"targetWriter" is a method parameter, and should not be used for synchronization.

Code Path:server/src/main/java/org/apache/iotdb/db/engine/compaction/writer/AbstractCompactionWriter.java

Synchronizing on a class field synchronizes not on the field itself, but on the object assigned to it. So synchronizing on a non-final field makes it possible for the field’s value to change while a thread is in a block synchronized on the old value. That would allow a second thread, synchronized on the new value, to enter the block at the same time.

The story is very similar for synchronizing on parameters; two different threads running the method in parallel could pass two different object instances in to the method as parameters, completely undermining the synchronization.

Start in server/src/main/java/org/apache/iotdb/db/engine/compaction/writer/AbstractCompactionWriter.java and inspect the method that uses targetWriter for synchronization. Confirm how concurrent calls can receive different parameter instances. Done means the synchronization no longer depends on that method parameter and the existing behavior remains safe for concurrent calls.

Written by the indexing model from the issue text.