridden methods

Small demo application: demo.zip

Environment Information

• Linux
• JDK 25

Example Application

demo.zip

Version

5.1.3

Suggestion

May I suggesting using CopyOnWriteArrayList? Here is the entire diff, because it is small:

diff --git a/data-tx/src/main/java/io/micronaut/transaction/impl/AbstractInternalTransaction.java b/data-tx/src/main/java/io/micronaut/transaction/impl/AbstractInternalTransaction.java
index cf79855071..b4eedf6885 100644
--- a/data-tx/src/main/java/io/micronaut/transaction/impl/AbstractInternalTransaction.java
+++ b/data-tx/src/main/java/io/micronaut/transaction/impl/AbstractInternalTransaction.java
@@ -23,7 +23,7 @@ import io.micronaut.transaction.support.TransactionSynchronization;
 import org.jspecify.annotations.NonNull;
 import org.jspecify.annotations.Nullable;
 
-import java.util.ArrayList;
+import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.List;
 
 /**
@@ -142,7 +142,7 @@ public abstract class AbstractInternalTransaction<C> implements InternalTransact
     @Override
     public void registerInvocationSynchronization(@NonNull TransactionSynchronization synchronization) {
         if (synchronizations == null) {
-            synchronizations = new ArrayList<>(5);
+            synchronizations = new CopyOnWriteArrayList<>();
         }
         synchronizations.add(synchronization);
         OrderUtil.sort(synchronizations);

In a way this is a continuation of issue #3983

Start with data-tx/src/main/java/io/micronaut/transaction/impl/AbstractInternalTransaction.java and inspect how synchronizations are stored, iterated, and sorted. Reproduce the registration of an afterCommit synchronization from beforeCommit, then run the relevant data-tx tests or demo; done means registration during a callback no longer causes ConcurrentModificationException while synchronization ordering remains intact.