FLINK-12481, the timer triggers are executed by the mailbox thread and passed to the mailbox with the maximum priority.
In case of operators that use mailbox.yield() (introduced in
FLINK-13248), current approach may execute timer triggers that belong to an upstream operator. Such timer trigger, may potentially call processElement|Watermark() which eventually would come back to the current operator. This situation may be similar to FLINK-13063.
To avoid this, the proposal is to set mailbox letters priorities of timer triggers with the priority of the operator that the trigger belongs to.