Here is the motivation use case. We are implementing event-time alignment across sources in Iceberg source. Basically, each Iceberg source/enumerator tracks its watermark using min/max timestamps captures in the column stats of the data files.
When the watermark from another source advances, notified source/enumerator can try `assignSplits` as constraints may be satisfied now. This callback is initiated from the coordinator thread from the other source. If we have `SplitEnumeratorContext.execute(Runnable r)` API, we can ensure that all the actions by enumerator and assigner are serialized by the coordinator thread. That can avoid the need of locks.